Experimental Side-Channel-Secure Quantum Key Distribution

Quantum key distribution can provide unconditionally secure key exchange for remote users in theory. In practice, however, in most quantum key distribution systems, quantum hackers might steal the secure keys by observing the side channels in the emitted photons, such as the photon frequency spectrum, emission time, propagation direction, spatial angular momentum, and so on. It is hard to prevent such kinds of attacks because side channels may exist in many dimensions of the emitted photons. Here we report an experimental realization of a side-channel-secure quantum key distribution protocol which is not only measurement-device independent, but also immune to all side-channel attacks to the photons emitted from Alice’s and Bob’s labs. We achieve a secure key rate of $1.73\times {10}^{-6}$ per pulse through 50 km fiber spools.

Figure 1

A schematic illustration of the side-channel-secure QKD protocol.

Figure 2

(a) Experimental setup. Alice and Bob modulate the phase-locked lasers with a phase modulator (PM) and three intensity modulators (IM1, IM2, IM3). The PM is used to encode the reference pulses; the intensity modulator IM1 is used to set sending or not sending of the signal, while IM2 and IM3 are used to set the intensities between the reference and signal pulses. Additional attenuators (ATTs) in the secure zones are calibrated to set the proper output photon intensity. Charlie interferes and measures the light from Alice and Bob with superconducting nanowire single-photon detectors. Polarization controller (PC), polarization beam splitter (PBS), circulator (CIR), beam splitter (BS). (b) Alice and Bob lock the wavelength of their lasers with a frequency-locking system [37]. The light transmits through an additional 50 km fiber. Acousto-optic modulator (AOM); Faraday mirror (FM); photodiode (PD); quarter wave plate (QWP); ultrastable cavity (USC).

Figure 3

Secure key rates. The magenta, blue, and red circles represent the experimental key rates over 0, 20, and 50 km fiber spools accordingly. The solid magenta, blue, and red curves show the simulated key rates under three fixed intensities which we applied in three experiments respectively. The dashed black curve illustrates the optimized simulation results point by point.