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中国科大首次实现纠缠系统波函数的直接测量 波函数是量子力学最核心的概念之一，不论是单体还是多体量子系统，其状态都可以用波函数完全刻画。目前最常用的测量波函数的方法是量子态层析，然而该方法随着待测系统规模的增加，对资源的消耗呈指数增长。2011年科学家们基于弱测量和弱值（Weak Value）提出单光子空间波函数的直接测量方法，避免了量子态层析中复杂的重构过程。然而对于多体特别是含有纠缠的量子系统的波函数直接测量，却一直未能取得突破，难点在于无法提取多体系统非局域可观测量的弱值。 李传锋研究组继首次实现非局域可观测量的量子测量[Phys. Rev. Lett. 122, 100405 (2019)]后，与Yaron Kedem一起另辟蹊径，通过巧妙构造哈密顿量，实现对多体系统局域可观测量之和的模量（Modular Value）的测量，然后利用它与非局域可观测量的弱值的数学关系，直接给出后者的取值。该方法成功解决了提取多体系统非局域可观测量的弱值这一难题，可以很方便地应用到多体非局域波函数的测量中。研究组在实验上利用双光子超纠缠成功演示了双光子非局域波函数的直接测量。他们将两个光子制备到偏振和路径分别处于最大纠缠态的超纠缠态上，再实现偏振和路径间的相互作用，最后通过路径指针在不同投影基下的计数直接测量出双光子偏振态的波函数。 该成果首次实现多体纠缠系统波函数的直接测量，《直接测量纠缠态》一文评论该成果对未来量子信息技术中大规模纠缠系统的探测提供高效的方法。该工作还澄清了波函数的直接测量技术源自于弱值而非弱测量，更为重要的是，对含有纠缠的多体量子系统波函数的直接测量，证明这是一项纯粹的量子技术，而非基于经典的干涉过程。该方法为量子物理基本问题的研究带来新的思路，并对量子信息技术的发展起到重要推动作用。  直接测量两光子非局域波函数的实验装置图 该论文共同第一作者为中科院量子信息重点实验室博士研究生潘维韦与特任副研究员许小冶。该工作得到科技部、国家自然科学基金委、中科院、安徽省以及博士后创新人才支持计划的资助。 论文链接：https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.150402 相关链接：https://physics.aps.org/articles/v12/110 分享到：
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最新评论
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谭健 2019-10-13 22:43波函数是量子力学最核心的概念之一
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wangjin001x 2019-10-13 23:12中国科大首次实现纠缠系统波函数的直接测量
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mam07 2019-10-13 23:58潘校长厉害~！
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lufan 2019-10-14 00:04中国科大首次实现纠缠系统波函数的直接测量
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dushunli 2019-10-14 00:16纠缠系统波函数！
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bairuizheng 2019-10-14 00:50对含有纠缠的多体量子系统波函数的直接测量，证明这是一项纯粹的量子技术，而非基于经典的干涉过程。
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mang2004 2019-10-14 01:35相关阅读：
Directly Measuring an Entangled State
Measuring the quantum state of a system is difficult. The common way, called tomography, involves measuring multiple copies of the system and using the statistics of that ensemble to algorithmically guess the closest possible quantum state [1]. A more direct method exists using so-called weak values, which are the result of weak (low-precision) measurements on a pre- and postselected quantum state [2–4]. However, the weak value approach is limited when it comes to measuring nonlocal (spatially separated) quantum states. In a new work, the group of Guang-Can Guo at the University of Science and Technology of China has shown that it is possible to directly measure the wave function of two spatially separated entangled photons [5]. Rather than weak values, the researchers employ modular values, which are characterizations of a quantum system obtained by making a strong measurement of a qubit, called the meter, that is coupled to the system [6]. This demonstration may lead to more efficient methods for probing large entangled systems, as are imagined in future quantum information technologies.
To understand the context of this new work, let us consider the state of a single electron. We can think about all the possible measurements that can be made on the electron. For example, we can measure its spin direction along the z axis (up or down) or along the x axis. We can observe its position or its momentum in a certain direction. Quantum theory restricts these measurements by limiting the precision of knowledge obtainable for two complimentary observables, such as position and momentum. Additionally, there is the problem of measurement disturbance, where a strong measurement of one observable (position) irreversibly alters the result of another observable (momentum). However, as many have discovered, it is possible to make weak measurements where the disturbance is minimal. In these situations, some limited information can still be gained about complimentary observables without violating the strict limits set by quantum theory [7]. As an example, scientists can measure the spin of an electron by deflecting it with a magnetic field. If the magnetic field is weak enough, spin-up and spin-down electrons will only be displaced by a small amount from their mean trajectories. Such a weak measurement can leave the original spin state of the electron intact.
To extract the most from a weak measurement, researchers have devised experiments where they preselect and postselect which quantum states they look at. The output of such a measurement is called a weak value [4]. The exact meaning of the weak value has been debated, with some researchers claiming that it gives a direct view of underlying quantum features and others suggesting that weak values can violate standard quantum limits. Still, it is clear that the study of weak values represents a bone fide advance in our understanding of quantum measurement, particularly in the direct measurement of wave functions.
The first direct measurement of a wave function with weak values occurred in 2011 when Jeff Lundeen and collaborators at the Institute for National Measurement Standards in Canada recovered the spatial wave function of an ensemble of single photons [2]. Although their experimental technique was still statistical, in that many identical copies of the photon were measured in succession, their method bypassed the standard tomographic reconstruction to directly measure the complex amplitudes of the photon’s quantum wave function [1]. Lundeen’s group followed these experiments with weak measurements of a two-state system—in this case, the two orthogonal polarization states of a photon [3] (see 12 September 2016 Viewpoint). That work showed that weak values can be used to directly measure the density matrix, which is a generalization of the wave function that can account for statistically mixed ensembles of pure states.
The next step in this development would presumably be to directly measure the wave function of a two-state system, in which the individual states are entangled and spatially separated. However, the standard weak value scheme cannot be applied to nonlocal states, as it requires information to instantaneously pass between the particles during the weak measurements [6]. Researchers have found a way to obtain the weak values for nonlocal states [8], but the method requires multiple (low-probability) weak measurements. The information obtained from such measurements is very limited, which means the experiments have to be repeated a large number of times and can be impractical for entangled photon experiments.
Guo’s team overcame the difficulties with weak values by switching to modular values, which also involve pre- and postselection of quantum states but are based on strong rather than weak measurements. The team first generated entangled photons using an ultraviolet (UV) laser and a beta barium borate (BBO) crystal that converted UV photons into lower-energy photon pairs. The photons could travel down one of four separate paths. Because of momentum conservation and the properties of the BBO crystal, the two photons in each pair were entangled in both their paths and in their polarization (Fig. 1). The team used the path state of the photons as the qubit meter in order to determine the polarization state of the entangled photons. Specifically, they postselected polarization states using wave plates and polarizing beam splitters in each path, and they used interference effects and single-photon detectors to strongly measure the paths of the photons. By analyzing the data, they could calculate the entangled polarization state. The team verified their results using standard tomographic techniques.
The measurement and characterization of entangled states is extremely important for quantum information applications such as cryptography, communication, and computing. As state-of-the-art research efforts continue to produce larger entangled quantum systems, methods to characterize those systems must similarly improve. Standard tomographic methods become difficult as system sizes increase, in part due to the requirement of global reconstruction of the full quantum state. Therefore, direct measurement methods—that scale reasonably—may provide a good option for entangled state characterization. I believe the work by Guo’s group could provide such an option. It will be interesting to see if their modular value method can be expanded to atomic or superconducting systems, which are widely being studied in quantum computing applications.
References
1.D. F. V. James, P. G. Kwiat, W. J. Munro, and A. G. White, “Measurement of qubits,” Phys. Rev. A 64, 052312 (2001).
2.J. S. Lundeen, B. Sutherland, A. Patel, C. Stewart, and C. Bamber, “Direct measurement of the quantum wavefunction,” Nature 474, 188 (2011).
3.G. S. Thekkadath, L. Giner, Y. Chalich, M. J. Horton, J. Banker, and J. S. Lundeen, “Direct measurement of the density matrix of a quantum system,” Phys. Rev. Lett. 117, 120401 (2016).
4.Y. Aharonov, D. Z. Albert, and L. Vaidman, “How the result of a measurement of a component of the spin of a spin-1/2 particle can turn out to be 100,” Phys. Rev. Lett. 60, 1351 (1988).
5.W.-W. Pan et al., “Direct measurement of a nonlocal entangled quantum state,” Phys. Rev. Lett. 123, 150402 (2019).
6.Y. Kedem and L. Vaidman, “Modular values and weak values of quantum observables,” Phys. Rev. Lett. 105, 230401 (2010).
7.M. Ozawa, “Universally valid reformulation of the Heisenberg uncertainty principle on noise and disturbance in measurement,” Phys. Rev. A 67, 042105 (2003).
8.J. S. Lundeen and K. J. Resch, “Practical measurement of joint weak values and their connection to the annihilation operator,” Phys. Lett. A 334, 337 (2005).
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redplum 2019-10-14 06:56真是牛逼啊
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likaihit 2019-10-14 06:58好深哦的
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蠊蠊 2019-10-14 07:55对未来量子信息技术中大规模纠缠系统的探测提供高效的方法。该工作还澄清了波函数的直接测量技术源自于弱值而非弱测量，更为重要的是，对含有纠缠的多体量子系统波函数的直接测量，证明这是一项纯粹的量子技术，而非基于经典的干涉过程。该方法为量子物理基本问题的研究带来新的思路，并对量子信息技术的发展起到重要推动作用。