Publications by authors named "Xuzong Chen"

23 Publications

  • Page 1 of 1

Extension of the Generalized Hydrodynamics to the Dimensional Crossover Regime.

Phys Rev Lett 2021 Mar;126(9):090602

Vienna Center for Quantum Science and Technology (VCQ), Atominstitut, TU Wien, Vienna, Austria.

In an effort to address integrability breaking in cold gas experiments, we extend the integrable hydrodynamics of the Lieb-Liniger model with two additional components representing the population of atoms in the first and second transverse excited states, thus enabling a description of quasi-1D condensates. Collisions between different components are accounted for through the inclusion of a Boltzmann-type collision integral in the hydrodynamic equation. Contrary to standard generalized hydrodynamics, our extended model captures thermalization of the condensate at a rate consistent with experimental observations from a quantum Newton's cradle setup.
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http://dx.doi.org/10.1103/PhysRevLett.126.090602DOI Listing
March 2021

Evidence of Potts-Nematic Superfluidity in a Hexagonal sp^{2} Optical Lattice.

Phys Rev Lett 2021 Jan;126(3):035301

State Key Laboratory of Surface Physics, Institute of Nanoelectronics and Quantum Computing, Department of Physics, Fudan University, Shanghai 200438, China.

As in between liquid and crystal phases lies a nematic liquid crystal, which breaks rotation with preservation of translation symmetry, there is a nematic superfluid phase bridging a superfluid and a supersolid. The nematic order also emerges in interacting electrons and has been found to largely intertwine with multiorbital correlation in high-temperature superconductivity, where Ising nematicity arises from a four-fold rotation symmetry C_{4} broken down to C_{2}. Here, we report an observation of a three-state (Z_{3}) quantum nematic order, dubbed "Potts-nematicity", in a system of cold atoms loaded in an excited band of a hexagonal optical lattice described by an sp^{2}-orbital hybridized model. This Potts-nematic quantum state spontaneously breaks a three-fold rotation symmetry of the lattice, qualitatively distinct from the Ising nematicity. Our field theory analysis shows that the Potts-nematic order is stabilized by intricate renormalization effects enabled by strong interorbital mixing present in the hexagonal lattice. This discovery paves a way to investigate quantum vestigial orders in multiorbital atomic superfluids.
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http://dx.doi.org/10.1103/PhysRevLett.126.035301DOI Listing
January 2021

A linewidth locking method to control the microwave power in optically pumped cesium-beam clocks.

Rev Sci Instrum 2020 Sep;91(9):094708

Institute of Quantum Electronics, School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China.

In this paper, we present a linewidth locking method to control the microwave power in optically pumped cesium-beam frequency standards. The responses of optically pumped cesium-beam tubes and classical cesium-beam tubes are analyzed and compared against the power of the microwave field. Due to the wide probability distribution of atomic velocity resulting from the optical state preparation and detection, the linewidth of the Ramsey pattern is sensitive to the microwave power. The results can be used to control the microwave power instead of using the traditional extremum method. The advantages of the new method are discussed, and we named this new method the linewidth locking method. When the microwave power is well controlled at a low level by the linewidth locking method, the frequency stability of cesium-beam clocks will be improved to a certain degree for the reduction of the Ramsey pattern linewidth. In experiment, using the linewidth locking method, the Allan deviation of our optically pumped cesium-beam frequency standard is 2.64×10/τ and continues until the averaging time exceeds 1 × 10 s, which is 17% better than that using the traditional extremum method.
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http://dx.doi.org/10.1063/1.5144478DOI Listing
September 2020

Frequency instability of a miniature optically pumped cesium-beam atomic frequency standard.

Rev Sci Instrum 2020 Jul;91(7):074705

Institute of Quantum Electronics, School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China.

This paper proposes a miniature optically pumped cesium-beam atomic frequency standard with a volume of 38.4 l and a weight of 28 kg and examines the main factors that affect its signal-to-noise ratio (SNR). Methods to improve the SNR are proposed, which improve the short-term frequency instability: installing a collimator at the exit of the cesium oven, using the beam fluorescence spectrum with the fiber-coupled output to stabilize the laser frequency, and using the 4-5 cycling transition of the cesium D line for the atomic detection. We also examine several frequency shifts that affect the long-term frequency instability and detail methods to reduce these shifts. At present, the frequency instability achieved by the Peking University miniature optically pumped cesium-beam frequency standard has reached 3.12×10/τ.
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http://dx.doi.org/10.1063/5.0001749DOI Listing
July 2020

The emergence of picokelvin physics.

Authors:
Xuzong Chen Bo Fan

Rep Prog Phys 2020 Jul 17;83(7):076401. Epub 2020 Apr 17.

Institute of Quantum Electronics, Department of Electronics, School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, People's Republic of China.

The frontier of low-temperature physics has advanced to the mid-picokelvin (pK) regime but progress has come to a halt because of the problem of gravity. Ultracold atoms must be confined in some type of potential energy well: if the depth of the well is less than the energy an atom gains by falling through it, the atom escapes. This article reviews ultracold atom research, emphasizing the advances that carried the low-temperature frontier to 450 pK. We review microgravity methods for overcoming the gravitational limit to achieving lower temperatures using free-fall techniques such as a drop tower, sounding rocket, parabolic flight plane and the International Space Station. We describe two techniques that promise further advancement-an atom chip and an all-optical trap-and present recent experimental results. Basic research in new regimes of observation has generally led to scientific discoveries and new technologies that benefit society. We expect this to be the case as the low-temperature frontier advances and we propose some new opportunities for research.
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http://dx.doi.org/10.1088/1361-6633/ab8ab6DOI Listing
July 2020

Asymmetric population of momentum distribution by quasi-periodically driving a triangular optical lattice.

Opt Express 2019 Sep;27(20):27786-27796

Ultracold atoms in periodical-driven optical lattices enable us to investigate novel band structures and explore the topology of the bands. In this work, we investigate the impact of the ramping process of the driving signal and propose a simple but effective method to realize desired asymmetric population in momentum distribution by controlling the initial phase of the driving signal. A quasi-momentum oscillation along the shaking direction in the frame of reference co-moving with the lattice is formed, causing the formation of the mix of ground energy band and first excited band in laboratory frame, within the regime that the driving frequency is far less than the coupling frequency between ground band and higher energy bands. This method avoids the construction of intricate lattices or complex control sequence. With a triangular lattice, we experimentally investigate the influence of the initial phase, frequency, amplitude of the driving signal on the population difference and observe good agreement with our theoretical model. This provides guidance on how to load a driving signal in driven optical lattice experiment and also potentially supplies a useful tool to form a qubit that can be used in quantum computation.
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http://dx.doi.org/10.1364/OE.27.027786DOI Listing
September 2019

Extraction and identification of noise patterns for ultracold atoms in an optical lattice.

Opt Express 2019 Apr;27(9):12710-12722

To extract useful information about quantum effects in cold atom experiments, one central task is to identify the intrinsic fluctuations from extrinsic system noises of various kinds. As a data processing method, principal component analysis can decompose fluctuations in experimental data into eigenmodes, and give a chance to separate noises originated from different physical sources. In this paper, we demonstrate for Bose-Einstein condensates in one-dimensional optical lattices that the principal component analysis can be applied to time-of-flight images to successfully separate and identify noises from different origins of leading contribution, and can help to reduce or even eliminate noises via corresponding data processing procedures. The attribution of noise modes to their physical origins is also confirmed by numerical analysis within a mean-field theory. As the method does not rely on any a priori knowledge of the system properties, it is potentially applicable to the study of other quantum states and quantum critical regions.
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http://dx.doi.org/10.1364/OE.27.012710DOI Listing
April 2019

Observation of a Dynamical Sliding Phase Superfluid with P-Band Bosons.

Phys Rev Lett 2018 Dec;121(26):265301

School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China.

Sliding phases have been long sought after in the context of coupled XY models, as they are of relevance to various many-body systems such as layered superconductors, freestanding liquid-crystal films, and cationic lipid-DNA complexes. Here we report an observation of a dynamical sliding phase superfluid that emerges in a nonequilibrium setting from the quantum dynamics of a three-dimensional ultracold atomic gas loaded into the P band of a one-dimensional optical lattice. A shortcut loading method is used to transfer atoms into the P band at zero quasimomentum within a very short time duration. The system can be viewed as a series of "pancake"-shaped atomic samples. For this far-out-of-equilibrium system, we find an intermediate time window with a lifetime around tens of milliseconds, where the atomic ensemble exhibits robust superfluid phase coherence in the pancake directions, but no coherence in the lattice direction, which implies a dynamical sliding phase superfluid. The emergence of the sliding phase is attributed to a mechanism of cross-dimensional energy transfer in our proposed phenomenological theory, which is consistent with experimental measurements. This experiment potentially opens up a novel venue to search for exotic dynamical phases by creating high-band excitations in optical lattices.
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http://dx.doi.org/10.1103/PhysRevLett.121.265301DOI Listing
December 2018

Realization of two-stage crossed beam cooling and the comparison with Delta-kick cooling in experiment.

Rev Sci Instrum 2018 Dec;89(12):123110

School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, People's Republic of China.

We report the first experimental realization of the two-stage crossed beam cooling (TSCBC) method that we proposed in 2013 [L. Wang , J. Phys. B: At., Mol. Opt. Phys. , 195302 (2013)]. With the Rb Bose-Einstein condensation apparatus and electromagnet coils providing the magnetic levitation to counteract the gravitation, we simulated the micro-gravity environment and realized the TSCBC with 4 × 10 Rb atoms. We estimated that the lowest temperature of atoms can be at 3.56 nK with a new method and verified that the cooling process is adiabatic enough with time-of-flight images. According to analysis, we believed that the noise of magnetic field was the main obstacle that hinders the further cooling of the atomic ensemble. Under the same experimental conditions, we carried out the Delta-kick cooling method and got a lowest temperature of 23.3 nK also with 4 × 10 Rb atoms. According to the results of comparing experiments, we can see that the TSCBC method is more effective.
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http://dx.doi.org/10.1063/1.5046815DOI Listing
December 2018

High precision calibration of optical lattice depth based on multiple pulses Kapitza-Dirac diffraction.

Opt Express 2018 Jun;26(13):16726-16735

The precise calibration of optical lattice depth is an important step in the experiments of ultracold atoms in optical lattices. The Raman-Nath diffraction method, as the most commonly used method of calibrating optical lattice depth, has a limited range of validity and the calibration accuracy is not high enough. Based on multiple pulses Kapitza-Dirac diffraction, we propose and demonstrate a new calibration method by measuring the fully transfer fidelity of the first diffraction order. The high sensitivity of the transfer fidelity to the lattice depth ensures the highly precision calibration of the optical lattice depth. For each lattice depth measured, the calibration uncertainty is further reduced to less than 0.6% by applying the Back-Propagation Neural Network Algorithm. The accuracy of this method is almost one order of magnitude higher than that of the Raman-Nath diffraction method, and it has a wide range of validity applicable to both shallow lattices and deep lattices.
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http://dx.doi.org/10.1364/OE.26.016726DOI Listing
June 2018

Deep cooling of optically trapped atoms implemented by magnetic levitation without transverse confinement.

Rev Sci Instrum 2017 May;88(5):053104

School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, People's Republic of China.

We report a setup for the deep cooling of atoms in an optical trap. The deep cooling is implemented by eliminating the influence of gravity using specially constructed magnetic coils. Compared to the conventional method of generating a magnetic levitating force, the lower trap frequency achieved in our setup provides a lower limit of temperature and more freedoms to Bose gases with a simpler solution. A final temperature as low as ∼6nK is achieved in the optical trap, and the atomic density is decreased by nearly two orders of magnitude during the second stage of evaporative cooling. This deep cooling of optically trapped atoms holds promise for many applications, such as atomic interferometers, atomic gyroscopes, and magnetometers, as well as many basic scientific research directions, such as quantum simulations and atom optics.
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http://dx.doi.org/10.1063/1.4982348DOI Listing
May 2017

Sinusoidal phase-modulating self-mixing interferometer with nanometer resolution and improved measurement velocity range.

Appl Opt 2015 Sep;54(26):7820-7

A new signal-processing method based on an electronic frequency down-conversion technique has been introduced into a sinusoidal phase-modulating, self-mixing interferometer. The developed interferometer employs an electro-optical crystal placed in the external cavity of a He-Ne laser to generate the sinusoidal phase modulation with high modulation rate and ultralow insertion loss. Phase quadrature signals which have been amplitude-modulated by the sine and cosine functions, respectively, of the measured displacement can be extracted from the high-density optical fringes through the use of dual-channel multiplier/filter circuits. Therefore, the displacement of the external target can be retrieved from the phase quadrature signals with nanometer resolution and high computational efficiency. Moreover, a much-improved measurement speed from 2.5 to 22  mm/s has been realized owing to the simplified signal-processing method. The performance of the proposed interferometer has been experimentally verified by comparison to an Agilent 5529A dual-frequency laser interferometer. The measurement results from the two instruments agree well, and we therefore expect that our new technique offers a powerful instrument for high-speed metrology sciences.
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http://dx.doi.org/10.1364/AO.54.007820DOI Listing
September 2015

Two-stage crossed beam cooling with ⁶Li and ¹³³Cs atoms in microgravity.

Opt Express 2015 May;23(9):11378-87

Applying the direct simulation Monte Carlo (DSMC) method developed for ultracold Bose-Fermi mixture gases research, we study the sympathetic cooling process of 6Li and 133Cs atoms in a crossed optical dipole trap. The obstacles to producing 6Li Fermi degenerate gas via direct sympathetic cooling with 133Cs are also analyzed, by which we find that the side-effect of the gravity is one of the main obstacles. Based on the dynamic nature of 6Li and 133Cs atoms, we suggest a two-stage cooling process with two pairs of crossed beams in microgravity environment. According to our simulations, the temperature of 6Li atoms can be cooled to T = 29.5 pK and T/TF = 0.59 with several thousand atoms, which propose a novel way to get ultracold fermion atoms with quantum degeneracy near pico-Kelvin.
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http://dx.doi.org/10.1364/OE.23.011378DOI Listing
May 2015

Excitation of atoms in an optical lattice driven by polychromatic amplitude modulation.

Opt Express 2015 Apr;23(8):10064-74

We investigate the mutiphoton process between different Bloch states in an amplitude modulated optical lattice. In the experiment, we perform the modulation with more than one frequency components, which includes a high degree of freedom and provides a flexible way to coherently control quantum states. Based on the study of single frequency modulation, we investigate the collaborative effect of different frequency components in two aspects. Through double frequency modulations, the spectrums of excitation rates for different lattice depths are measured. Moreover, interference between two separated excitation paths is shown, emphasizing the influence of modulation phases when two modulation frequencies are commensurate. Finally, we demonstrate the application of the double frequency modulation to design a large-momentum-transfer beam splitter. The beam splitter is easy in practice and would not introduce phase shift between two arms.
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http://dx.doi.org/10.1364/OE.23.010064DOI Listing
April 2015

Laser frequency stabilization using a dispersive line shape induced by Doppler Effect.

Opt Express 2015 Feb;23(3):2982-90

We report a simple and robust Doppler-free spectroscopic technique to stabilize a laser frequency to the atomic transition. By employing Doppler Effect on the atomic beam, we obtained a very stable dispersive signal with a high signal-to-noise ratio and no Doppler-background, which served as an error signal to electronically stabilize a laser frequency without modulation. For validating the performance of this technique, we locked a DFB laser to the (133)Cs D2 line and observed an efficient suppression of the frequency noise and a long-term reduction of the frequency drifts in a laboratory environment.
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http://dx.doi.org/10.1364/OE.23.002982DOI Listing
February 2015

Tunneling dynamics and band structures of three weakly coupled Bose-Einstein condensates.

Opt Express 2014 Aug;22(16):19219-34

We study the tunneling dynamics and energy bands of three Bose-Einstein condensates which are coupled weakly with each other. The study is carried out with both the mean-filed model and the second-quantized model. The results from these two models are compared and found to agree with each other when the particle number is large. Without interaction, this system possesses a Dirac point in its energy band. This Dirac point is immediately destroyed and develops into a loop structure with arbitrary small interaction. This loop structure has a strong effect on the tunneling dynamics. We find that the tunneling dynamics in this system is very sensitive to the system parameter, e.g., the interaction strength. This sensitivity is found to be caused by the chaos in the mean-field model and the avoided energy crossings with tiny gaps in the second-quantized model. This result gives a certain indication on how the classical dynamics and quantum dynamics are connected in the semi-classical limit. Our mean-field results are also valid for three mutually coupled optical nonlinear waveguides.
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http://dx.doi.org/10.1364/OE.22.019219DOI Listing
August 2014

A modified Bitter-type electromagnet and control system for cold atom experiments.

Rev Sci Instrum 2014 Feb;85(2):024701

Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, People's Republic of China.

We present a modified Bitter-type electromagnet which features high magnetic field, fine electronic properties and efficient heat removal. The electromagnet is constructed from a stack of copper layers separated by mica layers that have the same shape. A distinctive design of cooling channels on the insulating layers and the parallel ducts between the layers ensures low resistance for cooling water to flow. A continuous current control system is also made to regulate the current through the electromagnet. In our experiment, versatile electromagnets are applied to generate magnetic field and gradient field. From our measurements, a peak magnetic field of 1000 G and a peak gradient field of 80 G/cm are generated in the center of the apparatuses which are 7 cm and 5 cm away from the edge of each electromagnet with a current of 230 A and 120 A, respectively. With the effective feedback design in the current control system and cooling water flow of 3.8 l/min, the stability of the current through the electromagnets can reach 10(-5).
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http://dx.doi.org/10.1063/1.4864051DOI Listing
February 2014

Asymmetric superradiant scattering and abnormal mode amplification induced by atomic density distortion.

Opt Express 2013 Jun;21(12):14377-87

School of Electronics Engineering & Computer Science, Peking University, Beijing 100871, China.

The superradiant Rayleigh scattering using a pump laser incident along the short axis of a Bose-Einstein condensate with a density distortion is studied, where the distortion is formed by shocking the condensate utilizing the residual magnetic force after the switching-off of the trapping potential. We find that very small variation of the atomic density distribution would induce remarkable asymmetrically populated scattering modes by the matter-wave superradiance with long time pulse. The optical field in the diluter region of the atomic cloud is more greatly amplified, which is not an ordinary mode amplification with the previous cognition. Our numerical simulations with the density envelop distortion are consistent with the experimental results. This supplies a useful method to reflect the geometric symmetries of the atomic density profile by the superradiance scattering.
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http://dx.doi.org/10.1364/OE.21.014377DOI Listing
June 2013

Minimization of the temperature coefficient of resonance frequency shift in the coherent population trapping clock.

Opt Lett 2011 May;36(10):1740-2

Institute of Quantum Electronics, School of Electronics Engineering & Computer Science, Peking University, Beijing 100871, China.

We studied the relationship between the frequency shift of coherent population trapping resonance and the cell temperature of (85)Rb. Results show that the temperature coefficient of the frequency shift can be reduced by buffer gas pressure adjustment and light shift optimization. When the contribution of buffer gas collision to temperature coefficient of frequency shift is less than 0.3 Hz/K, the contribution of light shift to the temperature coefficient of frequency shift becomes obvious. Under this cancelling effect, we can reduce the rate of total frequency shift to near zero.
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http://dx.doi.org/10.1364/OL.36.001740DOI Listing
May 2011

Detection of saturated absorption spectroscopy at high sensitivity with displaced crossovers.

Opt Lett 2011 Feb;36(4):561-3

Institute of Quantum Electronics, School of Electronics Engineering Computer Science, Peking University, Beijing, China.

We present an unconventional experimental approach for detecting saturated absorption spectroscopy. Using this approach, crossover peaks are displaced, leaving out peaks corresponding to an atom's natural resonant frequencies. Sensitivity of detection can also be enhanced. Consequently, the spectrum could reflect the energy structure of atoms more explicitly. Without harmful influence from crossovers, the locking range of the error signal is significantly increased and the symmetry of the dispersion line shape is perfectly preserved, so reliability of frequency stabilization can be improved.
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http://dx.doi.org/10.1364/OL.36.000561DOI Listing
February 2011

Observation of a red-blue detuning asymmetry in matter-wave superradiance.

Phys Rev Lett 2010 Nov 23;105(22):220404. Epub 2010 Nov 23.

Physics Laboratory, National Institute of Standards & Technology, Gaithersburg, MD 20899, USA.

We report the first experimental observation of strong suppression of matter-wave superradiance using blue-detuned pump light and demonstrate a pump-laser detuning asymmetry in the collective atomic recoil motion. In contrast to all previous theoretical frameworks, which predict that the process should be symmetric with respect to the sign of the detuning of the pump laser from the one-photon resonance, we find that for condensates the symmetry is broken. With high condensate densities and red-detuned pump light the distinctive multiorder, matter-wave scattering pattern is clearly visible, whereas with blue-detuned pump light superradiance is strongly suppressed. However, in the limit of a dilute atomic gas symmetry is restored.
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http://dx.doi.org/10.1103/PhysRevLett.105.220404DOI Listing
November 2010

Detecting quantum coherence of Bose gases in optical lattices by scattering light intensity in cavity.

Opt Express 2010 Jul;18(15):15664-71

School of Electronics Engineering & Computer Science, Peking University, Beijing 100871, China.

We propose a new method of detecting quantum coherence of a Bose gas trapped in a one-dimensional optical lattice by measuring the light intensity from Raman scattering in cavity. After pump and displacement process, the intensity or amplitude of scattering light is different for different quantum states of a Bose gas, such as superfluid and Mott-Insulator states. This method can also be useful to detect quantum states of atoms with two components in an optical lattice.
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http://dx.doi.org/10.1364/OE.18.015664DOI Listing
July 2010

Optical phase locking with a large and tunable frequency difference based on a vertical-cavity surface-emitting laser.

Opt Lett 2008 Feb;33(4):357-9

Institute of Quantum Electronics, School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China.

We present a novel method to phase lock two lasers with a controllable frequency difference. A microwave frequency-modulated vertical-cavity surface-emitting laser is used to phase connect two diode lasers by a two-step injection locking. The phase fluctuations of the two lasers are measured to be 6.4 x 10(-4) rad2, corresponding to 99.94% phase coherence. The frequency difference of the two lasers is tunable up to tens of gigahertz. The sideband suppression of the slave laser is more than 30 dB at 30 microW seed power. A narrow linewidth spectrum of coherent population trapping in rubidium is achieved using such beams.
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http://dx.doi.org/10.1364/ol.33.000357DOI Listing
February 2008
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