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We demonstrate a client-server quantum key distribution (QKD) scheme. Large resources such as laser and detectors are situated at the server side, which is accessible via telecom fiber to a client requiring only an on-chip polarization rotator, which may be integrated into a handheld device. The detrimental effects of unstable fiber birefringence are overcome by employing the reference-frame-independent QKD protocol for polarization qubits in polarization maintaining fiber, where standard QKD protocols fail, as we show for comparison. This opens the way for quantum enhanced secure communications between companies and members of the general public equipped with handheld mobile devices, via telecom-fiber tethering.

Photons can carry spin angular momentum (SAM) and orbital angular momentum (OAM), which can be used to realize a qubit system and a high-dimension system, respectively. This spin-orbital system is very suitable for implementing one-dimensional discrete-time quantum random walks. We propose a simple scheme of quantum walks on the spin-orbital angular momentum space of photons, where photons walk on the infinite OAM space controlled by their SAM. By employing the recent invention of an optical device, the so-called "q-plate," our scheme is more simple and efficient than others because there is no Mach-Zehnder interferometer in the scheme.

The transverse structure of light is recognized as a resource that can be used to encode information onto photons and has been shown to be useful to enhance communication capacity as well as resolve point sources in superresolution imaging. The Laguerre-Gaussian (LG) modes form a complete and orthonormal basis set and are described by a radial index p and an orbital angular momentum (OAM) index . Earlier works have shown how to build a sorter for the radial index p or/and the OAM index of LG modes, but a scalable and dedicated LG mode sorter which simultaneous determinate p and is immature. Here we propose and experimentally demonstrate a scheme to accomplish complete LG mode sorting, which consists of a novel, robust radial mode sorter that can be used to couple radial modes to polarizations, an -dependent phase shifter and an OAM mode sorter. Our scheme is in principle efficient, scalable, and crosstalk-free, and therefore has potential for applications in optical communications, quantum information technology, superresolution imaging, and fiber optics.

An experiment is performed to demonstrate the temporal distinguishability of a four-photon state and a six-photon state, both from parametric down-conversion. The experiment is based on a multi-photon interference scheme in a recent discovered NOON-state projection measurement. By measuring the visibility of the interference dip, we can distinguish the various scenarios in the temporal distribution of the pairs and thus quantitatively determine the degree of temporal (in)distinguishability of a multi-photon state.PACS numbers: 42.50. Dv, 03.65.Mn, 42.50.St It has been well-known by now that quantum nonlocality is more dramatic in multi-particle entanglement [1]. It was shown that the amount of locality violation increases with the number of particles [2]. Experimental demonstrations of locality violation have thus been shifted from the traditional test of two-photon Bell's inequalities [3,4,5] to the test of generalized Bell's inequalities for three or four photons in various states [6,7,8,9,10]. While entangled two-photon states are produced naturally from parametric down-conversion, generation of three-and four-photon entangled states has to rely on simultaneous two-pair production in parametric downconversion. Since pairs are produced randomly in parametric down-conversion process, this raises a question, that is, are the two pairs really in an entangled fourphoton state or they are simply independent uncorrelated two pairs?This question was first attempted by Ou, Rhee, and Wang [11,12] in an experiment similar to the famous Hong-Ou-Mandel experiment [13] but with two pairs of photons. Recently, a number of experiments were performed to further address the problem of photon pair distinguishability in parametric down-conversion [14,15,16,17]. All the experimental schemes are more or less some sort of multi-photon interference (either twophoton or four-photon). More recently, a new scheme was proposed by Ou [18] that relies on a newly discovered NOON-state projection measurement process [19,20,21] to characterize quantitatively the degree of temporal distinguishability of an N-photon state. When it applies to photon pairs from parametric down-conversion, it shows various visibility of multi-photon interference for different scenarios in the temporal distributions of the photons [18].In this letter, we wish to report on an experimental implementation of the NOON-state projection measurement for characterizing the temporal distinguishability of photon pairs from parametric down-conversion. We find that the temporal distinguishability depends on the visibility of multi-photon interference in NOON state projection. When the pairs are indistinguishable from each other, we obtain the maximum visibility which starts to decrease as the pairs begin to separate from each other and becomes a nonzero minimum when they are well separated and completely distinguishable.The key idea in Ref.[18] for characterizing temporal distinguishability is the NOON-state projection measurement, as depicted in Fig.1 for N = 4 and 6. Th...

When a vortex beam with the spiral phase structure passes through a dynamic angular double slits (ADS), the interference pattern changes alternatively between destructive and constructive at the angular bisector direction of the ADS due to their phase difference. Based on this property, we experimentally demonstrate a simple method, which can precisely and efficiently determine the topological charge of vortex beams. What's more, this scheme allows determining both the modulus and sign of the topological charge of vortex beams simultaneously. [29][30][31]. Some other interference methods have also been proposed for the same purpose, for example, Padgett's group proposed a scheme to efficiently sort different OAM states under the single photon level based on a Mach-Zehnder interferometer with Dove prisms inserted into each arm [32,33]. Recently, there is a refractive beam copying method which can efficiently sort OAM states by a complex optical transformation [34,35]. For these methods, most of them are involved with a complexity of interferential patterns or a complicated experimental setup.In our previous study [36], we found that the interference pattern of the vortex beam after passing through an angular double slits (ADS) changes alternatively between darkness and brightness at the center of the ADS because of the phase difference. Moreover, it has opposite variation tendencies between the transition of constructive and destructive interference for opposite signs of the TC of vortex beams. So the modulus and sign of the TC of the vortex beam can be determined according to the periodic and moving direction of interference patterns. However, this scheme needs to collect a series of interference patterns and estimate the periodic of interference patterns, which is not precise. Based on this work, we describe a method by which the modulus and sign of the TC of vortex beams can be conveniently, precisely and simultaneously determined.

It is known that the cross-correlation function (CCF) of a partially coherent vortex (PCV) beam shows a robust link with the radial and azimuthal mode indices. However, the previous proposals are difficult to measure the CCF in practical system, especially in the case of astronomical objects. In this letter, we demonstrate experimentally that the Hanbury Brown and Twiss effect can be used to measure the mode indices of the original vortex beam and investigate the relationship between the spatial coherent width and the characterization of CCF of a PCV beam. The technique we exploit is quite efficient and robust, and it may be useful in the field of free space communication and astronomy which are related to the photon's orbital angular momentum.In 1992, L. Allen et al. 1 pointed out that beam with spiral phase distribution of exp(ilϕ) carries an orbital angular momentum (OAM) of l , where l is an integer which denotes the azimuthal mode index (topological charge), and ϕ is the azimuthal coordinate. Beam with this kind of phase structure is also called vortex beam. Vortex beam has been widely studied in the last two decades and found a lot of applications, such as optical tweezers 2 , spiral phase contrast microscopy 3 .Since the values of l is theoretically unlimited, OAM states construct an infinite dimensional Hilbert space, and it exhibits great potential for applications in the field of quantum information process 4,5 , free-space information transfer and communications 6-8 . Despite the extensive applications, determining the topological charge l of OAM state remains an intriguing problem, and a lot of methods has been proposed. Such as MachZehnder interferometer 9 , diffraction pattern with specific masks 10-13 , image reformatting 14 , intensity analysis 15,16 .Recently, more and more researches have focused on the partially coherent vortex (PCV) beam 17-31 . In 2004, Palacios et al. firstly verified that a robust phase singularity exists in the spatial coherence function when a vortex is presented 17 in the original beam. Meanwhile, in Refs. [21][22][23] , the authors revealed the linkage between the mode indices and the cross correlation function (CCF) of a PCV beam, and they also discussed the spatial coherence on determining the mode indices of a PCV beam.Also, there has been interest in the use of spatial modes, such beams carrying OAM, as an additional degree of freedom to increase the available information bandwidth for free-space communication. Because a large Hilbert space is helpful to improve the security of cryptographic keys transmitted with a quantum key distribution system 33 . Since the important application of free space communication, the effects of propagation through random aberrations (atmospheric turbua) Electronic mail: zhangpei@mail.ustc.edu.cn lence) on coherence for single-photon communication systems based on orbital angular momentum states are also investigated 32,34-36 . However, determining the mode indices of a PCV beam is still difficult. In Refs. 17,23 , the authors experim...

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