Probing and Harnessing Entanglement, Noise, and Dissipation across Quantum Metrology and Computing

Author: Direkci, Su

Year: 2027

Degree: Dissertation (Ph.D.)

Advisors: Chen, Yanbei; Endres, Manuel A.

Committee Members: McCuller, Lee P.; Chen, Yanbei; Preskill, John P.; Endres, Manuel A.

Option: Physics

DOI: 10.7907/c2gy-xk87

Abstract

Modern quantum systems have two facets: they are resources we would like to harness, and they contain physics we would like to probe. This thesis studies both through several case studies in quantum metrology and computing, unified by the interplay between entanglement, noise, and dissipation. We first probe entanglement in an optomechanical setting, where a mechanical oscillator is continuously driven and monitored by a light field. Assuming Gaussian dynamics and arbitrary, possibly non-Markovian noise, we derive necessary and sufficient conditions for entanglement. For a vacuum input, the entangling-disentangling transition is set by the environmental noise alone and is independent of the optomechanical coupling strength. Furthermore, frequency-dependent squeezing cannot entangle a system that is separable when driven by vacuum. In the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO), we find that the present level of classical noise, dominated by low-frequency seismic noise, makes significant stationary entanglement unlikely. We then turn to quantum frequency estimation and present two schemes. The first scheme harnesses entanglement as a metrological resource, approaching Heisenberg-limited precision up to a constant factor using blocks of Greenberger-Horne-Zeilinger (GHZ) states prepared with logarithmic-depth circuits and read out with local, adaptive measurements. The second scheme addresses the precision-bandwidth trade-off through a weak-measurement Ramsey protocol that recovers the classical precision limit over an extended dynamic range. Finally, we harness dissipation by introducing the 0-n Fock qubit, a noise-biased bosonic encoding stabilized by frequency-selective dissipation, with bit-flip errors exponentially suppressed in n (the highest Fock level). Used as a bias-preserving ancilla for cat-qubit architectures, it enables noise-biased controlled-X gates and low logical error rates with experimentally realistic parameters. Together, these results explore the regimes where entanglement and quantum advantage can survive noise, and show how measurement and dissipation can be engineered to protect them.

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