Abstract
Recently implemented quantum devices such as quantum processors and quantum simulators combine highly complicated quantum dynamics with high-resolution measurements. We present a passivity deformation methodology that sets constraints on the evolution of such quantum devices. The approach yields bounds that are often tighter, and thus more predictive, than the quantum microscopic analogue of the second law of thermodynamics. In particular, (i) it yields tight bounds even when the environment is microscopic; (ii) it successfully handles the ultracold limit; (iii) it enables one to account for constrained dynamics; and (iv) it bounds observables that do not appear in the second law of thermodynamics. Furthermore, this framework provides insights into nonthermal environments, correlated environments, coarse graining in microscopic setups, and the ability to detect heat leaks. Our findings can be explored and used in physical setups such as trapped ions, superconducting circuits, neutral atoms in optical lattices, and more.
- Received 5 March 2020
- Revised 4 September 2020
- Accepted 26 January 2021
DOI:https://doi.org/10.1103/PRXQuantum.2.010336
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
An intensive research effort is devoted to the development of quantum technologies, such as quantum computers or sensors. The quantum dynamics that one wishes to run on a quantum computer is often hard to simulate on a classical computer or to solve analytically. Motivated by ideas from thermodynamics, we offer a different, complementary, approach for the description of quantum processes. We present a framework for deriving inequalities that constrain the dynamics of quantum devices. The inequalities show what is and is not possible in a setup and what values observables can attain without the need to calculate its evolution.
Our framework is based on the notion of passivity. The possibility of finding different observables that are passive with respect to an initial condition is used to identify tight inequalities. Crucially, these bounds are tight even when the dynamics is thermodynamically irreversible. The framework can be used to shed light on fundamental properties like the zero-temperature limit in thermodynamics, erasure with small environments, or the changes in the energy of a subsystem that is not initially in a thermal state. Finally, violations of the inequalities can be used to detect imperfections of the setup, such as the existence of heat leaks to the environment that were unjustifiably ignored.
The flexibility of the framework makes it a useful tool for studying different potential realizations of quantum devices, such as superconducting circuits, or ion traps, whose spectrum is known. These ideas were recently applied to IBMs quantum experience platform.
