Experimental Quantum Communication without a Shared Reference Frame

We present an experimental realization of a robust quantum communication scheme [Phys. Rev. Lett. 93, 220501 (2004)] using pairs of photons entangled in polarization and time. Our method overcomes errors due to collective rotation of the polarization modes (e.g., birefringence in optical fiber or misalignment), is insensitive to the phase’s fluctuation of the interferometer, and does not require any shared reference frame including time reference, except the need to label different photons. The practical robustness of the scheme is further shown by implementing a variation of the Bennett-Brassard 1984 quantum key distribution protocol over 1 km optical fiber.

Figure 1

Experiment setup for robust QKD. A 400 mW Argon-ion laser beam (${\mathrm{Ar}}^{+}$) of 351 nm passes through a 2 mm beta-barium-borate (BBO) crystal and produces polarization-entangled photon pairs of 702 nm. POC1-POC4 are four Pockel cells; BS1 and BS2 are beam splitters; PBS1-PBS6 are six polarized beam splitters; FC1 and FC2 are two fiber couplers; HWP1 and HWP2 are two half-wave plates (HWP); a HWP inserting between two QWP constitutes a PR; IF is a interference filter; D1-D4 are four single photon detectors.

Figure 2

Interference pattern observed for the state $|H_T^{\prime }{V}_T^{\prime }⟩-|V_T^{\prime }{H}_T^{\prime }⟩$ after applying a Hadamard gate on each photon and by measuring the coincident counts. The zero delay position corresponds to the maximum interference visibility.

Figure 3

QBERs measured under different collective rotations and with a 4 m single mode fiber. The first collective rotation correspond to the identity and the fifth, to a bit flip. The angles in between are chosen as described in the text. The QBERs of each state was measured in 20 min without (a) or with random rotations (b). The average QBER over all states with and without the random rotations are compared in (c). In (d), we give the normalized coincidence counts in function of the angle of the collective rotation with or without random rotations.

Figure 4

QBER measured under different collective rotations (as in Fig. 3) and with a 1 km single mode fiber. The QBER of each states was measured in 3 h without (a) or with random rotations (b). The average QBER over all states with and without the random rotations are compared in (c). In (d), we give the normalized coincidence counts in function of the angle of the collective rotation with or without random rotations.