An Almost Deterministic Cooling by Measurements

IF 4.4 Q1 OPTICS Advanced quantum technologies Pub Date : 2024-07-02 DOI:10.1002/qute.202300435
Jia-shun Yan, Jun Jing
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Abstract

Nondeterministic measurement-based techniques are efficient in reshaping the population distribution of a quantum system but suffer from a limited success probability of holding the system in the target state. To save the experimental cost, a two-step protocol is proposed to cool a resonator down to the ground state with a near-unit probability by exploiting the state-engineering mechanisms of both conditional and unconditional measurements on an ancillary qubit. In the first step, the unconditional measurements on the ancillary qubit are applied to reshape the target resonator from a thermal state to a reserved Fock state. The measurement sequence can be efficiently optimized by reinforcement learning for maximum fidelity. In the second step, the population on the reserved state can be faithfully transferred in a stepwise way to the resonator's ground state with a near-unit fidelity by the conditional measurements on the qubit. Properly designing the projection operator and the measurement interval enables the Kraus operator to act as a lowering operator for neighboring Fock states. Through dozens of measurements in all, the initial thermal average population of the resonator can be reduced by five orders in magnitude with a success probability of over 95%.

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通过测量实现几乎确定性的冷却
基于非确定性测量的技术能有效重塑量子系统的种群分布,但将系统保持在目标状态的成功概率有限。为了节省实验成本,我们提出了一种两步协议,利用对辅助量子比特的条件测量和非条件测量的状态工程机制,以接近单位的概率将谐振器冷却到基态。第一步,对辅助量子比特进行无条件测量,将目标谐振器从热态重塑为保留的福克态。测量序列可以通过强化学习进行有效优化,以获得最大保真度。第二步,通过对量子比特的条件测量,保留态上的种群可以以接近单位的保真度逐步忠实地转移到谐振器的基态。适当设计投影算子和测量间隔,可使克劳斯算子成为邻近 Fock 状态的降低算子。通过总共几十次测量,共振器的初始热平均群体可以减少五个数量级,成功概率超过 95%。
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