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arXiv 0704.0758quant-ph
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时间测量法研究独立光子的纠缠

Entangling Independent Photons by Time Measurement

Matthaeus Halder, Alexios Beveratos, Nicolas Gisin, Valerio Scarani, Christoph Simon, Hugo Zbinden

摘要:

由两个或多个子系统组成的量子系统可以处于纠缠态,即全局系统的属性被很好地定义,但每个子系统的属性没有定义的状态。纠缠是量子物理学的核心,无论是其概念基础还是在信息处理和量子通信中的应用。值得注意的是,纠缠可以“交换”:如果准备两个独立的纠缠对A1-A2和B1-B2,对A1和B1的联合测量(称为“贝尔状态测量”,BSM)具有将A2和B2投射到纠缠态的效果,尽管这两个粒子从未相互作用或共享任何共同的过去[1,2]。使用自发参量下转换(SPDC)产生的孪生光子的实验已经证明了纠缠交换[3-6],但在这里我们提出了使用连续波(CW)源的第一个实现,正如最初提出的那样[2]。挑战是在BSM中实现光子的足够尖锐的同步。使用窄带滤波器,经历BSM的光子的相干时间显著增加,超过了探测器的时间分辨率。因此,脉冲源可以被CW源取代,CW源不需要任何同步[6,7],允许第一次使用完全自主的源。我们的实验利用了光子探测器的时间精度、SPDC与非线性晶体中的波导产生光子对的效率[8]以及窄带滤波器的稳定性方面的最新进展。这种方法与纠缠的形式无关;我们采用了电信波长的时间仓纠缠光子[9]。由于cm长的相干长度,我们的设置对光纤中的热或机械波动具有鲁棒性。

英文摘要:

A quantum system composed of two or more subsystems can be in an entangled state, i.e. a state in which the properties of the global system are well defined but the properties of each subsystem are not. Entanglement is at the heart of quantum physics, both for its conceptual foundations and for applications in information processing and quantum communication. Remarkably, entanglement can be "swapped": if one prepares two independent entangled pairs A1-A2 and B1-B2, a joint measurement on A1 and B1 (called a "Bell-State Measurement", BSM) has the effect of projecting A2 and B2 onto an entangled state, although these two particles have never interacted or shared any common past[1,2]. Experiments using twin photons produced by spontaneous parametric down-conversion (SPDC) have already demonstrated entanglement swapping[3-6], but here we present its first realization using continuous wave (CW) sources, as originally proposed[2]. The challenge was to achieve sufficiently sharp synchronization of the photons in the BSM. Using narrow-band filters, the coherence time of the photons that undergo the BSM is significantly increased, exceeding the temporal resolution of the detectors. Hence pulsed sources can be replaced by CW sources, which do not require any synchronization[6,7], allowing for the first time the use of completely autonomous sources. Our experiment exploits recent progress in the time precision of photon detectors, in the efficiency of photon pair production by SPDC with waveguides in nonlinear crystals[8], and in the stability of narrow-band filters. This approach is independent of the form of entanglement; we employed time-bin entangled photons[9] at telecom wavelengths. Our setup is robust against thermal or mechanical fluctuations in optical fibres thanks to cm-long coherence lengths.

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