Robust free-space optical frequency transfer in time-varying link distances conditions

Future inter-satellite clock comparison on high orbit will require optical time and frequency transmission technology between moving objects.Here,we demonstrate robust optical frequency transmission under the condition of variable link distance.This variable link is accomplished by the relative motion of a single telescope fixed on the experimental platform to a corner-cube reflector(CCR)installed on a sliding guide.Two acousto–optic modulators with different frequencies are used to separate forward signal from backward signal.With active phase noise suppression,when the CCR moves back and forth at a constant velocity of 20 cm/s and an acceleration of 20 cm/s^(2),we achieve the best frequency stability of 1.9×10^(-16) at 1 s and 7.9×10^(-19) at 1000 s indoors.This work paves the way for future studying optical frequency transfer between ultra-high-orbit satellites.

Delayed error verification in quantum key distribution

Quantum key distribution(QKD)provides an unconditional secure key generation method between two distant legitimate parties Alice and Bob based on the fundamental properties of quantum mechanics,in the presence of an eavesdropper Eve.Since key reconciliation cannot always assure that the reconciled keys between Alice and Bob are identical,error verification is an important step in QKD.In this paper,we propose a scheme of delayed error verification using extra keys gained by privacy amplification with an arbitrarily small failure probability.The proposed scheme simplifies the post-processing procedure in QKD,which can be applied in practical QKD systems.

Measurement-device-independent quantum key distribution for nonstandalone networks

Untrusted node networks initially implemented by measurement-device-independent quantum key distribution(MDI-QKD)protocol are a crucial step on the roadmap of the quantum Internet.Considering extensive QKD implementations of trusted node networks,a workable upgrading tactic of existing networks toward MDI networks needs to be explicit.Here,referring to the nonstandalone(NSA)network of 5G,we propose an NSA-MDI scheme as an evolutionary selection for existing phase-encoding BB84 networks.Our solution can upgrade the BB84 networks and terminals that employ various phase-encoding schemes to immediately support MDI without hardware changes.This cost-effective upgrade effectively promotes the deployment of MDI networks as a step of untrusted node networks while taking full advantage of existing networks.In addition,the diversified demands on security and bandwidth are satisfied,and network survivability is improved.

Polarization-insensitive interferometer based on a hybrid integrated planar light-wave circuit

Interferometers are essential elements in classical and quantum optical systems.The strictly required stability when extracting the phase of photons is vulnerable to polarization variation and phase shift induced by environment disturbance.Here,we implement polarization-insensitive interferometers by combining silica planar light-wave circuit chips and Faraday rotator mirrors.Two asymmetric interferometers with temperature controllers are connected in series to evaluate the single-photon interference.Average interference visibility over 12 h is above 99%,and the variations are less than 0.5%,even with active random polarization disturbance.The experiment results verify that the hybrid chip is available for high-demand applications like quantum key distribution and entanglement measurement.

Twin-field protocols:Towards intercity quantum key distribution without quantum repeaters

The main obstacle of realizing long-distance quantum key distribution(QKD)[1]is that the secret key rate(SKR)decreases with the loss of the quantum channel.Specifically,the transmittanceηof a quantum channel depends on its distance,namelyη=10^(−αl/10),whereα=0.2dB/km for a typical fiber channel,and l is the distance of the channel.For most single-photon-based QKD protocols,including the well-known BB84[1].