Lorraine Sadler

Spontaneous symmetry breaking in a quenched ferromagnetic spinor Bose–Einstein condensate

A central goal in condensed matter and modern atomic physics is the exploration of quantum phases of matter—in particular, how the universal characteristics of zero-temperature quantum phase transitions differ from those established for thermal phase transitions at non-zero temperature. Compared to conventional condensed matter systems, atomic gases provide a unique opportunity to explore quantum dynamics far from equilibrium. For example, gaseous spinor Bose–Einstein condensates (whose atoms have non-zero internal angular momentum) are quantum fluids that simultaneously realize superfluidity and magnetism, both of which are associated with symmetry breaking. Here we explore spontaneous symmetry breaking in 87Rb spinor condensates, rapidly quenched across a quantum phase transition to a ferromagnetic state. We observe the formation of spin textures, ferromagnetic domains and domain walls, and demonstrate phase-sensitive in situ detection of spin vortices. The latter are topological defects resulting from the symmetry breaking, containing non-zero spin current but no net mass current.

High-resolution magnetometry with a spinor Bose-Einstein condensate.

We demonstrate a precise magnetic microscope based on direct imaging of the Larmor precession of a 87Rb spinor Bose-Einstein condensate. This magnetometer attains a field sensitivity of 8.3 pT/Hz1/2 over a measurement area of 120 microm2, an improvement over the low-frequency field sensitivity of modern SQUID magnetometers. The achieved phase sensitivity is close to the atom shot-noise limit, estimated as 0.15 pT/Hz1/2 for a unity duty cycle measurement, suggesting the possibilities of spatially resolved spin-squeezed magnetometry. This magnetometer marks a significant application of degenerate atomic gases to metrology.

Direct nondestructive imaging of magnetization in a spin-1 bose-einstein gas

Polarization-dependent phase-contrast imaging is used to resolve the spatial magnetization profile of an optically trapped ultracold gas. This probe is applied to Larmor precession of degenerate and nondegenerate spin-1 87Rb gases. Transverse magnetization of the Bose-Einstein condensate persists for the condensate lifetime, with a spatial response to magnetic field inhomogeneities consistent with a mean-field model of interactions. In comparison, the magnetization of the non-condensed gas decoheres rapidly. Rotational symmetry implies that the Larmor frequency of a spinor condensate be density independent, and thus suitable for precise magnetometry with high spatial resolution.

Coherence-enhanced imaging of a degenerate Bose-Einstein gas

We present coherence-enhanced imaging, an in situ technique that uses Raman superradiance to probe the spatial coherence of an ultracold gas. Applying this technique, we identify the coherent portion of an inhomogeneous degenerate (87)Rb gas and obtain a spatially resolved measurement of the first-order spatial correlation function. We find that the decay of spin gratings is enhanced in high density regions of a Bose-Einstein condensate, and ascribe the enhancement to collective atom-atom scattering. Further, we directly observe spatial inhomogeneities that arise generally in the course of extended-sample superradiance.

Direct, non-destructive imaging of magnetization in a spin-1 bose gas

Polarization-dependent phase-contrast imaging is used to spatially resolve the magnetization of an optically trapped degenerate and nondegenerate spin-1 Rb gases. Transverse magnetization of the Bose-Einstein condensate persists for the condensate lifetime, while the noncondensed gas decoheres rapidly. Quantum fluids with a spin degree of freedom have been of longstanding interest, stimulated both by the complex phenomenology of superfluid [1] and by p-wave superconductivity [2]. Advances in ultracold atomic physics have now led to the creation of novel multicomponent quantum fluids including pseudospin-1/2 Bose-Einstein condensates (BECs) [3] and spin-1 and -2 condensates of Na [4,5] and Rb [6-8]. The internal state of a multi-component system is characterized by the populations in each of the components and the coherences among them. However, in all previous studies of the spin-1 or spin-2 spinor condensates, while the populations in each magnetic sublevel were measured, no information was obtained regarding the coherence between overlapping populations [4,6-10]. Moreover, although spatial patterns of longitudinal magnetization have been reconstructed from images of freely expanding spinor gases, the expansion process severely limits the resolution obtainable [9,11,12]. In this work, we exploit atomic birefringence to image the magnetization of an ultracold spin-1 Bose gas non-destructively with high spatial resolution. By varying the orientation of an applied magnetic field with respect to our imaging axis, we measure either longitudinal magnetization, which derives from the static populations in each of the magnetic sublevels, or transverse magnetization, which derives from time-varying ∆m=1 coherences. This probe is used to observe Larmor precession in both degenerate and non-degenerate spinor Bose gases. In particular, optical characterization of Larmor precession in a BEC provides a novel probe of the relative phases between condensates in different internal states with excellent temporal and spatial resolution (see Refs.[13] for other recent measurements of

Spontaneous symmetry breaking and defect formation in a quenched ferromagnetic spinor Bose-Einstein condensate

We observe spontaneous symmetry breaking in a 87Rb spinor Bose condensate quenched across a quantum phase transition. This quench causes inhomogeneous ferromagnetic regions and topological defects to form in a magnetic quantum fluid.

Periodically-dressed Bose- Einstein condensates in 87Rb

Using Raman coupling to induce spatially periodic coupling between atomic levels, novel quantum fluids with anisotropic critical velocities and highly-correlated ground states are created. Theoretical and experimental investigations of this system are presented.