Patent Application
20090242743
The present invention concerns a guided coherent atom source or matter-wave laser. The invention also concerns an atomic interferometer which can be used for inertial atom sensors.
Methods and apparatus have been developed for manipulating atoms. U.S. Pat. No. 5,274,232 describes an “atomic fountain” wherein the atoms are initially trapped in a magnetic trap and then launched vertically with a controlled velocity.
The general principle of magnetic trapping for cold atoms is known. Devices including permanent magnets have been used to produce high density Bose-Einstein Condensate (BEC). However, such devices do not allow to cancel the magnetic field, so they do not enable to extract atoms from the condensate.
Electromagnetic devices that produce magnetic trapping of cold neutral atoms have also been developed. For example, EP 1130949 describes a ferromagnetic structure with six-poles used to generate a trapping magnetic field. This setup allows continuous or pulsed operation with turn-off times of 100 ms. The electro-magnetic structure enables to adjust the magnetic fields produced by the various coils by adjusting the current flowing through the coils. Such an electro-magnetic device allows to generate high density cold neutral atoms condensate.
Hybrid magneto-optic trapping of cold neutral atoms has also been described (Guérin et al., Phys. Rev. Lett., 97, 200402 (2006), noted [PRL 97] below) by superimposing an optical laser beam (from a Nd:YAG laser, λ=1064 nm) to a magnetically trapped cold cloud of 87Rb atoms. Bose-Einstein Condensation is directly obtained at the intersection of the magnetic trap with an elongated optical trap.
After trapping, atoms can be released and dropped or launched in order to create a guided atom source. For use in atom interferometry, the atoms direction, velocity, and repetition rate must be extremely controlled.
The general principle of a coherent guided atom source, or “guided atom laser” in short, is also known. The publication [PRL97] reports the realization of a guided quasicontiuous atom laser, where the coherent source, i.e. the trapped BEC, and an optical waveguide are merged together in a hybrid configuration of a magnetic Ioffe-Pritchard trap and a horizontally elongated far off-resonance optical trap, constituting an atomic waveguide. The BEC, in a state sensitive to both trapping potentials (magnetic and optic), is submitted to an RF-outcoupler yielding atoms in a state sensitive only to the optical potential. The atoms are submitted to a repulsive potential due to interactions with the BEC that give a first kinetic energy to the atom beam. A coherent matter-wave is thus extracted, and the atoms propagate along the weak confining direction of the optical tweezer, resulting in an atom laser. This guided 87Rb atom laser presents a large and almost constant de Broglie wavelength ≧0.5 μm., with the atom-laser velocity ˜9 mm·s−1 and an atom flux of 5×105 at·s−1.
The advantage of such an atom laser is to provide a coherent beam of atoms extracted from a magnetic trap, wherein the atoms position and direction are well defined in space due to the optical waveguide. The guided atom coherent source also enables to adjust the atoms velocity, i.e. the atom laser wavelength, by adjusting the laser focus and RF power. The atom laser thus formed is equivalent to an optical laser source pigtailed to a fiber optic, wherein photons propagate along the fiber optic waveguide.
High precision inertial atom sensors in embedded systems are desirable for land or underwater navigation and geodesy. Another field of application is the use of inertial atom sensors in microgravity or in space for fundamental physics experiments or for inertial mapping.
Embedded inertial atom sensors would be improved with a compact, portable guided coherent atom source able to produce cold atoms with precise position, emission direction, velocity, high repetition rate, and high brilliance (flux×collimation) that was not available prior to the invention.
As a matter of fact, the setup disclosed in [PRL 97] cannot be used to make a compact and portable inertial sensor for various environments (navigation, space . . . ) because it uses electro-magnetic (ferromagnetic structure) and optical components (Nd:YAG laser) that are too bulky and energy-consuming to be embedded. The magnetic structure power consumption is around a few hundred Watts. The Nd:YAG laser output is around 2 W.
Besides, the setup disclosed in [PRL 97] does not allow high rate repeatability, due to experimental imperfections. The setup long term stability is limited by centering inaccuracy between the magnetic trap and optical waveguide. For high precision atomic interferometry applications, the atomic source must be positioned with ˜1 μm precision.
Prior art atomic fountains propose setups where atoms fall under gravity or are launched but with large position and direction uncertainty. The difficulty for high precision atomic interferometry lies not only in atoms trapping, but also in injecting into a waveguide and guiding them while maintaining coherency.
In order to miniaturize components for atom sources, integrated magnetic traps have been disclosed (for example see U.S. Pat. No. 7,126,112). Such magnetic traps use electric wires deposited on a substrate that generate magnetic fields. U.S. Pat. No. 7,126,112 reports the integration of a microchip in a sealed vacuum chamber used to confine, cool and manipulate cold atoms. The atom-chip is used to create an electro-magnetic field and produce a 87Rb BEC.
As outlined in U.S. Pat. No. 7,126,112 (Col 6 L 5-7), chip-scale atomic system require an unwieldy assembly of electronic, optical and vacuum instrumentation. U.S. Pat. No. 7,126,112 simplifies the vacuum system for BEC atom chip, by sealing the atom chip into the wall of a vacuum chamber. This vacuum chamber includes optical access for external light beams coming from UV lamps. A silver mirror can be transferred to the chip surface to create a MOT. However, such an optical beam is not sufficient for confining and guiding atoms. The device disclosed in U.S. Pat. No. 7,126,112 does not show how to couple and align the magnetic trap and the optical beam, and it does not form an atom laser. This device does not allow efficient atoms extraction for interferometry. Even if the system disclosed in U.S. Pat. No. 7,126,112 is more compact than previous system using solid ferromagnetic structures, it is still too bulky for embedded sensors. In addition, it does not solve the difficulty in alignment between the magnetic trap and the optical waveguide.
It is an object of the invention to propose a compact, light-weight, low energy-consuming coherent guided atom source, that provides cold atoms having precision controlled and adjustable position, direction and velocity at a high repeatability rate.
The guided coherent atom source according to the invention solves these difficulties by integrating onto a same substrate an electro-magnetic micro-chip and a solid-state laser source.
Concerning the application to cold-atom interferometry, prior art coherent atom sources provide insufficient measurement repetition rate. In addition, high gradient magnetic fields from bulk ferromagnetic trap structures induce perturbations that prevent high precision measurements. The atom source of the invention is compact enough so that coherent atoms can be used away from the magnetic trap, without being perturbed by residual magnetic fields. The atom source of the invention provides high repetition rate atom laser production thus allowing high precision interferometry measurements.
The invention concerns a guided coherent atom source comprising
characterized in that
In various embodiments the invention also concerns the following features, that can be considered alone or according to all possible technical combinations and each bring specific advantages:
The invention also concerns an atomic interferometer comprising
The above description is given as an example of the invention but can have various embodiments that will be better understood when referring to the following figures:
This guided coherent atom source 1 comprises means for generating neutral atoms in a gaseous state (not shown in
The atoms belong to the alkaline or alkaline earths atoms. In the example below 87Rb atoms are used for the atom source of the invention. Other convenient atoms (such as Ytterbium) could also be used.
The atom source 1 comprises means for generating a magnetic field 4, and more particularly an electro-magnetic micro-chip 6 capable of condensing the atoms in a BEC. The magnetic trap is obtained using wires on an micro-chip, providing a magnetic field pattern similar (considering gradients, intensity and field geometry) to the one obtained using a bulky ferromagnetic structure, but with reduced size. The electrically conductive wires 6 are patterned on a surface 18 of the substrate 14. Different wires patterns can be used.
In a first embodiment shown in
h=μ
0
I/(2πB0)
The radial confinement gradient is given by the equation
b′=B
0
/h=2πB02/(μ0I)
The confinement is thus stronger when electric current is small, and when the atoms cloud is close to the surface. So a process for producing the desired condensed atoms consists in creating the BEC in a magnetic trap confined close to the substrate surface, and then to control the condensed atoms position relatively to the surface by changing the current. In this way, the confinement is reduced as required to form a guided atom source (see [PRL 97]).
Typical parameters can be as follow:
B0=6 G; Bz=1 G; I=100 mA (high confinement): h=33 μm, ω=2π*1.6 kHz
B0=6 G; Bz=1 G; I=3 A (low confinement): h=1 mm, ω=2π*54 Hz
The condensed atoms form a Bose-Einstein Condensate (BEC). The distance between the BEC 30 and the substrate surface 18 can be adjusted by varying the applied electric current. More particularly, the BEC 30 is first formed in the vicinity of the substrate surface 18, and the electric current is progressively increased in order to increase the distance between the substrate 14 and the BEC 30 and to decrease the BEC radial confinement.
The atom source 1 of
The atom source 1 comprises a laser diode 20 for emitting and directing an optical coherent beam 12 toward the condensed atoms so that the condensed atoms acquire a velocity and are guided by the said optical coherent beam (12). The laser diode emission wavelength is selected to be off resonance for atoms internal transition. 87Rb has transitions at ˜780 nm. and 795 nm., so the laser wavelength is chosen above 780 nm. A diode laser emitting around ˜1.064 μm can be used, with an output power of a few hundred mW. The difference between resonance and guiding laser wavelength is noted Δ. The optical guiding force is proportional the laser intensity, and inversely to the laser waist dimention (w) and to Δ:
F=k·I/(w·Δ)
By varying the electric power supplied to the laser diode, the optical beam intensity can be adjusted. This enables to adjust a guiding force, and thus to adjust the atoms acceleration between 0 and 10 mm·s−2. After applying an RF-EM field, the atoms are still sensitive to the optical potential and thus propagate along the optical beam axis. The atoms are attracted toward the high intensity region and thus guided along the optical waveguide. The atoms propagate in one direction or in two opposed directions depending on adjustment of waist position relatively to the atoms.
As shown in
As shown in
In the configuration represented
In the configuration represented
A focusing microlens 24, can be used in order to adjust the diode focus position. The microlens 24 is preferably attached to the same substrate 14, or to the laser diode 20.
The microchip can include a reflecting layer deposited on the surface. The layer (or multilayer) surface treatment can be used to trap “hot” atoms into the BEC. Such a surface treatement is chosen to provide a high reflection coefficient at the “hot” atoms wavelength, and to be transparent at the optic/laser source wavelength.
When applying a magnetic field generated by the micro-chip and an optical beam from the laser diode, atoms are trapped at the intersection of the BEC and of the elongated optical waveguide. An RF-outcoupler at the boundary of the BEC and the waveguide enables to couple atoms from the BEC along the optical waveguide, thus producing a coherent guided atom source. The atoms are attracted by the lowest optical potential point in the optical beam, that is at the waist of the laser beam. By adjusting the distance between the BEC and the waist of the laser beam, one can adjust the atoms velocity.
The atoms propagate along the optical waveguide, in a coherent way, along distances ranging between 0.1 and 10 mm.
The de Broglie wavelength is comprised between 0.4 μm and 5 μm.
As illustrated in
As in
The BEC area is located in the central area between the two long branches of the two Z, at a distance h from the wires plane.
When choosing Bext=μ0
The following parameters can be used:
Bext=6 G; Bz=1 G; S=2 mm, I=3 A: h=1 mm, ω=2π*54 Hz.
By adjusting the electric current applied to the electric wires 6, the BEC position and confinement can be adjusted. The BEC position can even be located inside the substrate or in front of the substrate surface opposed to the patterned wire structure.
Since confinement is less strong with the two-wires configuration, it is advisable to make the condensate using only one wire (applying current only to one of the Z-shaped wires), and then to switch to a two-wires configuration (by applying electric current to the two wires) for coupling with the optical waveguide.
A laser source 22 emission axis 17 is directed toward the BEC area of the magnetic trap, in order to create a hybrid magneto-optic trap and a waveguide for the atoms. The laser source is in this example fixed onto the substrate 14, using conventional mechanical mountings. The substrate 14 may be formed in a transparent material such as glass or sapphire. A converging microlens can be etched into the substrate. The microlens can be made from multilayers that create a focusing effect.
The optical beam goes through the microlens.
Typical parameters are a working distance of a few hundred microns, for a millimeter size lens diameter. The transverse guide frequency can typically be around a few hundred Hertz.
The electro-magnetic micro-chip is patterned directly on the back-emitting surface of the VCSEL substrate. The micro-chip double Z wires are patterned around the laser source so that the laser beam and the magnetic trapping area have an intersection.
In the embodiment illustrated
The embodiment illustrated on
By adjusting the electric voltage applied to the electro-magnetic micro-chip and to the laser diode power and/or waist position, it is possible to adjust the atom laser repetition rate, and the atom laser velocity.
The invention thus provides a coherent guided atom source, the atoms being extracted from a magnetic trap, wherein the atoms direction and position are very well defined in space due to the optical waveguide. The device also enables to control precisely the atoms velocity, i.e. the de Broglie wavelength of the atom laser.
The velocity can be set to any arbitrary value between 0 and 10 mm·s−2 which allows to reduce significantly the setup overall dimensions, while maintaining a very high sensitivity. These features are very important for inertial sensor applications, for example atom rate gyros.
The compact atom laser enables to realize precise atomic interferometers. Indeed, large magnetic fields from bulk ferromagnetic structures are difficult to control due to the high gradients in the vicinity of the magnetic trap and they induce systematic bias errors disturbing precision measurements. The guided coherent atom source according to the invention enables to use the cold atoms away from the atom chip, where magnetic fields/gradients are low, and to use atoms in an internal state where they are not sensitive to magnetic field.
The guided atom laser made using an atom chip enables to manufacture small size inertial sensors using ultra-cold atom source.
An atomic interferometer according to the invention is shown in
The atoms emitted from the magneto-optic trap are coupled into the optical waveguide. The laser beam is then turned off, and the atoms are probed during their free fall due to gravity. The atoms are probed using a guided laser and series of Raman pulses (wherein internal atom states are manipulated together with external states), or Bragg pulses (wherein only external states are manipulated). The pulses can be either horizontal or vertical. The transparent area corresponds to a single beam for manipulating atomic states. The arrows correspond to the areas where the atoms are probed. The single illuminating area can be replaced with three separate light areas.
The probing time to maintain a vertical probing area (with atoms launched horizontally) is limited to around 10 ms.
For longer probing times, the atoms must be launched vertically.
An interferometer is placed on each side of the BEC, and probes atoms going in opposed directions.
This configuration allows common mode rejection, and acceleration/rotation decoupling.
The atom source according to the invention can be combined with other atom chip.
An interferometer is placed between the two atom sources and probes atoms going in opposed directions. This configuration allows improved common mode rejection (due to the use of the same laser beam), and acceleration/rotation decoupling
In the case where an interferometer uses Raman or Bragg pulses, interferences do not occur when the atoms are confined along two dimensions, that is along the optical waveguide 12.
The optical waveguide is then turned off to let the atoms propagate in free fall. When atoms are launched vertically, a small atom chip is necessary, so that the atoms do not fall on the substrate surface.
In an improved setup, shown in
The coherent guided atom source according to the invention enables to use efficiently coherent atom source.
The source of the invention provides increased brightness compared to conventional atom sources, which permits higher contrast and better measurements.
The improved optical coupling reduces the optical and electrical power required.
Atoms with lower velocity (higher de Broglie wavelength) permit compact setup.
The guided atoms provide higher performances, and avoid systematic effects due to magnetic traps.
| Number | Date | Country | Kind |
|---|---|---|---|
| 08305062.5 | Mar 2008 | EP | regional |
