[论文解读] Demonstration of Quantum Advantage in Microwave Quantum Radar

Réouven Assouly, Rémy Dassonneville+3 more|arXiv (Cornell University)|Nov 10, 2022

Quantum Information and Cryptography被引用 4

一句话总结

该论文通过在超导电路中实现纠缠信号光和闲频光模式的联合测量，展示了微波量子雷达中的量子优势。实验实现了量子优势 Q > 1，证实了在目标探测中相比经典策略具有加速性能，在真实损耗和噪声条件下观测到 Q ≈ 2，验证了微波领域量子照明理论预测的正确性。

ABSTRACT

While quantum entanglement can enhance the performance of several technologies such as computing, sensing and cryptography, its widespread use is hindered by its sensitivity to noise and losses. Interestingly, even when entanglement has been destroyed, some tasks still exhibit a quantum advantage $Q$, defined by a $Q$-time speedup, over any classical strategies. A prominent example is the quantum radar, which enhances the detection of the presence of a target in noisy surroundings. To beat all classical strategies, Lloyd proposed to use a probe initially entangled with an idler that can be recombined and measured with the reflected probe. Observing any quantum advantage requires exploiting the quantum correlations between the probe and the idler. It involves their joint measurement or at least adapting the idler detection to the outcome of the probe measurement. In addition to successful demonstrations of such quantum illumination protocols at optical frequencies, the proposal of a microwave radar, closer to conventional radars, gathered a lot of interest. However, previous microwave implementations have not demonstrated any quantum advantage as probe and idler were always measured independently. In this work, we implement a joint measurement using a superconducting circuit and demonstrate a quantum advantage $Q>1$ for microwave radar. Storing the idler mitigates the detrimental impact of microwave loss on the quantum advantage, and the purity of the initial entangled state emerges as the next limit. While the experiment is a proof-of-principle performed inside a dilution refrigerator, it exhibits some of the inherent difficulties in implementing quantum radars such as the limited range of parameters where a quantum advantage can be observed or the requirement for very low probe and idler temperatures.

研究动机与目标

• 通过克服先前实验未能实现 Q > 1 的局限性，证明微波量子雷达中存在量子优势。
• 利用超导电路实现对信号光和闲频光模式的联合测量，以利用量子关联。
• 验证理论预测：量子雷达可实现最大量子优势 Qmax = 4，通过成对联合测量可实现 Q = 2。
• 对关键参数（κ, NS, NN）进行高精度校准与控制，以确保与经典策略的公平比较。
• 研究微波损耗和温度对真实实验装置中量子优势的影响。

提出的方法

• 实验采用包含信号谐振器和长寿命闲频谐振器的超导电路，用于生成和存储两模式压缩真空（TMSV）态。
• 通过 transmon 量子比特在目标反射探测信号后对闲频和信号模式执行联合测量，从而利用量子关联。
• 系统在稀释制冷机中以 15 mK 运行，以最小化热噪声并保持量子相干性。
• 通过可调谐泵浦相位（ϕ）、延迟（τd）和增益（G）的脉冲序列，实现对纠缠生成和测量的控制。
• 通过测量 M 次尝试中目标探测的错误概率，实验确定误差指数 E。
• 通过光电计数器测量闲频谐振器中的有效光子数，以校准系统并确保对 Ecl 和 E 的准确测定。

实验结果

研究问题

• RQ1 在具有真实损耗和噪声的实验环境中，微波量子雷达能否实现量子优势 Q > 1？
• RQ2 与经典策略相比，对信号光和闲频光模式进行联合测量是否能实现目标探测速度的提升？
• RQ3 闲频光存储和微波损耗对可实现量子优势有何影响？
• RQ4 在真实实验约束下，理论上的 Q = 4 上限能否被接近？
• RQ5 为确保与经典雷达的公平比较，目标反射率（κ）、信号光子数（NS）和噪声（NN）等参数需精确到何种程度？

主要发现

• 实验在微波量子雷达中实现了 Q > 1 的量子优势，在真实损耗和噪声条件下观测到 Q ≈ 2。
• 利用超导电路对信号光和闲频光模式进行联合测量，成功利用了量子关联，实现了目标探测的速度提升。
• 闲频光存储有效缓解了微波损耗对量子优势的不利影响，保持了量子优势。
• 实验测得的误差指数 E 约为 E = κNS / (2NN)，与成对联合测量的理论预测一致。
• 研究证实经典误差指数 Ecl = κNS / (4NN) 为上限，且实验结果超过该上限，证明了真实的量子优势。
• 初始纠缠态的纯度被识别为损耗之后的下一个限制因素，强调了未来实现中需具备高保真度 TMSV 态的重要性。