Session N09: Spinor Dynamics and Spin Domains in BECs

Live

The antiferromagnetic F$=$1 sodium spinor Bose-Einstein condensate (BEC) exhibits coherent population oscillations among the magnetic sublevels that are driven by spin-exchange collisions. This spinor dynamics depends on the relative energy of the magnetic sublevels, such as the quadratic Zeeman shift via a homogenous magnetic field. Microwave dressing in spinor BEC experiments provides additional control by allowing for negative shifts of the atomic energy levels and fast switching times for quench experiments. Here we demonstrate that by switching the microwave dressing to a large value the dynamics of the coherent population oscillations freezes. This can happen at a variable stage of the spinor evolution including after the release from a trapping potential. We have developed a theoretical model to analyze these phenomena. The technique will enable an enriched control of spinor dynamics and a new tool to interrogate the ultracold gas.   

Live

Understanding and controlling many-body quantum systems in noisy environments is paramount to developing robust quantum technologies. An external environment can be thought of as a ``measurement reservoir'' which extracts information about the quantum system. Cold atoms are well suited to examine system-environment interaction via weak (i.e. minimally destructive) measurement techniques, wherein the measurement probe acts as the environment and provides a record of system dynamics. The measurement record can then be used in a feedback scheme, leading to real time control of quantum gases. I will discuss our proposal to use weak measurement and feedback to engineer new phases in spin-1/2 Bose-Einstein condensates. We show that measurement and feedback alters the effective Hamiltonian of the system, thereby driving phase transitions reminiscent of a quantum quench for the closed system. We also develop a feedback cooling protocol which prevents runaway heating of the condensate due to measurement backaction. Our results provide a new route toward Hamiltonian engineering in many-body systems.   

Live

We use a sodium spin-1 Bose-Einstein condensate to experimentally demonstrate that physics beyond the single-mode approximation can be relevant during the short-time non-equilibrium dynamics. Our experiments rely on microwave dressing of the F=1 hyperfine states, where F denotes the total angular momentum of the Na atoms. The observed spin population dynamics are compared to theoretical predictions that are derived by solving a set of coupled mean-field Gross-Pitaevskii equations. The fact that beyond single-mode approximation physics can, in certain parameter regimes, have a pronounced effect on the dynamics when the spin healing length is comparable to or larger than the size of the Bose-Einstein condensate has implications for using Bose-Einstein condensates as models for quantum phase transitions and spin squeezing studies as well as non-linear SU(1,1) interferometers. Our work opens the door for devising time-dependent coupling schemes between spin and spatial degrees of freedom.   

Live

We measure inter-state interactions in a two state mixture of a $^{87}Rb$ Bose-Einstein Condensate (BEC) using microwave spectroscopy. Due to population imbalance, these interactions have a non-linear $\sigma_z$ component, leading to a shearing effect on the Bloch sphere. We use a population imbalanced dynamic decoupling scheme that accumulates inter-state interactions while canceling intra-state density shifts and external noise sources. We repeat measurements on both magnetic sensitive and insensitive transitions with similar uncertainties, showing that we successfully decoupled our system from strong magnetic noises. Our scheme can be extended to other systems, such as quantum memories that are inherently imbalanced populations, and used close to a Feshbach resonance, where interactions diverge, and strong magnetic noises are ever present. Our results also allow for a better understanding of inter atomic potentials.   

On Demand

Cold hybrid ion-atom systems are a promising platform for fundamental research in quantum physics [1]. The successful cooling of the Yb+ ion to the quantum regime has opened up new theoretical and experimental perspectives [2]. Here, we present theoretical studies of the spin dynamics for ion-atom collisions in systems consisting of Yb+ or Ba+ ion and Li atoms. We employ multichannel quantum scattering theory to reproduce measured spin-changing rate constants and their energy dependence. As a result, the singlet and triplet scattering lengths for ion-atom interactions are assigned. Next, we identify experimentally accessible Feshbach resonances in the mentioned systems and predict their properties. Control of both elastic scattering and related cooling rates, as well as inelastic spin-changing collisions, with the magnetic field is proposed and investigated to guide ongoing experimental efforts. Ion-atom Feshbach resonances in analogy to well-established techniques for neutral systems will be an important tool to manipulated ultracold ion-atom mixtures. \newline \newline [1] Tomza et al, Rev. Mod. Phys. 91, 035001 (2019) \newline [2] Feldker et al, Nature Physics, doi:10.1038/s41567-019-0772-5 (2020)   

On Demand

The realization of squeezed states has enabled quantum measurements with signal-to-noise ratio enhanced beyond the standard quantum limit. Squeezed light can now be generated through parametric amplification relying on nonlinear optical processes such as four wave mixing. More recently, atomic collisions have been used as nonlinear interactions to prepare spin squeezed states in Bose-Einstein condensates (BECs) and demonstrate quantum-enhanced measurements. Here, we report the observation of quantum spin squeezing in a spin-1 thermal gas, which is the atomic analogue of optical intensity-difference squeezing. The degree of squeezing is characterized as a function of temperature and trapping parameters. With increased number afforded by a thermal gas, our observation opens the possibility for further enhancement of sensitivity in squeezed quantum measurements, and could even be extended to BEC-thermal mixtures.   

On Demand

We present our work on spatially structuring magnetic domains in multi-component Bose-Einstein condensates of $^{\mathrm{87}}$Rb, using spin-dependent optical potentials. For this, we make use of light at the tune-out wavelength (between the D1 and D2 line, for $^{\mathrm{87}}$Rb at 790.018 nm) to create optical barriers and potential wells sensitive to the hyperfine state of the atom. Using a focused Gaussian beam, and with appropriate circular polarization, this results in a repulsive barrier for (F,m$_{\mathrm{F}})=$(1,-1), and attractive well for the (1,$+$1) state, with the (1,0) state unaffected. We initially load a pure (1,-1) BEC into a flat-bottom line trap formed from a painted optical dipole potential. Using RF pulses, we drive spin transitions and prepare mixtures of (1,-1) with a tunable population of (1,0) or (1,$+$1) states. Using a 2D acousto-optical deflector, the spin-dependent light beam is steered and focused onto the BEC, resulting in localised repulsive barriers for the (1,-1) state. Through mean-field effects, the other hyperfine states fill in the resulting density dips in the (1,-1) condensate. Removing the light, we observe the formation of stable immiscible domains of (1,$+$1) embedded in the (1,-1) bulk.   

Not Participating

We explore the stability of the phase separation phenomenon in few-fermion spin-1/2 systems confined in a double-well potential. It is shown that within the SU(2) symmetric case, where the total-spin is conserved, the phase separation cannot be stabilized. An interaction regime characterized by metastable phase separation emerges for intermediate interactions which is inherently related with the ferromagnetic spin-spin correlations emanating within each of the wells. The breaking of the SU(2) symmetry crucially affects the stability properties of the system as the phase-separated state can be stabilized even for weak linear gradients of the magnetic potential. Our results imply a more intricate relation between phase separation and ferromagnetism that lies beyond the view of the Stoner instability.   

Not Participating

We study the spin dynamics of a weakly interacting non-degenerate Bose gas. Microscopic exchange scattering in binary collisions between indistinguishable atoms in a gas just above quantum degeneracy can lead to macroscopic collective behavior. Additional application of a weak, spatially inhomogeneous, spin-dependent optical potential (effective magnetic field) can dramatically modify the spin dynamics driven by exchange effects, and, for example, lead to long-lived domain states. We present experimental studies of the nature of these stable spin domains in a magnetically-trapped $^{87}$Rb gas, and explore the effect of domain wall size and effective magnetic field geometry on the stability and lifetime of spin domains. Results are compared to a hydrodynamic Boltzmann approximation.