肉眼观测纠缠

今天看到的一篇很cool的论文是讨论用肉眼能否看到纠缠，结论是多个人合作，用肉眼是可能”看到”纠缠的。这真是一个很有趣的题目，想人所不愿意想，把”不可能”变成可能。

Possible entanglement detection with the naked eye

Nicolas Brunner, Cyril Branciard, and Nicolas Gisin

Group of Applied Physics, University of Geneva, CH-1211 Geneva 4, Switzerland

The human eye can detect optical signals containing only a few photons. We investigate the possibility to demonstrate entanglement with such biological detectors. While one person could not detect entanglement by simply observing photons, we discuss the possibility for several observers to demonstrate entanglement in a Bell-type experiment, in which standard detectors are replaced by human eyes. Using a toy model for biological detectors that captures their main characteristic, namely, a detection threshold, we show that Bell inequalities can be violated, thus demonstrating entanglement. Remarkably, when the response function of the detector is close to a step function, quantum nonlocality can be demonstrated without any further assumptions. For smoother response functions, as for the human eye, postselection is required.

逆转量子测量

量子力学数学表示告诉我们，量子测量时，量子态塌缩，最后会稳定到系统的量子本征态上面，无法逆转。这实际是量子测量的冯.诺伊曼表示，对应的一种很强的量子测量过程。

但是实际测量一个量子系统没有这么简单。比如，我们通过测量也许只能得到量子系统的部分信息，量子态并没有完全塌缩，那么我们还是可以翻转这个量子测量过 程的。假设一个量子阱中束缚了一个电子，只有它自旋向上时才能被测量到，但是探测器的效率并不是100%。那么当探测器没有测量事件时，电子可能是自旋向 上，也可能向下，并没有完全塌缩。然后对这个量子态旋转180度，再次测量。如果仍旧没有测量事件发生，那么我们实际上就把量子态恢复到测量前的状态了。 当然，这个方案只能以一定的概率实现。

最近的物理评论快报报道了在超导量子比特中实现了这个方案。为了验证方案的可靠性，他们最后用了量子全息术来测量量子态，得出量子态的保真度。他们发现，当测量概率小于等于0.6时，量子态的保真度大于0.7。更高的测量成功率会破坏这个方案。更加详细的分析讨论，见下面的这个评述。

New paper!

Generating EPR beams in cavity optomechanical system

Authors: Zhang-qi Yin, Y.-J Han

Abstract: We propose a scheme to produce continuous variable entanglement between phase-quadrature amplitudes of two light modes in optomechanical system. For proper driving power and detuning, the entanglement is insensitive with bath temperature and Q of mechanical oscillator. Under realistic experimental conditions, we find that the entanglement could be very large even at room temperature.

You can read it in arXiv:0811.0424.

Update: the paper has been published in Phys. Rev. A 79, 024301 (2009).