Device for measuring a quantity representing the population of cold atoms and associated sensor

Abstract

The present invention relates to a method for measuring a quantity (Γ) representing the population number (N) of a cold atom. A ) device (1, 2), the cold atoms are located in a cold atom cloud (CL) to be analyzed, the device comprising: a microwave source (S), configured to generate an incident signal (SMWi) at a predetermined signal frequency (fs), a microwave waveguide (GO) configured to propagate the incident signal and an antenna (Ant) configured to transmit the incident signal to the cold atom cloud and its environment, the antenna and the microwave waveguide being further capable of recovering an atomic reflection signal (SMWra) generated by reflection of the incident signal by the cloud and its environment, and the atomic reflection signal propagating in the waveguide in a direction opposite to the incident signal, a power splitting device (SS), coupled to the microwave waveguide and configured to extract at least a portion of the atomic reflection signal, a detector (Det), configured to detect the atomic reflection signal extracted by the power splitting device, the quantity representing the population number (N) of the cold atoms being a detected value (S) from the atomic reflection signal (SMWra). A ) and is obtained from the detection value (S0) of the signal reflected by the environment in the absence of said cloud, called the reference reflected signal (SMWr0).

Description

Technical Field

[0001] The present invention relates to the field of sensors based on cold atomic clouds and, more particularly, to an apparatus for carrying out the steps of detecting the number of atoms in a predetermined atomic state, producing a measurement result of a desired quantity. Background Art

[0002] Cold atom sensors have demonstrated excellent performance in measuring time (clocks), gravity (gravimeters), acceleration (accelerometers), rotation (gyroscopes), and magnetic fields (magnetometers). They can also be used in spectroscopy.

[0003] To perform measurements, cold atom sensors require a cold atomic cloud in a vacuum chamber—a cloud of atoms that has been decelerated in three spatial dimensions. The atoms in the cloud typically have a temperature below 10 μK and are prepared in a specific hyperfine state.

[0004] For the sensor implementation, the differences are:

[0005] - a step of preparing atoms, comprising generating the above-mentioned cloud;

[0006] - Actual interferometric steps based on the cloud (clock, acceleration, rotation). The first generation of cold atom sensors uses atomic microcircuits or atomic chips, which include microwires for guiding / trapping one or more cold atom clouds close to their surface.

[0007] A detection step involves converting the interference phase accumulated by the atomic wave functions during the measurement into a population difference between two Zeeman sublevels (hyperfine levels), then measuring this population difference and finally deriving the desired physical quantity from it. According to the prior art, this reading is performed using a detection laser that illuminates the cold atomic cloud.

[0008] The atoms to be detected are cooled and then manipulated on the microcircuit of the atom chip. First, they are prepared in a given hyperfine ground state and guided or trapped on the microcircuit at a distance h between 3 and approximately 100 μm to create an atom interferometer for measuring inertia.

[0009] Figure 1 An example of an atomic chip Ach0 for an accelerometer / gyroscope inertial sensor is shown in FIG. Here, an atomic interferometer is created for measuring the inertia. Part of the surface of the chip forms a measurement plane 13. Figure 1A cross section of a magnetic guide G generated by a microcircuit MC is shown. The microcircuit is, for example, a collection of magnetic quadrupoles (created by a collection of three wires or even by five or more wires), a collection of coplanar microwave waveguides and wires, etc. In general, the microcircuit is suitably powered, for example by a voltage generator and / or a DC and / or AC (RF) generator external to the atomic chip or by an external magnetic field.

[0010] In the magnetic guide G, two atomic clouds CL1 and CL2 are guided so as to propagate in opposite directions along a path 16. The magnetic guide G is generated by a line of the chip (e.g., a magnetic guide quadrupole) to guide the two cold atomic clouds at a predetermined distance h from the measurement plane 13. The magnetic guide G is circular for the gyroscope and linear for the accelerometer. For the dedicated clock measurement, the clouds do not move. The rotational speed Ω is measured by the gyroscope about an axis Z perpendicular to the measurement plane 13 of the chip 1. The acceleration is in the direction of the linear magnetic guide along its axis ( Figure 1 Y) measurement in .

[0011] In a reference frame rotating with angular velocity Ω, the phase shift θ caused by the Sagnac effect between two counter-rotating matter waves is given by:

[0012]

[0013] where A is the area inscribed in the path, m is the mass of the atom and is the reduced Planck constant.

[0014] The phase shift Φ in acceleration measurement is given by:

[0015] Φ=2kaT 2 (2)

[0016] where k is the wave vector of the laser light used to separate (emit), deflect and recombine the atom cloud, 2T is the duration of the interferometer and a is the acceleration to be measured.

[0017] For acceleration measurements, the paths of the two clouds are one-dimensional, for rotation measurements the paths are two-dimensional, and for clock measurements the clouds do not move.

[0018] For example, cloud CL1 comprises cold atoms in state |a> with velocity v1 = +v, and cloud CL2 comprises cold atoms in state |b> with velocity v2 = -v. For example, in the case of rubidium 87, the atomic energy levels |a> and |b> are spaced according to velocity. But they correspond to the same internal state |F=2,mF=+2> (see below for details).

[0019] When the two clouds have completed (taken) a closed path at least once, or completed a round trip for the linear case, they recombine (phase shift) at the starting point and then form two clouds to be analyzed.

[0020] In the sensor, the atom chip 1 is placed in a vacuum chamber which is kept under vacuum, for example using an ion pump and which preferably includes magnetic shielding.

[0021] The vacuum chamber also includes a cold atom source, which includes:

[0022] an atom source (distributor), for example formed by a heated filament conveying rubidium vapor,

[0023] (Magneto-optical) primary atom trap for pre-cooling and placing the initial cold atom cloud near the chip (preparation step).

[0024] The vacuum chamber also comprises a magnetic field source GM outside the chip Ach0. This makes it possible to apply a uniform and stable magnetic field B0 over a thickness of at least the order of height h above the measurement plane 13. Advantageously, the direction of the uniform magnetic field is parallel to the measurement plane.

[0025] The atoms used in cold atom sensors are such that they have two fundamental atomic energy levels that are so-called "hyperfine," that is, at frequencies in the order of gigahertz. H These atoms are usually rubidium 87, whose f H = 6.834 GHz, but other alkali metal atoms such as rubidium 85 (f H =3.0GHz), cesium (f H =9.2GHz), sodium (f H =1.7GHz) or potassium 40(f H =1.3GHz) or rare earth ions such as ytterbium (f H =12.6GHz) have the same type of atomic structure and can be used. When two fundamental atomic energy levels are subjected to a uniform magnetic field B0, they split into Zeeman sublevels, each separated by a frequency f0 that depends on the applied magnetic field (typically several hundred kHz per gauss). It is well known that this bias field is useful for the operation of the sensor.

[0026] For rubidium, the lowest hyperfine level is called F=1, the highest level is called F=2, and Figure 2 Shown by the amount m F The values ​​of identify the various associated Zeeman sublevels. For F = 1, m F It can take values ​​of -1, 0, +1, and for F=2, m FThe values ​​can be -2, -1, 0, +1, +2. According to the value of F and m F The value of is used to identify the atomic state, for example |F=1,m F =-1>.

[0027] Figure 2 The three known atomic transitions of Rubidium 87, Tn, Tm and Tn (referred to as σ+), used for the operation of inertial sensors are also shown and have:

[0028] -|F=2,m F =+2> and |F=1,m F = +1> transition Tn, the frequency is f T =f H +3f0

[0029] -|F=2,m F =+1> and |F=1,m F =0> transition Tm, with a frequency of f T =f H +f0

[0030] -|F=2,m F =0> and |F=1,m F =-1> transition T1, the frequency is f T =f H -f0

[0031] exist Figure 1 During operation of the sensor in , the starting state of the atom is one of the Zeeman sublevels with zero velocity (e.g. |F=2, mF=+2>). It is selected based on the sensor and the type of measurement required.

[0032] Using a laser that produces a Bragg transition, the velocities of the atoms in the starting state are modified to create a state |a> with velocity v1 = +v and a state |b> with velocity v2 = -v. Thus, the internal state is still the same Zeeman sublevel, but there are now two states |a> and |b> that differ in their velocities.

[0033] The method for measuring a given physical quantity (clock, acceleration, rotation, magnetic field) by means of an atomic chip sensor comprises several of the above steps (preparation, measurement, detection). More specifically, Figure 1 The various steps in the implementation of the cold atomic sensor shown are detailed below and presented in Figure 3 Shown in.

[0034] Preparation step E1 comprises generating a cold atomic cloud, comprising emitting said atoms from a source, cooling said atoms, initializing said atoms to at least one atomic state for guiding the cloud at a minimum of the magnetic potential, for example |F=2,mF =+2>.

[0035] The measurement step consists of one (E2) or two (E2 and E3) sub-steps:

[0036] In step E2 , the atomic state is initialized by a coherent superposition between a first atomic state |a> with velocity v1 and a second atomic state |b> with velocity v2 of the cold atoms.

[0037] |a>=|F=2,m F =+2;v1=+v>;|b>=|F=2,m F =+2;v2=-v>.

[0038] This initialization is performed by a first pulse, called "π / 2," which prepares 50% of the atoms in state |a> and 50% in state |b>. After this pulse, two clouds, CL1 and CL2, are generated, consisting of atoms in states |a> and |b>, respectively, with opposite velocities, and spatially separated along predefined paths.

[0039] In step E3, the atoms' velocities are swapped using a second pulse, called "π," to return them to their starting point. Thus, the velocity of state |a> is -v, while the velocity of state |b> is +v. For the circular guide G, only their arrival at the starting point is expected, with the arrival time determined by the guide's radius and velocity v.

[0040] The detection step also includes multiple sub-steps.

[0041] Once each cloud has returned to its starting point, in step E4, a third "π / 2" pulse is used to reassemble the two atomic velocity states in order to mix the two propagation states. After the reassembly, the two clouds CL1 and CL2 in the free propagation mode separate again and then form the two clouds to be analyzed.

[0042] Generally speaking, after a "π / 2" pulse (a separation or recombination pulse):

[0043] 1) Cloud CL1 always has velocity v1 and contains N1 atoms.

[0044] 2) Cloud CL2 always has velocity v2 and contains N2 atoms.

[0045] Then, after the final "π / 2" pulse, in the recombination:

[0046] 1) Cloud CL1 contains N1 atoms, of which p% of the atoms are in cloud CL1 before the pulse and (100-p)% of the atoms are in cloud CL2 before the pulse.

[0047] 2) Cloud CL2 contains N2 atoms, of which (100-p)% of the atoms were in cloud CL1 before the pulse and p% of the atoms were in cloud CL2 before the pulse.

[0048] In other words, at the end of the measurement performed by the interferometer, before the last “π / 2” pulse, the number of atoms with velocity v1 and the number of atoms with velocity v2 are counted independently of their source (cloud CL1 or CL2 ).

[0049] In other types of sensors, such as the gyroscope described in document WO 2017089489, the atomic cloud is trapped (also known as parked) in its initial position during and after recombination, so that the two clouds to be analyzed are coincident, and only one cold atomic cloud is to be analyzed. Here, the microcircuit comprises two coplanar microwave waveguides and a conductor carrying a DC current, which causes the two clouds to move on a rectangular path.

[0050] During this recombination, the phase accumulated by the atomic wave function during the measurement is transferred into the difference between the populations of the two Zeeman sublevels |a> and |b>. Let N1 be the atomic population in state |a> after the phase transfer and N2 be the atomic population in state |b>. Thus, the two probabilities Prob1 and Prob2 that the atom occupies the two atomic energy levels |a> and |b>, respectively, are expressed using the following equations:

[0051]

[0052]

[0053] in is the desired phase, and is the phase that defines the working / operating point of the sensor.

[0054] In step E5', the number (population) N1 of atoms in state |a> of cloud CL1 and the number (population) N2 of atoms in state |b> of cloud CL2 are measured. This allows the calculation of probabilities Prob1 and Prob2. These are the ratios of the number of atoms in states |a> and |b>, respectively, to the total number of atoms participating in the measurement. The phase is then determined based on Prob1 and Prob2.

[0055] Finally, in step E6 , the required physical quantity is determined according to the phase.

[0056] According to the prior art, step E5 ′, ie detecting the signal at the output of the atom interferometer, is performed using a detection laser which illuminates the cold atom cloud to be analyzed, referred to herein as CL. Two methods are used.

[0057] Figure 4 The first method, shown, involves irradiating the cloud to be analyzed with a resonant laser beam FL (via optical transition excitation) and imaging the cloud on a detector. Each energy level absorbs radiation differently, and an image is obtained by detecting the transmission of the laser beam FL through the atoms on a CCD camera, based on the absorption of the cloud. This then gives the number of atoms in each energy level.

[0058] exist Figure 5 In the second method shown, the cloud of atoms is subjected to resonant laser excitation by illuminating it with beam FL' (excitation via optical transitions) and the fluorescence signal emitted by the cloud, which depends on the number of atoms in the fluorescing atomic level, is collected on a photodiode D0.

[0059] In both cases, atoms (for example, in state |a>) are excited at an optical level very far away from levels |a> and |b> (separated by several terahertz). The number of atoms in state |a> is then measured, causing them to vanish. The same applies to state |b>. Therefore, the corresponding population of each energy level is measured using this optical level (typically 780 nm).

[0060] The main disadvantage of both methods is that they are destructive. This means that a new cloud of cold atoms must be prepared to restart the measurement cycle. This leads to lag times in the measurement of the physical quantity. These lag times limit the stability of the sensor (Dick effect) and restrict the sensor's bandwidth (typically 1 Hz).

[0061] Furthermore, both methods require complex and very bulky optical systems to effectively detect atoms, making them incompatible with the development of compact, low-power inertial sensors.

[0062] An object of the present invention is to remedy the above-mentioned drawbacks by proposing a method for measuring a quantity representative of the population of atoms in a predetermined atomic state, said atoms being located in a cold atomic cloud, said measurement being non-destructive and non-optical. Summary of the Invention

[0063] The invention relates to a device for measuring a quantity representative of the population of cold atoms occupying predetermined atomic energy levels, said cold atoms being located in a cloud of cold atoms to be analyzed, the device comprising:

[0064] - a microwave source configured to generate an incident signal at a predetermined signal frequency,

[0065] - a microwave waveguide configured to propagate the incident signal, and an antenna configured to transmit the incident signal to the cold atomic cloud and its environment,

[0066] The antenna and the microwave waveguide are also capable of recovering an atomic reflection signal generated by the cloud and its environment reflecting the incident signal, and the atomic reflection signal propagates in the waveguide in a direction opposite to that of the incident signal.

[0067] - a power splitting device coupled to the microwave waveguide and configured to extract at least a portion of the atomic reflection signal,

[0068] - a detector configured to detect the atomic reflection signal extracted by the power splitting device,

[0069] The quantity representative of the population of cold atoms of the predetermined atomic energy level is obtained from the detected values ​​of the atomic reflection signal and from the detected values ​​of a signal reflected by the environment in the absence of the cloud, called a reference reflection signal.

[0070] According to one embodiment, the expressed amount is proportional to the population.

[0071] According to one embodiment, the representative quantity is called the atomic reflection coefficient and is defined as follows:

[0072]

[0073] Among them, S A is the detected value of the atomic reflection signal and S0 is the detected value of the reference reflection signal.

[0074] According to a variant, the detector is configured to detect a reflected atomic power and a reference power, the representative quantity being determined from the atomic power and the reference power.

[0075] According to another variant, the detector is configured to detect the reflected atomic amplitude and a reference amplitude, the associated representative quantity being determined from the amplitude, and / or to detect the atomic phase and a reference phase, the associated representative quantity being determined from the phase.

[0076] Preferably, the power splitting device is configured to extract a portion of the incident signal and the detector is configured to detect the portion of the incident signal and measure the atomic phase and the reference phase.

[0077] Preferably, the microwave waveguide and the antenna are integrated on an atom chip.

[0078] According to one embodiment, the signal frequency fs is such that:

[0079] fs≥fT+f0 / 2

[0080] where fT is the frequency of the highest detected transition among the possible transitions, and f0 is the separation frequency of the two Zeeman sublevels of the cold atom.

[0081] According to another aspect, the present invention relates to a cold atom sensor comprising:

[0082] - an atom chip comprising a microcircuit configured to generate a magnetic guide and an atom source, said atom chip and said atom source being placed in a vacuum chamber,

[0083] The sensor is configured to generate an initial cold atomic cloud comprising atoms having a first atomic energy level and a second atomic energy level, to generate and, where applicable, to move the cold atomic cloud having the first atomic energy level and the cold atomic cloud having the second atomic energy level in opposite directions along a predetermined path, and to recombine the cold atomic clouds to generate at least one cold atomic cloud to be analyzed,

[0084] The cold atomic sensor further comprises:

[0085] at least one measuring device according to the invention, configured to measure a first quantity representative of the population of said first atomic energy level and a second quantity representative of the population of said second atomic energy level, a physical quantity determined from said first and second quantities represented.

[0086] Preferably, the microwave waveguide and the antenna of the measuring device are integrated on the atom chip, and the antenna is close to the position of the cold atom cloud to be analyzed.

[0087] According to one embodiment, the power splitting device of the measuring device is also integrated on the atomic chip.

[0088] According to one embodiment, the sensor according to the present invention is configured to generate a first cold atomic cloud and a second cold atomic cloud for analysis in motion, and includes a first measuring device and a second measuring device, the first measuring device and the second measuring device respectively including a first waveguide and a second waveguide and a first antenna and a second antenna integrated on the chip, the first measuring device being configured to measure the first quantity representing the first cloud and the second measuring device being configured to measure the second quantity representing the second cloud.

[0089] According to a further aspect, the invention relates to a method for measuring a quantity representative of the population of cold atoms occupying predetermined atomic energy levels, said cold atoms being located in a cloud of cold atoms to be analyzed,

[0090] The method comprises the following steps:

[0091] A generates a microwave incident signal at a predetermined signal frequency,

[0092] B propagating the incident signal in a microwave waveguide and, where applicable, transmitting the incident signal to the cold atomic cloud,

[0093] C recovers a reflected signal, called an atomic reflected signal, generated by the reflection of the incident signal by the cloud and its environment, and propagating in the waveguide in a direction opposite to that of the incident signal,

[0094] D extracting at least a portion of the atomic reflection signal,

[0095] E detects and extracts the atomic reflection signal,

[0096] F. before or after performing steps A to E in the presence of said cold atomic cloud, performing steps A to E in the absence of said cloud, such that in the absence of said cloud, the reflection of the incident signal by the cloud-evolved environment produces a reflection signal referred to as a reference reflection signal,

[0097] G determines the quantity representative of the population of cold atoms of the atomic energy level to be analyzed based on the value of the atomic reflection signal and the value of the reference reflection signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0098] The following description presents several exemplary embodiments of the device of the invention: these examples do not limit the scope of the invention. These exemplary embodiments present essential characteristics of the invention and additional characteristics relevant to the embodiment under consideration.

[0099] The invention will be better understood and other features, objects and advantages thereof will become apparent in the following detailed description and with reference to the accompanying drawings, which are given by way of non-limiting examples and in which:

[0100] [ Figure 1 ]Already cited Figure 1 Shown is an Atom chip for an accelerometer / gyroscope inertial sensor.

[0101] [ Figure 2 ]Already cited Figure 2 The various Zeeman sublevels of rubidium 87 are shown.

[0102] [ Figure 3 ]Already cited Figure 3 The main steps achieved by the cold atom sensor are shown.

[0103] [ Figure 4 ]Already cited Figure 4 A first optical detection method is shown.

[0104] [ Figure 5 ]Already cited Figure 5A second optical detection method is shown.

[0105] [ Figure 6 ] Figure 6 An apparatus according to the invention for measuring a quantity representative of the population of cold atoms is shown.

[0106] [ Figure 7 ] Figure 7 The quantity Γ representing the population is shown A According to the change of population.

[0107] [ Figure 8 ] Figure 8 The quantity Γ obtained by interrogating a cold rubidium-87 atom cloud is shown as a function of the interrogation frequency. A The change in the absolute value of .

[0108] [ Figure 9 ] Figure 9 The Zeeman levels of rubidium 87 and the frequency offset to be added to the detection transition in order to determine the signal frequency are described.

[0109] [ Figure 10 ] Figure 10 A variation of a power splitter arrangement for extracting a portion of the incident signal is shown.

[0110] [ Figure 11 ] Figure 11 An example of a power splitter device integrated on an atom chip and coupled to a similarly integrated 3-wire coplanar microwave waveguide is shown.

[0111] [ Figure 12 ] Figure 12 A first example of a gyroscopic inertial sensor according to the present invention is shown.

[0112] [ Figure 13 ] Figure 13 A second example of an accelerometer inertial sensor according to the present invention is shown.

[0113] [ Figure 14 ] Figure 14 Shown Figure 13 An embodiment of the sensor described in Figure 11 The power splitter described in allows access to the phase measurement of the atomic reflection signal and the reference reflection signal. DETAILED DESCRIPTION

[0114] Figure 6A device 1 is shown for measuring a quantity representing the population number N of cold atoms occupying a predetermined atomic energy level, referred to as |e>, and located in a cold atom cloud CL to be analyzed. The device 1 comprises a microwave source S configured to generate an incident signal SMWi at a predetermined signal frequency fs, a microwave waveguide GO coupled to the source S and configured to propagate the incident microwave (MW) signal, and an antenna Ant configured to transmit the incident signal to the cold atom cloud CL to be analyzed. According to one preferred embodiment, it is the end of the microwave waveguide GO that serves as the antenna (the shape of the end of the waveguide, the manner in which the wire is connected to the end). According to another embodiment, the antenna Ant is an additional component coupled to the microwave waveguide GO. The term antenna therefore refers to the transmitting portion of the microwave waveguide GO or to the additional component that performs this task, if present.

[0115] The antenna Ant and the microwave waveguide GO are also capable of recovering the atomic reflection signal SMWra generated by the reflection of the incident signal by the cloud and its environment, and which propagates in the waveguide in the opposite direction to the incident signal. This reflected signal is generated by the change in radiation impedance caused by the presence of the atomic cloud. In fact, the microwave power emitted by the antenna depends on the medium to which the antenna radiates. This dependence is defined by the radiation impedance of the medium and is specific to it. Therefore, not all microwave power of the incident signal SMWi sent from the source S to the antenna will be absorbed by the medium surrounding the antenna. Part of the transmitted power is reflected and carried by the atomic reflection signal SMWra. This signal SWMra contains information about the medium to which the antenna radiates, and the inventors have confirmed and verified that this information specifically includes quantities representing the number of atoms and the atomic states in which the atoms are.

[0116] The sources of radiation impedance and therefore also of atomic reflection signals are explained as follows. Atoms are characterized in a highly simplified manner by magnetic dipole moments and they can be considered as magnets that interact with the magnetic component of the microwave field. This interaction depends on the internal state of the atoms. In the case of rubidium-87 atoms, it is the Zeeman magnetic state of the fundamental hyperfine state that is capable of coupling with the field. Thus, the microwave magnetic field of the signal frequency fs excites the magnetic dipole of a certain atomic transition T of the material (e.g. rubidium-87) that makes up the atoms, the efficiency of which depends, among other things, on the difference between the signal frequency fs and the transition frequency. This atomic magnetic dipole in turn emits a microwave field which, once picked up by the antenna, generates an atomic reflection signal SMWra. The incident signal SMWi and the reflected signal SMWra are superimposed in the waveguide GO and produce a reflection coefficient that quantifies the radiation impedance of the antenna in the presence of atoms. The physical origin of radiation impedance is discussed in the publication “An antenna model for the Purcell effect” by Alexander E. Krasnok et al. (Nature Scientific Reports, August 2015).

[0117] In other words, in the presence of atoms, the impedance seen by the microwave generator S is defined by the coupling between the magnetic component of the field and the magnetic moments of the atoms. This coupling depends on the total number of atoms, their internal state, the mismatch between the signal frequency (atomic interrogation frequency) and the atomic transitions between hyperfine states, the speed of the atoms and the power radiated by the antenna.

[0118] In order to access the reflected signal, the device according to the invention further comprises power splitting means SS coupled to the microwave waveguide GO and configured to extract at least a portion of the reflected signal, and a detector Det configured to detect the reflected signal extracted by the power splitting means.

[0119] In addition to the atomic reflection signal SMWa in the presence of the atomic cloud, a signal reflected by the environment in the absence of the cloud CL is detected in the same manner, and this signal is referred to as the reference reflection signal SMWr0. The inventors have confirmed that the quantity representing the population number N of the cold atoms of the predetermined atomic energy level is the detected value P from the atomic reflection signal. A The atomic population N is related to the radiation impedance in the presence and absence of atoms.

[0120] For optimal operation of the device, the atoms in the cloud are placed in the near field of the antenna, at a distance that allows the reflected signal to be observed with a good signal-to-noise ratio defined by the application under consideration. This distance is typically between a few micrometers and 500 μm.

[0121] The device according to the invention has many advantages over the above-mentioned prior art optical devices.

[0122] This makes it possible to directly obtain an electrical signal representing the number of atoms in a predetermined atomic state.

[0123] The device according to the invention can be used in different ways depending on the application.

[0124] For applications of the device according to the invention in spectroscopy, the quantity representative of the number of atoms at a given energy level is directly obtained as a function of the signal frequency fs, making it possible to more easily locate the frequency of the transition by varying the interrogation frequency (see below).

[0125] As for the use of the device in inertial sensors and more generally in atom interferometers, the production of these systems is facilitated by the integration of the device 1 according to the invention.

[0126] The detection is non-destructive, making it possible to perform multiple measurements (e.g., inertial measurements) using the same atomic cloud. Thus, the contribution of the preparation time (cooling) to the interferometer lag time can be reduced / eliminated over a duration comparable to the lifetime of the cold atomic sample. As explained above, the presence of lag time limits the stability of inertial sensors. Using non-destructive measurements, this limitation is overcome, enabling, for example, autonomous inertial navigation over relatively long durations.

[0127] Due to their intrinsic properties, prior art cold atom inertial sensors have a limited bandwidth, typically limited to 1 Hz, due to the cooling of the atoms. The measurement device according to the invention enables the possibility of performing multiple non-destructive measurements on the same cold atom sample in succession, thus increasing the bandwidth of the sensor.

[0128] The device according to the present invention is particularly suitable for systems using atom chips (such as the inertial chip sensor, an example of which is described above), in which the atoms to be detected are those that are cooled and then manipulated on the microcircuitry of the atom chip. In this embodiment, the microwave waveguide GO and the antenna Ant of the device according to the present invention are integrated on the atom chip.

[0129] As described above, cold atoms are first prepared in a given hyperfine ground state, and then guided or trapped on a microcircuit at a distance h to create an atomic interferometer for measuring inertia. The waveguide GO here is preferably a coplanar microwave waveguide microfabricated on a chip. The waveguide is formed by a minimum of three microwires separated by a given distance d. The microwires can follow straight lines, curves, or any other pattern required by the intended application. In the case of a minimum three-wire configuration, the two outer wires are connected to the ground of the microwave source, and the center wire carries the active signal.

[0130] However, other types of waveguides may also be used, in particular waveguides whose fabrication is compatible with deposition-based or etching-based microfabrication techniques, such as microstrip lines.

[0131] The fabrication process of the waveguide GO is essentially the same as that used to fabricate the microcircuit MC, thus simplifying the fabrication of the sensor.

[0132] The waveguide GO is usually produced in a different chip plane from the chip plane of the microcircuit generating the magnetic guide G, so that the antenna can be brought as close as possible to the cloud, for example by positioning the antenna (which can be the end of the waveguide GO) in line with the path of the cloud.

[0133] Detection performed by the device according to the present invention, integrated on a chip, makes it possible to significantly reduce the size of inertial sensors. This eliminates the complex and often relatively bulky optical systems required for efficient atom detection. This also eliminates the CCD camera, whose large size is limited by the electronics required for signal shaping (converting pixel intensity into voltage). This substantial size reduction, combined with simplified manufacturing, paves the way for industrial production of this type of sensor / interferometer.

[0134] The integrated nature of the measuring / detection device according to the invention allows for good reproducibility from one device to another.

[0135] By modeling the interaction between the incident signal and the atomic cloud, the inventors demonstrated that the number of atoms in the interrogated energy level is represented by the quantity Γ defined as A Function:

[0136]

[0137] Among them S A is the detected value of the atomic reflection signal and S0 is the detected value of the reference reflection signal. A is called the atomic reflection coefficient.

[0138] According to a first embodiment, the detected signal is the microwave power P.

[0139] In this case, the coefficient Γ A Written as Γ AP , is a real number and is written as:

[0140]

[0141] Among them, P A is the atomic reflected power and P0 is the reference reflected power.

[0142] The detector Det here is a microwave power detector.

[0143] According to the second embodiment, the detected signal is a voltage (amplitude and phase of the electrical signal). The waveguide GO propagates the microwave signal, and thus the voltage V A and V0 can be written as follows:

[0144] as well as

[0145]

[0146] in:

[0147] V A0 is the atomic reflection amplitude,

[0148] V 00 is the reference reflection amplitude,

[0149] is the relative phase between the incident signal and the atomic reflected signal, called the atomic phase, and

[0150] Φ0 is the relative phase between the incident signal and the reference reflected signal, called the reference phase.

[0151] These voltages V A and V O It can be represented by a phase vector (phasor).

[0152] Then Γ AV and Defined as the quantities associated with the measurement results of amplitude and phase respectively:

[0153]

[0154] The detector Det is here configured to detect the atomic reflection amplitude V A0 and the reference reflection amplitude V 00 , the associated representation Γ AV According to the amplitude V A0 and V 00 Determine and / or separately detect atomic phases and reference phase Associated representation According to the phase and Sure.

[0155] The detector Det for measuring the amplitude and phase of the detected (atomic or reference) microwave signal is a vector network analyzer or a phase-amplitude detector (see below).

[0156] Simultaneous measurement of Γ AV and So that the characteristic quantity Γ is obtained ATwo values ​​of are possible, thereby increasing the accuracy of determining the quantity.

[0157] The inventors have also confirmed that the quantity Γ defined in equation (6) (and equations (7) and (8), because they involve the same quantity measured in three different ways) A There is a first-order approximation that there is a proportional relationship between and the population number N. Figure 7 The curve 70 in FIG. 1 depicts the value Γ A An example based on the variation of N. The following applies:

[0158] Γ A = K. N (9)

[0159] where K is the scaling factor.

[0160] Curve 70 is a theoretical curve calculated using power as the measured quantity. The conditions of interest are: cloud size = 1 mm; cloud temperature = 2 μK; transition Tn|F = 2, mF = +2> to |F = 1, mF = +1>, signal frequency fs = 0.999f Tn =0.999.(f H +3f0).

[0161] The scaling factor K is a function of the chosen signal frequency fs, the atomic transitions coupled by this signal frequency, the atomic species, and the shape factor of the antenna near field (which depends on the chosen antenna geometry).

[0162] When it is necessary to determine the exact value of the population number N, this factor K can be determined once and for all for a given device according to the invention by means of calibration.

[0163] However, in many applications, the exact value of the population is not necessary, and the relevant information is derived from the quantity Γ representing the population N. A Extracted.

[0164] In spectroscopy, one can obtain the value of Γ as a function of the interrogation frequency fs. A The relevant information is derived from the changes in Γ obtained by interrogating the cold rubidium 87 atomic cloud at the set of interrogation frequencies fs. A An exemplary variation of (defined by equation (6)) is as follows Figure 8 (curve 80) A Determined from the reflected atomic power and the reference power measured by the detector Det. For this curve, the cloud consists of 10 4 atoms, and the distance from the cloud to the atom chip is h = 5 μm.

[0165] It can be seen that the well-known transitions Tl, Tm and Tn of rubidium 87 (see Figure 2) appear clearly and their respective frequencies are identified. For values ​​of frequency fs equal to the known transition frequency, Γ A The value of is modified: there is indeed an atomic reflection coefficient Γ A Sensitivity to incident frequency.

[0166] For inertial sensor applications, it is not necessary to determine the exact value of the population number. When it is necessary to analyze two clouds CL1 and CL2, the quantity Γ that determines the population number N1 of the atomic energy level |a> is A1 And determine the number of population N2 of the atomic energy level |b> A2 .

[0167] For this purpose, two measuring devices according to the invention are preferably used (see further below). A There is a scaling factor K between the numerator and denominator in equations (3) and (4) expressing the occupancy probability, and according to Γ A1 and Γ A2 Directly deduce prob1 and prob2:

[0168]

[0169] When two clouds CL1 and CL2 are present in free-propagation mode, the detection performed by the device according to the present invention is preferably performed as a "time-of-flight" detection. The location and time of the recombination are not the location and time of the detection. The cloud to be analyzed moves, approaches the detector, passes in front of it (detection maximum), and then moves away. The optimal detection time is not easy to determine. In this case, the atomic reflection signal and the reference reflection signal are detected as a function of time at a specific time interval Δt. To detect the atomic reflection signal, the specific time interval Δt covers the time when the atomic cloud passes close to the antenna. The integral of the atomic signal and the reference signal is then measured.

[0170] When the reassembly takes place on a captured cloud that is no longer in motion (e.g. the sensor described in the above-mentioned document WO2017089489), it is possible to measure two representative quantities Γ at two different times using a single device according to the invention A1 and Γ A2 The device's antenna Ant should be placed as close as possible to the location of the captured cloud to be analyzed.

[0171] In interferometer applications, when measuring Γ AP The interrogation frequency fs can be selected in various ways depending on the measurement purpose.

[0172] When a measurement with maximum signal-to-noise ratio is required, the signal frequency fs is chosen so as to resonate with a given atomic transition ( Figure 8The signal is thus clear, but this configuration will maximally perturb the populations N1 and N2, which will not allow the atoms to be reused for subsequent measurements.

[0173] When the destructiveness of the measurement needs to be reduced, a frequency fs further away from the resonance is chosen. The destructiveness of the measurement is understood as the change in the Zeeman state of the atoms after applying a detection signal of frequency fs, that is, the change in the number of atoms N1 or N2 (interferometer application).

[0174] Measuring Γ AV and / or Instead of Γ AP One benefit of this is that it allows for measurements that are both relatively non-destructive and sensitive. To this end, the signal frequency is chosen to avoid resonance and to lie at the transition peak ( Figure 8 This configuration produces a good signal-to-noise ratio and minimal population interference.

[0175] The choice of the signal frequency fs interrogating the cloud of atoms CL to be analyzed therefore depends on the application and the type of sensor considered.

[0176] For the use of the device according to the invention in spectroscopy, the frequency is scanned over a given range to identify transitions (cf. Figure 8 ).

[0177] For inertial sensor applications, and when low-destructive measurements are required, in order to be able to reuse atoms, the choice of frequency fs depends on the atomic state |e> to be measured and the transitions (called σ+) that are allowed by choosing the detection energy level |d> associated with |e>.

[0178] For example:

[0179] For |e>=|F=2,m F =+2>, select |d>=|F=1,m F =+1>( Figure 2 transition Tn).

[0180] For |e>=|F=2,m F =+1>, select |d>=|F=1,m F =0>( Figure 2 transition Tm).

[0181] For |e>=|F=2,m F =0>, select |d>=|F=1,m F =-1>be( Figure 2 transition T1 in the .

[0182] As explained above, the frequency fs is not equal to the transition frequency used for detection (resonance) in this case. The spacing is large enough, but not too large, to keep the reflected signal detectable. It is necessary to find a compromise between the destructiveness and sensitivity of the measurement. In all cases, the frequency fs should be greater than the frequency of the highest detectable transition among the possible transitions, called f T , in order to avoid the transfer of atoms from one hyperfine state to another. The following relationship should be satisfied:

[0183] fs ≥ f T + f0 / 2 (12)

[0184] where f0 is the separation frequency of the two Zeeman sublevels of the cold atom.

[0185] In order to meet this relationship, Figure 9 The frequency offset Δj to be added to the associated detection transition Tj in order to satisfy relation (12) is described.

[0186] For the transition Tn as the highest transition (measurement |e>=|F=2,m F =+2>), gap Δn≥f0 / 2.

[0187] For transition Tm, Δm≥2.f0+f0 / 2,

[0188] For Tl, Δl ≥ 4.f0 + f0 / 2

[0189] The condition for Δ therefore depends on the chosen detection energy level |d>.

[0190] The frequency fs can be determined by a calibration step in which fs is varied and Γ is observed as a function of fs. A The resonance changes of the oscilloscope and the identification of which interrogation frequency presents the best compromise consistent with the required measurement performance are analyzed. This calibration also makes it possible to characterize the destructive nature of the detection.

[0191] For inertial sensor applications, it is necessary to measure two quantities Γ representing N1 and N2 associated with the two atomic states |a> and |b> to be analyzed, respectively. A1 and Γ A2 .for Figure 1 For example of a sensor in , the two states |a> and |b> to be analyzed (with populations of N1 and N2 atoms, respectively) are (see above):

[0192] |a>=|F=2, mF=+2; v1=+v>; |b>=|F=2, mF=+2; v2=-v>.

[0193] The states |a> and |b> differ only in speed. Therefore, the selected detection state is |d>=|F=1,m F =+1>( Figure 2 The transition Tn of the intermediate frequency fn is τ(n), and the frequency fs=fn+f0 / 2. The distinction is made only by the velocity and therefore by the position of the atoms relative to the antenna.

[0194] As can be seen, in the device according to the invention, the interrogation for detection is performed using microwave frequencies, unlike the prior art, which uses optical frequencies. This makes it possible to integrate the detector directly on the atom chip and obtain an electrical signal corresponding to the population of the atomic states of the interferometer.

[0195] Figure 10 A variant of the device 1 according to the invention for measuring the atomic phase and the reference phase is shown. For this purpose, the power splitter means SS is configured to extract a portion SMWi' of the incident signal and the detector Det is configured to detect this portion of the incident signal and thus measure the phase between the incident signal and the atomic reflected signal (atomic phase ), and the phase between the incident signal and the reference reflected signal (reference phase ).

[0196] Figure 11 An example of a power splitter SS integrated on chip Ach and coupled to a similarly integrated 3-wire coplanar microwave waveguide, capable of measuring the amplitude and phase of a reflected signal, is shown. The center line WC carrying the incident and reflected signals is coupled to line 20, which is configured to collect a portion SMWI' of the incident signal and direct it to a detector Det, and to direct the reflected signal to the detector via link 21, on the other hand. The power splitter acts as a power splitter around a frequency f H (very close to fs) Optimized bidirectional microwave coupler.

[0197] The detector Det here is a vector network analyzer or a phase-amplitude detector, which is used to measure the amplitude and phase between the incident signal and the (atom or reference) reflected signal.

[0198] According to another aspect, the present invention relates to a cold atom sensor CAS that integrates at least one measuring device according to the invention. This can be any type of sensor based on the use of a cold atom cloud that contains information about the physical quantity to be measured, encoded by the population of atoms in a specific atomic energy level, and therefore requires the measurement of a quantity representative of this population.

[0199] According to a variant, the sensor CAS according to the invention comprises an atomic chip Ach, and preferably the antenna Ant, the waveguide GO and the power splitter device SS are integrated on the atomic chip Ach.

[0200] Atomic chip inertial sensors have been described in the prior art.

[0201] The sensor CAS according to this variant of the invention comprises an atom chip Ach including a microcircuit configured to generate a magnetic permeance G and an atom source AS, the atom chip and the atom source being placed in a vacuum chamber.

[0202] The sensor is configured to generate an initial cloud of cold atoms comprising atoms having a first atomic energy level |a> and a second atomic energy level |b>, to generate and, where applicable, move a cloud of cold atoms having the first atomic energy level and a cloud of cold atoms having the second atomic energy level in opposite directions along a predetermined path, and to reassemble the clouds of cold atoms to generate at least one cloud of cold atoms to be analyzed: a single capture cloud CL for the sensor described in document WO 2017089489; and two clouds CL1 and CL2 for use in free propagation mode. Figure 1 Sensors in.

[0203] The cold atom sensor further comprises at least one measuring device 1 according to the invention, which is configured to measure a first quantity Γ representing the population number N1 of the first atomic energy level |a> A1 and a second quantity Γ representing the population N2 of the second atomic energy level |b> A2 A physical quantity such as inertia is expressed in terms of a first representation Γ A1 and the second representation Γ A2 To this end, the device 1 measures Γ in sequence over time. A1 and Γ A2 .

[0204] Figure 12 A first example of a gyroscopic inertial sensor according to the present invention is shown, the operating principle of which is described in document WO2017089489. The microcircuit comprises two coplanar microwave waveguides CPW1 and CPW2 (powered by a generator GMW) and a plurality of conductors 17 (powered by a DC voltage or current generator GDC), which can guide and move the cloud along a substantially rectangular path 16. The device according to the present invention is integrated into the sensor, with the antenna Ant, waveguide GO, and power splitter SS all integrated on an atomic chip Ach, in a plane different from the plane in which the waveguides CPW1 and CPW2 are integrated and the conductors 17 are integrated. The antenna Ant is positioned close to, and as close as possible to, the cold atomic cloud CL to be analyzed, typically coinciding with the location of the CL.

[0205] Figure 13 Shown as Figure 1 A second example of an accelerometer inertial sensor CAS according to the invention is described in [ 1 ]. The microcircuit here comprises a magnetic quadrupole. The paths of the two clouds are straight along the axis Y, and the two clouds CL1 and CL2 to be analyzed are in motion (free propagation) during and after the recombination. The sensor comprises a first measuring device 1 according to the invention and a second measuring device 2 according to the invention. The first device 1 comprises a first microwave source S1, a first microwave waveguide GO1, a first antenna Ant1, a first power splitter SS1 (all three integrated on the chip Ach) and a first detector D1, and is configured to measure a first representative quantity Γ by analyzing the cloud CL1. A1 The second device 2 includes a microwave source S2, a second microwave waveguide GO2, a second antenna Ant2, a second power splitter SS2 (all three are also integrated on the chip Ach) and a second detector D2, and is configured to measure the second representative quantity Γ by analyzing the cloud CL2. A2 Γ is then measured in parallel using two devices 1 and 2. A1 and Γ A2 Preferably, the antennas are arranged symmetrically about the position in space where the recombination occurs and are located on the path 16, and the measurements are time-of-flight measurements.

[0206] according to Figure 14 One embodiment shown in Figure 13 The sensors described in include Figure 11 The two power splitter devices SS1 and SS2 (links 211 and 212) described in , allow access to the phase measurement of the atomic reflected signal and the reference reflected signal.

[0207] According to another aspect, the invention relates to a method 100 for measuring a quantity representative of the population number N of cold atoms occupying a predetermined atomic energy level, the cold atoms being located in a cloud of cold atoms CL to be analyzed, the method comprising the following steps:

[0208] A generates a microwave incident signal SMWi at a predetermined signal frequency fs,

[0209] B propagates the incident signal in a microwave waveguide and transmits the incident signal to the cold atomic cloud,

[0210] C recovers the reflected signal SMWra, called the atomic reflected signal, generated by the reflection of the incident signal by the cloud and its environment, and which propagates in the waveguide in the opposite direction to the incident signal,

[0211] D extracting at least a portion of the atomic reflection signal,

[0212] E detects and extracts the atomic reflection signal SMWra,

[0213] F. before or after performing steps A to E in the presence of said cold atomic cloud, performing steps A to E in the absence of said cloud, such that in the absence of said cloud, the reflection of the incident signal by the cloud-evolved environment produces a reflection signal referred to as a reference reflection signal,

[0214] G is based on the detection value S of the atomic reflection signal SMWra A The detected value S0 of the reference reflection signal SMWr0 determines the quantity representative of the population number N of cold atoms of the atomic level to be analyzed.

[0215] The implementation of steps A to F in the absence of a cloud is preferably performed after steps A to F in the presence of a cloud.

[0216] In the method of measuring inertia by means of a sensor, the measurement method 100 corresponds to implementing the detection step E5 instead of the step E5 ′ of implementing the light beam described in the prior art.

Claims

1. A quantity Γ for measuring the population number N of cold atoms occupying a predetermined atomic energy level A The device (1, 2), wherein the cold atoms are located in a cold atom cloud CL to be analyzed, comprises: a microwave source S configured to generate an incident signal SMWi at a predetermined signal frequency fs, - a microwave waveguide GO configured to propagate the incident signal, and an antenna Ant configured to transmit the incident signal to the cold atomic cloud and its environment, The antenna and the microwave waveguide are also capable of recovering an atomic reflection signal SMWra generated by the cloud and its environment reflecting the incident signal, and the atomic reflection signal propagates in the waveguide in a direction opposite to that of the incident signal. - a power splitter device SS coupled to the microwave waveguide and configured to extract at least a portion of the atomic reflection signal, - a detector Det, configured to detect the atomic reflection signal extracted by the power splitting device, The quantity representing the number N of cold atoms occupying the predetermined atomic energy level is the detected value S from the atomic reflection signal SMWra. A and obtained from the detection value S0 of the signal reflected by the environment in the absence of said cloud, called reference reflected signal SMWr0.

2. The device according to claim 1, wherein The expression amount is proportional to the population.

3. The device according to any one of claims 1 and 2, wherein The representation quantity Γ A is called the atomic reflection coefficient and is defined as follows: Among them, S A is the detected value of the atomic reflection signal, S0 is the detection value of the reference reflection signal.

4. The device according to any one of claims 1 and 2, wherein The detector is configured to detect a reflected atomic power and a reference power, the representative quantity being determined based on the atomic power and the reference power.

5. The device according to any one of claims 1 and 2, wherein: The detector is configured to detect a reflected atomic amplitude and a reference amplitude, the representative quantity being determined from the amplitude, and / or to detect an atomic phase and a reference phase, the representative quantity being determined from the phase.

6. The device according to claim 5, wherein The power splitting device SS is configured to extract a portion of the incident signal and the detector Det is configured to detect the portion of the incident signal and to measure the atomic phase and the reference phase.

7. The device according to any one of claims 1 and 2, wherein: The microwave waveguide and the antenna are integrated on an atom chip.

8. The device according to any one of claims 1 and 2, wherein: The signal frequency fs is such that: fs≥fT+f0 / 2 where fT is the frequency of the highest detected transition among the possible transitions, f0 is the separation frequency of the two Zeeman sublevels of the cold atom.

9. A cold atomic sensor (CAS), comprising: - an atom chip Ach comprising a microcircuit configured to generate a magnetic guide and an atom source AS, said atom chip and said atom source being placed in a vacuum chamber, The cold atom sensor is configured to: generate an initial cold atom cloud comprising atoms having a first atomic energy level |a> and a second atomic energy level |b>, generate and, where applicable, move the cold atom cloud having the first atomic energy level and the cold atom cloud having the second atomic energy level in opposite directions along a predetermined path, and recombine the cold atom clouds to generate at least one cold atom cloud CL, CL1, CL2 to be analyzed, The cold atomic sensor further comprises: - at least one device according to any one of claims 1 to 8, configured to measure a first quantity Γ representative of the population N1 of the first atomic energy level |a> A1 and a second quantity Γ representing the population N2 of the second atomic energy level |b> A2 , a physical quantity determined based on the first and second quantities represented.

10. The cold atomic sensor according to claim 9, wherein: The microwave waveguide GO and the antenna of the device are integrated on the atom chip, and wherein the antenna Ant is located close to the cold atom cloud to be analyzed.

11. The cold atomic sensor according to claim 10, wherein: The power splitter SS of the device is also integrated on the atom chip.

12. The cold atom sensor according to any one of claims 10 and 11, configured to generate a first cold atom cloud CL1 and a second cold atom cloud CL2 for analysis in motion, and comprising a first measuring device (1) and a second measuring device (2), wherein the first measuring device (1) and the second measuring device (2) respectively comprise a first waveguide GO1 and a second waveguide GO2 and a first antenna Ant1 and a second antenna Ant2 integrated on the chip, and the first measuring device (1) is configured to measure a first quantity Γ representing the first cold atom cloud CL1 A1 And the second measuring device (2) is configured to measure a second quantity Γ representing the second cold atomic cloud CL2 A2 .

13. A method for measuring a quantity representative of the population number N of cold atoms occupying a predetermined atomic energy level, said cold atoms being located in a cloud of cold atoms CL to be analyzed, The method comprises the following steps: A generates a microwave incident signal SMWi at a predetermined signal frequency fs, B propagating the incident signal in a microwave waveguide and, where applicable, transmitting the incident signal to the cold atomic cloud, C recovers a reflected signal SMWra, called an atomic reflected signal, generated by the cloud and its environment reflecting the incident signal, and propagating in the waveguide in a direction opposite to that of the incident signal, D extracting at least a portion of the atomic reflection signal, E detects and extracts the atomic reflection signal SMWra, F. performing steps A to E in the absence of said cloud, before or after performing steps A to E in the presence of said cold atomic cloud, such that in the absence of said cloud, reflection of said incident signal by the cloud-evolved environment produces a reflection signal referred to as a reference reflection signal, G is based on the value S of the atomic reflection signal SMWra A and the value S0 of the reference reflection signal SMWr0 determine the quantity representative of the population number N of the cold atoms of the atomic level to be analyzed.

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