We demonstrate enhancement of the information rate in continuous-variable quantum key distribution (QKD) by a factor of 20 compared to standard techniques (and potentially by 4-6 orders of magnitude) using broadband squeezed vacuum and broadband parametric homodyne detection with a single local oscillator. QKD enables a physically-secure exchange of cryptographic keys, relying on the principle of wave-function collapse. If an eavesdropper (Eve) attempts to measure the data sent from Alice to Bob, she will inevitably disturb the state of the light in a way that can be detected by the communicating parties, thus exposing the presence of the eavesdropper. QKD protocols are either discrete or continuous. In discrete QKD, single photons are manipulated, e.g. in polarization, but to ensure detection of the eavesdropper, single photon sources are required (or very weak), which limits the information rate drastically because of the low generation and detection rate of single-photon sources and detectors. In continuous variable QKD, information is modulated on a continuous quantity, such as the phase quadratures of squeezed vacuum light, and detected with homodyne measurement. Although broadband squeezed light (tens of THz) can be easily generated by spontaneous parametric down conversion, the maximum data rate is limited by the bandwidth of the homodyne measurement, which is in the MHz-to-GHz range. In our scheme, we measure the entire bandwidth of the light with a single local-oscillator using broadband parametric homodyne detection [1]. The phase of the local oscillator sets the measured quadrature, and hence the measurement basis. In the experiment, we generated a broad bandwidth of squeezed vacuum from a nonlinear crystal (LiNbO3), encode information on the light with a Fourier-domain pulse-shaper and simultaneously measure the chosen quadrature across the spectrum with parametric homodyne. We generate the squeezed light using a 6W CW pump laser at 780nm, producing broadband two-mode squeezed light in the range of 1520-1600nm, allowing future employment with standard telecom fibers. Deciphering the data is performed by passing the pump and the bi-photons in another nonlinear crystal: controlling the phase of the pump determines the measurement basis, and the measurement itself is implemented by recording the spectrum of light.
[1] Lifting the bandwidth limit of optical homodyne measurement with broadband parametric amplification / Yaakov Shaked, Yoad Michael, Rafi Z. Vered, Leon Bello, Michael Rosenbluh & Avi Pe’er, Nature Communications 9, (2018)
Quantum communication , Quantum optics and non-classical light sources
