Free-space dissemination of time and frequency with 10 instability over 113 km.

Networks of optical clocks find applications in precise navigation, in efforts to redefine the fundamental unit of the 'second' and in gravitational tests. As the frequency instability for state-of-the-art optical clocks has reached the 10 level, the vision of a global-scale optical network that achieves comparable performances requires the dissemination of time and frequency over a long-distance free-space link with a similar instability of 10. However, previous attempts at free-space dissemination of time and frequency at high precision did not extend beyond dozens of kilometres.

Chromatic interferometry with small frequency differences.

By developing a 'two-crystal' method for color erasure, we can broaden the scope of chromatic interferometry to include optical photons whose frequency difference falls outside of the 400 nm to 4500 nm wavelength range, which is the passband of a PPLN crystal. We demonstrate this possibility experimentally, by observing interference patterns between sources at 1064.4 nm and 1063.

Sending-or-Not-Sending with Independent Lasers: Secure Twin-Field Quantum Key Distribution over 509 km.

Twin-field (TF) quantum key distribution (QKD) promises high key rates over long distances to beat the rate-distance limit. Here, applying the sending-or-not-sending TF QKD protocol, we experimentally demonstrate a secure key distribution that breaks the absolute key-rate limit of repeaterless QKD over a 509-km-long ultralow loss optical fiber. Two independent lasers are used as sources with remote-frequency-locking technique over the 500-km fiber distance.

Color Erasure Detectors Enable Chromatic Interferometry.

By engineering and manipulating quantum entanglement between incoming photons and experimental apparatus, we construct single-photon detectors which cannot distinguish between photons of very different wavelengths. These color-erasure detectors enable a new kind of intensity interferometry, with potential applications in microscopy and astronomy. We demonstrate chromatic interferometry experimentally, observing robust interference using both coherent and incoherent photon sources.

Experimental Twin-Field Quantum Key Distribution through Sending or Not Sending.

Channel loss seems to be the most severe limitation on the practical application of long distance quantum key distribution. The idea of twin-field quantum key distribution can improve the key rate from the linear scale of channel loss in the traditional decoy-state method to the square root scale of the channel transmittance. However, the technical demands are rather tough because they require single photon level interference of two remote independent lasers.

Quantum Coherence Witness with Untrusted Measurement Devices.

Coherence is a fundamental resource in quantum information processing, which can be certified by a coherence witness. Due to the imperfection of measurement devices, a conventional coherence witness may lead to fallacious results. We show that the conventional witness could mistake an incoherent state as a state with coherence due to the inaccurate settings of measurement bases.

Experimental evidence of anomalously large superconducting gap on topological surface state of β-BiPd film.

Connate topological superconductor (TSC) combines topological surface states with nodeless superconductivity in a single material, achieving effective p-wave pairing without interface complication. By combining angle-resolved photoemission spectroscopy and in-situ molecular beam epitaxy, we studied the momentum-resolved superconductivity in β-BiPd film. We found that the superconducting gap of topological surface state (Δ ∼ 3.

Device-independent quantum random-number generation.

Randomness is important for many information processing applications, including numerical modelling and cryptography. Device-independent quantum random-number generation (DIQRNG) based on the loophole-free violation of a Bell inequality produces genuine, unpredictable randomness without requiring any assumptions about the inner workings of the devices, and is therefore an ultimate goal in the field of quantum information science. Previously reported experimental demonstrations of DIQRNG were not provably secure against the most general adversaries or did not close the 'locality' loophole of the Bell test.

Free-space quantum key distribution in urban daylight with the SPGD algorithm control of a deformable mirror.

Free-space quantum key distribution (QKD) is important to realize a global-scale quantum communication network. However, performing QKD in daylight against the strong background light noise is a major challenge. Here, we develop the stochastic parallel gradient descent (SPGD) algorithm with a deformable mirror to improve the signal-to-noise ratio (SNR).

Observation of Quantum Fingerprinting Beating the Classical Limit.

Quantum communication has historically been at the forefront of advancements, from fundamental tests of quantum physics to utilizing the quantum-mechanical properties of physical systems for practical applications. In the field of communication complexity, quantum communication allows the advantage of an exponential reduction in the transmitted information over classical communication to accomplish distributed computational tasks. However, to date, demonstrating this advantage in a practical setting continues to be a central challenge.

Experimental passive round-robin differential phase-shift quantum key distribution.

In quantum key distribution (QKD), the bit error rate is used to estimate the information leakage and hence determines the amount of privacy amplification-making the final key private by shortening the key. In general, there exists a threshold of the error rate for each scheme, above which no secure key can be generated. This threshold puts a restriction on the environment noises.

Measurement-device-independent quantum key distribution over 200 km.

Measurement-device-independent quantum key distribution (MDIQKD) protocol is immune to all attacks on detection and guarantees the information-theoretical security even with imperfect single-photon detectors. Recently, several proof-of-principle demonstrations of MDIQKD have been achieved. Those experiments, although novel, are implemented through limited distance with a key rate less than 0.