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Conference on Lasers and Electro-Optics

Beyond Werner: making optimally entangled arbitrary purity states

Andrew G. White, William J. Munro, Daniel F. V. James, Paul G. Kwiat

Published Aug 29, 2005• 0 citations
Publisher

Abstract

Summary form only given. Entangled states of multi-particle systems are arguably the quintessential feature of quantum mechanics. In addition to their central role in discussions of nonlocal quantum correlations, they form the basis of quantum information, and enable such phenomena as quantum cryptography, quantum dense coding, teleportation, and quantum computation. Yet surprisingly, to date only a limited class of entangled states has ever been produced. Via the process of spontaneous parametric downconversion in a pair of nonlinear crystals, it is now possible to produce, with high-efficiency, pairs of photons entangled in their polarization degree of freedom. Further, we can control both the extent of entanglement and the purity of the state.

DOI
10.1109/iqec.2000.907784

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Summary form only given. Entangled states of multi-particle systems are arguably the quintessential feature of quantum mechanics. In addition to their central role in discussions of nonlocal quantum correlations, they form the basis of quantum information, and enable such phenomena as quantum cryptography, quantum dense coding, teleportation, and quantum computation. Yet surprisingly, to date only a limited class of entangled states has ever been produced. Via the process of spontaneous parametric downconversion in a pair of nonlinear crystals, it is now possible to produce, with high-efficiency, pairs of photons entangled in their polarization degree of freedom. Further, we can control both the extent of entanglement and the purity of the state.
Summary form only given. Entangled states of multi-particle systems are arguably the quintessential feature of quantum mechanics. In addition to their central role in discussions of non-local quantum correlations, they form the basis of quantum information, and enable such phenomena as quantum cryptography, quantum dense coding, teleportation, and quantum computation. Via the process of spontaneous parametric downconversion in a pair of nonlinear crystals, it is now possible to produce, with high efficiency, pairs of photons entangled in their polarisation degree of freedom. We can also control both the extent of entanglement and the purity of the state. Density matrices for states generated by this method, and measured using tomographic techniques, are shown.
Entangled states are key ingredients to the new field of quantum information, including quantum dense coding, teleportation, and computation. However, only a relatively small class of entangled states has been investigated experimentally, or even discussed extensively. In particular, efforts to date have focused on two particles entangled in a single degree of freedom, for example polarization, or energy, or momentum direction. Novel phase-matching arrangements in spontaneous parametric down-conversion allow the preparation of pairs of photons that are simultaneously entangled in all of these. We shall call such a multiply-entangled state ‚hyper-entangled‘. In addition, an even more general state–-a non-maximally entangled state–-should be realizable, in which the amplitudes of the contributing terms are not equal.
Using a spontaneous parametric-downconversion source of photon pairs, we are working towards the creation of arbitrary 2-qubit quantum states with high fidelity. Currently, all physically allowable combinations of polarization entanglement and mixture can be produced, including maximally-entangled mixed states. The states are experimentally measured and refined via computer-automated quantum-state tomography, and this system has also been used to perform single-qubit and ancilla-assisted quantum process tomography.
Quantum entanglement is known to enable otherwise impossible feats in various communication protocols, such as quantum key distribution and super-dense coding. Here we describe efforts to further enhance the usual benefits, by incorporating quantum states that are simultaneously entangled in multiple degrees of freedom – “hyperentangled”. Via the process of spontaneous parametric down conversion, we have demonstrated photon pairs simultaneously entangled in polarization and spatial mode, and have used these to realize remote entangled state preparation, full polarization Bell-state analysis, and the highest reported capacity quantum dense coding.
For advanced quantum information technology, sources of photon pairs in quantum mechanically factorable states are of great importance for realizing high-fidelity photon-photon quantum gate operations. Here we experimentally demonstrate a technique to produce spectrally factorable photon pairs utilizing multi-order quasi-phase-matching (QPM) conditions in spontaneous parametric downconversion (SPDC). In our scheme, a spatial nonlinearity profile of a nonlinear optical crystal is shaped with current standard poling techniques, and the associated phase-matching function can be approximated to a Gaussian form. By the measurement of a phase-matching function and the second-order autocorrelation function, we demonstrate that telecom-band photon pairs produced by our custom-poled crystal are highly factorable with > 95% single-photon purity.
The development of intelligent Internet of vehicles (IoV) is based on communication technology, especially the quantum communication of hybrid entangled states based on orbital angular momentum (OAM). The quantum key distribution (QKD) method is demonstrated using hybrid entangled states with multi-degree of freedom and high purity based on OAM. First, the polarization entangled photon pairs are produced by two type I BBO crystals by means of spontaneous parametric down-conversion (SPDC). The polarization-OAM hybrid entangled states are created by q-plate (QP) to realize QKD. It is found that the polarization entangled photon pairs have the characteristics of OAM. A single mode fiber (SMF) is proposed to filter the topological charge non-zero photons and purify the polarization entangled photon pairs. Then the purification setup with M-Z interferometer and beam rotator (BR) is presented which can purify the polarization-OAM hybrid entangled states. The high purity polarization-OAM hybrid entangled states are implemented, and the QP conversion rate of 3 secondary feedbacks is 99.84% theoretically. Analysis indicates that precise purity polarization-OAM hybrid entangled state can increase the fidelity of information and improve the anti-interference ability in this program without mutually unbiased bases (MUBs). The photon utilization rate, energy-efficient and low-latency communication can be greatly improved by using the OAM carrying information.