Post-processing Free Quantum Random Number Generator Based on Avalanche Photodiode Array

Quantum random number generators adopting single negligible dead time of avalanche photodiodes （APDs） photon detection have been restricted due to the non- We propose a new approach based on an APD array to improve the generation rate of random numbers significantly. This method compares the detectors＇ responses to consecutive optical pulses and generates the random sequence. We implement a demonstration experiment to show its simplicity, compactness and scalability. The generated numbers are proved to be unbiased, post-processing free, ready to use, and their randomness is verified by using the national institute of standard technology statistical test suite. The random bit generation efficiency is as high as 32.8% and the potential generation rate adopting the 32× 32 APD array is up to tens of Gbits/s.

Fast and perfect state transfer in superconducting circuit with tunable coupler

In quantum computation and quantum information processing, the manipulation and engineering of quantum systems to suit certain purposes are an ongoing task. One such example is quantum state transfer(QST), an essential requirement for both quantum communication and large-scale quantum computation. Here we engineer a chain of four superconducting qubits with tunable couplers to realize the perfect state transfer(PST) protocol originally proposed in quantum spin networks and successfully demonstrate the efficient transfer of an arbitrary single-qubit state from one end of the chain to the other,achieving a high fidelity of 0.986 in just 25 ns. This demonstrated QST is readily to extend to larger chain and multi-node configurations, thus serving as a desirable tool for scalable quantum information processing.

Space-to-Ground Quantum Key Distribution Using a Small-Sized Payload on Tiangong-2 Space Lab

Quantum technology establishes a foundation for secure communication via quantum key distribution （QKD）. In the last two decades, the rapid development of QKD makes a global quantum communication network feasible. In order to construct this network, it is economical to consider small-sized and low-cost QKD payloads, which can be assembled on satellites with different sizes, such as space stations. Here we report an experimental demonstration of space-to-ground QKD using a small-sized payload, from Tiangong-2 space lab to Nanshan ground station. The 57.9-kg payload integrates a tracking system, a QKD transmitter along with modules for synchronization, and a laser communication transmitter. In the space lab, a 50MHz vacuum＋weak decoy-state optical source is sent through a reflective telescope with an aperture of 200mm. On the ground station, a telescope with an aperture of 1200mm collects the signal photons. A stable and high-transmittance communication channel is set up with a high-precision bidirectional tracking system, a polarization compensation module, and a synchronization system. When the quantum link is successfully established, we obtain a key rate over 100bps with a communication distance up to 719km. Together with our recent development of QKD in daylight, the present demonstration paves the way towards a practical satellite-constellation-based global quantum secure network with small-sized QKD payloads.

Quantum Science Satellite

Quantum Science Satellite is one of the first five space science missions, slated for launch in the framework of Chinese Academy of Sciences(CAS) Strategic Priority Research Program on space science. The project aims to establish a space platform with long-distance satellite and ground quantum channel, and carry out a series of tests about fundamental quantum principles and protocols in space-based large scale. The satellite will be launched at Jiuquan and on orbit for 2 years. The orbit will be circular and Sun-synchronous with an altitude of 600 km. It crosses the descending node at 00:00 LT. The satellite is under early prototype development currently.

Theoretical analysis of the coupling between Feshbach states and hyperfine excited states in the creation of 23Na40K molecule

We present an intensive study of the coupling between different Feshbach states and the hyperfine levels of the excited states in the adiabatic creation of 23Na40K ground-state molecules.We use coupled-channel method to calculate the wave function of the Feshbach molecules,and give the short-range wave function of triplet component.The energies of the hyperfine excited states and the coupling strength between the Feshbach states and the hyperfine excited states are calculated.Our results can be used to prepare a specific hyperfine level of the rovibrational ground state to study the ultracold collisions involving molecules.

Implementation of encoder and decoder for LDPC codes based on FPGA

This paper proposes a parallel cyclic shift structure of address decoder to realize a high-throughput encoding and decoding method for irregular-quasi-cyclic low-density parity-check(IR-QC-LDPC)codes,with a dual-diagonal parity structure.A normalized min-sum algorithm(NMSA)is employed for decoding.The whole verification of the encoding and decoding algorithm is simulated with Matlab,and the code rates of 5/6 and 2/3 are selected respectively for the initial bit error ratio as 6%and 1.04%.Based on the results of simulation,multi-code rates are compatible with different basis matrices.Then the simulated algorithms of encoder and decoder are migrated and implemented on the field programmable gate array(FPGA).The 183.36 Mbps throughput of encoder and the average 27.85 Mbps decoding throughput with the initial bit error ratio 6%are realized based on FPGA.

Superconducting phase qubits with shadow-evaporated Josephson junctions

We develop a fabrication process for the superconducting phase qubits in which Josephson junctions for both the qubit and superconducting quantum interference device（SQUID） detector are prepared by shadow evaporation with a suspended bridge. Al junctions with areas as small as 0.05 μm^2 are fabricated for the qubit, in which the number of the decoherencecausing two-level systems（TLS） residing in the tunnel barrier and proportional to the junction area are greatly reduced. The measured energy spectrum shows no avoided crossing arising from coherent TLS in the experimentally reachable flux bias range of the phase qubit, which demonstrates the energy relaxation time T1 and dephasing time Tφ on the order of 100 ns and 50 ns, respectively. We discuss several possible origins of decoherence from incoherent or weakly-coupled coherent TLS and further improvements of the qubit performance.

Terahertz Direct Detectors Based on Superconducting Hot Electron Bolometers with Microwave Biasing

Terahertz （THz） direct detectors based on superconducting niobium nitride （NbN） hot electron bolometers （HEBs） with microwave （MW） biasing are studied. The MW is used to bias the HEB to the optimum point and to readout the impedance changes caused by the incident THz signals. Compared with the thermal biasing method, this method would be more promising in large scale array with simple readout. The used NbN HEB has an excellent performance as heterodyne detector with the double sideband noise temperature （T N） of 403K working at 4.2K and 0.65THz. As a result, the noise equivalent power of 1.5pW/Hz 1/2 and the response time of 64ps are obtained for the direct detectors based on the NbN HEBs and working at 4.2K and 0.65THz.

Dynamics of bright soliton in a spin–orbit coupled spin-1 Bose–Einstein condensate

We have investigated the dynamics of bright solitons in a spin–orbit coupled spin-1 Bose–Einstein condensate analytically and numerically. By using the hyperbolic sine function as the trial function to describe a plane wave bright soliton with a single finite momentum, we have derived the motion equations of soliton's spin and center of mass, and obtained its exact analytical solutions. Our results show that the spin–orbit coupling couples the soliton's spin with its center-of-mass motion, the spin oscillations induced by the exchange of atoms between components result in the periodical oscillation of center-of-mass, and the motion of center of mass of soliton can be viewed as a superposition of periodical and linear motions. Our analytical results have also been confirmed by the direct numerical simulations of Gross–Pitaevskii equations.

Hong–Ou–Mandel interference linking independent room-temperature quantum memories

To realize a large-scale quantum network,both quantum memory and the interference of retrieved indistinguishable photons are essentially required to perform multi-photon synchronization and quantum-interference-mediated entanglement swapping.Significant progress has been achieved in low-temperature and well-isolated systems.However,linking independent quantum memories at room temperature remain challenging.Here,we present an experimental demonstration of Hong–Ou–Mandel interference between single photons from two independent room-temperature quantum memories.We manage to simultaneously operate two such quantum memories and individually obtain a memory-built-in quantum correlation of Stokes and anti-Stokes photons by a far-off-resonance Duan–Lukin–Cirac–Zoller protocol.We also successfully enhance the Hong–Ou–Mandel interference rate up to about 15 times by increasing each photon rate,which is achieved by coordinating two quantum memories with a repeat-until-success fashion.We observe the visibility of quantum interference up to 75.0%without reduction of any background noise,well exceeding the classical limit of 50%.Our results,together with its straightforward,broadband,and room-temperature features,open up a promising way towards realizing large-scale quantum networks at ambient conditions.

Momentum Spectroscopy for Multiple Ionization of Cold Rubidium in the Elliptically Polarized Laser Field

Employing recently developed magneto-optical trap recoil ion momentum spectroscopy(MOTRIMS)combined with cold atoms,strong laser pulse,and ultrafast technologies,we study momentum distributions of the multiply ionized cold rubidium(Rb)induced by the elliptically polarized laser pulses(35 fs,1.3×10^15 W/cm^2).The complete vector momenta of Rb^n+ions up to charge state n=4 are recorded with extremely high resolution(0.12 a.u.for Rb^+).Variations of characteristic multi-bands are displayed in momentum distributions because the ellipticity varies from the linear to circular polarization,are interpreted qualitatively with the classical overbarrier ionization model.Present momentum spectroscopy of cold heavy alkali atoms presents novel strong-field phenomena beyond the noble gases.

High-resolution three-dimensional atomic microscopy via double electromagnetically induced transparency

We aim to present a new scheme for high-dimensional atomic microscopy via double electromagnetically induced transparency in a four-level tripod system.For atom-field interaction,we construct a spatially dependent field by superimposing three standing-wave fields(SWFs)in 3D-atom localization.We achieve a high precision and high spatial resolution of an atom localization by appropriately adjusting the system variables such as field intensities and phase shifts.We also see the impact of Doppler shift and show that it dramatically deteriorates the precision of spatial information on 3D-atom localization.We believe that our suggested scheme opens up a fascinating way to improve the atom localization that supplies some practical applications in atom nanolithography,and Bose-Einstein condensation.

Observation of a Ubiquitous(π,π)-Type Nematic Superconducting Order in the Whole Superconducting Dome of Ultra-Thin BaFe_(2-x)Ni_(x)As_(2) Single Crystals

In iron-based superconductors,the(0,π) or(π,0) nematicity,which describes an electronic anisotropy with a fourfold symmetry breaking,is well established and believed to be important for understanding the superconducting mechanism.However,how exactly such a nematic order observed in the normal state can be related to the superconducting pairing is still elusive.Here,by performing angular-dependent in-plane magnetoresistivity using ultra-thin flakes in the steep superconducting transition region,we unveil a nematic superconducting order along the(π,π) direction in electron-doped BaFe_(2-x)Ni_(x)As_(2) from under-doped to heavily overdoped regimes with x=0.065- 0.18.It shows superconducting gap maxima along the(π,π) direction rotated by 45° from the nematicity along(0, π) or(π,0) direction observed in the normal state.A similar(π,π)-type nematicity is also observed in the under-doped and optimally doped hole-type Ba1-yKyFe2 As_(2),with y=0.2-0.5.These results suggest that the(π,π) nematic superconducting order is a universal feature that needs to be taken into account in the superconducting pairing mechanism in iron-based superconductors.

Advances in InGaAs/InP single-photon detector systems for quantum communication

Single-photon detectors(SPDs)are the most sensitive instruments for light detection.In the near-infrared range,SPDs based on III–V compound semiconductor avalanche photodiodes have been extensively used during the past two decades for diverse applications due to their advantages in practicality including small size,low cost and easy operation.In the past decade,the rapid developments and increasing demands in quantum information science have served as key drivers to improve the device performance of single-photon avalanche diodes and to invent new avalanche quenching techniques.This Review aims to introduce the technology advances of InGaAs/InP single-photon detector systems in the telecom wavelengths and the relevant quantum communication applications,and particularly to highlight recent emerging techniques such as high-frequency gating at GHz rates and free-running operation using negative-feedback avalanche diodes.Future perspectives of both the devices and quenching techniques are summarized.

Topological quantum walks:Theory and experiments

Inspired by recent breakthroughs with topological quantum materials,which pave the way to novel,high-efficiency,low-energy magnetoelectric devices[1-3]and fault-tolerant quantum information processing[4],inter alia,topological quantum walks have emerged as an exciting topic in its own right,especially due to the theoretical and experimental simplifications this approach offers[5-14].Motivated by impressive progress in topological quantum walks,we provide a perspective on theoretical studies and experimental investigations of topological quantum walks focusing on current explorations of topological properties arising for single-walker quantum walks.

12 superconducting qubits for quantum walks

Superconducting qubits are among the most promising candidates to build a quantum computer,which is beyond the current computers[1].Presently,the number of qubits for a superconducting quantum processor are around the order of 9-22 qubits[26],and devices with more superconducting qubits are reported under development.

Measurement-device-independent quantum key distribution protocol with phase post-selection

Measurement-device-independent quantum key distribution(MDI-QKD)protocol can remove all the loopholes of the detection devices and,thus,has attracted much attention.Based on the technique of single-photon interference,we propose a modified MDI-QKD protocol with phase post-selection.We prove the security of the announcement of the private phases in the X basis and show how to apply the phase post-selection method to the double-scanning four-intensity MDI-QKD protocol.The numerical results show that the phase postselection method can significantly improve the key rates at all distances.In the double-scanning method,two parameters need to be scanned in the calculation of the final key rate,and the global parameter optimization is pretty time-consuming.We propose an accelerated method that can greatly reduce the running time of the global parameter optimization program.This makes the method practically useful in an unstable channel.

Quantum advantage with membosonsampling

Quantum computer,harnessing quantum superposition to boost a parallel computational power,promises to outperform its classical counterparts and offer an exponentially increased scaling.The term“quantum advantage”was proposed to mark the key point when peo-ple can solve a classically intractable problem by artificially control-ling a quantum system in an unprecedented scale,even without er-ror correction or known practical applications.Boson sampling,a problem about quantum evolutions of multi-photons on multimode photonic networks,as well as its variants,has been considered as a promising candidate to reach this milestone.However,the current photonic platforms suffer from the scaling problems,both in pho-ton numbers and circuit modes.Here,we propose a new variant of the problem,membosonsampling,exploiting the scaling of the prob-lem can be in principle extended to a large scale.We experimentally verify the scheme on a self-looped photonic chip inspired by mem-ristor,and obtain multi-photon registrations up to 56-fold in 750,000 modes with a Hilbert space up to 10254.The results exhibit an inte-grated and cost-efficient shortcut stepping into the“quantum advan-tage”regimeina photonic systemfarbeyondpreviousscenarios,and provide a scalable and controllable platform for quantum information processing.

Erratum to‘Topologically protected polarization quantum entanglement on a photonic chip’[Chip 1,100003(2022)]

Given his role as Executive Editor of this journal,Xian-Min Jin had no involvement in the peer-review of the article titled‘Topologically pro-tected polarization quantum entanglement on a photonic chip’(Chip 1,100003),and had no access to information regarding its peer-review.Full responsibility for the editorial process for this article was delegated to Xibo Feng(fred.feng@chipress.org)at Chip Editorial Office.

Topologically Protected Polarization Quantum Entanglement on a Photonic Chip

Quantum entanglement,as the strictly non-classical phenomenon,is the kernel of quantum computing and quantum simulation,and has been widely applied ranging from fundamental tests of quantum physics to quantum information processing.Meanwhile,the topolog-ical phase is found inherently capable of protecting physical fields from unavoidable fabrication-induced disorder,which inspires the po-tential application of topological protection to quantum states.Here,we present the experimental demonstration of topologically protected quantum entangled states on a photonic chip.The process tomogra-phy shows that quantum entanglement can be well preserved by the topological states even when the chip material introduces disorder and relative polarization rotation in phase space.Our work links the fields of materials,topological science and quantum physics,opening the door to wide applications of topological enhancement in quantum regime.