Quantum Communication Systems Research Articles

A Quantum Repeater Based on Entanglement Purification and Entanglement Swapping

We discuss a long-distance quantum communication system based on entangled photon pairs, which apply entanglement as its fundamental resource. For distances longer than the coherence length of a counterpart noisy quantum channel, the fidelity of transmission is ordinarily so low that standard purification processes are not applicable. The quantum repeater stretches the length of the entangled photon pairs. And the high fidelity entanglement of photons between sender and receiver is obtained by entanglement purification and entanglement swapping. We compare the nested repeater with the common repeater and show that it outperforms the latter, which is built an EPR pair in less time.

A Security Real-time Privacy Amplification Scheme in QKD System

Quantum Key Distribution (QKD) technology, based on the laws of physic- s, can create unconditional security keys between communication parties. In recent years, researchers draw more and more attention to the QKD technology. Privacy am- plification is a very significant procedure in QKD system. In this paper, we propose the real-time privacy amplification (RTPA) scheme which converts the weak secret string to a uniform key that is fully secret from Eve. Our detailed proofs show the security of our RTPA scheme. In order to prevent the potential man-in-middle attacks, we em- ploy an authentication procedure to RTPA scheme (ARTPA) with the e-XOR almost universal hash functions. We implement our ARTPA scheme based on CLIP system, which is connected to the quantum communication system. Considering the privacy amplification and authentication overhead and the finite size effect on the security of final keys, we set the secret key length be 256k before privacy amplification and the authentication tag length be 60. Our experimental results show the efficiency of the proposed ARTPA scheme.

DEGREE OF DEPOLARIZATION OF QUANTIZATION HERMITE-GAUSSIAN BEAM IN A TURBULENT ATMOSPHERE

Based on quantum Stokes operators and non-Kolmogorov spectrum model of index-of-refraction ∞uctuation, the analytical for- mulas for the quantum degree of depolarization of quantization Hermite-Gaussian (QHG) beams propagating in a turbulent atmo- sphere slant channel are derived. The nonclassical polarization prop- erties of QHG beams propagating in turbulent atmosphere are stud- ied numerically. It is found that the polarization ∞uctuations of QHG beams are dependent of the turbulence factors such as spectrum power- law exponent, refractive index structure parameter at the ground and zenith angle. The degree of depolarization of QHG beams has a salta- tion and reaches the minimum value at spectrum power-law exponent fi = 11=3, the refractive index structure parameter at the ground of the turbulent atmosphere slightly afiects the polarization degree of QHG beams which have travelled a long distance, and the change of polariza- tion degree decreases with the increasing zenith angle. Furthermore, the numerical simulations show that QHG beams with higher photon- number level, lower beam order, shorter wavelength are less afiected by the turbulence. These results indicate that One can choose low-order QHG beams with wavelength ‚ = 690nm as optical carrier, increase photon number, set the size of transmitting aperture w0 as about 0.1m, and detect communication signals at the central region of beams to im- prove the performance of a polarization-encoded free-space quantum communication system.

High-speed polarization controller for all-fiber quantum communication systems

Polarization control is of vital importance in optical communications, which can affect the stability and bit error rate of a system. We demonstrate an all-fiber high-speed polarization control scheme based on a bidirectional Sagnac ring. The polarization direction can be accurately controlled by adjusting the phase difference of an electro-optic fiber phase modulator in the ring, meanwhile it is realized that only a single beam port outputs different polarization states. The phase can be adjusted accuratly to 10-3 rad, giving an extinction ratio as high as 30 dB, and the modulation speed can reach 2 GHz. The system has excellent stability, and as the optical circuit consists of simple, low cost components which can be easily integrated, there are good prospects for its application in quantum cryptography and other optical communcations fields.

Semiconductor Quantum Dot–Microcavities for Quantum Optics in Solid State

In this paper, exciting progress of quantum optics in solid state is reviewed. The focus is on semiconductor microcavities with self-assembled quantum dots embedded in the active layer. Due to enormous progress in semiconductor nanotechnology, such photonic structures have become a model system for the study of quantum optics on a scalable and integrable technology platform with high potential for future applications in quantum information technology. Quantum optical phenomena have become accessible due to 3-D confinement of light and matter on the length scale of their wavelength in state-of-the-art semiconductor micro- and nanostructures. This confinement leads to a quantization of the associated photonic and electronic energy levels and requires a quantum mechanical description of the system in the framework of cavity quantum electrodynamics (cQED). This approach considers the dipole interaction between two-level quantum emitters and discrete photonic states of a microcavity. Within the well-known Jaynes-Cummings model, the dipole interaction is described in terms of a coherent exchange of energy between the emitter and the resonator mode. This coherent interaction in the so-called strong coupling regime of cQED is reflected in a normal mode splitting, the vacuum Rabi splitting, of the involved modes and represents a central feature of quantum optics in solid state. Another important example of quantum optics in semiconductor nanostructures is the generation of nonclassical light in specific quantum devices. Of particular interest is the realization of Fock states which represent states containing a well-defined number of photons. Single-photon sources, for instance, allow for the generation of single photons on demand, which is highly desirable for quantum communication systems. In this context, this review paper will present recent experimental studies of quantum optics in solid state. This paper is meant for readers who would like to become familiar with this topic and for experts being interested in the progress in this field. It will cover a broad range of studies ranging from examples of fundamental light-matter interaction in the quantum limit to devices capable of emitting single photons and entangled photon pairs on demand.

Entangling Single Photons from Independently Tuned Semiconductor Nanoemitters

Quantum communication systems based on nanoscale semiconductor devices is challenged by inhomogeneities from device to device. We address this challenge using ZnMgSe/ZnSe quantum-well nanostructures with local laser-based heating to tune the emission of single impurity-bound exciton emitters in two separate devices. The matched emission in combination with photon bunching enables quantum interference from the devices and allows the postselection of polarization-entangled single photons. The ability to entangle single photons emitted from nanometer-sized sources separated by macroscopic distances provides an essential step for a solid-state realization of a large-scale quantum optical network. This paves the way toward measurement-based entanglement generation between remote electron spins localized at macroscopically separated fluorine impurities.

Exponentially enhanced quantum communication rate by multiplexing continuous-variable teleportation

A major challenge of today's quantum communication systems lies in the transmission of quantum information with high rates over long distances in the presence of unavoidable losses. Thereby the achievable quantum communication rate is fundamentally limited by the amount of energy that can be transmitted per use of the channel. It is hence vital to develop quantum communication protocols that encode quantum information as energy efficiently as possible. To this aim we investigate continuous-variable quantum teleportation as a method of distributing quantum information. We explore the possibility to encode information on multiple optical modes and derive upper and lower bounds on the achievable quantum channel capacities. This analysis enables us to benchmark single-mode versus multi-mode entanglement resources. Our research reveals that multiplexing does not only feature an enhanced energy efficiency, leading to an exponential increase in the achievable quantum communication rates in comparison to single-mode coding, but also yields an improved loss resilience. However, as reliable quantum information transfer is only achieved for entanglement values above a certain threshold a careful optimization of the number of coding modes is needed to obtain the optimal quantum channel capacity.

Single photon wavelength modulation absorption spectrum of acetylene for 1.5 m laser wavelength stabilization

We describe a kHz single photon wavelength modulation technology to realize the locking reference signal of the laser frequency. By detecting the single photon acetylene gas absorption spectrum and by amplifying the lock-in the discrete single photon response pulse, we achieve the laser frequency stabilized in real time. The 1.5 m distribution feedback semiconductor laser output frequency is stabilized at acetylene gas absorption peaks, and the typical laser frequency fluctuation is less than 25 MHz within 175 s. This single photon wavelength modulated absorption spectrum technology eliminates the high background noise in low frequency range and can be used in the quantum communication system and optical wavelength division multiplex.

Perspectives on Entangled Nuclear Particle Pairs Generation and Manipulation in Quantum Communication and Cryptography Systems

Entanglement between two quantum elements is a phenomenon which presents a broad application spectrum, being used largely in quantum cryptography schemes and in physical characterisation of the universe. Commonly known entangled states have been obtained with photons and electrons, but other quantum elements such as quarks, leptons, and neutrinos have shown their informational potential. In this paper, we present the perspective of exploiting the phenomenon of entanglement that appears in nuclear particle interactions as a resource for quantum key distribution protocols.

Study of Quantum Communication System with Coherent Light

In this paper， we introduced a system of coherent optical detection with homodyne， which implementing coherent optical communication，our own design based on the principle of coherent optical communication. IN the system, PCI6111E was useed as a data acquisition(DAQ) module. Alice and Bob’s control procedure by their computers under the LabVIEW. The synchronous detection and DAQ were carried out on the system. The signals were processed, displayed and measured. The Experimental results show that by light-synchronization and LabVIEW procedure control, each function module work together in cooperation, include Alice’s acousto-optic modulator and electro-optical modulator, Bob’s data acquisition and phase adjustment by PZT. The coherent optical information could be transitted by Alice and real-time demodulated by Bob.

Quantum phenomena explored with neutrons

Neutrons are suitable tools for quantum experiments since they are massive, experience nuclear, electromagnetic and gravitational interaction and are easy to manipulate and to detect. Perfect crystal interferometry opened new possibilities to explore quantum phenomena on a new ground. The 4π-symmetry of spinor wave functions, the spin-superposition law, topological quantum phases and various gravitational effects have been examined using this method. Experiments exploiting contextuality and Kochen–Specker phenomena exhibit intrinsic entanglement in single particle systems. This may have consequences for a deeper understanding of quantum physics and for applications in future quantum communication systems.

Sensitivity of entangled photon holes to loss and amplification

Energy-time entangled photon holes are shown to be relatively insensitive to photon loss due to absorption by atoms whose coherence times are longer than the time delays typically employed in nonlocal interferometry (a fraction of a nanosecond). Roughly speaking, the excited atoms do not retain any significant "which-path" information regarding the time at which a photon was absorbed. High-intensity entangled photon holes can also be amplified under similar conditions. Decoherence does occur from losses at beam splitters, and these results show that photon loss cannot always be adequately modeled using a sequence of beam splitters. These properties of entangled photon holes may be useful in quantum communications systems where the range of the system is limited by photon loss.

ABCD law of two-mode Gaussian-entangled light fields in linear optical systems

In this paper, we have theoretically investigated the propagation of two-mode Gaussian-entangled light fields (TGLFs) passing through a linear optical system. A general transformation formula of the TGLFs passing through the linear optical system has been derived under the paraxial approximation. Based on the derived formula, we have illustrated two typical examples: in the first example, the Einstein–Podolsky–Rosen (EPR) correlation of TGLFs propagating in a free space depends on the propagation distance and its value increases greatly with the propagation distance. In the second example, it is shown that the EPR correlation can be manipulated and controlled in the lens system. Moreover, we find that the von Neumann entropy of TGLFs does not change when TGLFs travel in these linear optical systems, verifying the entanglement of TGLFs is an intrinsic property. Our results may have potential applications in the first-order linear optical systems of quantum communication systems.

Intrinsic limitations to the quality of pulsed spontaneous parametric downconversion sources for quantum information applications

Spontaneous parametric downconversion (SPDC) sources are an essential element of quantum communication and quantum information processing systems. Their quality can be hampered by spectral, temporal, or spatial distinguishability of the two photons of a generated pair. Even when such defects have been corrected, the ultimate quality of the SPDC source is limited by the intrinsic multiple pair generation. In this paper, the effect of transmission, filtering and detection losses and asymmetrical spectral bandwidths in the photon pair paths on the visibility of the two-photon interference delivered by the SPDC source and its useful pair rate are investigated. In this investigation, two subcases are distinguished: deterministic and statistical splittings of the generated pairs, whatever the coherence of the multiple pairs. We show that the visibility is strongly degraded by a spectral bandwidth asymmetry, while the losses mainly affect the measured coincidence probability. The deterministic splitting configuration is shown to be generally more advantageous, although statistical splitting may present some advantages in applications with a narrow bandwidth requirement, such as those involving quantum memories. Moreover, while pump-induced optical noise only produces small visibility reductions, dark counts of the single-photon detectors are shown to limit the maximum possible visibility, particularly in the case of high-loss photon pair sources. Because our model quantifies the intrinsic limitations due to double pair emission in a simple way, the results presented can be used to optimize the design of SPDC sources and choose the optimum trade-offs between several key parameters such as visibility, generation probabilities, and spectral bandwidth, in particular for small bandwidth SPDC sources in the perspective of quantum networking and computing applications.

Up-conversion single-photon detector using multi-wavelength sampling techniques

The maximum achievable data-rate of a quantum communication system can be critically limited by the efficiency and temporal resolution of the system's single-photon detectors. Frequency up-conversion technology can be used to increase detection efficiency for IR photons. In this paper we describe a scheme to improve the temporal resolution of an up-conversion single-photon detector using multi-wavelength optical-sampling techniques, allowing for increased transmission rates in single-photon communications systems. We experimentally demonstrate our approach with an up-conversion detector using two spectrally and temporally distinct pump pulses, and show that it allows for high-fidelity single-photon detection at twice the rate supported by a conventional single-pump up-conversion detector. We also discuss the limiting factors of this approach and identify important performance-limiting trade offs.

Performance analysis of decoy state quantum key distribution using two kinds of photon sources

Since the single photon is easy to attenuate and be disturbed, its key generation rate and transmission distance in quantum communication systems are generally limited. In contrast, weak coherent pulse (WCP) and heralded single photon source (HSPS) exhibit higher feasibility than the single photon source in quantum key distributions (QKD). We compare the performances of QKD using these two kinds of photon sources and decoy state method, incorporating data-postprocessing methods including the Lütkenhaus scheme and Gottesman-Lo-Lütkenhaus- Preskill (GLLP) scheme. Simulation results indicate that QKD using HSPS can transmit longer distance than using WCP with lower overall quantum bit error rate (QBER) at the same transmission distance, albeit with relatively low key generation rate.

Fiber-based narrow-band single-photon source with high heralding efficiency

According to the correlated photon pairs produced by spontaneous four-wave mixing in dispersion-shifted fiber, we present a telecom-band single-photon source by using mode-locked laser. Single-photons with a -3 dB bandwidth of 1.1 nm are heralded with more than 50% probability. The measured second-order correlation g(2)(0) is equal to 0.090.01. Our source is well suit for quantum communication systems.

Error corrections of quantum key distribution of the quantum codes via optical wireless link

Quantum key generated by using a fiber optic Mach Zehnder interferometer incorporating a fiber optic ring resonator is proposed. The generated quantum key is distributed via an optical wireless link system, which is available for the free space link. The random quantum bits can be formed and used as the secret codes in quantum communication system. By adding an ancillary photon after the signal photon within the correlation time of the fiber noise and by performing quantum parity checking, the high fidelity to the noiseless quantum in free space is achieved by the technique known as the quantum error correction.

Spectral and spread-spectral teleportation

We report how quantum information encoded into the spectral degree of freedom of a single-photon state may be teleported using a finite spectrally entangled biphoton state. We further demonstrate how the bandwidth of the teleported wave form can be controllably and coherently dilated using a spread-spectral variant of teleportation. We calculate analytical expressions for the fidelities of spectral and spread-spectral teleportation when complex-valued Gaussian states are transferred using a proposed experimental approach. Finally, we discuss the utility of these techniques for integrating broad-bandwidth photonic qubits with narrow-bandwidth receivers in quantum communication systems.

Hiding keys could foil quantum hackers

Claims that quantum communications systems are unbreakable have been undermined in recent years. A new study suggests hiding the quantum key in a sea of noise to make it more difficult to intercept