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Hackers begone: Measurement-device-independent QKD increases clock rate and transmission distance while reducing failure

Posted December 2, 2014

In the ongoing effort to make communications secure, Quantum Key Distribution (QKD) theoretically provides a solution – but to the delight of increasingly sophisticated hackers, falls short in real-world systems due to implementation deviating from mathematical models. A number of QKD variants – including Device-Independent Quantum Key Distribution (DIQKD) and the more recently introduced Measurement-Device-Independent Quantum Key Distribution (MDIQKD) – attempt to close the gap between theory and practice with varying degrees of success. These two variants differ in several ways, the most important being that DIQKD requires but unlike MDIQKD cannot easily achieve very high detection efficiency and low channel loss to yield secure keys, while MDIQKD, unlike DIQKD, does not rely on any ideal devices and can close the most vulnerable QKD security hole by removing all side-channels from the measurement unit. Nevertheless, previous MDIQKD systems have had limitations as well, such as limited distance and a low key rate of less than 0.1 bit/s.

MDIQKD setup

(a) Schematic layout of our MDIQKD setup. Alice’s (Bob’s) signal laser pulses (1550 nm) are modulated into three decoy-state intensities by AM1. An AMZI, an AM2–4, and one PM are used to encode qubits. Charlie’s setup consists of a polarization stabilization system and a BSM system. The polarization stabilization system in each link includes an EPC, a PBS, and a SPAPD. The BSM system includes an interference BS and two SNSPDs. (b) Time calibration system. Two SynLs (1570 nm) are adopted, with the 500 kHz shared time reference generated from a crystal oscillator circuit (COC) and with the time delayed by a programmable delay chip (PDC). Alice (Bob) receives the SynL pulses with a PD and then regenerates a system clock of 75 MHz. WDM: wavelength division multiplexer, ConSys: control system. (c) Phase stabilization system. Circ: circulator, PC: polarization controller, PS: phase shifter. Credit: Tang, Yan-Lin et al. (2014) Measurement-Device-Independent Quantum Key Distribution over 200 km. Phys. Rev. Lett. 113:190501.

Recently, however, scientists at the University of Science and Technology of China devised a de novo MDIQKD protocol and have developed a 75 MHz clock rate, fully automatic and highly stable system and superconducting nanowire single-photon detectors with detection efficiencies of more than 40%.

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