Atom interferometer with improved common noise suppression

Abstract

The invention relates to an atom interferometer with a laser light source through which first laser light can be emitted at a fundamental frequency, and with at least one light emission device through which second laser light can be emitted at a frequency that deviates from the fundamental frequency by a difference frequency, and with an atom trap in which one or more atom ensembles can be captured and irradiated with the first and the second laser light.

Description

[0001] The invention relates to an atom interferometer with a laser light source through which first laser light can be emitted at a fundamental frequency, and with at least one light emission device through which second laser light can be emitted at a frequency that deviates from the fundamental frequency by a difference frequency, and with an atom trap in which one or more atom ensembles can be captured and irradiated with the first and the second laser light.

[0002] Atom interferometric gravimeters offer long-term stable, absolute gravity measurements. Compared to the widely used laser gravimeters (FG5X), their measurement principle allows for quasi-continuous data acquisition over extended periods and the prospect of higher accuracy. Portable and commercial instruments have already been developed and operated on mobile platforms such as ships. A crucial component of an atom interferometric gravimeter, or any atom interferometer for that matter, is the source of cold atoms. These are typically based on laser cooling in magneto-optical traps and polarization gradient cooling. A three-dimensional magneto-optical trap requires not only magnetic fields but also light fields, which are typically directed antiparallel along the three spatial axes to create an interaction zone for cooling the trapped atoms.This implies a comparatively complex setup with multiple beam-shaping optics and corresponding optical access points to the vacuum system in which the atoms are trapped and cooled. Specially designed pyramidal or lattice structures have been successfully used to create a beam geometry from a single incident laser beam that can effectively cool along all three spatial axes.

[0003] A key component of an atom interferometric gravimeter is the laser system, which generates the laser beams for the interferometry pulses. These pulses act on the atomic ensemble (i.e., the cloud of atoms trapped in an interaction zone) in two or more interaction zones within the vacuum system. The interferometry pulses cause the atoms to be transformed into quantum mechanical superposition states, which spatially separate and then recombine for interference. The interference signal is sensitive to accelerations, thus enabling the realization of a gravimeter. The laser beams typically require two different frequency components, which are emitted simultaneously. Various methods exist for generating these frequency components, each requiring compromises between volume, cost, and the purity of the frequency spectrum.A popular solution is the frequency modulation of a laser, for example using acousto-optical or electro-optical modulators (AOMs and EOMs), as this enables a compact and cost-effective laser system. However, generating the desired frequencies with optical modulators also introduces unwanted frequency components that impair signal quality. Since atom interferometric gravimeters require laser beams from both vertical spatial directions, the laser beam is often first directed at the atomic ensemble from one direction and then reflected back after passing through a mirror. This covers both spatial directions.

[0004] An important application of gravimeters is their use in gradiometers, where two spatially separated gravimeters are operated simultaneously. The first derivative of gravity, the gravitational gradient, can be determined from the difference signal. This difference signal benefits from common-noise rejection, meaning the gradiometer signal is only degraded by technical noise sources that affect the two gravimeters differently. It is particularly important that the two gravimeters are operated as symmetrically as possible to maximize the benefits of this noise rejection.

[0005] Interferometry essentially measures the phase relationships between two waves, in this case matter waves. For example, if one wants to measure the gradient of gravity with an atom interferometer, the upper atom interferometer contains the gravitational information about gravity at a height h1, and the lower atom interferometer contains the gravitational information at a height h2. The greater the distance between the atom interferometers, the larger the difference signal becomes. From atom interferometers 1 and 2, one obtains (simplified representation) φ1=φh1+φdisturbance(h1) and φ2=φh2+φdisturbance(h2).

[0006] If the disturbance is caused by an unwanted frequency component, it is periodic with the distance, so the disturbance has the same value at different distances. When the gradient is then calculated, the disturbance terms cancel out. Δφ=φ1−φ2=φh1+φdisturbance(h1)−φh2+φdisturbance(h2)=φh1−φh2

[0007] The state of the art in gradiometry with compact atom interferometers is well described in publication WO 2014 / 106811 A2. The gradiometer consists of two (or more) interaction zones. Patent WO 2014 / 106811 A2 focuses on the compact and simultaneous generation of multiple atom clouds using pyramid reflectors. Good noise suppression in gradiometers is achieved by ensuring that the laser beams for interferometry strike both atom clouds simultaneously. This is often accomplished by having a laser beam with the necessary frequency components pass through all atom clouds, be reflected by a mirror, and then strike the atom clouds again from the opposite direction.

[0008] If the frequency spectrum of the interferometric light contains not only the two necessary components but also additional frequency components, these frequency components can generate additional phase contributions during interferometer readout. These phase contributions can be determined and taken into account during signal analysis. However, uncertainties or noise in the operating parameters limit the characterization of these phase contributions, and these effects can therefore limit the sensitivity of the gravimeter.

[0009] In the gradiometer configuration, where two or more interferometers are operated at a distance from each other, each atom interferometer experiences its own phase contribution, which depends on the distance to the mirror. Since this contribution is not the same for both interferometers, the differential phase contribution in the gradiometer can be even larger than in the gravimeter and can also limit the sensitivity of the gradiometer.

[0010] In wave optics, the optical path length is the geometric path length I along a light ray multiplied by the refractive index n of the propagation medium. Because of the definition of the refractive index, the optical path length is thus the path length relative to a vacuum when light passes through a medium with refractive index n and length l. In other words, the optical path length is the distance that light travels in the same amount of time in a vacuum as it would take to travel a given path with a potentially different phase velocity (the speed at which the wavefronts of the light propagate). If there are no objects with varying refractive indices in the path of the first and second laser beams, the optical path length can also be defined by the geometric path length (hereinafter referred to as distance). Wherever the term distance is used below, it includes the optical path length of the respective light ray.

[0011] US patent 2014 / 0319329 A1 discloses measurement methods based on atom interferometry. US patent 2024 / 0134084 A1 discloses the preparation of cold atomic clouds for measuring gravitational gradients. US patent 2016 / 0178792 A1 discloses a gradiometer configuration that is invariant with respect to laser phase variations and sensor rotations.

[0012] The invention is based on the objective of providing an improved atom interferometer.

[0013] This task is solved in an atom interferometer of the type mentioned above by the fact that a) in the case of multiple atom ensembles, the optical path length between two atom ensembles that are assigned to each other as a pair and form a gradiometer arrangement corresponds to half the wavelength or an integer multiple of half the wavelength of the difference frequency or another disturbance frequency and / or b) the optical path length between an ensemble of atoms and a mirror of the atom interferometer reflecting the first and second light corresponds to half the wavelength or an integer multiple of half the wavelength of the difference frequency or another disturbance frequency.

[0014] If the distance between the two atom interferometers, or rather their two atom ensembles, in the gradiometer configuration is not arbitrary but optimally chosen, the differential phase shift between the interferometers can be suppressed. The phase contributions in the atom interferometer from unwanted frequency components exhibit periodic behavior when the distance between the two atom ensembles, or between the mirror and the atom ensemble, changes. The periodicity of the effect is given by half the wavelength of the unwanted modulation frequency. If this frequency is already known during the development phase of the gradiometer, the distance between the interferometers can be optimized. If the distance between the two atom interferometers is exactly a multiple of half the wavelength of the unwanted modulation frequency, the atom interferometers experience the same phase contribution at all times, so that it disappears in the differential analysis in the gradiometer.

[0015] The stated distance value or value of the optical path length does not necessarily have to be achieved exactly; even an approximate achievement of the desired value already results in a significant improvement in the accuracy and noise suppression of the atom interferometer.

[0016] The invention can be used not only to compensate for interference effects generated by the difference frequency, but also to compensate for interference effects generated by any other interference frequencies modulated onto the first laser light.

[0017] The invention is applicable analogously to a system with three or more atom interferometers or their atom ensembles located at a defined distance.

[0018] The invention can also be applied to individual atom interferometers of any type. If the optical path length between the atom ensembles and the mirror does not change or changes only minimally, as is the case, for example, with gyroscopes, the distance between the atom ensembles and the mirror can be selected to be a multiple of half the wavelength of the interfering frequency, so that the imposed phase shift is zero for all interferometry pulses. If the distances to the mirror change, the distances between the interaction zones can be selected to be a multiple of half the wavelength of the interfering frequency.

[0019] In an advantageous embodiment, the optical path length between the two atom ensembles (in the case of a pair of atom ensembles) or the optical path length between an atom ensemble and the mirror can be at least two, three, or four times half the difference frequency. This has the advantage that practical geometric dimensions can be achieved, with optical path lengths on the order of several centimeters. Accordingly, known atom interferometers do not need to be completely redesigned mechanically, but only moderately modified to achieve the desired distance value.

[0020] According to an advantageous embodiment of the invention, the atom interferometer is configured to perform Raman interferometry. For example, Raman light interferometry sequences can be executed by the atom interferometer. The first laser light and the second laser light, which differs from it by the difference frequency, serve to imprint different momentum states onto the atoms used. According to an advantageous embodiment of the invention, the atom interferometer is configured as a gradiometer, in which two pairs of atoms form the gradiometer arrangement. With such a gradiometer, measurements of the gravitational field can be performed with particularly good sensitivity, especially for determining the gravitational gradient of the gravitational field.

[0021] According to an advantageous embodiment of the invention, the atom interferometer is designed as a gravimeter. With such a gravimeter, long-term stable, absolute gravity measurements can be performed with particularly good sensitivity.

[0022] According to an advantageous embodiment of the invention, the light emission device is designed as an optical modulator, by which the first laser light can be converted into the second laser light by optical modulation. This allows for a simple and reliable provision of the second laser light, which deviates from the fundamental frequency of the first laser light by the difference frequency. The optical modulator can, for example, be designed as an electro-optical modulator or an acousto-optical modulator. The second laser light can thus be generated by irradiating an optical modulator with the first laser light from the laser light source or another light source that emits first laser light at the fundamental frequency, and by controlling the optical modulator with the desired difference frequency.It is also possible to generate the second laser light by a second laser light source separate from the first laser light source, which directly emits the second laser light that differs from the fundamental frequency by the difference frequency, i.e., without an additional optical modulator.

[0023] According to an advantageous embodiment of the invention, the optical path length between two atom ensembles assigned to each other as a pair is determined by the distance between the starting and / or ending locations of two interferometers formed by the atom ensembles.

[0024] According to an advantageous embodiment of the invention, the optical path length between two paired atom ensembles is determined by the distance between two interferometry interaction zones. Accordingly, the atom interferometer can be easily constructed with improved sensitivity. For example, two pyramid-shaped mirrors, such as those described in WO 2014 / 106811 A2, can be arranged such that the distance, or the resulting optical path length, corresponds to half the wavelength or an integer multiple of half the wavelength of the difference frequency or another interfering frequency.

[0025] In general, it can be said that the desired distance value can be determined by setting the distance between the interferometry interaction zones and / or between an interferometry interaction zone and the mirror.

[0026] According to an advantageous embodiment of the invention, the optical path length between two paired atom ensembles is determined by beam splitter operations (momentum transfer through atom-light interaction). In this way, for example, the sensitivity of a gravimeter can be improved in a simple manner.

[0027] According to an advantageous embodiment of the invention, the optical path length of an atom ensemble from a mirror reflecting the first and second light is determined by the distance of an interferometry interaction zone, e.g., a magneto-optical trap, from the mirror. For example, the desired distance value can be determined by the distance of a pyramid-shaped mirror from the mirror.

[0028] Interferometry interaction zones are defined as the locations where the respective atom ensembles are irradiated with the interferometry light pulses. The position of the atoms is initially determined by the position of the magneto-optical trap and may change during the interferometry sequence due to the free-fall motion of the atoms, thus also changing the location of the interferometry interaction zones.

[0029] The invention is suitable for various fields of application, e.g.: Geophysical applications: • Monitoring of volcanoes • Monitoring seismic activity • Hydrology Exploration: • Raw material exploration • Groundwater management Engineering: • Monitoring the stability of roads, dams and dikes • Creation of gravity field maps, detection of cavities in urban development and mining regions

[0030] The invention is explained in more detail below with reference to exemplary embodiments and drawings.

[0031] They show Fig. 1. A schematic representation of an atom interferometer, Fig. 2 light frequency bands in the atom interferometer, Fig. 3 a first embodiment of a distance-optimized atom interferometer, Fig. 4 a second embodiment of a distance-optimized atom interferometer.

[0032] The Fig. Figure 1 shows an atom interferometer 7, which can be configured, for example, as a gradiometer or gravimeter. The atom interferometer 7 has a laser light source 1 through which first laser light with a fundamental frequency f1 is emitted. The atom interferometer 7 also has a light emission device 2, for example with an optical modulator, through which second laser light with a frequency f2 can be emitted. The frequency f2 differs from the fundamental frequency f1 by a difference frequency Δf.

[0033] The atom interferometer 7 also has an atom trap 3 in which several atom ensembles 4, 5 are trapped at predefined positions, i.e., at interaction zones, and are irradiated with the first and second laser light, thus with the different frequencies f1, f2. The atom trap 3 can be designed, for example, as a magneto-optical trap (MOT), as described in detail in WO 2014 / 106811 A2.

[0034] The in Fig. The depicted irradiation of atom ensembles 4, 5 with laser light having only the frequencies f1, f2 is an idealized assumption that neglects the effect occurring in practice, namely that an optical modulator driven with the difference frequency Δf not only generates the one desired frequency sideband f2, but also further undesired sidebands f3, f4, f5,... etc., as shown in the Fig. Figure 2 shows that the sidebands are all spaced apart by the difference frequency Δf.

[0035] Rubidium, especially the isotope Rb-87, is an advantageous element for use in atom interferometric instruments. Light with a wavelength of 780 nm is required for the operation of the interferometric light pulses. This necessitates two frequencies, f1 and f2, separated by Δf of 6.835 GHz. One of these frequencies can, for example, be the fundamental frequency of laser source 1. The second frequency can be superimposed on the light from this laser 1 using an electro-optical modulator with a modulation at 6.835 GHz. This produces the sidebands described above, each separated by the selected difference frequency Δf and its multiples.

[0036] The unwanted frequency sidebands lead to a phase shift of the interferometric signal. In the gradiometer configuration, at an arbitrarily chosen distance, both interferometers experience a different phase shift, resulting in a differential phase shift. This differential phase shift is avoided, as demonstrated by the Fig. Figure 3 shows that if the optical path length d1 between the two atom ensembles 4, 5, which are paired and form a gradiometer arrangement, is chosen to be a multiple of half the wavelength λ / 2 of the frequency sideband of the 6.835 GHz frequency, i.e., λ / 2 = c / Δf = 21.9 mm, where c is the speed of light, then, for example, if an optical path length d1 of 219 mm is chosen, i.e., ten times the original value, the interferometers experience the same phase shift for all parameters, so that it does not affect the gradiometer signal.

[0037] As the Fig. As shown in Figure 3, the optical path length d2 between an atom ensemble 4, 5 and a mirror 6 of the atom interferometer 7 reflecting the first and second light can additionally or alternatively be set to such an optimal distance, namely a multiple of half the wavelength λ / 2 of the frequency sideband.

[0038] Additionally, as the Fig. Figure 4 illustrates that in an atom interferometer, where, for example, only an ensemble of atoms 4 is trapped in the atom trap 3, interference suppression can also be achieved through such distance optimization. Fig.Figure 4 shows a time sequence plotted against time t. At time t1, the atom ensemble 4 in the atom trap 3 is irradiated with laser light, as indicated by the arrows. In the subsequent steps of the sequence, an optimal optical path length d1 is established. For example, by a beam splitter operation at time t2, the atom ensemble 4 can be divided into two separate sub-ensembles 4a and 4b such that their separation d1 corresponds to the aforementioned optimal distance, i.e., a multiple of half the wavelength λ / 2 of the frequency sideband. The beam splitter operation can be performed, for example, by further incident laser light, as indicated by the arrows at time t2.

Claims

[1] Atom interferometer (7) comprising a laser light source (1) through which first laser light with a fundamental frequency (f1) can be emitted, and with at least one light emission device (2) through which second laser light with a frequency (f2) that deviates from the fundamental frequency (f1) by a difference frequency (Δf), and with an atom trap (3) in which one or more atom ensembles (4, 5) can be trapped and irradiated with the first and the second laser light, characterized by , that a) in the case of several atom ensembles (4, 5), the optical path length (d1) between two atom ensembles (4, 5) that are assigned to each other as a pair and form a gradiometer arrangement corresponds to half the wavelength (λ / 2) or an integer multiple of half the wavelength (λ / 2) of the difference frequency (Δf) or another disturbance frequency and / or b) the optical path length between an (d2) atom ensemble (4, 5) and a mirror (6) of the atom interferometer (7) reflecting the first and second light corresponds to half the wavelength (λ / 2) or an integer multiple of half the wavelength (λ / 2) of the difference frequency (Δf) or another disturbance frequency. [2] Atom interferometer according to claim 1, characterized by , that the atom interferometer (7) is designed as a gradiometer in which two atom ensembles (4, 5) assigned to each other as a pair form the gradiometer arrangement. [3] Atom interferometer according to any one of the preceding claims, characterized by , that the atom interferometer (7) is designed as a gravimeter. [4] Atom interferometer according to any one of the preceding claims, characterized by , that the light emission device (2) is designed as an optical modulator by which the first laser light can be converted into the second laser light by optical modulation. [5] Atom interferometer according to any one of the preceding claims, characterized by , that the optical path length (d1) between two atom ensembles (4, 5) assigned to each other as a pair is determined by the distance between the starting and / or ending locations of two interferometers. [6] Atom interferometer according to any one of the preceding claims, characterized by , that the optical path length (d1) between two atom ensembles (4, 5) assigned to each other as a pair is determined by the distance between two interferometry interaction zones. [7] Atom interferometer according to any one of the preceding claims, characterized by , that the optical path length (d1) between two atom ensembles (4, 5) assigned to each other as a pair is determined by beam splitter operations. [8] Atom interferometer according to any one of the preceding claims, characterized by, that the optical path length of an atom ensemble (4, 5) to a mirror (6) reflecting the first and second light is determined by the distance of an interferometry interaction zone from the mirror (6).

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