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arXiv:1207.6586 · Physical Review A

Cross time-bin photonic entanglement for quantum key distribution

Anthony Martin, Florian Kaiser, A. Vernier, A. Beveratos, Valerio Scarani, Sébastien Tanzilli

Published Feb 4, 2013• 52 citations
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Abstract

We report a fully fibered source emitting cross time-bin-entangled photons at 1540 nm from type-II spontaneous parametric down-conversion. Compared to standard time-bin-entanglement realizations, the preparation interferometer requires no phase stabilization, simplifying its implementation in quantum key distribution experiments. Bell-type tests of such a cross time-bin state are performed in the Franson configuration and lead to two-photon interference raw visibilities greater than 95$%$, which are only limited by the dark counts in the detectors and imperfections in the analysis system. Just by trusting the randomness of the beam splitters, the correlations generated by the source can be proved of nonclassical origin even in a passive implementation. The obtained results confirm the suitability of this source for time-bin-based quantum key distribution.

DOI
10.1103/physreva.87.020301

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We report a fully fibered source emitting cross time-bin-entangled photons at 1540 nm from type-II spontaneous parametric down-conversion. Compared to standard time-bin-entanglement realizations, the preparation interferometer requires no phase stabilization, simplifying its implementation in quantum key distribution experiments. Bell-type tests of such a cross time-bin state are performed in the Franson configuration and lead to two-photon interference raw visibilities greater than 95%, which are only limited by the dark counts in the detectors and imperfections in the analysis system. Just by trusting the randomness of the beam splitters, the correlations generated by the source can be proved of nonclassical origin even in a passive implementation. The obtained results confirm the suitability of this source for time-bin-based quantum key distribution.
We report the realization of a fiber-coupled time-bin entangled photon-pair source at 1538 nm. Unlike previous time-bin sources based on type-0 spontaneous parametric down conversion, our approach takes advantage of cross-polarized paired photons generated in a type-II periodically poled lithium niobate waveguide (PPLN/W). We use a fibered birefingent delay line (BDL) in a Michelson configuration to introduce a controllable delay between the paired photons. After the BDL, these photons are separated by a polarizing beam splitter (PBS) at 45° and sent to Alice and Bob. This way we create the non common time-bin entangled Bell state |Ψ+⟩ = 1/√2 [|s,l⟩ − |l,s⟩], where s and l represent the short and long time-bins, respectively.
We report the realization of a fiber-coupled time-bin entangled photon-pair source at 1538 nm. Unlike previous time-bin sources based on type-0 spontaneous parametric down conversion, our approach takes advantage of cross-polarized paired photons generated in a type-II periodically poled lithium niobate waveguide (PPLN/W). Here, we use a birefringent delay line (BDL) in a Michelson configuration to introduce a controllable delay between the paired photons. After the BDL, these photons are separated by a polarizing beam splitter (PBS) at 45° and sent to Alice and Bob. This way we create the non common time-bin entangled Bell state |Ψ ⟩ = 1/√2 [|s,l⟩ + |l,s⟩], where s and l represent the short and long time-bins, respectively.
Quantum networks that can perform user-defined protocols beyond quantum key distribution will require fully controllable entangled quantum states. To expand the available space of generated time-bin entangled states, we demonstrate a time-bin entangled photon source that produces qubit states $|{\psi}\rangle = \alpha |{00}\rangle + \beta |{11}\rangle $ with fully controllable phase and amplitudes. Eight different two-photon states have been selected and prepared from arbitrary states on the reduced two-qubit Bloch sphere. The photon pairs encoded in the time-bin scheme were generated at 2.4 MHz with a visibility of V=0.9475±0.0016V = 0.9475 \pm 0.0016V=0.9475±0.0016 , with a violation of the CHSH Bell’s inequality by 197 standard deviations. After entanglement distribution over 100 km of single-mode fibers, we obtained a visibility of V=0.9541±0.0113V = 0.9541 \pm 0.0113V=0.9541±0.0113 with a violation of the CHSH Bell’s inequality by 6 standard deviations. The prepared states had an average fidelity of 0.9540±0.00160.9540 \pm 0.00160.9540±0.0016 at the source and an average fidelity of 0.9353−0.0209+0.01000.9353 ^{+0.0100}_{-0.0209}0.9353−0.0209+0.0100​ after entanglement distribution, which shows that the quantum states generated by our time-bin entangled photon source can be fully controlled potentially to a level applicable to long-distance advanced quantum network systems.
We report an experimental demonstration of the distribution of time-bin entangled photon pairs over 100 km of optical fiber. In our experiment, 1.5-mum non-degenerated time-bin entangled photon pairs were generated with a periodically poled lithium niobate (PPLN) waveguide by using the parametric down conversion process. Combining this approach with ultra-low-loss filters to eliminate the pump light and separate signal and idler photons, we obtained an efficient entangled photon pair source. To detect the photons, we used single-photon detectors based on frequency up-conversion. These detectors operated in a non-gated mode so that we could use a pulse stream of time correlated entangled photon pairs at a high repetition frequency (1 GHz). Using these elements, we distributed time-bin entangled photon pairs over 100 km of dispersion shifted fiber and performed a two-photon interference experiment. We obtained a coincidence fringe of 81.6% visibility without subtracting any background noise, such as accidental coincidence or dark count, which was good enough to violate Bell's inequality. Thus, we successfully distributed time-bin entangled photon pairs over 100 km.
Generation of time-bin entangled photon pairs requires the use of the Franson interferometer which consists of two spatially separated unbalanced Mach-Zehnder interferometers through which the signal and idler photons from spontaneous parametric down-conversion (SPDC) are made to transmit individually. There have been two SPDC pumping regimes where the scheme works: the narrowband regime and the double-pulse regime. In the narrowband regime, the SPDC process is pumped by a narrowband cw laser with the coherence length much longer than the path length difference of the Franson interferometer. In the double-pulse regime, the longitudinal separation between the pulse pair is made equal to the path length difference of the Franson interferometer. In this paper, we propose another regime by which the generation of time-bin entanglement is possible and demonstrate the scheme experimentally. In our scheme, differently from the previous approaches, the SPDC process is pumped by a cw multi-mode (i.e., short coherence length) laser and makes use of the coherence revival property of such a laser. The high-visibility two-photon Franson interference demonstrates clearly that high-quality time-bin entanglement source can be developed using inexpensive cw multi-mode diode lasers for various quantum communication applications.
Quantum key distribution allows for a provably secure transmission of cryptographic keys over an optical channel. Encoded polarization states or time-bin degree of freedom have been used for successful demonstrations. However, photon losses in long fibers, slow single photon detectors, and detector dark counts significantly limit the overall bit rate. Improving key throughput and reducing the overhead of key reconciliation remain as major challenges. Methods which utilize multiple time bins allow for multiple key bits to be encoded in a single photon, thus increasing the fidelity of transmitted keys and decreasing the overhead of key reconciliation in real-world conditions. Previous implementations of these methods required that Alice and Bob share a time reference by sharing a dedicated classical channel used for synchronization. This work presents a technique that allows two parties to exchange time-bin encoded photons without the need for synchronized time references. Our technique uses a framing protocol which allows Alice to encode a time reference along with a key which is determined by Alice before transmission. Security can be achieved by monitoring the visibility of a pair of Franson interferometers, using decoy pulses and measuring the round trip time between Alice and Bob. The bit rate of this technique is limited only by the recovery time of the detector and the speed of the modulation electronics. We experimentally demonstrate a raw bit rate of 5Mb/s over an optical channel with 55dB of loss, which is competitive with current research. We also demonstrate absolute timing synchronization with an accuracy of 20ps.