Ultracold quantum matter

Abstract:

We outline the experimental concept and key scientific capabilities of AION (Atom Interferometer Observatory and Network), a proposed UK-based experimental programme using cold strontium atoms to search for ultra-light dark matter, to explore gravitational waves in the mid-frequency range between the peak sensitivities of the LISA and LIGO/Virgo/ KAGRA/INDIGO/Einstein Telescope/Cosmic Explorer experiments, and to probe other frontiers in fundamental physics. AION would complement other planned searches for dark matter, as well as probe mergers involving intermediate mass black holes and explore early universe cosmology. AION would share many technical features with the MAGIS experimental programme in the US, and synergies would flow from operating AION in a network with this experiment, as well as with other atom interferometer experiments such as MIGA, ZAIGA and ELGAR. Operating AION in a network with other gravitational wave detectors such as LIGO, Virgo and LISA would also offer many synergies.

Abstract:

Radiofrequency (RF)-dressed potentials are a promising technique for manipulating atomic mixtures, but so far little work has been undertaken to understand the collisions of atoms held within these traps. In this work, we dress a mixture of 85Rb and 87Rb with RF radiation, characterize the inelastic loss that occurs, and demonstrate species-selective manipulations. Our measurements show the loss is caused by two-body 87Rb+85Rb collisions, and we show the inelastic rate coefficient varies with detuning from the RF resonance. We explain our observations using quantum scattering calculations, which give reasonable agreement with the measurements. The calculations consider magnetic fields both perpendicular to the plane of RF polarization and tilted with respect to it. Our findings have important consequences for future experiments that dress mixtures with RF fields.

Abstract:

The understanding and precise prediction of non-equilibrium quantum many-body dynamics, in particular across a critical point, remains a difficult task due to the relevance of all length scales near the critical point. Furthermore, the number of parameters required to characterise the state of the system increases exponentially with the number of particles, making the numerical investigation of such a system extremely difficult.

In this thesis, we use ultracold ⁸⁷Rb atoms prepared in a bilayer two-dimensional (2D) trap to probe the Berezinskii-Kosterlitz-Thouless (BKT) phase transition in detail, both in and out of equilibrium. These experiments use a multiple-radiofrequency dressed trap, which allows dynamical control of the trapped atoms as well as the precise determination of the many-body wavefunction. For the characterisation of the 2D Bose gases using matter-wave interferometry, a novel technique was developed to obtain high contrast fringes by selective imaging of slices of the atomic cloud. This allows the observation of local fluctuations, such as phase correlation function, local vortex density and coherence full counting statistics. Utilising these observables, we have identified the BKT critical point and characterised microscopic features of harmonically-trapped 2D Bose gases in equilibrium. With this information about the system, we probe the non-equilibrium dynamics of 2D Bose gases following a quench across the BKT critical point. The system is quenched by a coherent splitting, which introduces a sudden reduction of density resulting in the quench from the superfluid to the thermal phase. We monitor the dynamics towards the vortex-proliferated state and find that the vortex-unbinding dynamics is well described by the real-time renormalisation group theory. Finally, we show preliminary results for a tunnel-coupled bilayer 2D gas, in which we probe the oscillations of the relative phase of the two layers of the superfluid.

The results presented in this thesis demonstrate that the multiple-RF dressing technique is a very powerful tool for investigating quantum many-body phenomena. This paves the way for future studies of non-equilibrium critical dynamics and their description with renormalisation-group theory.

Abstract:

The universal nature of phase transitions allows one to develop a unified description of microscopically different systems. A particularly interesting universality class, which includes the 2D Bose gas, is that of the 2D XY model. In these systems there exists a phase transition between a superfluid and a normal phase. While 2D Bose gases have been studied extensively theoretically and experimentally, there remain many open questions. This thesis explores the various aspects of universal behaviour of a 2D Bose gas in and out of equilibrium.

In this thesis, we prepare ultracold quantum gases of $^{87}$Rb in radio frequency (RF) dressed potentials. We describe a new experimental apparatus which utilises this technique and allows for a simple trap loading procedure from a cloud of laser-cooled atoms. The creation of a degenerate quantum gas is demonstrated, with evaporative cooling performed entirely in RF-dressed potential. We create a double well potential using multiple radio frequencies and perform coherent splitting to form bilayer 2D Bose gases. We use matter-wave interferometry to access two important quantities, the local relative phase of the clouds and the interference contrast. Using these methods, we characterise the full counting statistics of interference contrast across the superfluid transition and demonstrate its convergence to a universal functional form. We also study the effect of disordered potentials on 2D Bose gases and show that the system loses coherence with increasing disorder strength and crosses into the normal phase. Our results suggest that this disorder-induced transition is likely to be still within the 2D XY universality class. We show preliminary results for experiments where the BKT transition is crossed dynamically and compare the time-evolution of the system with universal scaling laws. Finally, we propose an RF dressed trap in which homogeneous 2D systems can be created.