One of the greatest challenges in modern physics is to understand the behaviour of an ensemble of strongly interacting particles. A class of quantum many-body systems (such as neutron star matter and cold Fermi gases) share the same universal thermodynamic properties when interactions reach the maximum effective value allowed by quantum mechanics, the so-called unitary limit. This makes it possible in principle to simulate some astrophysical phenomena inside the highly controlled environment of an atomic physics laboratory. Previous work on the thermodynamics of a two-component Fermi gas led to thermodynamic quantities averaged over the trap, making comparisons with many-body theories developed for uniform gases difficult. Here we develop a general experimental method that yields the equation of state of a uniform gas, as well as enabling a detailed comparison with existing theories. The precision of our equation of state leads to new physical insights into the unitary gas. For the unpolarized gas, we show that the low-temperature thermodynamics of the strongly interacting normal phase is well described by Fermi liquid theory, and we localize the superfluid transition. For a spin-polarized system, our equation of state at zero temperature has a 2 per cent accuracy and extends work on the phase diagram to a new regime of precision. We show in particular that, despite strong interactions, the normal phase behaves as a mixture of two ideal gases: a Fermi gas of bare majority atoms and a non-interacting gas of dressed quasi-particles, the fermionic polarons.

We investigate the low-lying compression modes of a unitary Fermi gas with imbalanced spin populations. For low polarization, the strong coupling between the two spin components leads to a hydrodynamic behavior of the cloud. For large population imbalance we observe a decoupling of the oscillations of the two spin components, giving access to the effective mass of the Fermi polaron, a quasiparticle composed of an impurity dressed by particle-hole pair excitations in a surrounding Fermi sea. We find m*/m = 1.17(10), in agreement with the most recent theoretical predictions.

Orbital angular momentum (OAM) of light represents a fundamental optical freedom that can be exploited to manipulate quantum state of atoms. In particular, it can be used to realize spinorbital-angular-momentum (SOAM) coupling in cold atoms by inducing an atomic Raman transition using two laser beams with differing OAM. Rich quantum phases are predicted to exist in manybody systems with SOAM coupling. Their observations in laboratory, however, are often hampered by the limited control of the system parameters. In this work we report, for the first time, the experimental observation of the ground-state quantum phase diagram of the SOAM coupled Bose-Einstein condensate (BEC). The discontinuous variation of the spin polarization as well as the vorticity of the atomic wave function across the phase boundaries provides clear evidence of firstorder phase transitions. Our results open up a new way to the study of phase transitions and exotic quantum phases in quantum gases. arXiv:1806.06263v2 [cond-mat.quant-gas]

In a three-level atomic system coupled by two equalamplitude laser fields with a frequency separation 2δ, a weak probe field exhibits a multiple-peaked absorption spectrum with a constant peak separation δ. The corresponding probe dispersion exhibits steep normal dispersion near the minimum absorption between the multiple absorption peaks, which leads to simultaneous slow group velocities for probe photons at multiple frequencies separated by δ. We report an experimental study in such a bichromatically coupled three-level Λ system in cold 87 Rb atoms. The multiple-peaked probe absorption spectra under various experimental conditions have been observed and compared with the theoretical calculations.42.50. Gy, 32.80.2t A resonant laser beam can pass through an opaque atomic medium without attenuation due to the quantum interference effect between the dressed states created by a coupling laser field. This phenomenon is known as electromagnetically induced transparency (EIT) [1][2][3]. In recent years, many studies on EIT and related phenomena have been carried out, which reveal the importance of EIT in understanding the fundamental physics involving interactions between light field and resonant medium [4][5][6][7][8]. It has been shown that EIT may have applications in a variety of research topics such as quantum optics with slow photons [9-13], quantum information processing [14], atomic frequency standard [15][16][17][18], and quantum nonlinear optics [19,20].Recently, Lukin et al proposed a mechanism to entangle two photons in an EIT medium based on obtaining slow photons at different frequencies [21]. Since the EIT created by a monochromatic field only provides the steep dispersion near the resonant frequency, sophisticated schemes are proposed to obtain slow photons at different frequencies [21,22]. Here, we show that EIT in a Λ type level configuration created by a bichromatic laser field may be used to slow down photons at different frequencies. The three-level Λ system coupled by a bichromatic field and a probe field is depicted in Fig. 1(a). The dressed states created by the bichromatic field consist of an infinite ladder with an equal-spacing separation δ between the neighboring levels when the average frequency of the bichromatic field with equal amplitudes of the two frequency components matches the atomic transition frequency. The dressed states are the superposition of the atomic states |2>, |3>, and the photon number states with the amplitude determined by the Rabi frequency (Ω c = Ω c1 = Ω c2 ) and the frequency separation 2δ. Such dressed states and the fluorescence spectrum of the two-level atoms coupled by a bichromatic field have been extensively studied before [23][24][25][26]. The dressed state picture of the bichromatic driven three-level system is depicted in Fig. 1(b). It is expected that the probe absorption spectrum will exhibit multiple peaks corresponding to the dressed transitions |1 >→ |m > and transparent windows with minimum absorption located near the middle separation of the d...

It is well-known that the magnetic Feshbach resonances of cold atoms are sensitive to the magnitude of the external magnetic field. Much less attention has been paid to the direction of such a field. In this work we calculate the scattering properties of spin polarized fermionic atoms in reduced dimensions, near a p-wave Feshbach resonance. Because of spatial anisotropy of the p-wave interaction, the scattering has nontrivial dependence on both the magnitude and the direction of the magnetic field. In addition, we identify an inelastic scattering process which is impossible in the isotropic-interaction model; the rate of this process depends considerably on the direction of the magnetic field. Significantly, an EPR entangled pair of identical fermions may be produced during this inelastic collision. This work opens a new method to manipulate resonant cold atomic interactions. Introduction.-Unlike electrons in condensed matter and nucleons inside a nucleus, ultracold atoms have tunable interactions thanks to the magnetic Feshbach resonances [1]. The direction of the magnetic field plays little role in isotropic s-wave interactions, other than providing a quantization axis for the hyperfine states. For p-wave interactions, however, the resonance positions for different orbital magnetic quantum numbers may be different due to the anisotropic magnetic dipole-dipole interaction [2,3]. The physical implication of this split for cold atomic scatterings in reduced dimensions has not been explored theoretically. In previous theoretical work on these scatterings, such split was not taken into account [4,5]. While this omission is reasonable for atoms with small magnetic dipoles such as 6 Li [6], we have to consider the effect of the split for other atoms such as 40 K which show large splits [2,3].In reduced dimensions, new two-body scattering resonances known as confinement-induced resonances (CIRs) [7] appear. Recently an impressive amount of work was devoted to the s-wave CIRs [8][9][10][11].In this Letter we study the scattering of two spinpolarized fermionic atoms near p-wave Feshbach resonances, confined in low dimensions. We will assume that the oscillator length in the confinement directions is much larger than the tiny Van der Waals length scale. Because of the split of resonance positions for different orbital magnetic quantum numbers, we find that 1) one can tune the scattering properties continuously by changing the direction of the magnetic field, 2) a new inelastic scattering process where one of the two atoms is excited in the confined direction by ω is now possible, where ω is the angular frequency for the confinement, and 3) in quasi-one-dimension (quasi-1D), by choosing the magnitude and direction of the field and the collision energy

We report an experimental investigation of electromagnetically induced transparency in a multilevel cascade system of cold atoms. The absorption spectral profiles of the probe light in the multi-level cascade system were observed in cold 85 Rb atoms confined in a magneto-optical trap, and the dependence of the spectral profile on the intensity of the coupling laser was investigated.The experimental measurements agree with the theoretical calculations based on the density matrix equations of the rubidium cascade system.Electromagnetically induced transparency (EIT)[1] is a quantum interference effect that permits propagation of light through an opaque atomic medium without attenuation, it was first proposed in 1989 [2] and experimentally verified in 1991 [3]. Since then, theoretical and experimental studies of EIT have attracted great attentions due to their potential applications in many fields, such as low light nonlinear optics [4], quantum information [5], atomic frequency standard [6], and so on. Early studies were carried out with hot atoms in vapor cells. In the hot atomic medium, the interaction time between the atoms and the laser fields is short which leads to the transient broadening. Also, the collisions in the hot atomic medium may severely shorten the coherence decay time. Recently, many groups explored the EIT phenomena using the laser cooled atoms. There are several advantages in the cold atoms [7]. Firstly, because of the low temperature of the cold atoms, the Doppler broadening effect is effectively minimized, which renders it possible to explore EIT-type nonlinear optical phenomena involving odd number of photons. Secondly, the lower collision rates in the cold atomic sample reduce the decoherence rate.Early experimental studies of EIT in the cold atoms were mainly carried out in rubidium atoms [8,9,10]. Subsequently, the EIT based nonlinear optical phenomena were studied [4,11], which led to the recent experiments on the resonant nonlinear optics at low light intensity. A very steep slope of refractive index and the extremely low group velocity of probe light have been obtained in the cold EIT mediums [12], which have been used to demonstrate light storage and recall based on the coherent excitation transfer between the photons and the atoms [13]. Recently, electromagnetically induced grating (EIG) [14,15] was realized in the cold atoms. Jason et al. experimentally compared the EIT phenomena between the hot atoms and the cold atoms [16], and Ahufinger et al. compared the EIT phenomena between the cold atoms above and below the transition temperature for Bose-Einstein condensation [17]. These studies on EIT and the related phenomena in the cold atoms provided intensive understanding of the atomic coherence and interference in the fundamental interaction between the light field and the atoms [18,19,20,21,22,23].EIT in the simple three-level system have been extensively studied, but EIT in the multilevel cascade systems and their possible applications have not been fully explored. Although essenti...

We investigate an active Raman gain scheme for significant group velocity reduction. We show that this scheme, which is fundamentally different from the electromagnetically induced transparency scheme, is capable of achieving ultraslow and distortion-free propagation of a pulsed probe field. We demonstrate the group velocity behavior that is drastically different from the conventional electromagnetically induced transparency scheme, and we show that the new scheme can be used to accurately determine the decoherence rate of a long-lived state. In addition, the Raman gain scheme has the advantage of being broadly tunable, an important feature that may have potential applications.

We investigate the simultaneous formation and propagation of coupled ultraslow optical soliton pairs in a cold, lifetime-broadened three-state double-Lambda atomic system. Starting from the equations of motion of atomic response and two-mode probe-control electromagnetic fields, we derive coupled nonlinear Schrödinger equations that govern the nonlinear evolution of the envelopes of the probe fields in this four-wave mixing scheme by means of the standard method of multiple scales. We demonstrate that for weak probe fields and with suitable operation conditions, a pair of coupled optical solitons moving with remarkably slow propagating velocity can be established in such a highly resonant atomic medium. The key elements to such a shape preserving, well matched yet interacting soliton pair is the balance between dispersion effect and self- and cross-phase modulation effects of the system.