Speaker
Description
Time-bin encoding is a promising platform for long-distance, optical fibre-based quantum key distribution (QKD) because it can support high communication rates and is robust to polarization instability which is present in commercial single-mode fibres. Practical time-bin encoding requires the bin separation time to be small enough to maintain sufficient phase stability in interferometric measurement schemes, while being large enough for the bins to still be distinguishable.
The use of ultrafast optics and fast, low-jitter detectors can enable the preparation and measurement of time-bins separated by only a few picoseconds. However, an optical fibre deployed in a real-world environment may be exposed to fluctuating temperatures, which in turn results in a fluctuating effective refractive index due to thermal expansion. This fluctuation can be large enough to create ambiguity in whether a photon is detected in a “early” or “late” time-bin. Disambiguating the time-bins requires the use of a suitable clock or signal that can maintain a consistent time delay relative to the time-bin states, regardless of absolute fluctuations in the fibre effective index.
In this work, we propose the use of wavelength-division multiplexing in a single-mode fibre to co-propagate O-band time-bin states with a C-band optical pulse train acting as a reference clock. In our setup, a 1040 nm femtosecond pulsed laser is used to generate linearly-polarized photons at 1345 nm via spontaneous four-wave mixing in a birefringent fibre; these photons are subsequently converted into corresponding time-bin states by inducing a polarization-dependent delay using a birefringent crystal (α-BBO). The laser simultaneously pumps an optical parametric oscillator which generates pulses at 1550 nm synchronously with the 1345 nm photons. Both fields are then co-propagated through a 10-km spool of SMF-28 fibre. The two fields are demultiplexed after the fibre, where the 1550 nm pulses are used to trigger/gate the fast detectors used to measure the 1345 nm time-bin states. We characterize the stability of the relative signal-clock delay under real-world conditions by varying the temperature of the fibre spool, and assess the feasibility of using this procedure for synchronization in time-bin QKD.
| Keyword-1 | quantum communication |
|---|---|
| Keyword-2 | optical fibre |
| Keyword-3 | ultrafast optics |