A solid block of semiconductor nanostructure, chilled to near absolute zero and isolated from its surroundings, should stop behaving like normal matter and enter a strange new quantum state called Many Body Localization (MBL). This matters because MBL breaks a core assumption of physics—ergodicity, the idea that a system’s behaviour over time matches its average behaviour across space. In an MBL state, particles become trapped by disorder and refuse to spread out and thermalise, even at finite temperatures. This contradicts standard statistical mechanics and opens a door to phenomena that cannot exist in ordinary materials. The problem is that no one has yet built a solid-state system that can sustain MBL long enough to study it properly. The researchers will fabricate free-standing semiconductor nanostructures that drastically reduce heat leakage from the environment, then probe them with electrical and thermal measurements at millikelvin temperatures. If they succeed, they will create a laboratory testbed for a fundamentally new phase of matter. The project is primarily fundamental science. But MBL states could eventually provide a form of quantum protection—a way to preserve fragile quantum information without active error correction—which would be transformative for quantum computing and topological physics.
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The subject of electron transport when states are localized by disorder has been an important topic in physics for a considerable time. It was first realized in 2006 that a closed quantum system in which there is both disorder and many body interactions shows a completely new regime of behaviour termed Many Body Localization, MBL. This regime is characterised by a breakdown of equilibrium statistical mechanics, it predicts a zero conductance state at a finite temperature, entanglement can spread although there is a lack of thermalisation due to the breakdown of ergodicity expressed as a violation of the Eigenstate Thermalisation Hypothesis, ETH. Ergodicity is assumed in many areas of condensed matter science, namely that a sub-system of the whole is typical of the whole and that the behaviour averaged over time is identical to that averaged over space. Consequently the fact that it does not hold in this situation allows new phenomena as does the lack of equilibration due to the ETH no longer holding. Possible new states can be formed by the application of high frequencies to MBL and these will be investigated in the project. To date there has been no sustained experimental investigation of these predictions in condensed matter systems although there is considerable activity using cold atoms which naturally form a closed quantum system. Enormous theoretical interest has been expressed in the hundreds of papers published on the topic. It is in the area of condensed matter that this new state of matter would have a major impact if realised - which is the purpose of the project. We will comprehensively investigate this regime of behaviour using semiconductor technology and the fabrication techniques used in investigating mesoscopic devices and semiconductor nanostructures. By fabricating free standing nanostructures we will ensure a closed system by drastically reducing the coupling to the phonons which act as a heat bath. The temperature of measurements will be down the milliKelvin region and the length scale of the disorder will be varied as will other parameters such as dimensionality. Electrical and thermal techniques will be utilised as probes of the MBL state. In addition to the importance for basic physics this work will be extremely significant in quantum information and topological physics as this new state provides a means of quantum protection not presently available.
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