Zong-Wen Yu

Protecting quantum states from decoherence of finite temperature using weak measurement

We show how to optimally protect quantum states and quantum entanglement under non-zero temperature based on measurement reversal from weak measurement. In particular, we present explicit formulas of the protection.

Making the decoy-state measurement-device-independent quantum key distribution practically useful

The relatively low key rate seems to be the major barrier to its practical use for the decoy-state measurement-device-independent quantum key distribution (MDI-QKD). We present a four-intensity protocol for the decoy-state MDI-QKD that hugely raises the key rate, especially in the case in which the total data size is not large. Also, calculations show that our method makes it possible for secure private communication with fresh keys generated from MDI-QKD with a delay time of only a few seconds.

Three-intensity decoy-state method for measurement-device-independent quantum key distribution

We study the measurement device independent quantum key distribution (MDI-QKD) in practice with limited resource, when there are only 3 different states in implementing the decoy-state method. We present a more tightened explicit formula to estimate the lower bound of the yield of two-single-photon pulses. Moreover, we show that the bounding of this yield and phase flip error of single photon pulse pairs can be further improved by using other constraints which can be solved by a simple and explicit program. Results of numerical simulation for key rates with both the improved explicit formula and the program are presented. It shows that the results obtained with our methods here can significantly improve the key rate and secure distance of MDI QKD with only three intensities.

Measurement-device-independent quantum key distribution with source state errors and statistical fluctuation

We show how to calculate the secure final key rate in the four-intensity decoy-state MDI-QKD protocol with both source errors and statistical fluctuations with a certain failure probability. Our results rely only on the range of only a few parameters in the source state. All imperfections in this protocol have been taken into consideration without any unverifiable error patterns.

Measurement-device-independent quantum key distribution with source state errors in photon number space

The existing decoy-state MDI-QKD theory assumes the perfect control of the source states which is a an impossible task for any real setup. In this paper, we study the decoy-state MDI-QKD method with source errors without any presumed conditions and we get the final security key rate only with the range of a few parameters in the source state.

Efficient tomography of quantum-optical Gaussian processes probed with a few coherent states

An arbitrary quantum-optical process (channel) can be completely characterized by probing it with coherent states using the recently developed coherent-state quantum process tomography (QPT) [Lobino et al., Science 322, 563 (2008)]. In general, precise QPT is possible if an infinite set of probes is available. Thus, realistic QPT of infinite-dimensional systems is approximate due to a finite experimentally feasible set of coherent states and its related energy-cutoff approximation. We show with explicit formulas that one can completely identify a quantum-optical Gaussian process just with a few different coherent states without approximations like the energy cutoff. For tomography of multimode processes, our method exponentially reduces the number of different test states, compared with existing methods.

Reexamination of decoy-state quantum key distribution with biased bases

State Key Laboratory of Low Dimensional Quantum Physics, Tsinghua University, Beijing 100084, People’s Republic of China Data Communication Science and Technology Research Institute, Beijing 100191, People’s Republic of China 3 Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China Hefei, Anhui 230026, People’s Republic of China 4 Shandong Academy of Information and Communication Technology, Jinan 250101, People’s Republic of China

Three-intensity decoy state method for device independent quantum key distribution

We study the measurement device independent quantum key distribution (MDI-QKD) in practice with limited resource, when there are only 3 different states in implementing the decoy-state method. We present a more tightened explicit formula to estimate the lower bound of the yield of two-single-photon pulses. Moreover, we show that the bounding of this yield and phase flip error of single photon pulse pairs can be further improved by using other constraints which can be solved by a simple and explicit program. Results of numerical simulation for key rates with both the improved explicit formula and the program are presented. It shows that the results obtained with our methods here can significantly improve the key rate and secure distance of MDI QKD with only three intensities.

Measurement-device-independent quantum key distribution via quantum blockade

Efficiency in measurement-device-independent quantum key distribution(MDI-QKD) can be improved not only by the protocol, but also single-photon sources. We study the behavior of MDI-QKD with statistical fluctuation using quantum blockade source. Numerical simulation for a type of 4-intensity protocol shows that, after parameter optimization, this source can improve the final key rate by 100 times compared with traditional weak coherent state sources.

Entanglement-distribution maximization over one-sided Gaussian noisy channels

We present an upper bound of the entanglement evolution for two-mode Gaussian pure states under a one-sided Gaussian map. Based on this, the optimization of entanglement evolution is studied. Even if complete information about the one-sided Gaussian noisy channel does not exist, one can still maximize the entanglement distribution by testing the channel with only two specific states.