Wenyuan Wang

We propose a fully-passive twin-field quantum key distribution (QKD) setup where basis choice, decoy-state preparation and encoding are all implemented entirely by post-processing without any active modulation. Our protocol can remove the potential side-channels from both source modulators and detectors, and additionally retain the high key rate advantage offered by twin-field QKD, thus offering great implementation security and good performance. Importantly, we also propose a post-processing strategy that uses mismatched phase slices and minimizes the effect of sifting. We show with numerical simulation that the new protocol can still beat the repeaterless bound and provide satisfactory key rate.

Passive quantum key distribution (QKD) has been proposed for discrete variable (DV) protocols to eliminate side channels in the source. Unfortunately, the key rate of passive DV-QKD protocols suffers from sifting loss and additional quantum errors. In this work, we propose the general framework of passive continuous variable quantum key distribution. Rather surprisingly, we find that the passive source is a perfect candidate for the discrete-modulated continuous variable quantum key distribution (DMCV QKD) protocol. With the phase space remapping scheme, we show that passive DMCV QKD offers the same key rate as its active counterpart. Considering the important advantage of removing side channels that have plagued the active ones, passive DMCV QKD is a promising alternative. In addition, our protocol makes the system much simpler by allowing modulator-free quantum key distribution. Finally, we experimentally characterize the passive DMCV QKD source, thus showing its practicality.

Photons play an important role in quantum information processing as they are easy to manipulate locally and transfer over a long distance. Photonic qubits are widely used in tasks such as linear optical quantum computing (LOQC), quantum sensing/metrology, and quantum communication. However, to date, efficient high-speed single photon sources are still difficult to make. Here, we propose that we can use "classical" phase-randomized coherent states, combined with post-processing, to perform various quantum information processing tasks. Specifically, we divide the tasks into two scenarios: ones with a circuit of a known Hilbert space dimension, describable by a unitary matrix, such as LOQC and metrology, as well as ones with an unknown channel, such as quantum communication. We propose methods that can hugely improve the numerical precision and applicable dimensions in both scenarios, including a machine learning method for the former and a linear interpolation method for the latter, opening up a wide variety of applications that can be implemented with easily attainable coherent light sources and threshold detectors, such as quantum metrology or universal fully-passive state preparation.