With the widespread use of complex electrical parts composed of nanomaterials in optoelectronic and energy field, the evolution of electrical properties for these components in real operation condition should be mastered. In this work, we demonstrate a quantum-based magnetic imaging apparatus that can directly monitor the nanoscale current dynamics of complex networks noninvasively. We investigate the change of current distribution of silver nanowire networks during direct current (DC) and alternating current (AC) electrical annealing. We observe some reported phenomena at nanoscale, such as winner-takes-all dominated network under low DC voltage stress and “life” in crack during breakdown. Besides, by AC electrical annealing the stability difference of networks with high and low density are directly distinguished. Our technique is well suited for various complex network to image the dynamics of nanoscale current paths.
Due to the inevitable loss of single photon transmission in optical fiber, a quantum repeater scheme is needed to establish large-scale quantum networks. The existing elementary quantum repeater links are all based on emissive quantum memories, with entangled photons emitted by the memory itself. This architecture is difficult to support deterministic photon emission and multiplexing storage simultaneously, which fundamentally limits the rate of entanglement distribution. In this talk, I will present our recent work about the realization of heralded quantum entanglement between two absorptive quantum memories based on rare-earth-ion-doped crystals. This work confirmed the feasibility of constructing quantum repeater based on absorptive quantum memories and demonstrated the acceleration effect of multiplexing in quantum repeater for the first time, which lays a solid foundation for the construction of practical high-speed quantum networks.
Recently, a source-independent quantum random number generator (SI-QRNG), which can generate secure random numbers with untrusted sources, has been realized. However, the measurement loopholes of the trusted but imperfect devices used in SI-QRNGs have not yet been fully explored. Here, we point out and evaluate the security loopholes of practical imperfect measurement devices in SI-QRNGs. We also provide corresponding countermeasures to prevent these information leakages by recalculating the conditional minimum entropy and adding a monitor. Furthermore, by taking into account the finite-size effect, we show that the influence of the afterpulse can exceed that of the finite-size effect with the large number of sampled rounds. Our protocol is simple and effective, and it promotes the security of SI-QRNG in practice as well as the compatibility with high-speed measurement evices, thus paving the way for constructing ultrafast and security-certified commercial SI-QRNG systems.
We investigate a quantum random number generation (QRNG) based on backward spontaneous Raman scattering (SpRS) in standard single-mode fiber, where the randomness of photon wavelength superposition and arrival time are simultaneously utilized. The experiment uses four avalanche photodiodes working in gated Geiger mode to detect backward Raman scattering photons from four different wavelength channels and a time-to-digital converter placed behind the detectors to record their arrival time.
There is an increasing demand for multiplexing of quantum key distribution with optical communications in a single fiber in consideration of high costs and practical applications in the metropolitan optical network. Here, we realize the integration of quantum key distribution and Optical Transport Network of 80 Gbps classical data at 15 dBm launch power over 50 km of the widely used standard (G.652 Recommendation of the International Telecom Union Telecom Standardization Sector) telecom fiber. A secure key rate of 11 kbps over 20 km is obtained. By tolerating a high classical optical power up to 18 dBm of 160 Gbps classical data on single mode fiber our result shows the potential and tolerance of quantum key distribution being used in future large capacity transmission systems, such as metropolitan area networks and data center. The quantum key distribution system is stable and practical which is insensitive to the polarization disturbance of channels by using phase coding system based on Faraday-Michelson interferometer. We also discuss the fundamental limit for quantum key distribution performance in the multiplexing environment.
Benefit from the recent nanotechnology process, people can integrate different nanostructures on a
single chip. Particularly, quantum dots (QD), which behave as artificial atoms, have been shown to
couple with a superconducting resonator, indicating that quantum-dot based quantum chip has a highly
scalable possibility. Here we show a quantum chip architecture by combining graphene quantum dots
and superconducting resonators together. A double quantum dot (DQD) and a microwave hybrid
system can be described by the Jaynes-Cummings model, while a multi-quantum-dots system is
conformed to the Tavis-Cummings model. These simple quantum optics models are experimentally
realized in our device, providing a compelling platform for both graphene study and potential
applications.
The ultimate goal of quantum information science is to build a global quantum network, which enables quantum resources to be distributed and shared between remote parties. Such a quantum network can be realized using only fiber elements, thus deriving the advantages of low transmission loss, low cost, scalability, and integrability through mature fiber communication techniques such as dense wavelength division multiplexing. Hence high-quality entangled-photon sources based on fibers are in high demand. Here we report multiplexed polarization- and time-bin-entangled photon-pair sources based on the dispersion-shifted fiber operating at room temperature. The associated high quality of entanglement is characterized using interference, Bell’s inequality, and quantum state tomography. The simultaneous presence of both types of entanglement in multi-channel pairs of a 100-GHz dense wavelength division multiplexing device indicates a great capacity in distributing entangled photons over multiple users. Our design provides a versatile platform and takes a big step toward constructing an all-fiber quantum network.
Lithium niobate on insulator platform, with excellent light confinement and second order nonlinearity, has recently attracted great interest for applications towards next-generation wavelength conversion systems that are highly efficient and can be densely fabricated. Here we propose and experimentally demonstrated efficient quasi-phase matched second harmonic generation in periodically-grooved lithium niobate waveguides with sub-micron dimensions. We show that, an additional momentum kick induced by periodically modulating the waveguide width could be used to compensate for the phase mismatch between the two fundamental modes at pump and second harmonic wavelengths. We measure normalized conversion efficiencies as high as 7.0% W-1cm-2 from the fabricated devices. This system is promising for future on-chip quantum wavelength conversion.
In 1935, Einstein, Podolsky and Rosen published their influential paper proposing a now famous paradox (the EPR paradox) that threw doubt on the completeness of quantum mechanics. Two fundamental concepts: entanglement and steering, were given in the response to the EPR paper by Schrodinger, which both reflect the nonlocal nature of quantum mechanics. In 1964, John Bell obtained an experimentally testable inequality, in which its violation contradicts the prediction of local hidden variable models and agrees with that of quantum mechanics. Since then, great efforts have been made to experimentally investigate the nonlocal feature of quantum mechanics and many distinguished quantum properties were observed. In this work, along with the discussion of the development of quantum nonlocality, we would focus on our recent experimental efforts in investigating quantum correlations and their applications with optical systems, including the study of entanglement-assisted entropic uncertainty principle, Einstein-Podolsky-Rosen steering and the dynamics of quantum correlations.
Quantum Key Distribution (QKD) networks allow multiple users to generate and share secret quantum keys with
unconditional security. Although many schemes of QKD networks have been presented, they are only concentrated on
the system realization and physical implementations. For the complete practical quantum network, a succinct theoretic
model that systematically describes the working processes from physical schemes to key process protocols, from
network topology to key management, and from quantum communication to classical communication is still absent. One
would hope that research and experience have shown that there are certain succinct model in the design of
communication network. With demonstration of the different QKD links and the four primary types of quantum
networks including probability multiplexing, wavelength multiplexing, time multiplexing and quantum multiplexing, we
suggest a layer model for QKD networks which will be compatible with different implementations and protocols. We
divide it into four main layers by their functional independency while defining each layer's services and responsibilities
in detail, orderly named quantum links layer, quantum networks layer, quantum key distribution protocols process layer,
and keys management layer. It will be helpful for the systematic design and construction of real QKD networks.
The imperfect components of a quantum key distribution (QKD) system will degrade its security. The highly attenuated
weak coherent laser source used in today's QKD system is vulnerable to PNS attacks due to multi-photon optical pulses.
Decoy State QKD, which is capable of beating PNS attacks and providing unconditional security, has been an attractive
scheme recently. We have used a one way Faraday-Michelson phase modulated QKD system to implement 123km
decoy state QKD in the lab. Furthermore a field decoy state QKD experiment of 32km dark fiber in Beijing is fulfilled.
The optical pulse repetition rate is 1MHz and the final secure key is 9.59×10-7 (6.784×10-6) bit per pulse for 123km
(32km). The experiment result indicates that the decoy state scheme can be successfully used in real-life metropolitan
QKD sessions.
We have demonstrated a gated-mode single-photon detector at 1550 nm using two thermoelectrically cooled InGaAs/InP
avalanche photodiodes (APDs). Balanced outputs from the two APDs were used to cancel the charge and discharge
spikes, which were attributable to capacitive behavior in a gated mode. The avalanche signals were not attenuated during
the spike cancellation, which enable one to reduce the bias voltage applied to the APDs and thus reduce the dark count
probability. We obtained a quantum efficiency of 10.5% with a dark count probability of 4.8E-5 per gate at 212 K. A
single photon detector module that integrated APD and all necessary circuits into a compact bin has been performed.
In the paper, the foundation requirement of a quantum key distribution network is described. According to the requirement, a kind of star topology quantum key distribution network is introduced. The core of this quantum network is a "router" which is composed of less than N wavelength division multiplexers. Based on the "router", a four nodes quantum key distribution network has been set up, the measurement results shown us that it is suitable for simple purpose of quantum key distribution over many users and the crosstalk is weak enough.
To find whether a set of reduced density matrixes come from a common multi-party state is a hard and important problem. In this paper, (1) we introduce a method to find out some polytopes in one-party eigenvalue-space which are sufficient conditions of this problem. (2) We point out that there are some relations between the compatible conditions and the entanglement of pure states. And we show this idea more clearly in the three-qubit case. (3) We investigate the relations between the compatibility problem and the invariants of a matrix-set under some groups. Furthermore, we show that it is one of the reasons why the compatibility problems which involve the multi-party density matrixes are much more difficult than the one-party case.
Remote state preparation and remote operation are entanglement assisted protocols in quantum information process, here we present a practical and general scheme of remote preparation for pure and mixed state, in which an auxiliary qubit and controlled-NOT gate are used. We give an experimental scheme of the quantum remote operation on single photons, where the unitary operation is restricted to the sets Ucom or Uanti. We discuss the remote state preparation (RSP) in two important types of decoherent channel (depolarizing and dephaseing). In our experiment, we realize RSP in the dephaseing channel by using spontaneous parametric down conversion (SPDC), linear optical elements and single photon detector.
We report experimental generation of nonclassically correlated photon pairs from collective emission in a room temperature atomic vapor cell. The nonclassical feature of the emission is demonstrated by observing a violation of the Cauchy-Schwarz inequality. Each pair of correlated photons are separated by a controllable time delay up to 2 microseconds. This experiment demonstrates an important step towards the realization of the Duan-Lukin-Cirac-Zoller scheme for scalable long-distance quantum communication.
An experimental setup for quantum key distribution (QKD) in special optical fiber at the wavelength of 850nm was presented. The system employs the B92 protocol to establish a secret key between Alice and Bob. The key is encoded in the phase of very weak laser of average photon number 0.1 per pulse. The measured error rate is lower than 8%, the effective transmission rate is about 17bit/s.
After analyzing the main quantum secret sharing protocol based on the GHZ-state, we propose the idea to directly encode the qubit of quantum key distributions, and then present a quantum secret sharing scheme where only product states are employed. As entanglement, especially the inaccessible multi-entangled state, is not necessary in the present quantum secret sharing protocol, it may be more
applicable when the number of the parties of secret sharing is large. Its theoretic efficiency is also doubled to approach 100%.
Although Mayers, Lo and Chau successfully showed that unconditionally secure quantum bit commitment (QBC) is impossible, we present a secure QBC protocol which is quite different from the model used in their proofs. If Alice wants to commit a bit b, she first encodes b into a series of classical bit strings of the same length. The requirement is that the modulo 2 sum of all bits in every bit string
equals to b. Then Alice uses quantum one-way function to produce quantum states which are corresponding to the encoded classical bit strings and will be sent to Bob as evidence. The quantum one-way function used here is just that was successfully applied in Gottesman and Chuang's quantum digital signatures. When it comes to the unveilingtime, Alice only sends the bit strings to Bob. Finally, Bob
generates quantum states form the bit strings by the same quantum one-way function and uses controlled-swap circuit to check whether the quantum states are identical with the evidence.
The aim of this paper is to try to find out the possibility of reducing the photon number noise in an optically pumped three-level atomic system. Consider a three-level atomic system. The atomic transition between level 1 and level 3 is forbidden. Each atom is incoherently excited to upper level 3 from level 1, transits to level 1 through intermediate level 2, and emits photons at frequency (omega) 1 and (omega) 2. We place the atoms with the above feature into an oscillator and may obtain two coherent light beams, whose frequency are (omega) 1 and (omega) 2, respectively. There may be some correlation between these two light beams. We make one beam to control the pump source by a feedback loop and expect to reduce the noise of photon number of the other beam.
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