1

Frames of Visibility in Public Places

Jean-Paul Thibaud Places. A Forum of Environmental Design. Vol. 14, n°1, 2001, pp. 42-47 A place is generally considered to be public when it is accessible to all, when every person can be physically present and circulate freely within it. Conversely, a place is considered to be private when access is controlled, reserved to certain people.

Yet physical access is simply one mode of access among others, since our body experiences space through each of its senses: sight, of course, but also hearing, touch and smell. A place can provide partial accessibility without the actual presence of one's body since « the actual senses which measure proximity, which qualify presence, are senses at a distance. »

1 For example, looking through an office window at what is happening in the street or listening to a conversation taking place in an adjacent room are potential modes of access to public places.

Public places can thus be characterized according to their degree of porosity, or according to the possibilities they offer for perceiving objects and people at a distance. Rather than considering the publicness of a place solely as a function of its architectural and spatial form or its degree of openness, it is appropriate to question a full range of sensory qualities of a place 2 .

Between Light and Sight

This paper is based on two fundamental assumptions. First, it is necessary to consider the ordinary experience of city dwellers. While urban planning tends to objectify places by being removed from them 3 , we are instead interested in the exercise of vision the way it occurs at the eye-level, in situ, for pedestrians, in their day-to-day practices. In this sense, the built environment organizes the various viewpoints of passers-by; it can be considered as a context that orients our ways of seeing and interacting in the street. As Deleuze put it: « If architectural structures, for example, are visible, places of visibility, it is because they are not only figures of stone, orderings of things and combinations of qualities, but first and foremost, forms of light which organize the clear and the obscure, the opaque and the transparent, the seen and the unseen, etc. » 4 .

Second, an interdisciplinary approach that involves both architecture and sociology is essential. Public space is simultaneously a built environment and a social setting. Analyzing the visual qualities of urban public space can improve our understanding of the relationship between spatial forms and social interactions. Interaction in public space requires the possibility of seeing, and being seen by, other people. In return, it requires rules of conduct that regulate the exchange of glances between passerby 5 .

On the other hand, buildings can increase or decrease the luminosity of places; they modify light by directing, reflecting, absorbing or bouncing it. Thus, the built environment conditions interpersonal observation, producing diverse circumstances of reduced, contrasted or hypertrophied visibility. How does the luminous dimension of the urban environment relate to the visual dimension of social interactions?

Frames of Visibility

Exposure is one of the basic categories for characterizing people's visual experience of each other in public. It involves being visible and observable by others, and behaving accordingly. Richard Sennett has argued that « fear of exposure » is the main problem of modern public space and that city dwellers have lost the ability to expose themselves and interact with each other 6 .

Although the idea of exposure helps to qualify the nature of interpersonal visibility in public, it is a generic term that fails to account for the full range of conditions that can occur in the luminous urban environment. The notion of frame of visibility can help to distinguish different types of exposure, specify the way that architecture conditions seeing in public, and better explain the relationships between light and sight in urban space.

A frame of visibility is a methodological device that helps describe the various luminous contexts in which interpersonal observation occurs. It enables us to specify the basic conditions in which people see and appear to each other. This notion focuses not on what people see in public places but how they see, depending on the place they are in. Each frame of visibility stands between two extreme cases that prevent any form of vision: complete brightness and complete darkness.

The five frames of visability that occur most frequently in the visual experience of city dwellers are overexposure, enclosure, filtering, blurring and silhouetting. These basic phenomena are not exclusive from each other, they sometimes combine or overlapp according to the spatial position of the observer, and nor is the list is exhaustive.

Overexposure

Overexposure involves a creating differentiation between and a hierarchy among objects in the visual world. It consists of increasing the visibility of a specific object, such as a monument or an individual. This frame of visibility displays passers-by, attracts their visual attention and points out what can or should be seen by anybody.

Such a phenomenon can be produced in two different ways:

-either by making use of contrast between lit space and obscure space (as for example at bus shelters, under awnings or in telephone booths at night), -or by offering a view from above of what is happening underneath (as, for example, at the terrace of the Rockefeller Center skating-rink or at the belvedere entrance of the Louvre Pyramid). In this kind of situation, people are more or less observable depending on where they are located.

Whatever interpretation we give to this phenomenon, it involves a splitting of status between the passers-by: in one case there are actors (in the « box » or below the terrace) and in another there are spectators (outside the « box » or on the terrace). These urban devices increase the awareness of being potentially watched and intensify the impression of being on stage. They tend to enforce the rules of conduct in public places as well as emphasize the scenic character of the place.

Enclosure

Enclosure involves the delimitation and fragmentation of what can been seen in the built environment. Its function is to structure and direct the visual field of passers-by, to shroud a portion of the place while revealing other parts and unifying what is visible. Enclosure both reveals and hides, depending on the spatial position of the observer. This phenomenon introduces a differentiation between areas that could be considered upstage and backstage.

Passageways, narrow streets without shops and subway corridors are places characterized by a strong sense of enclosure; they tend to orient people to what is directly in front of them by preventing views to the side. Places like these make people visible to each other for extended periods of time.

Windows and other types of penetrations in walls that offer restricted vistas of a place also emphasize enclosure. People look at each other as in a picture frame; they appear when they are situated in the frame and disappear when they walk out of it. Such a phenomenon brings people together for only a brief moment. It produces short glances between passers-by and a limited period of time of interpersonal observation.

Enclosure is possibly one of the most basic experiences of living in a city. It enables us to understand how the built environment conditions the way people temporarily relate to each other and offers specific views of the urban landscape.

Filtering

Filtering involves the quality and the propagation of light in the built environment. By passing through a physical milieu (such as glass or foliage), natural light can be refracted, absorbed or reflected. Filtering produces an ambiguous relationship between the inside and the outside, and creates various types of luminous surroundings depending on the weather, the season or the time of day.

This phenomenon occurs frequently in places with glass roofs, such as atriums or train stations, and along arcades or shaded paths. In places like these, the lighting of the place is neither completely bright nor totally obscure; instead, the light produces a mottled atmosphere. Such an impressionistic surrounding enhances and transfigures the shapes and the colors of the place.

This frame of visibility creates the sensation of bathlight or a luminous envelope shared by everybody. Such a diffuse light increases the coherence and the unity of the place. People located in this kind of surrounding feel physically bonded with the environment and can also sense the time passing by. The way people appear to each other is constantly changing, depending on the light and shade projected onto their own body.

Blurring

Blurring involves the reducing of visibility of people, making it difficult to perceive the contours and the shapes of objects and bodies. Such a frame relies primarily on a rather problematic relationship between the figure and the ground: the former tends to merge with the latter. Blurring emphasizes the dilution of the visible forms and limits the perception of depth.

This phenomenon occurs naturally in certain weather conditions, such as fog, mist or smog. Some urban waterfalls, tinted glass windows and other types of translucent screens interposed between people can produce a similar effect. In this case, the observer can barely identify the presence of someone else located at the opposite side of the screen. In places that are dimly lit at nighttime, passers-by can have difficulty distinguishing the location of obstacles.

Blurring enables people reduce their visual interaction with other, even to hide; it can be used as what Goffman called an « involvement shield ». A feeling of insecurity, mystery or surprise can result from this luminous context: the pedestrian cannot really anticipate what will be in his path. Such a frame may also be interpreted as a means to enhance the secretive character of a place and dramatize the experience of urban space.

Silhouetting

Silhouetting emphasizes the contour of objects or individuals instead of the details of their surfaces. This frame involves a particularly pronounced figure-ground relationship; it produces a clear differentiation between several juxtaposed planes or visual elements. Backlighting is the most common example.

The transition from a dark, artificially lit place to a bright, naturally lit place, such as the exit from an underground place into the daylight, is the most common context for experiencing silhouetting.

Such a frame reduces the visibility of people's faces, expecially their expressions, and tends to make passers-by anonymous, since visual recognition is difficult. However, the perception of the outline occurs only one way around, when the brighter area is in front of, not behind, the perceiver. Thus, this phenomenon involves a non-reciprocal visual, relationship between passers-by, a completely different experience depending on the location and orientation of the subject. From an architectural point of view, silhouetting makes it possible to strongly accentuate the transition between two places and clearly differentiate the foreground from the middleground and foreground.

Conclusion

The notion of frame of visibility is an attempt at linking the design of an urban space to the social relation that occur there. The aim of this paper was not to advance one particular frame of visibility to the detriment of others; it does not argue that any particular frame should be systematically sought out or avoided. Rather, my purpose was to point out some basic criteria that could be useful to architects and urban planners. Three main issues have been suggested:

The relationship between the built and the visible forms. In terms of visual perception, architecture is not merely a juxtaposition of buildings to be seen, it also helps establish the conditions of visual reception. For instance, the built space can open or block vistas (enclosure), offer a glimpse of specific objects or places (overexposure), emphasize their contours (silhouetting), dilute the visible forms (blurring) or transform them (filtering). Thus, one of the issues in the design of urban space is to consider the patterns of ambient light and the viewing conditions that buildings create.

The reciprocity of interpersonal observation. Architecture can be analyzed as a device that structures the way people relate to each other visually. Each frame of visibility mediates the way that people see each other: differentiation between actors and spectators (overexposure), short glances between passers-by (enclosure), creation of a shared luminous milieu (filtering), reduced mutual visibility (blurring), asymmetrical visibility between passers-by (silhouetting). The goal for architecture should be to incorporate and promote the « civility of the eye » in the design of public places.

The variability of the urban scenery. Too often, places or buildings are described as if they were always experienced the same way, as if they had no temporal dimension. Each frame of visibility emphasizes factors that can change in time and contribute to the visual diversity of a place: body orientations and spatial positions (overexposure, enclosure), weather conditions and time of the day or night (blurring, filtering), directions of walking and transitions from a place to another one (silhouetting). The goal for architecture should be to integrate the plurality of conditions into the singularity of a place, the temporal to the spatial dimensions of the urban environment.

Notes:

1 Mikel Dufrenne, L'oeil et l'oreille (Paris, Jean-Michel Place, 1991).

2 Grégoire Chelkoff & Jean-Paul Thibaud, « L'espace public, modes sensibles » in Les Annales de la recherche urbaine 57-58 (1993), 7-16.

3 Michel de Certeau, The Practice of Everyday Life (Berkeley: University of California Press, 1984). 4 Gilles Deleuze, Foucault (Paris: Minuit, 1986). 5 Erving Goffman, Behavior in Public Places (New York: The Free Press, 1963).

6 Richard Sennett, The Conscience of the Eye (New York: Knopf, 1990).


Jean-Paul Thibaud

Pascal Amphoux

Silencing the city?

The notion of silence must be handled very carefully. In addition, its use reveals the way we deal with the urban environment as well as social life. What does the notion of silence convey about the current state of the urban sonic environment? How can we clarify the various meanings and the stakes involved in silence? Three themes are developed in order to answer these questions: silence as a research topic presents three complementary perspectives (acoustic, sociocultural, technological); silence as a polysemous notion emphasises the ideas of keeping quiet, tranquillity and pause; silence as a design issue relies on basic properties and principles in order to orient the design of the urban sonic environment.

The complexity of the urban sonic environment is now recognised fairly generally and numerous developments in research have shown that:

-It can no longer be tackled without addressing the myriad factors and interactions at play and, in particular, without comparing and contrasting physical or acoustic with sociological or cultural data.

-It can no longer be tackled purely as a nuisance: the issues of annoyance and noise pollution are now being supplanted by reflections on sonic quality and aural comfort.

i We wish to deal with a number of pointers that may be used as an alternative to excessively simplistic arguments that tend to reduce silence to what it is not: a monovalent concept.

ii Certainly, silence is a very ambivalent notion which leads to various paths, sometimes contrasted or even opposed. On the one hand, silence is to be associated with death or boredom. Closely related to horror vacui of modern times, it involves a negative connotation. On the other hand, silence is used as a criterion of sonic quality denoting a positive value. We look for it and we appreciate it. François Michel (1986, p. 54) summarises this phenomenon: 'Ambivalence of the fundamental notions: silence, place of any blessings, or place of all the fears' (personal translation).

Obviously, silence is a notion that must be handled very carefully and its use reveals the way we deal with the urban environment as well as social life. What does the notion of silence convey about the current state of the urban sonic environment? How can we clarify the various meanings and the stakes involved in silence? Three themes will be developed in order to answer these questions:

-Silence as a research topic -Silence as a polysemous notion -Silence as a design issue

Silence as a research topic

The notion of silence is very often applied -at least implicitly -in research on the sonic urban environment. Hence, silence emerges as a significant and multivalent theme in current culture. Although delicate, it is situated at the crossroads of a set of very diverse issues: issues of well-being when it comes to setting up norms of acoustic comfort; issues of public health when it comes to stress or hearing disabilities; issues of economic stakes when it comes to estimating the value of real property situated in quiet zones; issues of sociability when it comes to solving problems of disputes between neighbours. The complexity of such questions requires learning the lessons of various scientific disciplines and perspectives.

A) We know that from an acoustic perspective the historic evolution of urban noise has witnessed extreme intensities before being reduced to 'average levels'. While there may be less noise now than in the late nineteenth century, at least in Western countries, we cannot say that there is more 'silence(s)'. For example, the reduction in high-intensity industrial noise (resulting from decentralisation, degrowth and regulation of the industrial sector) has been accompanied by a decline in areas or periods of silence (think of urban sprawl, peri-urbanisation, metropolisation and development of nocturnal activities).

At the same time, the discontinuousness and strongly rhythmical nature of yesterday's noises have been replaced by the continuity of contemporary noises and by what Gillo [START_REF] Dorfles | L'intervalle perdu[END_REF] has so aptly termed 'the loss of interval' (as illustrated by the prevalence of urban drone -loud and incessant background noise from traffic -the persistence of nocturnal movements and activities 24/7, or the relentless use of electrical or electronic equipment etc.).

So silence would appear to be a relevant issue in aural and urban planning as well as in the recognition of the acoustic identities of cities in terms of the low intensity or discontinuity that it reintroduces into the soundscape.

B) We also know that from a sociocultural perspective our recent heightened sensitivity to noise is an important phenomenon. There are many reasons for this, including the popularisation of ecology, the individualisation of society or more effective techniques for controlling noise production. These reasons have to be set against the emergence of a strong argument in the development of environmental policies, namely combating noise pollution. And an ideological trend highlighted in our research, tends -either implicitly or explicitly -to link the notion of silence to this argument.

In other words, we may contend that 'the right to silence' -an underlying legal notion harnessed by political authorities or associations to combat noise pollution iiiis progressively turning into a 'duty of silence', a social norm that is gradually creeping in on the back of the increasing individualisation of habits and lifestyles described by sociologists. The totalitarian threat of such an argument is based on a confusion and lumping together of hazy notions: public tranquillity, aural comfort and acoustic silence. So the manner in which silence is handled, either in institutional policy or in user behaviour, would appear to be a delicate area in the social management of the urban sonic environment.

C) Finally, from a technical and technological perspective we should not underestimate the economic and industrial riches at stake in the soundproofing market; take the emergence of new 'soundproofing' materials and techniques such as double glazing, sound barriers or active noise absorption. This is also borne out in a different way in changing advertising pitches in various different markets. 30 years ago, a powerful engine had a powerful roar ('Put a tiger in your engine!'). Today, power is much more low profile ('Mitsubishi, Silence, Power').

Here, once again, silence as a virtual objective of technical progress appears as an implicit argument around which a general consensus appears to be emerging, even though the proliferation and systematisation of such developments are likely to generate paradoxical side effects. Take the proliferation of sound barriers along motorways or large infrastructures that frequently have deleterious interruptive impacts on both social and urban life and the landscape on a European level. Whether in terms of the development of new technologies or spatial devices, the question of silence is a serious issue in the economic and technical management of the urban sonic environment.

These three major perspectives -acoustic, sociocultural, technological -make silence a contemporary topic and a complex research object that cannot merely be dealt with in a simplistic, mono-disciplinary or dualistic manner. Therefore, a dual representation that tends to treat silence as the opposite of noise, thus implicitly defining the former as the absence of the latter, is increasingly being used to analyse the urban soundscape. In other words, silence is perceived by default, and although it crystallises 'ideal' representations of the environment, it does so in both a monovalent and decontextualised way: monovalent in the sense that silence is defined only in relation to noise; decontextualised in the sense that silence per se is regarded as an absolute quantity. In other words, such a dualistic approach to representations appears to be an obstacle to achieving silence: Fighting against noise pollution may not be the same as fighting for silence.

Silence as a polysemous notion

Silence. The polysemous nature of the word evokes three overlapping notions. The first two are certainly the most obvious. They refer to the Latin distinction between taceo and sileo. As Roland Barthes (2002) showed in his course devoted to 'The Neutral', the word silence involves both the idea of keeping quiet, of absence of talk (tacere), and the idea of tranquillity, of absence of movement and noise (silere). The third notion -the pause -emphasises the temporal dimension of silence and its articulatory power. In this case, silence enables the unfolding process of the sonic environment: 'The time grants no privilege to sounds on the silences: they both have in common the dimension of duration' (Charles, 1978, p. 64, personal translation).

A) First and foremost, silence denotes the absence of noise, the state of a place in which no sound can be heard. The silence of the city under this first meaning indicates that the urban space tends towards an absence of any noise emissions. References to this type of silence suppose that extreme situations can be generated: by regulations (a 'zone of silence' near a hospital); by fantasy ('silent neighbourhoods' demanded by certain complainants); or by ritual (a minute's silence for victims of a disaster).

But this meaning also runs through more banal situations demanding 'peace' or 'tranquillity', and it is as if silence is a guarantor or even a catalyst for such notions. This would make the 'peace' of the city just a euphemism for silence in its dreamed-of radicality.

And yet we quickly realise that as soon as we start listening closely to the city, this absence of emanating sounds never really exists: 'Silence, like music, is non-existent.

There always are sounds' [START_REF] Cage | Silence[END_REF]. So instead of an absence, silence denotes the presence of 'almost no' noise, the state of a place in which the slightest sounds can be heard. Hearing one's own footsteps, the hum from somewhere near, or the tolling of a bell in another neighbourhood are diverse ways of perceiving urban space. By allowing different sounds -both near and not so near -to be heard, silences reveal the territorialising power of the sonic environment [START_REF] Augoyard | Répons pour voix discrètes et trois silences[END_REF].

So this first semantic interpretation is dominated by a spatial connotation and by the territorial dimension of silence. The issue is the absence of noise, which can generate feelings of either insecurity or public tranquillity, and this dimension may be linked primarily to topological principles for organising urban space: isolation, barriers, islands, sectioning, zoning etc.

However, this absence of noise is fundamentally relative insofar as these topological principles for organising the urban space cannot be treated as planning recipes or universal solutions. They need to be studied and designed in their own specific contexts in accordance with the aural scale that, in the case of the city, is determined by the relationship between near and far, here and there, and between the local and the global, etc.

B) The next meaning of silence is to say nothing -remaining speechless. Under this second meaning, the silence of the city signifies that the city has ceased 'speaking to us', that it has lost its identity or its 'aural signature'. The urban drone swallows up noises, empties them of their meaning and renders mundane the noises as well as the areas or the moments of silence that are supposed to counteract this.

Homogenisation, standardisation, repetition, normalisation and a continuous stream of noise and music; functionalisation and mediatisation of transmission or communication processes; jamming of any listening or reception create the condition for neutralising the expressive power of sounds [START_REF] Attali | Noise. The Political Economy of Music[END_REF]. Neither noisy nor silent, the sonic environment becomes mute.

And yet silence is not always synonymous with an absence of expression within this second meaning. Very often it helps to qualify everyday social situations. It can denote distrust or reserve, threat or complicity, prudence or connivance, depending on the situation; it all depends on how, when and in what circumstances silence occurs.

'Silence imposed', resulting from an order to be quiet, is supplemented by 'silence acted', which modulates relations with others. Silence becomes a manifestation of a way of being together. Hence, it is important not to equate silence with absence of communication. Following Watzlawick, Beavin and Jackson's theory of communication (1967), it is not possible not to communicate. Even silence, which could be interpreted as a refusal to communicate, is in itself a form of communication. Moreover, in the classical period, Abbot Dinouart (1771) suggested clarifying the principles of the art of keeping silent and speaking in company. Thus, instead of opposing silence and communication, it is more relevant to consider silence as a form of communication.

It is also possible to analyse the relationship between silence and political power.

The control of sound by power aims to limit speech and verbal expression. Everything must not be publicly expressed; it is a question of establishing diverse mechanisms settling the said and the unspoken. The analysis of the power of the Church and its evolution in the eighteenth century reveals various usages of silence:

In the distraught or trembling silence in front of God, substitutes itself gradually as an art to keep silent, to 'guard one's tongue' in good Christian form and in virtuous subject, as the civil practices become detached from religious behavior.

The momentum of the silent faith give way to an education of the 'virtues' by the Jesuits, by taking place deliberately in the field of the civil practices and by introducing 'civility', 'honesty' and 'duty of state' (Courtine & Haroche, 1987, p. 33, personal translation).

Censorship, which finds its extreme application in the Inquisition, condemns the excess of words. It is the golden age where the Church has the power to silence by strength. The rule of silence is explicitly ordered here. Then, with the development of the political strengths and the new philosophical currents which dispute the authority of the Church, we observe a movement which lessens the faith in customs. Civil practices are less influenced by religious behaviour. Silence, until then compulsory, is transformed into a code of civility. The citizen owes it respect, no longer in blind faith, but as testimony to knowing how to live in society. More recently, the legal texts concerning the ban on disturbance of peace at night explicitly mention the argument of law and order. Please note, with Pierre Sansot, that it is not so much the noise as such that is considered here, but its conditions of production and its meaning: 'On the pretext of taking care of the rest of the inhabitants, we shall ban, we shall pursue the "disturbance of the peace at night". The noise which is tolerated when it accompanies the effort of production is prohibited when it is a sign of enjoyment or tumult without purpose ' (1976, p. 23, personal translation).

To summarise, this second semantic level is dominated by the social connotation of silence. The issue here is mutism (communication or the absence of communication between people), or more specifically, types of mutism -or forms of communication between people. For Georg [START_REF] Simmel | The Sociology of Georg Simmel[END_REF] silence is a fundamental dimension of secrets and operates as a basic condition for society to function.

C) Lastly, silence has a third meaning: the interval of time during which sound is interrupted, suspended or pending. This time the silence of the city denotes that the activity will continue, i.e. that there are pauses and that these indicate key moments in the unfolding of the sonic urban environment. The individual is placed in a state of hyperesthesia insofar as impressions are accentuated and perceived much more acutely during the silent 'pause'.

But here again silence, understood in this third meaning, does not just belong to the here and now. It also encompasses a longer period that brings into play past memories and anticipation of the future. As the city rises in the morning, the background noise gets increasingly louder. Even though it is gone, the peace of the night is still there in people's ears and serves as a reference in relation to the changing ambiance they are experiencing. Conversely, the gradual disappearance of the sounds of human activity in the late afternoon announces a silence that is not yet there but can be sensed. Therefore, silence also triggers a 'movement of protention and retention' (Husserl, 1990) in a city's aural state.

This third semantic level is dominated by a temporal connotation. The issue is the pause, or more precisely, the dynamic that underpins the pause and which takes us into the realm of perception and aural aesthetics. And this dimension ultimately harks back to principles of composition and the temporal organisation of the urban sonic environment -permanence, cyclicality, discretion etc. In that sense, Brandon LaBelle accurately argues that 'silencing comes to perform a sort of domesticating arrest onto the dynamics of the social ' (2010, p. 47).

Silence as a design issue

Based on what we have just seen and by way of a conclusion, we may attribute three basic properties to the notion of silence that in different ways point out the determining role of the spatial, social or temporal context in the conception of silence.

But before completing this article, let us listen to Mikel Dufrenne (1991, p. 89) who very accurately emphasises the fact that silence is not a state but an ongoing process:

Certainly, we can wish for silence; against troublesome or aggressive noises, we try to protect ourselves, and the teacher asks its class for silence. But then we look for calm more than for silence: the lessening of a certain violence, the possibility of a finer listening. It also happens that at certain hours the world around us makes silence: when noon reign over the savanna, when the night reigns over the city, and when what tears away this peace is surprise and worries. But saying that silence is made, it is to recognize that it is not first, that it is in a way unnatural, as when we impose it or when we produce it artificially (Dufrenne, 1991, p. 89, personal translation).

How then can the notion of silence be clarified and help us to better design the sonic environment of cities?

The notion of silence is both relative and fictitious

It is relative because silence does not exist per se, but only in relation to a different level or type of sound, action or perception. And it is fictitious because there is no such thing as absolute silence (a total absence of external noise does not eliminate the enteropathic sounds made by the body) and because 'the perfect silence' also appears to be an unachievable pipe dream. Vladimir Jankélévitch refers to 'corridors of silence'

(coulisse de silence): 'As silence takes hold, tiny noises hidden away in the corridors of silence emerge and rise up out of the obscure depths ' (1978, p. 188, personal translation). In a way, silence brings people to face with themselves and it brings inhabitants' relationship to their soundscape into focus.

2. There is no one silence, but a diversity of silences

While political or institutional initiatives to combat noise pollution tend to assign silence a simplistic 'one-way' value (treating it as an ideal minimal acoustic level), we have tried to show how the uses, practices and representations of silence vary markedly depending on the social, spatial or temporal context in which it is analysed.

We need to counter the monosemous and simplistic treatment of the notion with a more polysemous analysis and, in a more concrete vein, to highlight the myriad ways that silence may be experienced in the city.

For example, if there is such a thing as 'empty' silence that simply means 'emptying' the noise of others (traffic or a neighbour) and keeping it at a distance from oneself or one's house, there must also be 'full' silences that offer multiple possible meanings and differential textures. Every silence has its own specific texture that needs to be teased out and described. The nocturnal silence of the city is not the same silence as the one found when the city is deserted during holidays. Silence may be sought after or feared; it may play a repellent role or trigger angst; it may also attract, etc.

Silence acts as a revealer of sonic identities

Silence is created not so much by the level or absence of noise, as by the change from one qualitative level to another (a change or break in intensity, continuity or direction). In other words, we need to distinguish between the object and the principle of silence. As we have seen, depending on how we analyse it, 'silence as object' may relate to a pause in a conversation ('communicational and social' silence), an absence of emanating noise ('spatial' silence) or discontinuity ('temporal' silence).

'Silence as principle', on the other hand, resides in the transition from one to the other. Take some examples: The silence of a crowd reveals the noises of the city; the

silence of the city reveals the noise of neighbours; or alternatively, 'social silence' reveals 'spatial silence', which may reveal 'temporal silence', and vice versa.

We may also use these basic properties of silence to propose certain principles for designing the urban sonic environment: 'relativity' (keeping a sense of proportion and balance), 'pluralising' and 'revealing'. However, none of these principles would be suitable or transposable from a regulatory perspective. The suitability of implementing them and the weight assigned to them may only be gauged in accordance with a project's specific context.

The principle of relativity assumes that we differentiate between different levels and textures of sonic space (inside a dwelling, at neighbourhood level or in the design of an urban itinerary), but it does not prejudge the sonic texture of each one.

In a similar vein, the principle of plurality assumes that we intervene not only in spatial configurations, but in the nature of social exchanges or the rhythm of urban activities. However, it does not prejudge what form these respective actions should take: creating protected areas, reintroducing courtyard-type designs, reinterpreting spatial forms ('pointed', 'elbow-shaped' or 'funnel-shaped') in street design, mixing or separating activities, temporary traffic bans, alternating certain urban activities, etc.

The revealing principle can only be implemented after a complex expert analysis of the potential that may be unlocked in the local, social and circumstantial context: Closing a street may generate new types of social interaction; reducing the level of population density may provide the possibility for a break during the day; regulating the duration at which public spaces are used may provide an opportunity for showcasing the city's existence. However, depending on the context, each of these actions could have the exact opposite effect. These three principles (relativity, plurality and the revealing principle) are not general solutions applicable in any situation; they are design tools which always need to be interpreted on the basis of the specific context.

iii In 1969 The International Music Council of UNESCO agreed on a resolution announcing the right to silence. Thanks to Jacob Kreutzfeldt for communicating us this information.


Bruno Lamas

Mathias L Richard

Harry Sokol
email: harry.sokol@aphp.fr

Harry Pr

Sokol

Caspase recruitment domain 9, microbiota and tryptophan metabolism: dangerous liaisons in Inflammatory bowel diseases

Keywords: CARD9, IBD, microbiota, tryptophan

come

INTRODUCTION

The gastro-intestinal tract is a complex milieu in which coexist host epithelial and immune cells, a wide array of microorganisms and molecules from food or microbiota metabolism.

A functional intestinal barrier is essential to maintain the balance between health and disease. This is accomplished by a tightly regulated process involving multiple cell types.

The single layer of epithelial cells forms a physical barrier between the intestinal lumen and the host's lamina propria (LP). Specialized intraepithelial lymphocytes (IELs), such as TCR γδ cells, are located between epithelial cells and represent a critical first line of defense. In the LP, a variety of immune cells are present (B cells, T cells, macrophages, dendritic cells, and innate lymphoid cells (ILCs)) that participate in the maintenance of intestinal homeostasis. The intestinal immune system plays a crucial role in shaping the microbiota so it can be tolerated by the host.

The human intestinal microbiota is a complex ecosystem composed of more than 10 11 microorganisms per gram of feces. It plays a crucial role in wide variety of vital functions such as digestion, immune system development, resistance against intestinal pathogens, etc. Indeed, germ-free (GF) mice have defects in production by ILC leading to production of antimicrobial peptides by intestinal epithelial cells which provide resistance for opportunistic pathogens such as Candida albicans [START_REF] Zelante | Tryptophan catabolites from microbiota engage aryl hydrocarbon receptor and balance mucosal reactivity via interleukin-22[END_REF].

In correlation, tryptophan-free diet in mice decreases the production of intestinal antimicrobial peptides that affects the gut microbiota composition and diversity, suggesting that tryptophan metabolism by the gut microbiota play an important role in mucosal homeostasis [START_REF] Hashimoto | ACE2 links amino acid malnutrition to microbial ecology and intestinal inflammation[END_REF]. Therefore, the complex bilateral interaction between the host and its microbiota has a crucial role in human health. In some circumstances, the tolerance toward the intestinal microbiota is broken, leading to inappropriate immune response and intestinal or extra-intestinal inflammation. This is the case in inflammatory bowel diseases (IBD).

Inflammatory bowel disease (IBD)

IBD include Crohn's disease (CD) and ulcerative colitis (UC) and are characterized by pathological inflammation of the digestive tract. In CD, inflammation affects the entire digestive tract while only the rectum and colon are affected in UC. These diseases have periods of active illness followed by periods of remission whose duration and frequency are patient dependant. IBDs appear most often between the age of 20 and 30 years old.

These pathologies modify for the long-term the health-related quality of life of patients and currently these chronic diseases do not have curative treatment. The highest occurrence of IBD is found in the developed countries of North America and Europe but IBD has also emerged in newly industrialized countries as well as Asia and South and Central America.

The pathogenesis of IBD is unknown but involves an activation of the gastro-intestinal immune system toward the gut microbiota in genetically susceptible hosts and under the influence of environment. As pointed out above, the composition of the intestinal microbiota affects the host as a whole. In recent studies, anti-or pro-inflammatory microorganisms have been identified: in the mice, segmented filamentous bacteria induce T helper 17 (Th17) cells in the small intestine which have pro-inflammatory effects [4*].

Other bacteria like Bacteroides fragilis [START_REF] Telesford | A commensal symbiotic factor derived from Bacteroides fragilis promotes human CD39(+)Foxp3(+) T cells and Treg function[END_REF] and Feacalibacterium prausnitzii [6,7*] have anti-inflammatory effects mediated by metabolites production and/or regulatory T cells recruitment. Thus, loss of the fragile equilibrium within this complex ecosystem termed dysbiosis often characterized by a decreased biodiversity, overgrowth of potentially harmful microorganisms and disappearance of protective ones can eventually trigger numerous pathologies, including IBD [8*,9*]. However, it is still unclear whether dysbiosis is a cause or a consequence of these diseases. In addition of the dysbiosis, genetic factors are also suspected in the IBD pathogenesis. More than 160 IBD susceptibility loci have been identified and among them several in the gene CARD9 [10*].

Caspase Recruitment Domain 9 (CARD9)

CARD9 is signaling protein that is highly expressed in myeloid cells and particularly in macrophages and dendritic cells. It is an adapter protein playing a central role for the integration of signals downstream of Pattern Recognition Receptors (PRRs). CARD9 play a major role in the sensing of mycobacteria, bacteria, fungi and virus via different receptors (Mincle, TLR, NOD2 and Dectin respectively). Downstream of CARD9, several pathways are involved including NF-kB and p38/JNK leading to cytokines production and participating to the elimination of the detected microorganisms. Card9 is thus a key adapter protein for innate immunity toward a wide range of microorganisms including many intestinal commensals and pathogens [11,12,13*,14]. We showed in a previous study that Card9 knockout mice (Card9 -/-) are more susceptible to dextran sulfate sodium (DSS)induced colitis which is used to mimic IBD in mice [START_REF] Sokol | Card9 mediates intestinal epithelial cell restitution, T-helper 17 responses, and control of bacterial infection in mice[END_REF]. Card9 -/-mice exhibited an impaired immune response with defective expression of IL-6, IL-17, IFN-γ and IL-22 in the colon [START_REF] Sokol | Card9 mediates intestinal epithelial cell restitution, T-helper 17 responses, and control of bacterial infection in mice[END_REF]. IL-22 is involved in mucosal wound healing and it mediates innate antimicrobial resistance in mice by inducing production of antimicrobial peptides, such as Reg3γ and Reg3β, by intestinal epithelial cells [15*,16]. Moreover, we have recently showed that intestinal epithelial cell proliferation is reduced and apoptosis is increased after DSS-induced colitis in Card9 -/-mice [17*]. In these mice, we also observed a decreased colonic expression of Reg3γ and Reg3β and an alteration of the bacterial and fungal microbiota [17*]. The major role of IL-22 and some of its target genes Reg3γ and Reg3β in the response to bacterial infections suggested that Card9 -/-gut microbiota might be deeply impacted.

Impact of the Card9 -/-microbiota in the susceptibility to colitis

Using the linear discriminant analysis effect size (LEfSe) pipeline, we observed several differences in the composition of the fecal bacterial microbiota at baseline in WT and Card9 -/-mice, including decreases in Adlercreutzia (genus), Allobaculum (genus), and Lactobacillus reuteri in Card9 -/-mice [17*]. These data confirmed that CARD9 plays an important role in shaping the microbiota but raises questions about the specific role of the microbiota in the hyper-susceptibility to colitis that is observed in Card9 -/-mice. To address this question, we colonized GF WT mice with the microbiota of WT (WT→GF) or Card9 - /-(Card9 -/-→GF) mice and exposed them to DSS. The transfer of Card9 -/-microbiota in GF WT mice was sufficient to recapitulate the phenotype observed in Card9 -/-mice. Indeed, we observed an increased susceptibility to colitis with decreased proliferation and increased apoptosis in intestinal epithelial cell of Card9 -/-→GF mice. Transcriptomic analysis in the colon showed also a down-expression of IL-22, Reg3γ and Reg3β in Card9 - /-→GF mice. In these animals, decreased production of IL-22 in the colon and in the mesenteric lymph nodes was also observed. No differences in IL-22 amounts were observed in splenocytes, suggesting a gut-limited defect in Card9 -/-→GF mice [17*].

Several sources of IL-22 have been identified in the gut, including ILCs, natural killer cells, T helper 17 (Th17) and Th22 cells, TCR γδ cells, and lymphoid tissue inducer (LTi) cells [15*,18]. Flow cytometry analysis of these different cells demonstrated that the microbiota of Card9 -/-mice is defective in inducing IL-22 production by Th22, NKp46 + ILCs, LTi cells, and CD3 -CD4 -NKp46 -cells in the colon, leading to increased susceptibility to colitis

[17*].

Tryptophan metabolism is impaired in Card9 -/-→GF mice

Recent data have suggested that tryptophan catabolites generated by the microbiota metabolism have a role in mucosal immune responses via the aryl hydrocarbon receptor (AhR) by modulating production of IL-22 [START_REF] Zelante | Tryptophan catabolites from microbiota engage aryl hydrocarbon receptor and balance mucosal reactivity via interleukin-22[END_REF][START_REF] Rutz | IL-22, not simply a Th17 cytokine[END_REF]. AhR is a ligand-activated nuclear transcription factor widely expressed in the body and evolutionarily conserved from invertebrates onwards [START_REF] Stockinger | The aryl hydrocarbon receptor: multitasking in the immune system[END_REF]. In the absence of a ligand, AhR is retained in an inactive complex in the cytoplasm but after ligand binding it translocates into the nucleus where it binds genomic regions containing the dioxin response element (DRE) that regulates expression of target genes. For many years AhR was almost exclusively studied for its role in mediating the toxicity of xenobiotics, and notably of a side product in industrial organic synthesis of herbicides, 2,3,7,8-tetrachlorodibenzo-p-dioxin. Recently, AhR signaling has been shown to be a key player of the immune response, particularly at barrier sites such as →GF mice. Using mice hepatoma cell lines containing a stably integrated DRE-driven firefly luciferase reporter plasmid we found that feces from Card9 -/-→GF mice were defective in their ability to activate AHR, to a similar extent as that for feces from GF mice [17*]. Lactobacillus reuteri can catabolize tryptophan to indole derivatives with AhR agonistic activity and there is preliminary evidence that other species can also produce AhR agonists via unknown pathways [START_REF] Zelante | Tryptophan catabolites from microbiota engage aryl hydrocarbon receptor and balance mucosal reactivity via interleukin-22[END_REF]. In line with these data, culture supernatants of L. reuteri and Allobaculum sp., two bacteria with decreased abundance in the Card9 -/-mice microbiota, strongly activate AhR [17*]. These results suggested that impaired tryptophan metabolism by the microbiota of the Card9 -/-mice could be associated with, or even be responsible for, the hypersusceptibility of Card9 -/-mice to colitis. In WT mice, bacterial microbiota analysis allowed us to isolate several strains producing high amounts of AhR ligands including three strains of Lactobacillus (L. murinus, L. reuteri and L. taiwanensis).

In Card9 -/-→GF mice, susceptibility of colitis, and IL-22 defect were rescued after treatment with AHR agonist (6-formylindolo(3,2-b)carbazole), or inoculation with the three Lactobacillus strains with strong AHR agonist activity. These effects were mediated by AHR, as they were abrogated in the presence of an AHR antagonist (CH223191) [17*].

These results showed that impaired tryptophan metabolism in the microbiota of the Card9 - /-mice lead to defective AhR activation which contributes to the susceptibility of mice to colitis by reduction of IL-22 production.

Impaired AhR activity and tryptophan metabolites in patients with IBD

To determine whether these findings were relevant to human disease, we analyzed fecal samples from individuals with IBD and healthy subjects for their ability to activate AHR.

Impaired AhR activity associated with decreased concentrations of IAA and tryptophan were observed in feces of patients with IBD. These patients were also genotyped for an IBD-associated single-nucleotide polymorphism (SNP) within CARD9 (rs10781499) [22] . We observed that the risk allele was associated with reduced AhR activation by microbiota-derived metabolites extracted from fecal samples. No association was observed among other major IBD-associated SNP, including those in NOD2 [17*]. These results suggest a connection between IBD, CARD9, and the ability of the microbiota to produce AHR agonists in humans.

Conclusion

These results in human should be confirmed in an independent cohort but indole derivatives, or the microorganisms that produce them, could be used as a supportive therapy in individuals with intestinal dysbiosis to override the defect of tryptophan metabolism by the gut microbiota. In a more general concept, our findings support the notion that abnormal innate immunity, such as defect in CARD9, can shape an altered microbiota, which can then modify the host immune response and amplify the dysbiosis in a vicious cycle leading to the loss of intestinal homeostasis. Therefore, the respective roles of the host factors and the gut microbiota in IBD pathogenesis cannot be completely distinguished. Thus, dysbiosis should neither be considered a cause nor a consequence of IBD, but both simultaneously.

Key points

 Alteration of intestinal microbiota is observed in IBD.

 CARD9 is one of the identified IBD susceptibility genes.

 CARD9 has a role in shaping the bacterial and fungal gut microbiota and it is required for the production of AhR ligands by the microbiota.

 Impaired ability of the microbiota to catabolize tryptophan into AhR ligands increased sensitivity to colitis by altering the IL-22 signaling pathway.

 Impaired microbial production of AHR ligands is observed in patients with IBD and correlates with an IBD-associated SNP within CARD9.

  intestinal immune system development and function, including in development of gut-associated lymphoid tissue and production of Immunoglobulin A [1*]. Moreover, metabolites derived from commensal bacteria, as the short-chain fatty acids (SCFA), the polyamines and the aryl hydrocarbon receptor (AhR) ligands may regulate immune cells functions via indirect and direct mechanisms [1*]. Lactobacilli spp. can metabolize dietary tryptophan in AhR ligands that induce IL-22

  the skin and the intestinal mucosa [20,21**]. Tryptophan can be metabolized either by the gut bacteria into indole derivatives, such as indole-3-acetic acid (IAA) or by host cells into kynurenine (Kyn) via indoleamine 2,3-dioxygenase 1 (IDO1) [2,21**]. Indole derivatives are AhR ligands known to promote local IL-22 production by immune cells [1*,21**]. To determine whether the modulation of AhR activation by the intestinal microbiota is involved in the decreased IL-22 production in our model, we determined the concentration of AhR ligands in the feces of our mice. Production of IAA was impaired in feces of Card9 - /-

Acknowledgements

We thank the members of the ANAXEM germ-free platform, the members of the animal facilities of INRA, and T. Ledent of the animal facilities of Saint-Antoine Hospital for their assistance in mouse care; M. Moroldo and J. Lecardonnel from the CRB GADIE core facility for technical assistance in performing the microarray analyses; S. Dumont for technical help in histology and immunochemistry; and C. Aubry, N. M. Breyner, F. Chain, S. Le Guin, C. Cherbuy, N. Lapaque, S. Taleb, B. Ryffel and D. Skurnik for fruitful discussions and technical help. We also thank E. Drouet and the Clinical Research Assistant team of Unité de Recherche Clinique de l'Est Parisien for their help in obtaining samples from patients with IBD. The cell reporter system to determined AhR activity was provided by Michael S. Denison (University of California, Davis, CA, USA). Financial Support and Sponsorship Funding was provided by Equipe ATIP-Avenir 2012, INSERM-ITMO SP 2013 and ECCO grant 2012. HS is an awardee of the European Research Council (ERC-2016-StG-715776).

Conflicts of Interest

None.


A Jeantet

Y Chassagneux

T Claude

P Roussignol

J S Lauret

J Reichel

C Voisin

Exploiting one-dimensional exciton-phonon coupling for tunable and efficient single-photon generation with a carbon nanotube

Condensed-matter emitters offer enriched cavity quantum electrodynamical effects due to the coupling to external degrees of freedom. In the case of carbon nanotubes a very peculiar coupling between localized excitons and the one-dimensional acoustic phonon modes can be achieved, which gives rise to pronounced phonon wings in the luminescence spectrum. By coupling an individual nanotube to a tunable optical micro-cavity, we show that this peculiar exciton-phonon coupling is a valuable resource to enlarge the tuning range of the single-photon source while keeping an excellent exciton-photon coupling efficiency and spectral purity. Using the unique flexibility of our scanning fiber cavity, we are able to measure the efficiency spectrum of the very same nanotube in the Purcell regime for several mode volumes. Whereas this efficiency spectrum looks very much like the freespace luminescence spectrum when the Purcell factor is small (large mode volume), we show that the deformation of this spectrum at lower mode volumes can be traced back to the strength of the exciton-photon coupling. It shows an enhanced efficiency on the red wing that arises from the asymmetry of the incoherent energy exchange processes between the exciton and the cavity. This allows us to obtain a tuning range up to several hundred times the spectral width of the source.

Light-matter interaction can be controlled at the ultimate level where a single quantum emitter interacts with a single photon by means of optical micro-cavities [1]. The small mode volumes and high quality factors achieved in such cavities provide a new means to tailor the optical properties of the emitter through the so-called cavity quantum electrodynamical (CQED) effects. In the weak coupling regime where the losses remain larger than the interaction strength between the emitter and the electromagnetic mode, it is possible to control the spontaneous emission rate of the emitter and to funnel efficiently the photons into a single photonic mode by reshaping the local density of states (Purcell effect [2]). This effect has been demonstrated with a number of solid-state emitters [3] and more recently with carbon nanotubes [4][5][6]. In this regime, a figure of merit of CQED effects is the Purcell factor F p defined as the ratio of the radiative emission rate into the cavity mode to the radiative rate in free-space. In addition, in the case of a broad emitter, the Purcell effect also allows to reshape the spectral properties of the emitter through the so-called cavity feeding effect. In particular for solid-state emitters, the environment induced broadening is a new resource that can be exploited to enhance the tuning range of the photon source, while keeping a narrow spectral output and large efficiency [7]. This approach has been implemented in quantum dots where local electrostatic fluctuations and the three dimensional phonons of the matrix induce some level of pure dephasing of the excitonic line [8]. Nevertheless, this effect remains limited to an energy window of the order of a few cavity line-widths ( 800µeV). In carbon nanotubes at low temperature the situation is radically different because of the coupling of localized excitons to the truly one-dimensional acoustic phonon bath. This gives rise to a diverging effective exciton-phonon coupling for low-energy vibrational modes resulting in phonon wings extending over several meV even though the zero-phonon line (ZPL) is typically 10 to 100 times narrower [9][10][11][12]. As first noted by Galland et al. the strong asymmetry of the wings is the signature of non-Markovian decoherence processes due to the one-dimensional confinement of acoustic phonons [9]. In addition, this dephasing is to some extent adjustable by means of proper nanotube/matrix coupling resulting in variable exciton and phonon localization lengths [10]. When the nanotube is coupled to a tunable micro-cavity, this spectral broadening enlarges considerably the tuning range of the photon source while keeping excellent anti-bunching properties [5].

In this work, we show that this peculiar excitonphonon coupling results in an asymmetric incoherent energy exchange between the cavity mode and the emitter. This enables an enhanced cavity feeding effect on the red wing, possibly by-passing the intrinsic limit for the efficiency of the source. We investigate this effect ex-perimentally by scanning continuously the working frequency of a fiber micro-cavity coupled to a nanotube, thereby measuring the single-photon generation spectral efficiency. Beside a globally enhanced brightness, we show that the spectral efficiency of the source at lower mode volumes does not map out the free-space PL spectrum of the emitter anymore. In particular, the efficiency of the source on the red phonon wing is more enhanced than at the ZPL. This effect is interpreted quantitatively in the framework of the Jaynes-Cummings model where the exciton-phonon coupling is described in the spinboson approximation [13][14][15][16] with an effective phonon mode taking into account the coupling to all acoustic branches [17].

The experimental setup was described in Ref. [5]. In brief, PFO coated CoMoCat nanotubes embedded in a 120 nm thick layer of polystyrene [18] are deposited on a flat dielectric mirror. All the measurements are conducted at 15 K. The individual nanotubes are selected by means of a micro-photoluminescence (PL) setup using a cw excitation wavelength near 800 nm. After a full free-space characterization the nanotube is inserted into a micro-cavity by approaching the top mirror (radius of curvature of ∼ 10µm) which is engineered at the apex of an optical fiber by laser ablation [19]. The fiber is mounted on a three-dimensional piezo stage in order to adjust the spatial mode matching and the cavity length. The luminescence collected from the cavity or from the PL microscope (free-space emission) is dispersed in a 500 mm spectrometer and detected with a nitrogen-cooled CCD camera.

The free-space luminescence spectrum of a carbon nanotube at 15 K is presented in Fig. 1 (a). Its characteristic profile consists of a sharp ZPL together with a low intensity blue wing and a higher intensity red wing. This profile is well understood in the framework of onedimensional acoustic phonon side bands of a localized exciton [10][11][12]. As a consequence of phonon confinement in PFO-wrapped nanotubes, the ZPL becomes largely decoupled from the phonon wings [10,12].

When phonon side-bands represent a sizable fraction of the transition oscillator strength, the CQED effects modify the recombination dynamics both for the ZPL and for the sidebands, which makes it possible to tune the source over a broad spectral range with a large efficiency (Fig. 1 (b)). We checked that the anti-bunching level remains below 0.03±0.02 both at the ZPL frequency and on the phonon wings (Fig. 1 (c-d)). In addition this antibunching is very robust to an increased pumping rate since it remains below a few percents up to a pumping intensity of the order of the saturation intensity.

In order to quantify the strength of the exciton-photon coupling in the cavity feeding regime, we traced back the coupling strength hg to the dependence of the efficiency of the photon source on the detuning hω cav -hω ZP L . This coupling g (equal to half the vacuum Rabi split-

(a) (b) (c) (d) time (ns) g (2)
FIG. 1. (a) Free-space PL spectrum of a single carbon nanotube recorded at 15 K for a cw excitation at λ =800 nm. (b) Spectra of the single-photon source obtained with this nanotube for several cavity detunings showing the high spectral purity and high efficiency of the single-photon source on a broad tuning range. The excitation density is kept constant for all the spectra. (c) and (d) Intensity correlation functions g (2) (t) measured under pulsed excitation for a nanotube coupled to a cavity in resonance with the ZPL (c) or in resonance with the red phonon wing at a detuning of 5 meV (d). (For technical reasons, these data were not recorded on the nanotube studied in the rest of the paper and correspond to the data published in [5].)

ting) corresponds to the interaction energy between the transition dipole and the electrical field of the zero-point quantum fluctuations of the optical mode. The singlephoton source efficiency β(f ) (where f is the optical frequency) is defined as the probability for a photon to be emitted by the cavity for each single excitation launched into the nanotube. For a given excitation density, this efficiency is proportional to the output intensity. In fact, for a fixed cavity length, the output of the source I(f ) is a narrow line as shown in Fig. 1 (b) which simply reads

I(f ) ∝ S cav fc (f ) • β(f c )
, where S cav is the normalized Lorentzian profile of the cavity centered at f c . In order to get the variation of the efficiency with the detuning, we scan the cavity length continuously at a frequency Ω = 25 Hz and record the time-average spectrum of the output (Fig. 2 (a)). The cavity scanning range spans a window slightly broader than the nanotube emission spectrum (Fig. 2 (b)) so that the dwell time of the cavity is constant all over the spectrum. A strong enhancement of the signal is observed at resonance with the ZPL, whereas the intensity remains sizable when the cavity probes the phonon side bands (Fig. 2 (c)). Note that the full scanning window ( 20 meV) corresponds to a relative change in the cavity length of 1.4% and thus to a change in the mode volume of about 2%, which is negligible (see SI in [5]). This efficiency spectrum can also be measured for several values of the mode volume by changing the length of the cavity by steps of λ/2, i.e. by changing the longitudinal mode order. Overall, the mode volume can be varied by at least of factor of 2. Basically, the output intensity increases when decreasing the mode volume as expected from the Purcell effect (Fig. 3 (a)). However, the spectra are not homothetic to each other and the intensity of the phonon wings grows at a faster pace than that of the ZPL when decreasing the mode volume (Fig. 3 (b)).

. (a)

A nanotube is in the cavity formed by a planar mirror and a concave mirror engineered at the tip of an optical fiber. The cavity length oscillates at frequency Ω to cover the full nanotube spectrum. (b) Sketch of the spectrum of the source for selected detunings (e.g. solid red line when the cavity is in resonance with the ZPL). The dotted line represents the efficiency spectrum as obtained from the envelope of the spectra when the detuning is scanned continuously (see main text). (c) Experimental efficiency spectrum (blue dots) for the lowest mode volume together with the normalized free-space spectrum (dashed black) of the same nanotube and a fit to the data (solid red) according to eq.( 1). Note that the data from all the figures 2-5 were taken on this same nanotube, as well as the values discussed in the main text.

This behavior can be captured quantitatively by a generalized Jaynes Cummings model where the coupling to the phonon bath is treated in the spin-boson framework within the NIBA approximation [16]. In this approach, an elementary excitation is launched in the exciton population and the efficiency of the source is calculated as the time-integrated output of the cavity mode. The outcome of the model is that the efficiency of the source reads :

β(f cav ) = g 2 κ Sem (f cav ) g 2 κ Sem (f cav ) + g 2 γ Sabs (f cav ) + κγ (1)
where κ stands for the cavity decay rate, γ for the exciton free-space recombination rate, Sem (f ) and Sabs (f ) are normalized ( S(f )df ≡ 1) emission and absorption free-space spectra convoluted by the normalized Lorentzian cavity mode profile. Note that when the cavity mode is much narrower than the ZPL, the convolution can be omitted and Sem (f ) represents the probability for the free-space radiative recombination to occur within a frequency band df around f when one exciton is launched in the nanotube. 1) and the data of panel (d). The solid blue line is a guide to the eye. (c) Purcell factor of an individual nanotube coupled to a micro-cavity tuned at the ZPL frequency for several cavity lengths, extracted from lifetime measurements (eq. ( 6)). The blue dashed line shows the 1/V behavior. (d) Coupling factor gZP L for different mode volumes, extracted from lifetime measurements (red crosses) and from the fit of the efficiency spectrum according to eq. ( 1) (black open dots). The dashed blue line shows the 1/V behavior. The mode volume is deduced from the cavity free spectral range (FSR) [5].

Although this formula is established in a rigorous way within the spin-boson model [16], it is enlightening to infer it in a heuristic way by considering incoherent energy exchanges between the emitter and the cavity mode (Fig. 4). In this picture, one can show that when the cavity is detuned, the energy exchange rate becomes g 2 Sem for the emission process whereas it becomes g 2 Sabs for the absorption process (SI). As a consequence, the total decay rate of the emitter becomes γ + g 2 Sem whereas the total decay rate of the cavity mode becomes κ + g 2 Sabs .

Hence, the probability for a photon in the cavity mode to reach the detector reads :

p cav→det = κ/[κ + g 2 Sabs ]
(2)

the probability for the photon to be reabsorbed by the emitter reads :

p cav→N T = g 2 Sabs /[κ + g 2 Sabs ]
(3)

and the probability for the excited emitter to emit into the cavity mode reads :

p N T →cav = g 2 Sem /[γ + g 2 Sem ] (4)
Finally, the total probability for a photon to be emitted by the source when a single excitation is launched in the emitter (which is the above defined efficiency of the source) reads : (5) which yields eq. ( 1). This heuristic approach shows that the combined effect of the exciton-phonon and exciton-photon couplings boils down to asymmetric energy exchanges between the cavity and the nanotube. In addition, the full derivation shows that this conclusion remains valid even when the coupling g becomes larger than the loss rates γ and κ (strong coupling regime in a two-level picture) [16].

β = p N T →det = p N T →cav [1 + p cav→N T p N T →cav + ... + (p cav→N T p N T →cav ) n ]p cav→det
The emission spectrum Sem can be directly obtained from the measurement of the free-space photoluminescence spectrum. The absorption spectrum of the nanotube can be deduced from a mirror symmetry with respect to the ZPL as is well known for emitters showing Huang-Rhys phonon coupling profiles [START_REF] Henderson | Optical spectroscopy of inorganic solids[END_REF].

Nanotube Cavity

FIG. 4. Heuristic model of the dynamics of the exciton/cavity/phonon system based on incoherent energy exchanges between the emitter and the cavity.

Figure 2 (c) shows the efficiency spectrum measured for the smallest mode volume of ∼ 2λ 3 obtained for a cavity length of 6λ/2 together with the free-space emission spectrum normalized to the same ZPL intensity. Obviously, the relative efficiency of the single-photon source at the phonon wings is boosted by the cavity as compared to the intensity at the ZPL frequency. This behavior is quantitatively reproduced by the model as shown by the red line which is a fit according to eq. ( 1) with a single free parameter, the coupling strength g (all the other parameters : loss rates, emission profile,... are deduced from independent measurements, see SI). In addition, the ratio of the efficiency of the source at the red phonon wing maximum over the one at the ZPL shows a dependence as a function of the cavity length (Fig. 3 (b)) which is well accounted for by eq. ( 1), using the fact that g 2 ∝ 1/V . We obtain values ranging from hg = 23 ± 4µeV to hg = 40 ± 4µeV for the largest and smallest mode volumes respectively (obtained for longitudinal mode orders ranging from 6 to 10).

Interestingly, these values of g can be benchmarked to those deduced from time-resolved photo-luminescence. In this case, we focus on measurements done at the ZPL. Using pulsed excitation and a fast photon counting module, we obtain the life-time τ of the exciton both in freespace and in the cavity for several mode volumes (SI). The change of lifetime ∆τ gives access to the effective Purcell factor F p provided that the PL quantum yield η of the emitter is known [START_REF] Faraon | [END_REF] :

F p = ∆τ ητ (6)
In the present case represents the effective quantum yield of the ZPL which includes the Debye-Waller factor. It is obtained from saturation measurements in pulsed excitation [5]. As expected, the Purcell factors we deduce from this method follow a 1/V law (Fig. 3 (c)), where V is the mode volume, in agreement with the Purcell formula F p = 3/(4π 2 )(λ/n) 3 Q e /V where λ is the emission wavelength, n is the optical index and Q e is an effective quality factor such as

Q -1 e = Q -1 cav + Q -1
ZP L [22]. Notably, these values of F p , up to F p = 60 are much larger than our previous data (of the order of 5 to 10) [5]. This arises from the much reduced radius of curvature of the top mirror leading to a mode-volume reduction of the order of 5 in line with the observed increase of F p . Such high values of F p are particularly valuable for a dim emitter like carbon nanotubes (that show an intrinsic quantum yield η of the order of 1%) because it directly translates into an enhanced brightness of the source even for a pumping intensity well below the saturation. Here, we infer that the effective quantum yield of the nanotube coupled to the cavity amounts up to ηF p /(1 + ηF p ) ∼ 40% ± 5, which is in qualitative agreement with the values deduced from the fitting of the efficiency spectrum with eq. ( 1). Given the short life-time of nanotubes (100 ps or below) and the nearly perfect extraction efficiency, we anticipate that the single-photon generation rate could reach up to several GHz. This cavity induced reduction of life-time is a direct consequence of the strength of the exciton-photon coupling g ZP L . In a simple Lorentzian picture, which is reasonably applicable for the ZPL, the coupling g ZP L can be traced back to the change of lifetime through g ZP L = 1/2 (1/τ cav -1/τ fs )ω ZP L /Q e , where τ cav (resp. τ fs ) stands for the lifetime of the emitter coupled to the cavity tuned at the angular frequency ω ZP L (resp. the freespace lifetime). Interestingly, g ZP L can be measured experimentally with higher accuracy than F p since it does not rely on any calibration of the setup detection efficiency (needed to assess the quantum efficiency used in ( 6)). Finally, the coupling deduced from this life-time measurements can be related to g deduced from the fitting of the efficiency spectra using g ZP L = g * √ DW , where DW = 0.4 is the Debye-Waller factor (SI). The two approaches agree within the error bars as shown in Fig. 3 (d). It is interesting to examine the asymptotic behavior of the system when the coupling strength g is increased (eq. ( 1)). The efficiency spectrum becomes more and more distorted, the red phonon wing becoming eventually higher than the ZPL (Fig. 5). This striking effect arises from the asymmetry between the absorption and emission processes on the red phonon wing at low temperature, the former being much less likely than the latter on the red side. Thus, the nanotube cavity coupling becomes favorably asymmetric with enhanced emission probability of the nanotube into the cavity mode and reduced reabsorption probability. This allows to bypass the natural limit κ/(κ + γ) observed at the ZPL where both processes are equiprobable. For hg values of the order of 40 µeV and for nanotubes showing a ZPL width of the order of 900 µeV the tuning range of the source can already reach up to 10 meV (2.5 THz). Using a stabilized cavity, the source width (itself set by the cavity mode width) can be reduced down to below 10 GHz [19]. Thus, the tuning range represents up to several hundred times the spectral width of the source, opening the way to widerange multiplexing in quantum communications. This is in contrast to the plasmonic approach where the large tuning range is obtained at the expense of the brightness [23].

In conclusion we have shown that the peculiar excitonphonon coupling in carbon nanotubes is a valuable resource to expand the bandwidth of a single-photon source. The most original feature of this system is the asymmetry in the energy exchange processes that allows to bypass the intrinsic limit for the efficiency of the source and ultimately to reach a close to 100% yield, provided that the coupling is strong enough. This could be achieved in future integrated devices using photonic crystal resonators that have much reduced mode volumes. In addition, the use of carbon nanotubes of diameters of the order of 1.1 nm or of functionalized carbon nanotubes would bring the single-photon source in the telecom bands [24]. The indistinguishability of our source is currently estimated to 0.25 (see SI), but could be sensibly increased by tuning the coupling and finesse of the cavity [25] thus paving the way to the tunable generation of indistinguishable single photons using carbon nanotubes.

  FIG. 2. (a)A nanotube is in the cavity formed by a planar mirror and a concave mirror engineered at the tip of an optical fiber. The cavity length oscillates at frequency Ω to cover the full nanotube spectrum. (b) Sketch of the spectrum of the source for selected detunings (e.g. solid red line when the cavity is in resonance with the ZPL). The dotted line represents the efficiency spectrum as obtained from the envelope of the spectra when the detuning is scanned continuously (see main text). (c) Experimental efficiency spectrum (blue dots) for the lowest mode volume together with the normalized free-space spectrum (dashed black) of the same nanotube and a fit to the data (solid red) according to eq.(1). Note that the data from all the figures 2-5 were taken on this same nanotube, as well as the values discussed in the main text.

FIG. 3 .

 3 FIG. 3. (a) Evolution of the spectral efficiency of the same emitter for selected mode volumes labeled by longitudinal mode number (zoom on the red phonon wing). (b) Ratio of the intensity at the maximum of the red wing to the maximum of the ZPL as a function of the longitudinal mode number (red dots). Theoretical values (blue open dots) as obtained from eq. (1) and the data of panel (d). The solid blue line is a guide to the eye. (c) Purcell factor of an individual nanotube coupled to a micro-cavity tuned at the ZPL frequency for several cavity lengths, extracted from lifetime measurements (eq. (6)). The blue dashed line shows the 1/V behavior. (d) Coupling factor gZP L for different mode volumes, extracted from lifetime measurements (red crosses) and from the fit of the efficiency spectrum according to eq. (1) (black open dots). The dashed blue line shows the 1/V behavior. The mode volume is deduced from the cavity free spectral range (FSR)[5].

FIG. 5 .

 5 FIG.5. Simulated evolution of the efficiency (for different values of the coupling parameter g) as a function of the cavity detuning δ = ω -ωZP L expressed in units of the cavity linewidth κ. This representation shows readily the remarkable tuning capabilities of the source. The red line corresponds to the asymptotic efficiency β ∞ while the dashed black one corresponds to the intrinsic limit for markovian decoherence.

This work was supported by the C'Nano IdF grant "ECOQ" and the ANR grant "NC2". JSL is partly funded by "Institut Universitaire de France". We thank A. Leclercq for the mechanical fabrication, N. Izard for helping in sample preparation.