Atomic vapor chamber and configuration of atomic optical clock implementing said atomic vapor chamber

Abstract

A system (30) for controlling, regulating or authenticating a timer comprises means (31) for measuring the frequency signal and / or rate of one or more timers based on a time reference provided by an optical atomic clock (10) comprising an atomic vapor chamber (1), the atomic vapor chamber (1) comprises a hermetically sealed enclosure (8) defining a volume containing a vapor of a reference atom (2), the hermetically sealed enclosure (8) comprising an optical inlet (3) allowing transmission of a probe beam adapted to excite an optical transition of the reference atom (2) and an optical outlet (4) allowing transmission of a fluorescence signal from the reference atom (2), the hermetically sealed housing (8) further comprises a wall (5) through which the fluorescent signal can penetrate, said wall (5) being coated on its outer side with a coating (6) which is reflective to the fluorescent signal.

Description

Technical Field

[0001] This invention belongs to the field of optical atomic clocks. More specifically, the invention relates to a vapor chamber configured for optimal collection of fluorescence emitted by the vapor of reference atoms contained within the chamber. The invention also relates to a method of manufacturing the chamber and a configuration of an optical atomic clock using the vapor chamber as an atomic frequency reference. Furthermore, the invention relates to a system for controlling, adjusting, or authenticating a timer. Background Technology

[0002] Atomic clocks are highly stable frequency standards and are increasingly used in applications such as navigation, telecommunications, and high-end scientific instruments. Unlike first-generation microwave atomic clocks, optical atomic clocks use atomic optical transitions as a frequency reference.

[0003] Several schemes for optical atomic clocks based on two-photon transitions have been proposed. Advantageously, the two-photon transitions can be observed via a fluorescence signal that can be separated from the probe laser spectrum. The detection of the two-photon transitions can also be advantageously configured in a Doppler-free scheme, thereby producing a narrow absorption band with high-frequency accuracy.

[0004] An example of a two-photon optical atomic clock can be found in the following scientific publication by Perrella et al.: “Dichroic Two-Photon Rubidium Frequency Standard”, PHYSICAL REVIEW APPLIED 12,054063 (2019) (hereinafter referred to as “Perrella”). This paper discloses a two-photon scheme based on monitoring the fluorescence of rubidium atoms, which is generated by the two-photon absorption of two probe lasers of different wavelengths in this case. In this paper, the authors clearly reveal a common problem encountered in this type of scheme: the relatively low intensity of the collected fluorescence signal, which leads to a small photocurrent at the detector and translates into shot noise that significantly affects clock stability. According to the authors, this low photocurrent is partly attributed to poor collection efficiency, which they estimate to be approximately 0.2% of the total emitted fluorescence.

[0005] Clearly, an optical scheme is needed that provides higher fluorescence collection efficiency for atomic reference atoms in the vapor chamber.

[0006] Another known problem related to the problem of collection efficiency commented in "Perrella" is that, in order to capture the maximum possible number of fluorescent photons, their photodetector (photomultiplier tube, PMT) is positioned in close proximity to the rubidium cell, which is kept at a temperature of 90°C to increase the Rb vapor density. This proximity increases the temperature of the PMT and, consequently, its dark noise, which also has a decreasing effect on the frequency stability of the clock. Therefore, there is a need for an optical solution that provides high collection efficiency while reducing the effects of vapor cell heating on the temperature-sensitive photodetector. SUMMARY

[0007] According to a first aspect of the present application, the above object is achieved by providing an atomic vapor cell defined by the following proposal.

[0008] 1. An atomic vapor cell (1) comprising a hermetically sealed enclosure (8) defining a volume containing a vapor of reference atoms (2), said hermetically sealed enclosure (8) comprising an optical inlet (3) allowing the transmission of a probe beam adapted to excite an optical transition of said reference atoms (2) and an optical outlet (4) allowing the transmission of a fluorescence signal from said reference atoms (2); characterized in that said hermetically sealed enclosure (8) further comprises a wall (5) permeable to said fluorescence signal, said wall (5) being coated on its outer side with a coating (6) reflective to said fluorescence signal.

[0009] 2. The atomic vapor cell (1) according to proposal 1, wherein said wall (5) occupies more than 50% of the outer surface of said hermetically sealed enclosure (8), preferably more than 80% of the outer surface of said hermetically sealed enclosure (8).

[0010] 3. The atomic vapor cell (1) according to proposal 1 or 2, wherein said coating (6) reflects more than 70%, preferably more than 80% of said fluorescence signal averaged over all possible angles of incidence.

[0011] 4. The atomic vapor cell (1) according to any one of proposals 1 to 3, wherein said optical inlet (3), said optical outlet (4) and said wall (5) are integrally constructed to comprise a material or a combination of materials permeable to both said probe signal and said fluorescence signal.

[0012] 5. The atomic vapor cell (1) according to any one of proposals 1 to 4, wherein said coating (6) comprises a metallic layer or an interference mirror.

[0013] 6. The atomic vapor cell (1) according to any one of proposals 1 to 5, wherein said outer side of said wall (5) comprises a textured region (13).

[0014] 7. The atomic vapour cell (1) according to proposal 6, wherein said textured area (13) comprises micro- or macroscopic facets, for example frusto-conical facets, arranged to preferentially reflect an incident fluorescence signal in the direction of said optical exit (4).

[0015] 8. The atomic vapour cell (1) according to any one of proposals 1 to 7, wherein said optical entrance (3) and optical exit (4) are the same element.

[0016] According to a second aspect of the present application, there is provided a method for manufacturing such an atomic vapour cell, defined by the following proposal.

[0017] 9. A method for manufacturing an atomic vapour cell according to any one of proposals 1 to 8, comprising the following steps:

[0018] (S1) providing a cell body comprising a housing (8) and a connector (9), said housing (8) comprising a penetrable wall (5), said connector (9) defining an opening in said housing (8);

[0019] (S2) cleaning the inner surfaces of said cell body by a process comprising outgassing and plasma cleaning;

[0020] (S3) introducing a vapour of reference atoms (2) into said housing (8) through said connector (9);

[0021] (S4) hermetically sealing said connector (9) to obtain a hermetically sealed housing (8) defining a volume containing a vapour of reference atoms (2); and

[0022] (S5) depositing on at least part of the surface of said penetrable wall (5) a coating apt to reflect a fluorescence signal of said reference atoms (2).

[0023] 10. The method according to proposal 9, further comprising a step (S6) comprising texturing an outer region of said penetrable wall (5), said step (S6) being performed at any stage before said step (S5).

[0024] 11. The method according to proposal 9 or 10, wherein said step (S5) comprises depositing a metal layer, in particular by physical vapour deposition (PVD) or by chemical vapour deposition (CVD) or by atomic layer deposition (ALD).

[0025] According to a third aspect of the present application, the above mentioned objects are further solved by providing an optical atomic clock comprising a signal collection scheme according to any one of the following proposals.

[0026] 12. An optical atomic clock (10) comprising an atomic vapor chamber (1) according to any one of proposals 1 to 8.

[0027] 13. The optical atomic clock (10) according to proposal 12, comprising:

[0028] - a laser source adapted to generate a laser probe light beam (11) adapted to excite a two-photon optical transition of the reference atoms (2),

[0029] - a beamsplitter (12) adapted to distinguish the probe light beam (11) and a fluorescence signal (9) of the reference atoms (2), and

[0030] - a photodetector (17).

[0031] 14. The optical atomic clock (10) according to proposal 13, wherein the probe light beam (11), the atomic vapor chamber (1) and the beamsplitter (12) are configured such that, in operation:

[0032] - the probe light beam (11a) enters the atomic vapor chamber (1) through an optical inlet (3) of the atomic vapor chamber (1), passes through a defined optical path through a volume body containing a vapor of reference atoms (2) until the optical outlet (4), passes through the optical outlet (4), reaches the beamsplitter (12) and is back-reflected into the optical outlet (4), passes through the defined optical path back to the optical inlet (3), and

[0033] - the fluorescence signal (9) of the reference atoms (2) is transmitted through the optical outlet (4) towards the photodetector (17).

[0034] 15. The optical atomic clock (10) according to proposal 13, wherein the atomic vapor chamber (1) is according to proposal 8, the probe light beam (11), the atomic vapor chamber (1) and the beamsplitter (12) are configured such that, in operation:

[0035] - the probe light beam (11a) is transmitted through the beamsplitter (12), enters the atomic vapor chamber (1) through an optical inlet (3) of the atomic vapor chamber (1), passes through a defined optical path through a volume body containing a vapor of reference atoms (2) until a mirror and is back-reflected through the defined optical path towards the optical inlet (3), and

[0036] - the fluorescence signal (9) of the reference atoms (2) is transmitted through the optical outlet (4), reaches the beamsplitter (12) and is reflected towards the photodetector (17).

[0037] 16. The optical atomic clock (10) according to any one of the preceding claims 12 to 15, further comprising a light pipe (14) configured to capture the fluorescence signal (9) emitted from the atomic vapor chamber (1), in particular through the optical exit (4), and to transport the fluorescence signal (9) to the photodetector (17).

[0038] 17. The optical atomic clock (10) according to claim 16, wherein the light pipe (14) has a tubular form defining a pipe entrance face (15), a pipe exit face (16) and reflective walls, wherein the shape of the pipe entrance face (15) is substantially equal to or larger than the shape of the optical exit (4) of the atomic vapor chamber, and wherein the shape of the pipe exit face (16) is substantially equal to or smaller than the shape of the photodetector (17).

[0039] According to a fourth aspect, the present application also relates to a system for controlling, regulating or authenticating a timepiece according to any one of the appended claims 1 to 13.

[0040] More specifically, according to the first aspect of the present application, there is provided an atomic vapor chamber comprising a gas-tight enclosure defining a volume containing a vapor of reference atoms. In the gas-tight enclosure, there is provided an optical entrance allowing the transmission of a probe light beam adapted to excite an optical transition of the reference atoms, and an optical exit allowing the transmission of a fluorescence signal from the reference atoms. A further distinctive feature of the atomic vapor chamber is that the gas-tight enclosure further comprises a wall, which is permeable to the fluorescence signal, coated on its outside with a coating having a reflectivity for the fluorescence signal.

[0041] Unlike the process of stimulated emission, fluorescence is a spontaneous process that can occur in any direction. In prior art atomic vapor chambers, only fluorescence photons spontaneously emitted in the direction of the photodetector are detected. Since photodetectors usually have a relatively small sensitive surface, this results in a very low collection efficiency. Within the chamber disclosed here, fluorescence photons emitted in any direction (except for the photodetector) will most likely reach the permeable chamber wall and be reflected back into the chamber by the external coating. Multiple reflections of the photons can occur, all of which increase the probability that the photons eventually reach the optical exit and thus the photodetector.

[0042] For example, if the chamber comprises a silicon substrate or a metallic element as part of the sealed enclosure, some parts of the sealed enclosure of the chamber can comprise a non- penetrable material. Moreover, some penetrable parts of the gas-tight enclosure can be exempted from an external coating; for example, the optical outlet is preferably defined in a penetrable part of the chamber enclosure which typically does not comprise a fluorescent reflective coating. As for the optical inlet, in many cases it will also not comprise a reflective coating. The gas-tight enclosure of the chamber can also comprise some features for the introduction of reference atoms during the manufacturing process, such as sealed gas connectors, or some mounting and alignment structures which do not necessarily come with an external coating.

[0043] On the other hand, it is advantageous for the efficiency of the fluorescence collection to maximize the surfaces of the sealed enclosure which are reflective to the fluorescent photons. Advantageously, the mentioned penetrable walls which are coated on their outside can represent at least 50% of the outer surface of the gas-tight enclosure. Preferably, the mentioned penetrable walls which are coated on their outside can represent at least 80% of the outer surface of the gas-tight enclosure.

[0044] Since the concepts of penetrability and reflectivity are not absolute, it is convenient to stipulate in the context of the present document that a material is considered to be penetrable when the transmission of an optical signal is achieved with a loss of less than 50%, preferably less than 10%, or in other words, to allow the transmission of said optical signal.

[0045] The concept of reflectivity is somewhat more difficult to define since the reflectivity can strongly depend on the angle of incidence, especially in interference coatings. In the context of the present document, a coating will be considered to be reflective to an optical signal when its reflectivity to said optical signal, averaged over all possible angles of incidence, is at least 70%.

[0046] In the present case, the external coating of the mentioned walls is preferably reflective to at least 80% of the fluorescent signal, averaged over all possible angles of incidence.

[0047] In terms of fluorescence collection efficiency, it is possible and even advantageous to provide a reflective coating on the inner surface of the chamber walls. However, this is not convenient in the case of an atomic vapor chamber where it is desirable to avoid the presence of any atomic or molecular species inside the chamber other than the reference vapor. Providing the coating outside the sealed enclosure advantageously excludes any such contamination.

[0048] According to an advantageous embodiment, the atomic vapor chamber can be integrally manufactured from a penetrable material, such as silicate or borosilicate glass, sapphire or fused quartz, and can be provided as a monolithic structure. By "monolithic" is meant a structure in which all parts are firmly joined without any joints or glue. The monolithic structure can for example result from a glass welding process. The monolithic structure can advantageously comprise a single material that is penetrable, which facilitates the step of possibly welding the different parts. On the other hand, the monolithic structure can also comprise elements of different materials if some particular properties are required for example at the optical inlet and outlet, while the chamber walls seek other optical or mechanical properties.

[0049] As to the properties of the external coating, it can be advantageous to provide it in the form of a metallic layer. Metallic coatings can be applied on the surface of glasses and other transparent materials by methods well known in the photonics industry, such as physical vapor deposition (PVD). Metallic coatings are reflective for a broad range of wavelengths, and their reflectivity advantageously exhibits a relatively low dependence on the angle of incidence.

[0050] According to an advantageous embodiment, the coating can comprise an aluminum layer having a thickness of at least 100 nm. Aluminum is a material that has excellent reflectivity in the ultraviolet spectrum, which is conveniently adapted to the fluorescence of rubidium atoms having a characteristic wavelength of 420 nm.

[0051] Alternatively, the external coating can comprise an interference mirror. Interference mirrors are generally formed from a plurality of thin layers of dielectric materials having different refractive indices. The width and refractive indices of these layers can be adjusted to enhance the reflection of selected wavelengths. Advantageously, this type of mirror can provide very high reflectivity for specific wavelengths; for example, in this case, it can be adapted to the high reflectivity of the fluorescence signal. On the other hand, this type of filter is harder and more expensive to produce than a metallic coating, and can have a strong dependence on the reflectivity as a function of the angle of incidence. Some known methods of producing thin dielectric layers for photonics applications include ion beam sputtering (IBS), chemical vapor deposition (CVD) or atomic layer deposition (ALD).

[0052] Advantageously, the optical inlet and outlet can be provided on planar portions of the gas-tight enclosure that are different from the curved wall. Planar windows have the advantage of (substantially) preserving the collimated distribution of the laser beam used to excite the reference atoms.

[0053] It can further be advantageous to provide the optical inlet and outlet in two planar windows separated by a peripheral chamber wall that hermetically seals the volume between the two windows.

[0054] Advantageously, the outer surface of the coated wall can be textured. The texturing can comprise random irregularities, as in a sanded glass surface, or can comprise a pattern specifically configured to promote preferential reflection of randomly oriented incident light rays in the direction of the optical exit. Such a pattern can comprise micro- or macro-features. By "micro" is meant features having a characteristic dimension on the order of the optical wavelength (e.g. less than 5 microns) that are configured to deviate an incident light beam in a given direction due to diffraction. By "macro" is meant features having a characteristic dimension that is at least 10 times the optical wavelength (e.g. greater than 5 microns). In an advantageous implementation, such macro-features can be provided in the form of sawteeth. The dimension is preferably the distance between a peak and a valley.

[0055] In an advantageous implementation, the reference atoms housed in the chamber can be alkali atoms, for example rubidium (Rb) atoms. Alkali metals are commonly used as a reference in atomic clocks (both optical and microwave clocks); several atomic transitions are known that conveniently correspond to wavelengths available with commercial lasers and detectors.

[0056] According to a second aspect of the application, a chamber as disclosed above can be manufactured by a method comprising the steps of:

[0057] (S1) providing a chamber body comprising an enclosure and a connector, the enclosure comprising a penetrable wall, the connector defining an opening in the enclosure;

[0058] (S2) cleaning the inner surface of the chamber body by a process comprising outgassing and plasma cleaning;

[0059] (S3) introducing a vapor of reference atoms into the enclosure through the connector;

[0060] (S4) hermetically sealing the connector to obtain a hermetic enclosure defining a volume containing the vapor of reference atoms; and

[0061] (S5) depositing on at least part of the surface of the penetrable wall a coating having reflectivity for the fluorescence signal of the reference atoms.

[0062] It is clear that the penetrability of the wall mentioned above is defined at least with respect to the wavelength of the fluorescence signal and / or the wavelength of the interrogation signal.

[0063] Advantageously, the disclosed method can also comprise a step (S6) comprising texturing the outer surface of the penetrable wall. The step S6 can be performed at any stage prior to the application of the reflective coating in step S5. For example, step S6 can be performed on the filled and sealed chamber after step S4. Alternatively, step S6 can also be performed after step S1 or even before step S1. For example, the chamber body can be provided by assembling different chamber parts, wherein some of the chamber parts can be textured prior to the assembly of the chamber.

[0064] According to a third aspect of the present application, there is provided an identifiable optical atomic clock, in that it comprises an atomic vapor chamber as described above.

[0065] The atomic vapor chamber provided in the present application has the advantage that it can more efficiently collect the fluorescence signal generated by the reference atoms. For a given power of the interrogation beam (also referred to as the probe beam), a higher collection efficiency results in a higher detected signal and thus in a lower shot noise and a higher clock stability. While it is in principle possible to compensate for a low collection efficiency by increasing the power of the probe beam, this is not desirable as a stronger probe beam enhances the light shift instability of the clock. In fact, one of the main sources of instability of an optical atomic clock comes from the AC Stark shift affecting the transition frequency of the atoms caused by the optical field of the interrogation laser itself. Power variations of the interrogation laser produce fluctuations in the energy of the reference transition, resulting in an instability of the clock frequency, which is commonly referred to as light shift instability. Since the light shift coefficient depends on the square of the excitation beam intensity, it is convenient to implement a probe beam with a low intensity but still sufficient to produce a sensible fluorescence signal. Therefore, an optical atomic clock comprising the chamber disclosed herein that achieves a good collection efficiency is advantageous in that it can be operated with a relatively low probe power, resulting in a reduced light shift instability, while still delivering a large fluorescence signal at the detector, resulting in a low shot noise instability.

[0066] Advantageously, the optical atomic clock can be based on the detection of two-photon absorption by the fluorescence signal from the reference atoms, and the optical scheme can be arranged according to a Doppler-free configuration. As previously mentioned, the Doppler-free configuration results in a narrow spectral absorption band, which provides a high frequency accuracy for a two-photon optical atomic clock.

[0067] According to an advantageous embodiment, the optical atomic clock can be provided, at the optical exit side of the atomic vapor chamber, with a light pipe configured to capture the fluorescence signal from the reference atoms and to transport said fluorescence signal to a photodetector, which can be located at a safe distance from the chamber. By "safe distance" it is meant that the photodetector and the chamber are sufficiently spaced apart so that the effect of the heating, usually applied to the atomic vapor chamber, on the temperature of the photodetector is small. For example, the atomic vapor chamber and the photodetector can be placed at a distance of 1 cm or more. According to this embodiment, this separation does not result in a loss of fluorescence signal, since the light pipe can effectively convey the fluorescence photons from the chamber optical exit to the photodetector.

[0068] The light pipe can be configured as a light transport element, having a pipe entrance face, a pipe exit face and reflective walls. The light pipe can be provided as a hollow pipe, for example, with internally reflective walls, where the entrance face and the exit face are just the openings of said hollow pipe. Alternatively, the light pipe can be provided as a pipe of a bulk penetrable material. The reflection of light at the walls of such a light pipe can advantageously result from internal reflection of light at the interfaces of the penetrable pipe. The pipe can additionally comprise a reflective coating to reduce the loss of light impinging on the interfaces at a relatively large angle with respect to the surface. The light pipe can also be provided as a curved or flexible light guide, such as a multimode optical fiber.

[0069] The light pipe can conveniently be adjusted so that the shape of the pipe exit is substantially equal to or smaller than the shape of the photodetector. By "shape of the photodetector" it must be understood in this context the shape of the light sensitive element or the shape of the optical window at the entrance of the photodetection device. This matching of shapes aims at ensuring that every photon conveyed by the light pipe to the pipe exit effectively reaches the photodetector.

[0070] Additionally, the light pipe can conveniently be adjusted so that the shape of the pipe entrance is substantially equal to or larger than the shape of the atomic vapor chamber exit. This matching of shapes aims at ensuring that every fluorescence photon emitted from the chamber effectively enters the light pipe.

[0071] In this text, the relationship between shapes is defined as follows. A first shape is larger than a second shape if all points of the second shape can be orthogonally projected on the first shape. A first shape and a second shape are equal if all points of the contour of the second shape can be orthogonally projected on the contour of the first shape.

[0072] When the atomic vapor chamber optical exit and the photodetector have significantly different shapes, the light pipe can advantageously have a conical tubular form and act as a shape-matching light coupling element.

[0073] According to a fourth aspect of the application, a system for controlling, regulating or authenticating a timekeeper is provided, characterized in that it comprises an optical atomic clock as disclosed above. Such a system comprises means for measuring the frequency signal and / or the rate of a timekeeper or of a plurality of timekeepers. BRIEF DESCRIPTION OF DRAWINGS

[0074] Further details of the application and other advantageous embodiments will become apparent when the following description is read in conjunction with the accompanying drawings, which show:

[0075] Figure 1 Atomic vapour cell according to one embodiment of the application.

[0076] Figure 2 Details of an atomic vapour cell according to another embodiment of the application.

[0077] Figure 3 Flow chart of a method for manufacturing an atomic vapour cell according to the second aspect of the application.

[0078] Figure 4 Illustration of some steps of the method according to a possible embodiment. Figure 3

[0079] Figure 5 Schematic view of a first example of a part of an optical atomic clock according to the third aspect of the application.

[0080] Figure 6 Schematic view of a second example of a part of an optical atomic clock according to the third aspect of the application.

[0081] Figure 7 Schematic view of a system for controlling, regulating or authenticating a timekeeper according to the fourth aspect of the application.

[0082] Reference signs used in the drawings

[0083] 1 - Atomic vapour cell; 2 - Reference atoms; 3 - Optical inlet; 4 - Optical outlet; 5 - Permeable wall; 6 - Coating; 7 - Condensate of reference atoms; 8 - Gas-tight enclosure; 9 - Ray of fluorescent signal; 10 - Optical atomic clock; 11 - Probe light beam (11a: incident light beam, 11b: reflected light beam); 12 - Beam splitter; 13 - Surface texture; 14 - Light pipe; 15 - Tube inlet face; 16 - Tube outlet face; 17 - Photodetector; 20 - Cell body; 21 - Flat window; 22 - Flat window; 23 - Cell wall; 24 - Connector; 25 - Reservoir; 26 - Vacuum pump; 27 - Valve; 28 - Removable protection; 30 - System for controlling, regulating or authenticating a timekeeper; 31 - Means for measuring the frequency signal and / or the rate of a timekeeper. ​​​​​​​​Detailed Implementation

[0084] Figure 1 An atomic vapor chamber 1 according to one embodiment of the present invention is shown. The atomic vapor chamber 1 includes:

[0085] -Airtight housing 8, especially airtight sealed housing 8, and

[0086] - Vapor of reference atom 2 contained within a volume defined by the hermetically sealed outer shell 8 of chamber 1.

[0087] The airtight housing 8 includes:

[0088] - Optical entrance 3, which defines a pathway allowing light to transmit into the cavity (e.g., a probe beam suitable for exciting an optical transition of reference atom 2), and

[0089] - A permeable wall 5 coated with a reflective coating 6 on its outer side. In this case, the permeability of the wall 5 and the reflectivity of the coating 6 are related to the wavelength of the fluorescence signal emitted by the reference atom 2.

[0090] Furthermore, as in the illustrated embodiment example, the hermetic housing 8 includes an optical outlet 4 that defines a pathway allowing fluorescence signals emitted by the reference atom 2 to be transmitted out of the chamber.

[0091] In the illustrated embodiment example, the remainder of the airtight housing 8 (excluding the optical inlet and optical outlet) is provided as a permeable wall 5 with a reflective coating 6 coated on its outer side.

[0092] Despite Figure 1 In the example, the optical inlet 3 and the optical outlet 4 are located on opposite sides of the hermetic housing 8, but in possible alternative implementations, they can be located at different positions, or even coincide on the same side of the atomic vapor chamber 1. In fact, the atomic vapor chamber 1 can include a single optical path within the hermetic housing 8, which serves both as the inlet 3 for the probe beam and as the outlet 4 for the fluorescence signal. In this case, considering that the fluorescence and probe beams typically have different wavelengths (especially in two-photon schemes), the two signals can be spectrally separated using, for example, a beam splitter.

[0093] Not in Figure 1 As shown, the atomic vapor chamber 1 may additionally include a sealed connector element that can be used to introduce atomic vapor during manufacturing.

[0094] Figure 1 The image further shows possible residual condensate 7 of reference atoms 2 in vapor phase equilibrium at the inner surface of the hermetically sealed shell 8, which may be generated by the chamber filling process, as will be explained later.

[0095] Figure 2 Details of the penetrable wall 5 of the chamber according to an advantageous embodiment are shown. Here, the penetrable wall 5 is provided with a textured outer region, where the texture is configured to promote the reflection of randomly oriented incident light rays 9 preferentially in the direction of the optical exit 4 (not shown, but assumed on the right side of the figure, as Figure 1 indicated). For illustration purposes, the figure depicts the trajectories of fluorescent rays emitted by the reference atom 2 inside the chamber.

[0096] In this example, the outer region of the wall is textured according to a geometry that appears as a sawtooth pattern in the cross-section shown Figure 2 The geometry of the outer region can be an alternating sequence of:

[0097] - planar (perpendicular to the direction of the probe beam), and

[0098] - frustoconical faces.

[0099] On the above, each tooth of the pattern can be, for example: Figure 2

[0100] - the intersection between a frustoconical face or groove around the cylindrical chamber wall and the cross-sectional plane of Figure 2 , and

[0101] - the intersection between a planar face (perpendicular to the direction of the probe beam) and the cross-sectional plane of Figure 2

[0102] The frustoconical face or groove is oriented towards the optical exit 4 of the chamber to promote the reflection of the fluorescence in this preferential direction. The depth and period of the sawtooth pattern can advantageously be between 0.2 and 2 mm. Advantageously, all geometries that allow to improve the deflection and / or concentration of the fluorescence signal towards the optical exit 4 can be used.

[0103] Figure 3 A block diagram of a method for manufacturing an atomic vapor chamber according to one embodiment of the second aspect of the invention is shown, while Figure 4 some stages of the manufacturing method are shown.

[0104] In step S1, a chamber body 20 is provided, as shown in Figure 4 A. The chamber body 20 comprises a housing and a connector 24 that defines an opening in said housing. The housing comprises two planar windows 21, 22 and an open cylindrical wall 23.

[0105] In this example, all parts of the chamber body 20 are advantageously made of a single penetrable material (e.g. optical glass) and assembled together as a monolithic structure by a glass soldering process. ​​

[0106] In an optional step (S6) before assembly of the chamber, some of the parts, for example the cylindrical wall 23, can be textured on their outer surface, according to an advantageous embodiment.

[0107] The texturing can be provided by a sandblasting step, resulting in a random texture. Alternatively, the texturing can comprise regular design features, such as a sawtooth pattern, which can be produced by mechanical or laser machining processes, photolithography or hot stamping processes.

[0108] Once the chamber is assembled, a cleaning step S2 comprising outgassing and plasma cleaning is performed, in order to remove any impurities, in particular from the inner surfaces of the chamber body 20. Outgassing can be achieved for example by heating the enclosure 20 at a temperature higher than 100°C while keeping it at high vacuum (<10 -5 mbar). Plasma cleaning can be performed by injecting a low pressure gas (about 10 mbar) inside the chamber and ionizing the gas by an RF field so that a plasma is generated. The high energy bombardment from the plasma breaks molecular bonds, removing contaminants from the surface. The now volatile gas and contaminants are then removed by vacuum suction.

[0109] In a next step S3, a vapor of reference atoms is introduced into the chamber body.

[0110] This can be done in several possible ways. In Figure 4 Example B, the connector 24 is connected to a glass piping system. On one branch of said piping system, a reservoir 25 is provided, comprising a condensate of reference atoms, for example a condensate of pure Rb atoms. Another branch of said piping system is connected to a high vacuum pump 26. The piping system further comprises a valve 27 for opening or closing the connection between the chamber enclosure and different branches of the piping system.

[0111] Initially, both valves can be open and any gas of the entire piping system, including the chamber enclosure, is evacuated by the pump 26.

[0112] In Figure 4 The following substep, illustrated in Example B, is to close the path to the vacuum pump 26. The reservoir 25 is heated, for example at a temperature between 70°C and 100°C, to produce a vapor of Rb atoms which diffuses through the piping into the chamber enclosure. Advantageously, the surfaces of the chamber enclosure can be cooled, for example by circulating a cooling liquid or applying cold fingers to said surfaces, resulting in the formation of a condensate 7 of said reference atoms inside the enclosure.

[0113] In an alternative embodiment of this sub-step, the reservoir 25 can contain a precursor compound of the reference atom, rather than a pure condensate. For example, the reservoir can contain rubidium metal azide (RbN3), which can be decomposed into rubidium atoms and nitrogen vapour by heating or by the application of ultraviolet radiation.

[0114] In Figure 4 In an optional subsequent sub-step, illustrated in Figure C, the path to the vacuum pump 26 is reopened and the chamber is again evacuated of any vapours. This step can be used, for example, to expel any nitrogen gas or residue of any solvent that can have been used to initially dispense the rubidium metal azide into the reservoir 25. After this sub-step, the pure condensate 7 of the reference atom remains within the enclosure.

[0115] After step S3, step S4 is performed to hermetically seal the connector, thereby obtaining a hermetically sealed enclosure defining a volume containing vapour of the reference atom. In Figure 4 D, this step can comprise burning the glass connector 24 to collapse its internal passage, seal the cavity, and finally disconnect the sealed connector from the glass tubing used in the filling step S3.

[0116] As Figure 4 D shows, the condensate 7 will generate vapour of the reference atom within the hermetically sealed chamber. The density of this vapour can be increased by heating the chamber. Such heating can be applied continuously, for example, during operation of the optical atomic clock comprising the chamber.

[0117] In a final step S5, a coating reflecting the fluorescent signal of the reference atom is applied on at least part of the surface of the enclosure.

[0118] A convenient method of applying said coating can be by a physical vapour deposition process, which results in a reflective metal layer deposited around the outside envelope of the chamber.

[0119] Advantageously, as Figure 4 E shows, removable protectors 28 can be provided at specific locations of the envelope, at the planar windows 21, 22 in this example, to prevent the coating from being deposited at said specific locations. The protectors can for example be removable resins or any other adhesive polymer, which can subsequently be removed without damaging the windows or leaving solid residues on their surface. Once the coating has been applied, the protectors 28 can be removed to expose the uncoated portion of the penetrable surface, which can then be used as optical inlet and outlet of the atomic vapour chamber.

[0120] Figure 5 A first example of a portion of an optical atomic clock 10 according to the third aspect of the application is illustrated. The optical atomic clock 10 comprises an atomic vapour chamber 1 according to the present disclosure. The optical atomic clock can further comprise:

[0121] - a laser source adapted to generate a probe light beam 11,

[0122] - a beamsplitter 12,

[0123] - a final optical guide or light pipe 14, and

[0124] - a final photodetector 17.

[0125] The generated probe light beam 11 is provided at a wavelength adapted to excite an optical transition, e.g. a two-photon optical transition, of the reference atoms in the chamber 1.

[0126] Advantageously, in this embodiment, the optical path of the probe light beam through the reference vapor is configured with a double pass 11a, 11b that enables a Doppler-free absorption. The probe light beam 11a enters the chamber 1 through the optical inlet 3, passes through the volume containing the vapor, and reaches the optical outlet 4 where it exits the chamber. The beamsplitter 12, adapted to reflect the probe light beam while transmitting the fluorescence signal from the reference atoms, retroreflects the probe light beam 11b through the same optical path, reentering the chamber 1 through the outlet 4 and exiting the chamber through the inlet 3.

[0127] In Figure 5 , the paths of the incoming light beam 11a and of the retroreflected light beam 11b are represented as if they were spatially shifted. This is only for illustrative purposes. In fact, the optical paths of the light beams in the two directions 11a, 11b are exactly the same. This leads to a preferential absorption of the two counter-propagating photons, which is not affected by the Doppler effect, independently of the direction and velocity of the absorbing atoms in the vapor phase.

[0128] Although Figure 5 the beamsplitter 12 in is depicted as a standalone optical element, it is clear that it can also be implemented as an interference coating provided at the outer surface of the optical outlet 4. Other combinations of spectral separation between the fluorescence signal and the probe light beam can also be envisaged by the skilled person without departing from the scope of the invention, before retroinjecting the probe light beam back into the cavity.

[0129] Figure 6 A second example of a portion of an optical atomic clock 10 according to the third aspect of the invention is shown. The second example mainly or only differs from the first example in that:

[0130] - the optical inlet 3 and the optical outlet 4 of the atomic vapor chamber 1 it uses are coincident on the same side, and

[0131] - the beamsplitter 12 is arranged in a non-normal configuration with respect to the probe light beam at the outlet of the optical pass 3, 4. In such an embodiment, the beamsplitter is configured to reflect the fluorescence signal towards the photodetector 17.

[0132] As in the example of Figure 6 , the single optical pass having both the entrance 3 for the probe beam and the exit 4 for the fluorescent signal does not only constitute an alternative configuration of the optical atomic clock 10. This solution advantageously allows a more efficient magnetic shielding of the atomic vapor chamber 1. It is known that magnetic fields affect the energy of atomic transitions. Therefore, in many atomic clocks, the atomic chamber is protected with a magnetic shield comprising, for example, a high magnetic permeability magnetic shield alloy, such as In the configuration of the optical atomic clock depicted in Figure 5 , such a magnetic shield would have at least two holes, allowing the transmission of light through the optical entrance 3 and exit 4. By contrast, Figure 6 The configuration depicted in allows to provide a magnetic shield having only one hole corresponding to the single optical pass 3, 4 of the chamber, thus improving the magnetic shielding.

[0133] Figure 5 and Figure 6 Another aspect shown in the examples of Figs. 1 and 2 (principally independent from the Doppler-free configuration discussed above) is to advantageously provide a light pipe 14 optically coupling the photodetector 17 with the chamber exit 4, or in the present example with the inserted beamsplitter 12. The light pipe 14 has a tubular form, comprising a pipe entrance face 15, a pipe exit face 16 and a reflective wall along its length.

[0134] In the optical atomic clock 10, in particular in the examples of Figure 5 and Figure 6 , the photodetector 17 can advantageously be a photomultiplier tube (PMT). The light pipe 14 allows an efficient transmission of the fluorescent photons towards the photodetector 17, while respecting a safety distance d. Advantageously, said distance d between said photodetector 17 and the chamber 1 can be at least 1 cm. This distance keeps the temperature-sensitive photodetector 17 free from heating, which is usually applied to the chamber 1 to increase and control the atomic vapor density.

[0135] In these examples, the shape and size of the pipe entrance face 15 are substantially equal to the shape and size of the chamber optical exit 4. Moreover, the shape and size of the pipe exit face 16 are substantially equal to the shape and size of the input window of the photodetector 17.

[0136] Figure 7 A system 30 for controlling, regulating or authenticating a timekeeper according to the fourth aspect of the present application is shown.

[0137] The system 30 comprises an optical atomic clock 10 provided in the present disclosure. The system 30 further comprises a device 31 for measuring the frequency signal and / or the rate of one or more timekeepers.

[0138] Such a device 31 can be, for example, a device that measures the rate by recording and analyzing the acoustic signals emitted by the chronograph, in particular the acoustic signals emitted by the function of the escapement and the balance spring oscillator in a mechanical chronograph. Alternatively or complementarily, such a device 31 can be a device that measures the rate by optically measuring the oscillations of the balance spring oscillator in a mechanical chronograph. Alternatively or complementarily, such a device 31 can be a device that measures the rate by visually determining the times indicated by the chronograph at at least two different moments in time.

[0139] Advantageously, the measurements of the frequency signal and / or the rate of one or more chronographs by the device 31 are performed using the optical atomic clock 10 disclosed above as a reference clock.

Claims

1. A system (30) for controlling, regulating or authenticating a timekeeper, comprising means (31) for measuring the frequency signal and / or the rate of a timekeeper or of a plurality of timekeepers based on a time reference provided by an optical atomic clock (10), said optical atomic clock (10) comprising an atomic vapor chamber (1), said atomic vapor chamber (1) comprising a hermetically sealed enclosure (8) defining a volume containing a vapor of reference atoms (2), said hermetically sealed enclosure (8) comprising an optical inlet (3) allowing the transmission of a probe light beam adapted to excite an optical transition of said reference atoms (2) and an optical outlet (4) allowing the transmission of a fluorescence signal from said reference atoms (2), said hermetically sealed enclosure (8) further comprising a wall (5) that said fluorescence signal can penetrate, said wall (5) being coated on its outer side with a coating (6) that is reflective to said fluorescence signal.

2. The system (30) according to claim 1, wherein said wall (5) occupies more than 50% of the outer surface of said hermetically sealed enclosure (8), preferably more than 80% of the outer surface of said hermetically sealed enclosure (8).

3. The system (30) according to claim 1 or 2, wherein said coating (6) reflects more than 70%, preferably more than 80% of said fluorescence signal averaged over all possible angles of incidence.

4. The system (30) according to any one of claims 1 to 3, wherein said optical inlet (3), said optical outlet (4) and said wall (5) are integrally configured to comprise a material or a combination of materials that both the probe signal and said fluorescence signal can penetrate.

5. The system (30) according to any one of claims 1 to 4, wherein said coating (6) comprises a metallic layer or an interference mirror.

6. The system (30) according to any one of claims 1 to 5, wherein said outer side of said wall (5) comprises a textured area (13).

7. The system (30) according to claim 6, wherein said textured area (13) comprises micro- or macroscopic facets, such as frustoconical facets, arranged to preferentially reflect incident fluorescence signals in the direction of said optical outlet (4).

8. The system (30) according to any one of claims 1 to 7, wherein said optical inlet (3) and optical outlet (4) are the same element.

9. The system (30) according to any one of claims 1 to 8, wherein said optical atomic clock (10) comprises: - a laser source adapted to generate a laser probe light beam (11) adapted to excite a two-photon optical transition of said reference atoms (2), - a beamsplitter (12) adapted to distinguish said probe light beam (11) from a fluorescence signal (9) of said reference atoms (2), and - a photodetector (17).

10. The system (30) according to claim 9, wherein said probe light beam (11), said atomic vapor chamber (1) and said beamsplitter (12) are configured such that, in operation: - said probe light beam (11) is transmitted through said optical inlet (3) and said beamsplitter (12) and into said atomic vapor chamber (1), - the probe light beam (11a) enters the atomic vapor cell (1) through an optical inlet (3) of the atomic vapor cell (1), passes through a defined optical path through the volume containing the vapor of reference atoms (2) until the optical outlet (4), passes through the optical outlet (4), reaches the beamsplitter (12) and is retro-reflected back into the optical outlet (4), passes through the defined optical path back to the optical inlet (3), and - the fluorescence signal (9) of the reference atoms (2) is transmitted through the optical outlet (4) towards the photodetector (17).

11. The system (30) according to claims 8 and 9, wherein the probe light beam (11), the atomic vapor cell (1) and the beamsplitter (12) are configured so that in operation: - the probe light beam (11a) is transmitted through the beamsplitter (12), enters the atomic vapor cell (1) through an optical inlet (3) of the atomic vapor cell (1), passes through a defined optical path through the volume containing the vapor of reference atoms (2) until a mirror and is retro-reflected through the defined optical path towards the optical inlet (3), and - the fluorescence signal (9) of the reference atoms (2) is transmitted through the optical outlet (4), reaches the beamsplitter (12) and is reflected towards the photodetector (17).

12. The system (30) according to any one of claims 1 to 11, wherein the optical atomic clock (10) further comprises a light pipe (14) configured to capture the fluorescence signal (9) emitted from the atomic vapor cell (1), in particular through the optical outlet (4), and to transport the fluorescence signal (9) to the photodetector (17).

13. The system (30) according to claim 12, wherein the light pipe (14) has a tubular form, defines a pipe inlet face (15), a pipe outlet face (16) and reflective walls, wherein the pipe inlet face (15) has a shape substantially equal to or greater than the shape of the optical outlet (4) of the atomic vapor cell, and wherein the pipe outlet face (16) has a shape substantially equal to or smaller than the shape of the photodetector (17).

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