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HIGHLIGHT - Universal Bell-state Synthesizer using Filtering-free Entanglement Concentration
Fig. 1: Experimental setup. (a) mixed state generation, (b) filtering-free entanglement concentrator, (c) detectors. |
It has been recently shown that revolutionary advances can be expected in many areas of sciences if entangled quantum states are used in place of classical states. Such areas include information science, communication, metrology, lithography, etc. Entangled two-qubit states (Bell-states) are especially important because of their simplicity, direct applications in quantum cryptography, and as building blocks of multi-particle entangled states. However, a universal source of two-qubit entanglement, which can be operated effectively at both cw and ultrafast pulsed domain, had not been available.
We have recently achieved a universal source of two-qubit entanglement using filtering-free entanglement concentration scheme. No photons are wasted in our scheme and all qubit pairs, which are initially in a mixed state, exit the entanglement concentrator as entangled qubit pairs. The experimental setup is shown in Fig.1. The initial mixed state is generated via type-II spontaneous parametric down-conversion pumped either by a cw Argon laser or an ultrafast pulsed laser. The entanglement concentrator (linear optical devices) shown in Fig.1(b) performs the entangling operations and the quality of entanglement is evaluated by the detection system. The experimental data demonstrating the entangling operation is shown in Fig.2. It is evident that the initial mixed two-qubit state become entangled when the interferometric entanglement concentrator is properly aligned (0 delay). High-quality entanglement is confirmed by observing both the space-time quantum interference and the polarization interference as shown in Fig.2. We expect that this new universal Bell-state synthesizer will be essential in generating true multi-photon entangled states, quantum cryptography and teleportation experiments.
Fig. 2: Experimental data demonstrating the entanglement concentration process. |
Another advantage the Bell-state synthesizer using interferometric entanglement concentration scheme is that the degree of entanglement can be continuously varied as evidenced in Fig.2. One can therefore easily introduce controllable decoherence factors into the experiment. This enables us to study the effects of decoherence in a controlled environment. Since multi-photon entanglement can be built using two-photon entanglement, this source also allows us the systematic study of the effects of decoherence in a multi-qubit system. Such understanding offers new opportunities for manipulating multi-qubit entanglement, which is necessary, for example, quantum computing, quantum cryptography, quantum metrology, quantum lithography, etc.
References:
Y.-H. Kim, S.P. Kulik, M.V. Chekhova, W.P. Grice, and Y. Shih, "Experimental Entanglement Concentration and Universal Bell-state Synthesizer," submitted to Phys. Rev. Lett. (2002).
Y.-H. Kim and W.P. Grice, Generation of Pulsed Polarization Entangled Two-Photon State via Temporal and Spectral Engineering," to appear in J. Mod. Opt. (2002).
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