Liu-Jun Wang

Metropolitan all-pass and inter-city quantum communication network

We have demonstrated a metropolitan all-pass quantum communication network in field fiber for four nodes. Any two nodes of them can be connected in the network to perform quantum key distribution (QKD). An optical switching module is presented that enables arbitrary 2-connectivity among output ports. Integrated QKD terminals are worked out, which can operate either as a transmitter, a receiver, or even both at the same time. Furthermore, an additional link in another city of 60 km fiber (up to 130 km) is seamless integrated into this network based on a trusted relay architecture. On all the links, we have implemented protocol of decoy state scheme. All of necessary electrical hardware, synchronization, feedback control, network software, execution of QKD protocols are made by tailored designing, which allow a completely automatical and stable running. Our system has been put into operation in Hefei in August 2009, and publicly demonstrated during an evaluation conference on quantum network organized by the Chinese Academy of Sciences on August 29, 2009. Real-time voice telephone with one-time pad encoding between any two of the five nodes (four all-pass nodes plus one additional node through relay) is successfully established in the network within 60 km.

Singlet-triplet relaxation in multivalley silicon single quantum dots

We investigate the singlet-triplet relaxation due to the spin-orbit coupling together with the electron-phonon scattering in two-electron multivalley silicon single quantum dots, using the exact-diagonalization method and the Fermi golden rule. The electron-electron Coulomb interaction, which is crucial in the electronic structure is explicitly included. The multivalley effect induced by the interface scattering is also taken into account. We first study the configuration with a magnetic field in the Voigt configuration and identify the relaxation channel of the experimental data by Xiao et al. [Phys. Rev. Lett. 104, 096801 (2010)]. Good agreement with the experiment is obtained. Moreover, we predict a peak in the magnetic-field dependence of the singlet-triplet relaxation rate induced by the anticrossing of the singlet and triplet states. We then work on the system with a magnetic field in the Faraday configuration, where the different values of the valley splitting are discussed. In the case of large valley splitting, we find the transition rates can be effectively manipulated by varying the external magnetic field and the dot size. The intriguing features of the singlet-triplet relaxation in the vicinity of the anticrossing point are analyzed. In the case of small valley splitting, we find that the transition rates are much smaller than those in the case of large valley splitting, resulting from the different configurations of the triplet states.

A scheme for spin transistor with extremely large on/off current ratio

Quantum wires with periodic local Rashba spin-orbit couplings are proposed for a higher performance of spin field-effect transistor. Fano–Rashba quantum interference due to the spin-dependent modulated structure gives rise to a broad energy range of vanishingly small transmission. Tuning Rashba spin-orbit couplings can provide the on or off currents with extremely large on/off current ratios even in the presence of a strong disorder.

Long-distance copropagation of quantum key distribution and terabit classical optical data channels

Quantum key distribution (QKD) generates symmetric keys between two remote parties, and guarantees the keys not accessible to any third party. Wavelength division multiplexing (WDM) between QKD and classical optical communications by sharing the existing fibre optics infrastructure is highly desired in order to reduce the cost of QKD applications. However, quantum signals are extremely weak and thus easily affected by the spontaneous Raman scattering effect from intensive classical light. Here, by means of wavelength selecting and spectral and temporal filtering, we realize the multiplexing and long distance co-propagation of QKD and Terabit classical coherent optical communication system up to 80km. The data capacity is two orders of magnitude larger than the previous results. Our demonstration verifies the feasibility of QKD and classical communication to share the resources of backbone fibre links, and thus taking the utility of QKD a great step forward.

Hole spin relaxation in p-type (111) GaAs quantum wells

Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics,University of Science and Technology of China, Hefei, Anhui, 230026, China(Dated: March 20, 2012)Hole spin relaxation in p-type (111) GaAs quantum wells is investigated in the case with onlythe lowest hole subband, which is heavy-hole like in (111) GaAs/AlAs and light-hole like in (111)GaAs/InP quantum wells, being relevant. The subband L¨owdin perturbation method is applied toobtain the effective Hamiltonian including the Dresselhaus and Rashba spin-orbit couplings. Undera proper gate voltage, the total in-plane effective magnetic field in (111) GaAs/AlAs quantumwells can be strongly suppressed in the whole momentum space, while the one in (111) GaAs/InPquantum wells can be suppressed only on a special momentum circle. The hole spin relaxation due tothe D’yakonov-Perel’ and Elliott-Yafet mechanisms is calculated by means of the fully microscopickinetic spin Bloch equation approach with all the relevant scatterings explicitly included. For(111) GaAs/AlAs quantum wells, extremely long heavy-hole spin relaxation time (upto hundreds ofnanoseconds) is predicted. In addition, we predict a pronounced peak in the gate-voltage dependenceof the heavy-hole spin relaxation time due to the D’yakonov-Perel’ mechanism. This peak originsfrom the suppression of the unique inhomogeneous broadening in (111) GaAs/AlAs quantum wells.Moreover, the Elliott-Yafet mechanism influences the spin relaxation only around the peak area dueto the small spin mixing between the heavy and light holes in quantum wells with small well width.We also show the anisotropy of the spin relaxation. In (111) GaAs/InP quantum wells, a mild peak,similar to the case for electrons in (111) GaAs quantum wells, is also predicted in the gate-voltagedependence of the light-hole spin relaxation time. The contribution of the Elliott-Yafet mechanismis always negligible in this case.

Singlet-triplet relaxation in SiGe/Si/SiGe double quantum dots

We study the singlet-triplet relaxation due to the spin-orbit coupling assisted by the electron-phonon scattering in two-electron SiGe/Si/SiGe double quantum dots in the presence of an external magnetic field in either Faraday or Voigt configuration. By explicitly including the electron-electron Coulomb interaction and the valley splitting induced by the interface scattering, we employ the exact-diagonalization method to obtain the energy spectra and the eigenstates. Then, we calculate the relaxation rates with the Fermi golden rule. We find that the transition rates can be effectively tuned by varying the external magnetic field and the interdot distance. Especially in the vicinity of the anticrossing point, the transition rates show intriguing features. We also investigate the electric-field dependence of the transition rates and find that the transition rates are almost independent of the electric field. This is of great importance in the spin manipulation, since the lifetime remains almost the same du...

Design considerations of high-performance InGaAs/InP single-photon avalanche diodes for quantum key distribution

InGaAs/InP single-photon avalanche diodes (SPADs) are widely used in practical applications requiring near-infrared photon counting such as quantum key distribution (QKD). Photon detection efficiency and dark count rate are the intrinsic parameters of InGaAs/InP SPADs, due to the fact that their performances cannot be improved using different quenching electronics given the same operation conditions. After modeling these parameters and developing a simulation platform for InGaAs/InP SPADs, we investigate the semiconductor structure design and optimization. The parameters of photon detection efficiency and dark count rate highly depend on the variables of absorption layer thickness, multiplication layer thickness, excess bias voltage, and temperature. By evaluating the decoy-state QKD performance, the variables for SPAD design and operation can be globally optimized. Such optimization from the perspective of specific applications can provide an effective approach to design high-performance InGaAs/InP SPADs.

Experimental integration of quantum key distribution and gigabit-capable passive optical network

Classical optical communications may be still the main communications technology for the foreseeable future, so integration of the quantum communication network with existing classical optical communication network is necessary because existing telecommunications infrastructure will be shared. This means multiplexing of quantum key distribution (QKD) and strong classical data signals, delivering quantum signals and classic signals in one fiber. Optical splitters are employed to access each user in a gigabit-capable passive optical network (GPON). In a 4-user network the splitter adds at least 6 dB of optical loss to the quantum channel, a 64-user network the splitter adds 18 dB of optical loss to the quantum channel. The optical splitters restrict the transmission distance and performance of QKD. We propose a new integration program of QKD and GPON based on wavelength-division multiplexing (WDM). At the optical splitting point, we use filters to separate the quantum signals and bypass the optical splitter, avoiding losses produced by the optical splitters. This increases the counting rate of the quantum signals states and the signal to noise ratio (SNR) improves, so a higher key generation rate and a longer transmission distance can be obtained with QKD.Quantum key distribution (QKD) ensures information-theoretic security for the distribution of random bits between two remote parties. To extend QKD applications to fiber-to-the-home optical communications, such as gigabit-capable passive optical networks (GPONs), an effective method is the use of wavelength-division multiplexing. However, the Raman scattering noise from intensive classical traffic and the huge loss introduced by the beam splitter in a GPON severely limits the performance of QKD. Here, we demonstrate the integration of QKD and a commercial GPON system with fiber lengths up to 14 km, in which the maximum splitting ratio of the beam splitter reaches 1:64. By placing the QKD transmitter on the optical line terminal side, we reduce the Raman noise collected at the QKD receiver. Using a bypass structure, the loss of the beam splitter is circumvented effectively. Our results pave the way to extending the applications of QKD to last-mile communications.

Experimental Realization of Measurement Device Independent Quantum Key Distribution

We report a complete experimental realization of measurement device independent quantum key distribution system with decoy method, which closes loopholes in both source and detection. 25-kbit secure key is generated over a 50-km fiber link.