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arXiv:1006.3250 · Physical Review Letters

Observation of Collective Atomic Recoil Motion in a Degenerate Fermion Gas

Pengjun Wang, L. Deng, E. W. Hagley, Zhengkun Fu, Shijie Chai, Jing Zhang

Published May 23, 2011• 31 citations
arXivHTMLPDF

Abstract

We demonstrate collective atomic recoil motion with a dilute, ultracold, degenerate fermion gas in a single spin state. By utilizing an adiabatically decompressed magnetic trap with an aspect ratio different from that of the initial trap, a momentum-squeezed fermion cloud is achieved. With a single pump pulse of the proper polarization, we observe, for the first time, multiple wave-mixing processes that result in distinct collective atomic recoil motion modes in a degenerate fermion cloud. Contrary to the case with Bose condensates, no pump-laser detuning asymmetry is present.

DOI
10.1103/physrevlett.106.210401

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•May 30, 2006•0 citations
We demonstrate collective atomic recoil motion with a dilute, ultracold, degenerate fermion gas in a single spin state. By utilizing an adiabatically decompressed magnetic trap with an aspect ratio different from that of the initial trap, a momentum-squeezed fermion cloud is achieved. With a single pump pulse of the proper polarization, we observe, for the first time, multiple wave-mixing processes that result in distinct collective atomic recoil motion modes in a degenerate fermion cloud. Contrary to the case with Bose condensates, no pump-laser detuning asymmetry is present.
Collective scattering of light due to atomic recoil, also known as collective atomic recoil lasing (CARL), occurs when a gas of atoms coherently scatters light from a pump laser via a collective instability. The scattering process can become unstable due to the atoms in the gas recoiling under the influence of the dynamic optical forces which arise due to the interference of the pump and scattered fields. The atoms move to form a spatially periodic density grating, which in turn amplifies the scattered field. This paper presents a theoretical analysis of CARL phenomena in degenerate ultracold gases of bosons and fermions. In the case of bosons i.e. in a Bose-Einstein Condensate (BEC),the effect of atom-atom interactions due to collisions on the CARL instability in the case of both attractive and repulsive condensates, is investigated. In the case of fermionic gases, the effect of Fermi pressure, is also presented. The linear and nonlinear evolution of the optical field and the atomic momentum distribution are considered for both cases.
We review our recent experimental realization and investigation of a spin-orbit (SO) coupled Bose-Einstein condensate (BEC) and quantum degenerate Fermi gas. By using two counter-propagating Raman lasers and controlling the different frequency of two Raman lasers to engineer the atom-light interaction, we first study the SO coupling in BEC. Then we study SO coupling in Fermi gas. We observe the spin dephasing in spin dynamics and momentum distribution asymmetry of the equilibrium state as hallmarks of SO coupling in a Fermi gas. To clearly reveal the property of SO coupling Fermi gas, we also study the momentum-resolved radio-frequency spectroscopy which characterizes the energy-momentum dispersion and spin composition of the quantum states. We observe the change of fermion surfaces in different helicity branches with different atomic density, which indicates that a Lifshitz transition of the Fermi surface topology change can be found by further cooling the system. At last, we study the momentum-resolved Raman spectroscopy of an ultracold Fermi gas.
We study spin asymmetry of the ground states for the trapped spin-degenerate (two-component) gases of the fermionic atoms in case that interparticle interaction between different spin components is repulsive. Density distributions are algebraically obtained from the Thomas–Fermi equation. We show that, for the system with a larger particle number than a critical value, the asymmetric (ferromagnetic) states are more stable than the spin-symmetric (paramagnetic) ones. The density profiles show that the spin-asymmetry occurs in the central regions of the trapped gas, and grows up with increasing particle number. We also discuss the realization of these ferromagnetic states in atom gases trapped in the optical potentials and suggest experimental conditions to perform it.
Subradiance, i.e., the cooperative inhibition of spontaneous emission by destructive interatomic interference, can be realized in a cold atomic sample confined in a ring cavity and lightened by a two-frequency laser. The atoms, scattering the photons of the two laser fields into the cavity mode, recoil and change their momentum. Under proper conditions the atomic initial momentum state and the first two momentum recoil states form a three-level degenerate cascade. A stationary subradiant state is obtained after the scattered photons have left the cavity, leaving the atoms in a coherent superposition of the three collective momentum states. Both a semiclassical description of the process and the quantum subradiant state with its Wigner function are given. Antibunching, quantum correlations, and entanglement between the atomic modes of the subradiant state are demonstrated.
We demonstrate a novel dual-beam atom laser formed by outcoupling oppositely polarized components of an all-optical F = 1 spinor Bose-Einstein condensate whose Zeeman sublevel populations have been coherently evolved through spin dynamics. The condensate is formed through all-optical means using a single-beam running-wave dipole trap. We create a condensate in the magnetic field-insensitive m(F) = 0 state, and drive coherent spin-mixing evolution through adiabatic compression of the initially weak trap. Such dual beams, number-correlated through the angular momentum-conserving reaction 2m(0) ?m(+1) +m(-1), have been proposed as tools to explore entanglement and squeezing in Bose-Einstein condensates, and have potential use in precision phase measurements.
Optical trapping as a viable means of exploring the physics of ultracold dilute atomic gases has revealed a new spectrum of physical phenomena. In particular, macroscopic and sudden occupation of the ground state below a critical temperature—-a phenomenon known as Bose-Einstein condensation—-has become an even richer system for the study of quantum mechanics, ultracold collisions, and many-body physics in general. Optical trapping liberates the spin degree of the BEC, making the order parameter vectorial (‘spinor BEC’), as opposed to the scalar order of traditional magnetically trapped condensates. The work described within is divided into two main efforts. The first encompasses the all-optical creation of a Bose-Einstein condensate in rubidium vapor. An all-optical path to spinor BEC (as opposed to transfer to an optical trap from a magnetic-trap condensate) was desired both for the simplicity of the experimental setup and also for the potential gains in speed of creation; evaporative cooling, the only known path to dilute-gas condensation, works only as efficiently as the rate of elastic collisions in the gas, a rate that starts out much higher in optical traps. The first all-optical BEC was formed elsewhere in 2001; the years following saw many groups worldwide seeking to create their own version. Our own all-optical spinor BEC, made with a single-beam dipole trap formed by a focused CO₂ laser, is described here, with particular attention paid to trap loading, measurement of trap parameters, and the use of a novel 780 nm high-power laser system. The second part describes initial experiments performed with the nascent condensate. The spinor properties of the condensate are documented, and a measurement is made of the density-dependent rate of spin mixing in the condensate. In addition, we demonstrate a novel dual-beam atom laser formed by outcoupling oppositely polarized components of the condensate, whose populations have been coherently evolved through spin dynamics. We drive coherent spin-mixing evolution through adiabatic compression of the initially weak trap. Such dual beams, nominally number-correlated through the angular momentum-conserving collision m=0 + m=0 ⇌ m=+1 + m=-1 have been proposed as tools to explore entanglement and squeezing in Bose-Einstein condensates.