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Journal of Lightwave Technology

Long-Distance Bell-State Analysis of Fully Independent Polarization Weak Coherent States

Thiago Ferreira da Silva, Douglas Vitoreti, GUILHERME BARRETO XAVIER, Guilherme P. Temporão, Jean Pierre von der Weid

Published Jul 24, 2013• 18 citations

Abstract

Here we report the experimental implementation of a linear-optics partial Bell-state analyzer following the propagation of single-photons through long telecommunication optical fibers. Polarization encoded weak coherent states were sent from independent remote continuous wave faint laser sources over two 8.5 km long optical fiber spools. Automatic polarization stabilization systems were used in each spool to compensate the random polarization drift. We demonstrate stable two-photon interference with a dip visibility of 47.8% over a 40 minute time-interval, close to the theoretical maximum of 50% for weak coherent states. We successfully show that it is possible to carry out remote long-distance Bell-state analysis of polarization weak coherent states in spite of random residual birefringence fluctuations in optical fibers. These results pave the way for important applications in quantum communications with polarization qubits such as measurement device-independent quantum key distribution and quantum repeaters.

DOI: 10.1109/jlt.2013.2274615

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Here we report the experimental implementation of a linear-optics partial Bell-state analyzer following the propagation of single-photons through long telecommunication optical fibers. Polarization encoded weak coherent states were sent from independent remote continuous wave faint laser sources over two 8.5 km long optical fiber spools. Automatic polarization stabilization systems were used in each spool to compensate the random polarization drift. We demonstrate stable two-photon interference with a dip visibility of 47.8% over a 40 minute time-interval, close to the theoretical maximum of 50% for weak coherent states. We successfully show that it is possible to carry out remote long-distance Bell-state analysis of polarization weak coherent states in spite of random residual birefringence fluctuations in optical fibers. These results pave the way for important applications in quantum communications with polarization qubits such as measurement device-independent quantum key distribution and quantum repeaters.

We perform a proof-of-principle demonstration of the measurement-device-independent quantum key distribution protocol using weak coherent states and polarization-encoded qubits over two optical fiber links of 8.5 km each. Each link was independently stabilized against polarization drifts using a full-polarization control system employing two wavelength-multiplexed control channels. A linear-optics-based polarization Bell-state analyzer was built into the intermediate station, Charlie, which is connected to both Alice and Bob via the optical fiber links. Using decoy states, a lower bound for the secret-key generation rate of 1.04$\ifmmode\times\else\texttimes\fi{}

10

{}^{\ensuremath{-}6}$ bits/pulse is computed.

We perform a proof-of-principle demonstration of the measurement-device-independent quantum key distribution (MDI-QKD) protocol using weak coherent states and polarization-encoded qubits over two optical fiber links of 8.5 km each. Each link was independently stabilized against polarization drifts using a full-polarization control system employing two wavelength-multiplexed control channels. A linear-optics-based polarization Bell-state analyzer was built into the intermediate station, Charlie, which is connected to both Alice and Bob via the optical fiber links. Using decoy-states, a lower bound for the secret-key generation rate of 1.04x10^-6 bits/pulse is computed.

To show the feasibility of a long-distance partial Bell-state measurement, a Hong-Ou-Mandel experiment with coherent photons is reported. Pairs of degenerate photons at telecommunication wavelength are created by parametric down-conversion in a periodically poled lithium niobate waveguide. The photon pairs are separated in a beam splitter and transmitted via two fibers of

25\phantom{\rule{0.3em}{0ex}}\mathrm{km}

. The wave packets are relatively delayed and recombined on a second beam splitter, forming a large Mach-Zehnder interferometer. Coincidence counts between the photons at the two output modes are registered. The main challenge consists in the trade-off between low count rates due to narrow filtering and length fluctuations of the $25\text{\ensuremath{-}}\mathrm{km}$-long arms during the measurement. For balanced paths a Hong-Ou-Mandel dip with a net visibility of 47.3% is observed, which is close to the maximal theoretical value of 50% developed here. This proves the practicability of a long-distance Bell-state measurement with two independent sources, as, e.g., required in an entanglement swapping configuration in the scale of tens of kilometers.

We demonstrate storage of polarization-entangled photons for 125 micros, a record storage time to date, in a 25-km-long fiber spool, using a telecommunications-band fiber-based source of entanglement. With this source we also demonstrate distribution of polarization entanglement over 50 km by separating the two photons of an entangled pair and transmitting them individually over separate 25-km fibers. The measured two-photon fringe visibilities were 82% in the storage experiment and 86% in the distribution experiment. Preservation of polarization entanglement over such long-distance transmission demonstrates the viability of all-fiber sources for use in quantum memories and quantum logic gates.