Bingjie Xu

Non-Gaussian postselection and virtual photon subtraction in continuous-variable quantum key distribution

Photon subtraction can enhance the performance of continuous-variable quantum key distribution (CV QKD). However, the enhancement effect will be reduced by the imperfections of practical devices, especially the limited efficiency of a single-photon detector. In this paper, we propose a non-Gaussian postselection method to emulate the photon substraction used in coherent-state CV QKD protocols. The virtual photon subtraction not only can avoid the complexity and imperfections of a practical photon-subtraction operation, which extends the secure transmission distance as the ideal case does, but also can be adjusted flexibly according to the channel parameters to optimize the performance. Furthermore, our preliminary tests on the information reconciliation suggest that in the low signal-to-noise ratio regime, the performance of reconciliating the postselected non-Gaussian data is better than that of the Gaussian data, which implies the feasibility of implementing this method practically.

Experimental quantum-key distribution with an untrusted source

The photon statistics of a quantum-key-distribution (QKD) source are crucial for security analysis. We propose a practical method, with only a beam splitter and a photodetector, to monitor the photon statistics of a QKD source. By implementing in a plug and play QKD system, we show that the method is highly practical. The final secure key rate is 52 bit/s, compared to 78 bit/s when the source is treated as a trusted source.

Source monitoring for continuous-variable quantum key distribution

The noise in optical source preparation needs to be characterized for the security of practical continuous-variable quantum key distribution (CVQKD). Two feasible schemes, based on either an active optical switch or a passive beamsplitter, are proposed to monitor the variance of the source noise. We derive the security bounds for both schemes against collective attacks in the asymptotical limit and find that the passive scheme performs better. The finite-size effect is also discussed briefly for both schemes.

Efficient rate-adaptive reconciliation for continuous-variable quantum key distribution

Using the tool of concatenated stabilizer coding, we prove that the complexity class QMA remains unchanged even if every witness qubit is disturbed by constant noise. This result may not only be relevant for physical implementations of verifying protocols but also attacking the relationship between the complexity classes QMA, QCMA and BQP, which can be reformulated in this unified framework of a verifying protocol receiving a disturbed witness. While QCMA and BQP are described by fully dephasing and depolarizing channels on the witness qubits, respectively, our result proves QMA to be robust against 27% dephasing and 18% depolarizing noise.By the Gottesman-Knill Theorem, the outcome probabilities of Clifford circuits can be computed efficiently. We present an alternative proof of this result for quopit Clifford circuits (i.e., Clifford circuits on collections of

The Security of SARG04 Protocol with An Untrusted Source

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Passive-scheme analysis for solving the untrusted source problem in quantum key distribution

-level systems, where

Passive scheme with a photon-number-resolving detector for monitoring the untrusted source in a plug-and-play quantum-key-distribution system

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Efficient rate-adaptive reconciliation for CV-QKD protocol.

is an odd prime) using Feynmans sum-over-paths technique, which allows the amplitudes of arbitrary quantum circuits to be expressed in terms of a weighted sum over computational paths. For a general quantum circuit, the sum over paths contains an exponential number of terms, and no efficient classical algorithm is known that can compute the sum. For quopit Clifford circuits, however, we show that the sum over paths takes a special form: it can be expressed as a product of Weil sums with quadratic polynomials, which can be computed efficiently. This provides a method for computing the outcome probabilities and amplitudes of such circuits efficiently, and is an application of the circuit-polynomial correspondence which relates quantum circuits to low-degree polynomials.Determining whether a given integer is prime or composite is a basic task in number theory. We present a primality test based on quantum order finding and the converse of Fermats theorem. For an integer

Continuous-variable measurement-device-independent quantum key distribution with virtual photon subtraction

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Experimental Implementation of High-speed Balanced Homodyne Detector

, the test tries to find an element of the multiplicative group of integers modulo