University of Waterloo
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What can detectors detect?
We congratulate Laura Henderson, who was awarded the Pearson Medal for her doctoral thesis in Relativistic Quantum Information. Entitled “What can detectors detect?”, Laura’s thesis advanced our understanding of the relationship between quantum information, space, and time in novel ways.
It has long been known that the quantum vacuum has fluctuations, and that these fluctuations can be entangled. However, only recently has it come to be appreciated that this quantum entanglement can be extracted (or harvested) by physical objects. Such objects are most easily modelled as 2-level detectors -- atoms (or quantum dots) with only 2 energy levels that, upon exposure to the vacuum (or `turning them on’) for a period of time, can become entangled.
Laura’s thesis advanced our understanding of this phenomenon in novel ways, showing how such detector entanglement provides us with a diagnostic of the structure of spacetime, and even quantum gravity. It describes several major accomplishments. One was a discovery that spacetime, with the right curvature properties, has “islands of separability”: isolated regions of space where, unexpectedly, the detectors cannot extract entanglement. Laura also described the first investigation of entanglement extraction near black holes, and found that black holes inhibit such extraction the closer the detectors are placed near the hole. If they are too close, extraction is impossible. Laura investigated for the first time the implications of how quantum gravity will affect vacuum entanglement. A full theory of quantum gravity is expected to allow situations where the order of events is not definite: it is possible to have a quantum state of “event A before event B” plus “event B before event A”, even if signals can travel between the events. This is known as “indefinite causal order”, and such a situation could have a significant effect on the entanglement structure of quantum fields. Laura found that, if this idea is correct, detectors can extract entanglement in regions that are otherwise forbidden, providing an in-principle novel test of quantum gravity. A final accomplishment of the thesis was to show that if quantum gravity provides an upper limit to the wavelength of quantum fields, this “bandlimit” can, in principle, be detected, and in fact exploited to extract entanglement.
Laura’s work resulted in 5 papers, one in the prestigious Physical Review Letters. She has 4 more papers, two of which are fast-track communications in the Journal Classical and Quantum Gravity. Her thesis significantly advanced our understanding of the quantum vacuum and its quantum information properties. To quote Miles Blencowe, the external examiner at her defence, it is a ``tour-de-force in mathematical analysis [that] represents a very thorough and considered investigation”, and ``ranges widely and deeply concerning the harvesting of entanglement”.
Laura had many fine collaborators to work with on her projects, not only from Canada, but from Austria, Australia, and China. She is grateful for the stimulating scientific exchange and the insights that emerged. As it turns out, detectors can detect many interesting things!
The University of Waterloo acknowledges that much of our work takes place on the traditional territory of the Neutral, Anishinaabeg and Haudenosaunee peoples. Our main campus is situated on the Haldimand Tract, the land granted to the Six Nations that includes six miles on each side of the Grand River. Our active work toward reconciliation takes place across our campuses through research, learning, teaching, and community building, and is centralized within our Indigenous Initiatives Office.