Experimental Low-Latency Device-Independent Quantum Randomness

Applications of randomness such as private key generation and public randomness beacons require small blocks of certified random bits on demand. Device-independent quantum random number generators can produce such random bits, but existing quantum-proof protocols and loophole-free implementations suffer from high latency, requiring many hours to produce any random bits. We demonstrate device-independent quantum randomness generation from a loophole-free Bell test with a more efficient quantum-proof protocol, obtaining multiple blocks of 512 random bits with an average experiment time of less than 5 min per block and with a certified error bounded by $2^{-64}\approx 5.42\times {10}^{-20}$.

Figure 1

Diagram of the entangled photon-pair source. A 775-nm-wavelength picosecond Ti:sapphire laser operating at a 79.3 MHz repetition rate pumps a 20-mm-long periodically poled potassium titanyl phosphate (PPKTP) crystal, to produce degenerate photons at 1550 nm with a per-pulse probability of 0.0045. The pump is transmitted through a polarization-maintaining single-mode fiber (SMF). The PPKTP crystal is cut for type II phase matching and placed in a polarization-based Mach-Zehnder interferometer constructed using half-wavelength plates (HWPs) and three beam displacers (BD1, BD2, and BD3). Tuning the polarization of the pump by a polarizer and HWP allows us to create the nonmaximally entangled state $|\psi ⟩=0.967|HH⟩+0.254|VV⟩$, where $H$ and $V$ denote the horizontally and vertically polarized single-photon states. The photons, along with a synchronization signal, are then distributed via optical fiber to Alice and Bob. The synchronization signal is generated by a fast photodiode (FPD) and divider circuit which divides the pump frequency by 800, and is used as a clock to determine the start of a trial and to time the operation of Alice’s and Bob’s measurements. This leads to a trial rate of approximately 100 kHz.

Figure 2

Locations of Alice ($\mathsf{A}$), Bob ($\mathsf{B}$), and the source ($\mathsf{S}$). Alice and Bob are separated by $194.8\pm 1.0\mathrm{m}$ (this is slightly further than in Refs. [7, 35]). Faint gray lines indicate the paths that the entangled photons take from the source to Alice and Bob through fiber optic cables. The light-green quarter circles are the 2D projections of the expanding light spheres containing the earliest available information about the random bits used for Alice’s and Bob’s setting choices at the trial. When Bob finishes his measurement, the radius of the light sphere corresponding to the start of Alice’s QRNG has expanded to $127.3\pm 0.5\mathrm{m}$, after which it takes an additional $222.3\pm 3.8\mathrm{ns}$ before the light sphere will intersect Bob’s location. Similarly, when Alice completes her measurement, the light sphere corresponding to the start of Bob’s QRNG has only reached a radius of $98.3\pm 0.5\mathrm{m}$, and it will take $315.5\pm 3.8\mathrm{ns}$ more to arrive at Alice’s station. In this way, the actions of Alice and Bob are spacelike separated. Inset: Alice’s and Bob’s measurement apparatuses both consist of a Pockels cell (PC), operating at approximately 100 KHz, and a polarizer, constructed using two half-wavelength plates (HWPs), a quarter-wavelength plate (QWP) and a polarizing beam displacer, in order to make fast polarization measurements on their respective photons. The measurement setting is controlled by a QRNG, the photon is detected by a high-efficiency superconducting nanowire single-photon detector, and the resulting signal is recorded on a time tagger, where a 10 MHz oscillator is used to keep Alice’s and Bob’s time taggers synchronized.