No-Switching Quantum Key Distribution Using Broadband Modulated Coherent Light

We realize an end-to-end no-switching quantum key distribution protocol using continuous-wave coherent light. We encode weak broadband Gaussian modulations onto the amplitude and phase quadratures of light beams. Our no-switching protocol achieves high secret key rate via a post-selection protocol that utilizes both quadrature information simultaneously. We establish a secret key rate of $25\mathrm{Mbits}/\mathrm{s}$ for a lossless channel and $1\mathrm{kbit}/\mathrm{s}$ for 90% channel loss, per 17 MHz of detected bandwidth, assuming individual Gaussian eavesdropping attacks. Since our scheme is truly broadband, it can potentially deliver orders of magnitude higher key rates by extending the encoding bandwidth with higher-end telecommunication technology.

Figure 1

Schematic of experiment. LO: local oscillator; RNG: random number generators; AM/PM: amplitude/phase modulators; PBS: polarizing beam splitter; $\lambda /2$: half wave plate; BS: $50/50$ beam splitter; $|0⟩$: vacuum state. (inset) (i) Bob’s detected noise spectra of the broadband modulation encoding shown with respect to the quantum noise limit (ii). Grey region denotes the 17 MHz sideband frequency spectrum used in our analysis.

Figure 2

(a) The “global” perspective of Alice’s ($x_A$) and Bob’s ($x_B$) data, represented in a scatter-plot diagram, for transmission losses of 54%. Dotted lines: “banded information channels”; green points: data that have error free binary encoding; blue points: data that have bit-flip errors; red points: data that have a negative net information rate. (b) Bob’s perspective of his and Alice’s data. (c) The global perspective and (d) Bob’s perspective of the theoretical net information rate contour plots.

Figure 3

Secret key rate for varying channel losses. Solid line: theoretical net Shannon information rate; circle symbols: experimental secret key rate after post selection; square symbols: secret key rate after privacy amplification. (inset) Bob’s mutual information with Alice (normalized to the Shannon’s capacity) for increasing number of banded information channels with 54% channel loss.