Abstract
We drastically improve the mode overlap between independently seeded, gain-switched laser diodes operating at gigahertz repetition rates by implementing a pulsed light seeding technique. Injecting pulsed light reduces the emission time jitter and enables frequency chirp synchronization while maintaining random optical phases of the emitted laser pulses. We measure interference of these pulsed sources both in the macroscopic regime, where we demonstrate near perfect mode overlap, and in the single photon regime, where we achieve a Hong-Ou-Mandel dip visibility of 0.499 ± 0.004, thus saturating the theoretical limit of 0.5. The measurement results are reproduced by Monte-Carlo simulations with no free parameters. Our light source is an ideal solution for generation of high rate, indistinguishable coherent pulses for quantum information applications.
Highlights
We drastically improve the mode overlap between independently seeded, gain-switched laser diodes operating at gigahertz repetition rates by implementing a pulsed light seeding technique
For the measurement-device-independent (MDI) quantum key distribution (QKD) protocol,[5] which relies on two-photon interference to gain immunity from detector vulnerabilities, independent lasers are required to transmit light pulses that can interfere with high visibility
Semiconductor laser diodes can be gain-switched to produce naturally phase-randomized, short light pulses at gigahertz clock rate,[14] but these light pulses do not interfere with sufficient visibility required by applications such as MDI-QKD
Summary
We drastically improve the mode overlap between independently seeded, gain-switched laser diodes operating at gigahertz repetition rates by implementing a pulsed light seeding technique. The irregular, broad spectral lineshapes are a result of frequency chirp arising from the carrier density variation in the laser active medium during pulse emission.[14] In Fig. 2(c) we plot the second-order correlation of the interference output, computed from the oscilloscope acquisition, as a function of temporal overlap between the two light sources.
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