Prof Dieter Jacksch

 Prof Martin Plenio

Impurity probes in ultracold gases

Some of the most striking manifestations of macroscopic quantum behaviour can be found in modern atomic physics experiments, where a gas of atoms is cooled to ultracold temperatures: some billionths of a degree above absolute zero. At such temperatures, novel quantum phases of matter appear, such as superfluids which flow with no resistance. In addition, a menagerie of new particles can emerge from collective excitations of the system, whose bizarre properties have no classical analogue. Ultracold gases have manifold applications in quantum technologies, ranging from quantum-enhanced metrology to simulations of complex condensed matter physics. However, in order to properly understand and characterise these systems, new experimental tools are required.

In this project, we investigate the application of a simple quantum system, e.g. a single atom of a different species (an impurity), as an experimental probe of ultracold gases. Such impurity probes could offer tunable, non-destructive, high-resolution alternatives to many conventional experimental techniques. We are currently examining the potential usefulness of such devices for thermometry [1, 2] and density measurements [3] in ultracold gases.

Autonomous quantum thermal machines

The development of classical thermodynamics in the 19th century underpinned the Industrial Revolution, and the enormous economic growth and social changes that followed. Now, in the 21st century, the burgeoning quantum technological revolution promises unprecedented advances in our computation and communication capabilities, enabled by harnessing quantum coherence. As machines are scaled down into the quantum regime, it is of prime importance to understand how quantum mechanics affects the operation of these devices. This problem has attracted great interest to the field of quantum thermodynamics over the last few years.

In this project, we try to understand how to design and optimise autonomous thermal machines based on the principles of quantum mechanics. Such autonomous machines perform useful tasks without any source of external control required from the experimenter: only a source of heat is required for the operation of the device. Examples of systems that we are interested in include heat engines, refrigerators [4], clocks and switches. Beyond the obvious technological applications, this research has the potential to elucidate foundational issues in quantum thermodynamics, and even the quantum nature of time itself.

Quantum effects in energy transport

In conventional approaches to quantum control, one tries to preserve the quantum coherent (wave-like) character of a system, typically by isolating it from its environment as far as possible. In questions of transport, however, quantum coherence may actually inhibit the transfer of excitations due to destructive interference. Electron transport in nanometre-scale systems can therefore be assisted by a small amount of decoherence, where interactions with the system’s environment induce fluctuations that suppress the wave-like nature of the electrons. Over the past several years, a collaborative effort by biologists, chemists and quantum physicists has revealed that plants and bacteria have evolved to use this effect, which allows them to photosynthesise sugar with remarkable efficiency in low light levels.

This project aims to understand how the interplay between environmental noise and coherent quantum interactions can give rise to more efficient energy transport. We study simple theoretical models of primitive devices such as quantum wires [5,6] and heat engines [4]. We try to optimise transport in order to enhance the operation of these systems, which may one day serve as primitive constituents of future quantum technologies.

[1] D. Hangleiter, M. T. Mitchison, T. H. Johnson, M. Bruderer, M. B. Plenio, and D. Jaksch, Phys. Rev. A 91, 013611 (2015).
[2] T. H. Johnson, M. T. Mitchison, F. Cosco, D. Jaksch and S. R. Clark, in preparation.
[3] M. T. Mitchison, T. H. Johnson, M. B. Plenio and D. Jaksch, in preparation.
[4] M. T. Mitchison, M. P. Woods, J. Prior and M. Huber, arXiv:1504.01593 [quant-ph] (2015).
[5] J. J. Mendoza-Arenas, M. T. Mitchison, S. R. Clark, J. Prior, D. Jaksch and M. B. Plenio,  New. J. Phys. 16, 053016 (2014).
[6] P.-L. Giscard, Z. Choo, M. T. Mitchison, J. J. Mendoza-Arenas and D. Jaksch, arXiv:1402.1421 [math-ph] (2014).