Experimental quantum data locking

Classical correlation can be locked via quantum means: quantum data locking. With a short secret key, one can lock an exponentially large amount of information in order to make it inaccessible to unauthorized users without the key. Quantum data locking presents a resource-efficient alternative to one-time pad encryption which requires a key no shorter than the message. We report experimental demonstrations of a quantum data locking scheme originally proposed by D. P. DiVincenzo et al. [Phys. Rev. Lett. 92, 067902 (2004)] and a loss-tolerant scheme developed by O. Fawzi et al. [J. ACM 60, 44 (2013)]. We observe that the unlocked amount of information is larger than the key size in both experiments, exhibiting strong violation of the incremental proportionality property of classical information theory. As an application example, we show the successful transmission of a photo over a lossy channel with quantum data (un)locking and error correction.

Figure 1

Schematic of experimental quantum data locking. Alice pulses a distributed feedback laser diode (LD) at $\lambda =1560$ nm with a pulse width of 10 ns at 100 kHz. After passing through an erbium-doped fiber amplifier (EDFA), the laser pulses are up converted to 780 nm via second-harmonic generation (SHG) in an in-line periodically poled lithium niobate (PPLN) waveguide. The residual long wavelengths are removed with a wavelength-division multiplexer (WDM) and a 945-nm low-pass (LP) filter. Alice focuses the pump pulses at 780 nm into a periodically poled potassium titanyl phosphate (PPKTP) crystal to create pairs of orthogonally polarized photons that are degenerated at 1560 nm via spontaneous parametric down conversion. The photon pairs are separated by a polarizing beam splitter (PBS). Alice uses dichroic mirrors (DMs) to remove the residual pump light at 780 nm and fluorescence. The pairs of signal and idler photons are collected into single-mode optical fibers. Alice detects the idler photons with a superconducting nanowire single-photon detector (SNSPD) to herald the presence of signal photons. The heralded signal photons are encoded by Pockels cells. After encoding, the single photons are sent to Bob via a fiber spool. In the meantime, a control signal is sent to Bob to prepare his bases accordingly to decode the incoming single-photon signals, which are received by two transition-edge sensors (TES) after a PBS. A polarization controller (PC) is applied wherever it is needed to maximize transmittance of photons in the right polarization and the extinction ratio. System synchronization is controlled by a field-programmable gate array (FPGA).

Figure 2

(a) Data-locking efficiency of the FHS scheme with tailored single-photon transmittance. (b) Sending a photo with data (un)locking and error correction. (c) Communication rate in a quantum erasure channel.