Reference-Frame-Independent Quantum-Key-Distribution Server with a Telecom Tether for an On-Chip Client

We demonstrate a client-server quantum key distribution (QKD) scheme. Large resources such as laser and detectors are situated at the server side, which is accessible via telecom fiber to a client requiring only an on-chip polarization rotator, which may be integrated into a handheld device. The detrimental effects of unstable fiber birefringence are overcome by employing the reference-frame-independent QKD protocol for polarization qubits in polarization maintaining fiber, where standard QKD protocols fail, as we show for comparison. This opens the way for quantum enhanced secure communications between companies and members of the general public equipped with handheld mobile devices, via telecom-fiber tethering.

Figure 1

Experimental setup for client-server rfiQKD. The server side holds a telecom wavelength (1550 nm) laser with a 1 MHz pulse generator (PG) and fixed polarizer, to send light pulses to the client through a polarization maintaining fiber (PMF). At the client side, an integrated polarization controller (PC) encodes qubits into the polarization of the attenuated (Att.) light. A fiber beam splitter (FBS) and photodetector (PD) continuously monitor power for malicious attacks. Qubits received back at the server side are measured with a similar PC, fiber polarizing beam splitter (FPBS), and superconducting single photon detectors (SSPDs), all controlled by an electronic board synchronization (Sync.), function programmable gate array (FPGA), and processor. The Bloch sphere illustrates the effects of an unstable environment on polarization.

Figure 2

Experimental data for secret key rate fraction $r$ showing robustness to drift. Data are initially collected in the situation of well aligned client-server reference frames, but the unfixed PMF quantum channel is subject to ambient environmental influences, which effect a slowly varying reference frame. While the key rate for the rfiQKD protocols is constant, for BB84 it suffers a fall as the alignment drifts. Lower bounds on the secret key rate from the urfiQKD analysis are shown.

Figure 3

Experimental data for secret key rate fraction $r$ showing automatic recovery from rapid noise. During the initial 60 s, the unaligned and slowly varying reference frames result in BB84 failure while the rfiQKD protocols operate. Between $t=60\mathrm{s}$ and $t=120\mathrm{s}$ we deliberately introduced rapid PMF deformations to force an rfiQKD failure. However, an automatic and passive revival of $r$ for the rfiQKD protocols is observed during the subsequent calm period.