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Abstract
Noise and fluctuations play a crucial role in quantum physics. They are not only important from a fundamental perspective—noise is ubiquitous in the quantum regime due to the Heisenberg uncertainty principle—but also interesting for practical purposes, as understanding and suppression of unwanted noise are key to constructing quantum information processors. While they are commonly treated as adversarial elements for quantum information processing, it is interesting to ask if noise and fluctuations can be exploited to make quantum-based technologies more powerful. An answer to this question can lead to applications that are directly compatible with state-of-the-art experimental platforms, where system decoherence is small but non-negligible. Further, it can also shed light on how quantum fluctuations are uniquely different from their classical counterparts.
In this thesis, we present a variety of theoretical advances in this direction. We start by examining how fundamental limits on quantum noise impose constraints on dynamics of open quantum systems. We consider the microscopic origin of Markovian dissipation, one of the most common types of quantum decoherence, and show that the Heisenberg uncertainty principle, and the resulting quantum noise inequalities, directly manifest as restrictions on the dissipative dynamics. Based on this insight, we propose a general recipe for designing non-Hermitian quantum dynamics without using any dissipation. Next, we turn to the question of distinguishing quantum versus classical noises. We study this in two scenarios: characterization of non-Gaussian quantum noise, and identifying genuinely quantum Markovian dephasing processes. For the former problem, we present a general characterization method based on quantum noise bispectra, and find a surprising quantum fluctuation-induced breaking of detailed balance in the case of driven photon shot noise. For the second question, we adopt an experimentally motivated approach, and define quantum dephasing environments as the ones having entangling power over the systems to which they are coupled. Focusing on purely dephasing environments, we identify necessary and sufficient conditions for entanglement generation, which have implications for, e.g., dissipative generation of entanglement.
Finally, we discuss applications inspired by the aforementioned insights. First, we consider quantum sensing protocols that exploit qubit dephasing to probe noise properties of an unknown environment. We show a general sensing modality that uses inadvertent quenches imparted by the probe to its environment, which allows one to measure the environmental spectral function. Our method unlocks a new class of quantum sensors beyond standard dephasing-based sensing protocols, which is directly compatible with numerous quantum platforms ranging from defect-based solid-state spin qubits to superconducting circuits. Second, we present a general strategy, based on inherent dissipative gauge symmetries of continuous Markovian processes, to design fully nonreciprocal dynamics in generic bipartite quantum systems. Intriguingly, this new mechanism for obtaining nonreciprocal interactions also leads to a general method for dissipative, deterministic realizations of unitary quantum gates.