Dynamic Cooling on Contemporary Quantum Computers

Lindsay Bassman Oftelie

doi:10.1103/PRXQuantum.5.030309

2024-07-16

Lindsay Bassman Oftelie *

Antonella De Pasquale

Michele Campisi

DOI：10.1103/PRXQuantum.5.030309

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摘要

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We study the problem of dynamic cooling whereby a target qubit is cooled at the expense of heating up N - 1 further identical qubits by means of a global unitary operation. A standard back-of-the-envelope high-temperature estimate establishes that the target qubit temperature can be dynamically cooled by at most a factor of 1/./N. Here we provide the exact expression for the minimum temperature to which the target qubit can be cooled and reveal that there is a crossover from the high initial temperature regime, where the scaling is 1/./N, to a low initial temperature regime, where a much faster scaling of 1/N occurs. This slow, 1/./N scaling, which was relevant for early high-temperature NMR quantum computers, is the reason dynamic cooling was dismissed as ineffectual around 20 years ago; the fact that current low-temperature quantum computers fall in the fast, 1/N scaling regime, reinstates the appeal of dynamic cooling today. We further show that the associated work cost of cooling is exponentially more advantageous in the low-temperature regime. We discuss the implementation of dynamic cooling in terms of quantum circuits and examine the effects of hardware noise. We successfully demonstrate dynamic cooling in a three-qubit system on a real quantum processor. Since the circuit size grows quickly with N, scaling dynamic cooling to larger systems on noisy devices poses a challenge. We therefore propose a suboptimal cooling algorithm, whereby relinquishing a small amount of cooling capability results in a drastically reduced circuit complexity, greatly facilitating the implementation of dynamic cooling on near-future quantum computers.

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