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A promising avenue for the preparation of Gibbs states on a quantum computer is to simulate the physical thermalization process. The Davies generator describes the dynamics of an open quantum system that is in contact with a heat bath. Crucially, it does not require simulation of the heat bath itself, only the system we hope to thermalize. Using the state-of-the-art techniques for quantum simulation of the Lindblad equation, we devise a technique for the preparation of Gibbs states via thermalization as specified by the Davies generator.In doing so, we encounter a severe technical challenge: implementation of the Davies generator demands the ability to estimate the energy of the system unambiguously. That is, each energy of the system must be deterministically mapped to a unique estimate. Previous work showed that this is only possible if the system satisfies an unphysical 'rounding promise' assumption. We solve this problem by engineering a random ensemble of rounding promises that simultaneously solves three problems: First, each rounding promise admits preparation of a 'promised' thermal state via a Davies generator. Second, these Davies generators have a similar mixing time as the ideal Davies generator. Third, the average of these promised thermal states approximates the ideal thermal state.
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Control of open quantum systems: The nonequilibrium Green’s function perspective
Manipulations with open quantum systems (such as qubits) are fundamental for any quantum technology. They are the focus of studies involving optimal control theory. Usually, control is achieved through the use of time-dependent external fields when driven system evolution is simulated employing the Davies construction (second-order Markov quantum master equation formulation). As a weak (second order) coupling scheme, the Davies construction is limited in its ability to account for bath-induced coherences. To overcome the limitation, we utilize the nonequilibrium Green’s function method and demonstrate that accounting for the coherences makes a qualitative impact on quantum control studies. We find that accounting for the coherences is especially important when dealing with system evolution involving mixed states.
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- Award ID(s):
- 2154323
- PAR ID:
- 10579060
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- APL Quantum
- Volume:
- 2
- Issue:
- 1
- ISSN:
- 2835-0103
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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