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Distributing entanglement and single photons through an intra-city, free-space quantum channel

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We have distributed entangled photons directly through the atmosphere to a receiver station 7.8 km away over the city of Vienna, Austria at night. Detection of one photon from our entangled pairs constitutes a triggered single photon source from the sender. With no direct time-stable connection, the two stations found coincidence counts in the detection events by calculating the cross-correlation of locally-recorded time stamps shared over a public internet channel. For this experiment, our quantum channel was maintained for a total of 40 minutes during which time a coincidence lock found approximately 60000 coincident detection events. The polarization correlations in those events yielded a Bell parameter, S=2.27±0.019, which violates the CHSH-Bell inequality by 14 standard deviations. This result is promising for entanglement-based free-space quantum communication in high-density urban areas. It is also encouraging for optical quantum communication between ground stations and satellites since the length of our free-space link exceeds the atmospheric equivalent.

©2005 Optical Society of America

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Figures (1)

Fig. 1.
Fig. 1. The layout for free-space entanglement distribution in Vienna. Alice has the single-mode fibre coupled polarization-entangled photon source (DC) and sending telescope and is located in the 19th century observatory, Kuffner Sternwarte. Bob has a receiver telescope and is located on the 46th floor of the Millennium Tower skyscraper 7.8km away. Alice measures the photons in mode A from each entangled pair using a four-channel detector, made with a 50/50 beam-splitter (BS), a half-wave plate (HWP), and polarizing beam-splitters (PBS), which measures the photon polarization in either the H/V or +/- basis. She sends the other photon in mode B, after polarization compensation (Pol.), via her telescope and free-space link, to Bob. Bob’s receiver telescope is equipped with a similar four-channel detector and can measure the polarizations in the same bases as Alice or, by rotating an extra HWP, measure another pair of complementary linear polarization bases. Alice and Bob are both equipped with time-tagging cards which record the times at which each detection event occurs. Rubidium atomic clocks provide good relative time stability. Both stations also embed a 1pps signal from the global positioning system (GPS) into their time-tag data stream to give a well-defined zero time offset. During accumulation, Bob transmits his time tags in blocks over a public internet channel to Alice. She finds the coincident photon pairs in real time by maximizing the cross-correlation of these time tags. Which of the four detector channels fired is also part of each time tag and allows Alice and Bob to determine the polarization correlations between their coincident pairs. Alice uses her polarization compensators to establish singlet-like anti-correlations between her measurements and Bob’s.

Tables (2)

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Table 1. Experimentally-measured coincidence rates.

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Table 2. The polarization correlations between the different bases.

Equations (3)

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ψ = 1 2 ( H A V B V A H B ) ,
S = E ( φ A , φ B ) E ( φ A , φ ˜ B ) + E ( φ ˜ A , φ B ) + E ( φ ˜ A , φ ˜ B ) ,
QBER = 1 2 ( 1 S EXP S QM ) ,
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