美国物理学联合会《科学之光》（AIP Scilight）专访报道：上海交大Nana Liu 教授团队关于非破坏性地探测量子系统的热力学最新研究成果

　　

近期，上海交通大学自然科学研究院和密西根学院副教授Nana Liu及其合作者团队发表题为“Nondestructively probing the thermodynamics of quantum systems with qumodes”的研究论文。该研究成果被美国物理学联合会出版社（AIP）杂志AVS Quantum Science作为编委推荐（Editor’s Pick）发表（图一），并被美国物理学联合会“科学之光”（Scilight）专题文章采访报道（图二）。

图一 

Scilight创办于2017年6月，是美国物理联合会出版社（AIP publishing）出版的网络周刊，致力于挑选AIP发表的物理领域最新的、最具有代表性的文章，简要总结其研究成果，并强调其在该领域的创新性和突破性。Scilight每年从AIP旗下30多个刊物中仅挑选300余篇物理领域内最值得关注的研究成果进行报道。

图二

研究真实量子系统的一个关键问题是如何测量它们的特性。 通常，测量过程是单次的，例如飞行时间方法，其中量子系统在单次测量后就被完全破坏。 这意味着量子系统必须为每次测量重新准备。 此外，对于某些类型的观测，这种直接测量方法甚至不可用。

为了解决这个问题，刘博士和她的团队提出了一种破坏性较小的方法，并考虑仅间接探测感兴趣的量子系统的外部探针。 这是一种确定某些系统性质同时保持系统完整性的实用方法。 该方法提出了由量子模(qumodes)制成的探针， qumodes是一种无限维量子态，可以通过例如激光束来实现。 使用适当的初始 qumode 状态并允许其与感兴趣的量子系统相互作用后，所得的 qumode 状态实际上包含有关要探测的量子系统的统计数据的重要信息。 这里特别重要的信息是感兴趣的量子系统的能谱。 因此，仅对 qumode 进行后续测量就可以确定系统的能谱以及各个能态的密度分布。 这使得能够对系统的可观察量进行全面表征，就像直接从系统中对可观察量进行采样一样。 这里关键的是，人们只需要直接测量量子模本身，从而使感兴趣的量子系统本身完好无损。

该量子模探针可用于研究量子系统的热力学。 这里，感兴趣的量子系统的统计数据可以在相互作用后被刻印到qumode探针上，因此只需要对探针的测量，而无需进一步破坏原始的量子系统。 qumode可以用作温度计来测量平衡状态下的量子系统的温度，也可以重构配分函数。 从配分函数中，对于处于平衡状态的系统，几乎所有热力学感兴趣的量都可以恢复。 它还可用于探测非平衡热力学，并用于测量哈密顿量不同参数范围的基态重叠。 从实验的角度来看，该协议也可以是实用的。

接下来的步骤是通过实验努力实现量子模的相互作用，从而超出目前可实现的范围，特别是对于大规模量子系统。 更多的理论工作应该针对不同应用的 qumode 探针的更有效的变体，并与探针本身的更有效的测量相结合。 该协议与相位估计协议密切相关，这表明传感和计算问题之间存在更深入的联系，可以进一步探索。

该工作得到了上海市科技重大专项资助。

AVS Quantum Scienc 论文链接：https://doi.org/10.1116/5.0139099

AIP Scilight专访报道链接：https://doi.org/10.1063/10.0020997

Dr. Nana Liu from the Institute of Natural Sciences and the Joint Institute at Shanghai Jiao Tong University and her team of collaborators recently published a work titled `Nondestructively probing the thermodynamics of quantum systems with qumodes’, appearing in AVS Quantum Science as an Editor’s Pick and selected for a featured article in Scilight, which highlights important work in AIP journals across the physical sciences.

Scilight was founded in June 2017. It is published weekly online by the American Institute of Physics (AIP) Publishing. It is committed to selecting the latest and most representative articles in the field of physics published by AIP, briefly summarizing their research results, and emphasizing its innovativeness and breakthrough in this field. Every year, Scilight selects only more than 300 of the most noteworthy research results in the field of physics from more than 30 publications under AIP for reporting.

A key problem in studying real quantum systems is how to measure their properties. Typically, a measurement process is single-shot, like the time-of-flight method, where the quantum system is completely destroyed after a single measurement. This means that the quantum system must be reprepared for every measurement. Furthermore, for some kinds of observations, this direct measurement method is not even available.

To address this problem, Dr. Liu and her team propose a less destructive method and consider an external probe that only indirectly probes the quantum system of interest. This constitutes a practical method to determine certain system properties whilst leaving the system intact. This method proposes probe made from a qumode, which is an infinite-dimensional quantum state that can be realized by, for example, a laser beam. Using an appropriate initial qumode state and after allowing it to interact with the quantum system of interest, the resulting qumode state actually contains important information about the statistics of the quantum system to be probed. In particular, the important information is the energy spectrum of the quantum system of interest. Thus, subsequent measurement of the qumode alone allows the spectrum of the system to be determined, along with the populations of the respective energy states. This enables a full characterisation of the system with respect to its observables, as though one had directly sampled the observable from the system. Crucially, one only needs to directly measure the qumode itself, thus leaving the quantum system of interest itself intact.

This qumode probe can be used to study the thermodynamics of quantum systems. Here the statistics of the quantum system of interest can be imprinted onto the qumode probe after interaction, thus only measurement of the probe is necessary without further destruction to the original quantum system. The qumode can be used as a thermometer to measure the temperature of quantum systems in equilibrium, and partition functions can also be reconstructed. From the partition function, virtually every quantity of thermodynamical interest, for a system in equilibrium, can be recovered. It can also be used to probe out-of-equilibrium thermodynamics and be used to measure the overlaps of ground states of different parameter regimes of a Hamiltonian. This protocol can also be practical from an experimental viewpoint.

The next steps are in the experimental efforts to realise interactions of the qumode beyond those that are currently accessible, in particular for large-scale quantum systems. More theoretical effort should be directed towards more efficient variants of the qumode probe for different applications, and combined with more efficient measurement of the probe itself. This protocol is intimately related to phase estimation protocols and this suggests deeper connections between sensing and computational problems that can be further explored.  

This work was sponsored by the Shanghai Municipal Science and Technology Major Project.

Source for paper: Nondestructively probing the thermodynamics of quantum systems with qumodes, Thomas Elliott, Mile Gu, Jayne Thompson, Nana Liu, AVS Quantum Science, Vol 5, 034402, 2023. Article can be accessed at https://doi.org/10.1116/5.0139099

Scilight article: “Probing quantum systems non-destructively”. The article can be accessed at Scilight 2023, 361105 (2023)

https://doi.org/10.1063/10.0020997

作者：

刘海彤

供稿单位：

自然科学研究院