arXiv subjects

Alexander L. Gaunt

Publications and source records attributed to Alexander L. Gaunt.

24 records · Page 2Linked to original sources

arXiv2014

Observing Properties of an Interacting Homogeneous Bose--Einstein Condensate: Heisenberg-Limited Momentum Spread, Interaction Energy and Free-Expansion Dynamics

We study the properties of an atomic Bose--Einstein condensate produced in an optical-box potential, using high-resolution Bragg spectroscopy. For a range of box sizes, up to $70~μ$m, we directly observe Heisenberg-limited momentum uncertainty of the condensed atoms. We measure the condensate interaction energy with a precision of $k_B \times 100$ pK and study, both experimentally and numerically, the dynamics of its free expansion upon release from the box potential. All our measurements are in good agreement with theoretical expectations for a perfectly homogeneous condensate of spatial extent equal to the size of the box, which also establishes the uniformity of our optical-box system on a sub-nK energy scale.

cond-mat.quant-gas↗

arXiv2014

Quantum Joule-Thomson Effect in a Saturated Homogeneous Bose Gas

We study the thermodynamics of Bose-Einstein condensation in a weakly interacting quasi-homogeneous atomic gas, prepared in an optical-box trap. We characterise the critical point for condensation and observe saturation of the thermal component in a partially condensed cloud, in agreement with Einstein's textbook picture of a purely statistical phase transition. Finally, we observe the quantum Joule-Thomson effect, namely isoenthalpic cooling of an (essentially) ideal gas. In our experiments this cooling occurs spontaneously, due to energy-independent collisions with the background gas in the vacuum chamber. We extract a Joule-Thomson coefficient $μ_{\rm JT} > 10^9$ K/bar, about ten orders of magnitude larger than observed in classical gases.

cond-mat.quant-gas↗

arXiv2013

Stability of a unitary Bose gas

We study the stability of a thermal $^{39}$K Bose gas across a broad Feshbach resonance, focusing on the unitary regime, where the scattering length $a$ exceeds the thermal wavelength $λ$. We measure the general scaling laws relating the particle-loss and heating rates to the temperature, scattering length, and atom number. Both at unitarity and for positive $a \ll λ$ we find agreement with three-body theory. However, for $a<0$ and away from unitarity, we observe significant four-body decay. At unitarity, the three-body loss coefficient, $L_3 \propto λ^4$, is three times lower than the universal theoretical upper bound. This reduction is a consequence of species-specific Efimov physics and makes $^{39}$K particularly promising for studies of many-body physics in a unitary Bose gas.

cond-mat.quant-gas↗

arXiv2012

A superheated Bose-condensed gas

Our understanding of various states of matter usually relies on the assumption of thermodynamic equilibrium. However, the transitions between different phases of matter can be strongly affected by non-equilibrium phenomena. Here we demonstrate and explain an example of non-equilibrium stalling of a continuous, second-order phase transition. We create a superheated atomic Bose gas, in which a Bose-Einstein condensate (BEC) persists above the equilibrium critical temperature, $T_c$, if its coupling to the surrounding thermal bath is reduced by tuning interatomic interactions. For vanishing interactions the BEC persists in the superheated regime for a minute. However, if strong interactions are suddenly turned on, it rapidly "boils" away. Our observations can be understood within a two-fluid picture, treating the condensed and thermal components of the gas as separate equilibrium systems with a tuneable inter-component coupling. We experimentally reconstruct a non-equilibrium phase diagram of our gas, and theoretically reproduce its main features.

cond-mat.quant-gas↗

arXiv2012

A compact single-chamber apparatus for Bose-Einstein condensation of $^87$Rb

We describe a simple and compact single-chamber apparatus for robust production of $^87$Rb Bose-Einstein condensates. The apparatus is built from off-the-shelf components and allows production of quasi-pure condensates of > $3\times 10^5$ atoms in < 30 s. This is achieved using a hybrid trap created by a quadrupole magnetic field and a single red-detuned laser beam [Y.-J. Lin et al., Phys. Rev. A 79, 063631 (2009)]. In the same apparatus we also achieve condensation in an optically plugged quadrupole trap [K. B. Davis et al., Phys. Rev. Lett. 75, 3969 (1995)] and show that as little as 70 mW of plug-laser power is sufficient for condensation, making it viable to pursue this approach using inexpensive diode lasers. While very compact, our apparatus features sufficient optical access for complex experiments, and we have recently used it to demonstrate condensation in a uniform optical-box potential [A. Gaunt et al., arXiv:1212.4453 (2012)].

cond-mat.quant-gas↗

arXiv2012

Bose-Einstein condensation of atoms in a uniform potential

We have observed Bose-Einstein condensation of an atomic gas in the (quasi-)uniform three-dimensional potential of an optical box trap. Condensation is seen in the bimodal momentum distribution and the anisotropic time-of-flight expansion of the condensate. The critical temperature agrees with the theoretical prediction for a uniform Bose gas. The momentum distribution of our non-condensed quantum-degenerate gas is also clearly distinct from the conventional case of a harmonically trapped sample and close to the expected distribution in a uniform system. We confirm the coherence of our condensate in a matter-wave interference experiment. Our experiments open many new possibilities for fundamental studies of many-body physics.

cond-mat.quant-gas↗