FET: Small: Weak and Continuous Quantum Measurements with Feedback

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One of the strangest aspects of quantum mechanics is quantum measurement. Rather than simply revealing information about a system, a quantum measurement causes an unpredictable disturbance in the system's state, and it is impossible to gain information about a quantum system without disturbing it. There are weak measurements that cause little disturbance, but these must inevitably also reveal very little information. A sequence of weak measurements, one after another, produces a continuous measurement, where both information and disturbance accumulate over time. This project concerns weak and continuous quantum measurements, and combines them with feedback, where the accumulated information is used to adapt the measurements while they are being done. These methods have many applications, to quantum computers, high-precision measurements (or metrology) and other quantum technologies. They also provide a powerful theoretical tool for understanding quantum theory.This award will study many aspects of weak and continuous quantum measurements, but will focus on three areas in particular. 1) Destructive (or lossy) weak measurements: many physical systems---such as light---cannot be easily measured except by being absorbed. Current technology includes only a limited set of quantum measurements of light; anything that extends this set would be an exciting development. 2) Continuous quantum error correction: quantum computers are highly susceptible to noise, or decoherence. These techniques would continuously monitor a quantum computer and reverse errors as they occur, perhaps even preventing them by the quantum Zeno effect. 3) Quantum resource theories: resource theories study properties of quantum systems, like entanglement, coherence and free energy, that can be exploited for useful tasks. Continuous measurement theory can quantify resources using monotones, which diminish or increase steadily as the resource is consumed, analogous to how entropy grows with time in the second law of thermodynamics. Many more interesting properties and uses of weak quantum measurements are expected to appear in the course of this project.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.