量子通信中单光子探测器的实验研究

为了提高单光子探测系统的灵敏度,实验采用InGaAs/InP雪崩光电二极管作为量子通信中的单光子探测器件,以门控脉冲模式实现了更高精度的单光子探测器的偏压生成电路、单光子信号放大电路、单光子信号检测电路和温度控制模块,并通过选用高精度前置放大器OP37和精密比较器AD8561,将量子效率提高到18.3%,暗计数控制小于4.1%×10~(-6)/ns.     

Single photon sources with single semiconductor quantum dots

In t.his contribution, we briefly recall the basic concepts of quantum optics and properties of semicon- ductor quantum clot. （QD） which a.re necessary to the nnderstanding of the physics of single-photon generation with single QDs. Firstly, we address the theory of quantmn emitter-cavity system, the fluorescence and optical properties of semiconductor QDs, and the photon statistics as well as opti- cal properties of the QDs. We then review the localizatioll of single semiconductor QDs in quantum confined optical microcavity systems to achieve their overall optical properties and perfornances in terms of strong coupling regime, elfieiency, directionality, and polarization control. Furthermore, we will discuss the recenl, progress on the fabrication of single photon sources, and various a.pproaehes for embedding single QDs into mieroca,vities or photonic crystal nanoeavities and show how to ex- tend the wavelength range. We focus in part;icular on new generations of electrically driven QD single photon source leading to high repetition rates, efficiencies at elevated temperature operation. Besides strong eoupling regime, and high collection new development;s of room temperature sin- gle photon emission in the strong coupling regime are reviewed. The generation of indistinguishable photons and remaining challenges for pract ical single-photon sources are also discussed.     

Single photon absorption by a single quantum emitter

We show that a three-level lambda quantum emitter with equal spontaneous emission rates on both optically active transitions can absorb an incident single-photon pulse with a probability approaching unity, provided that the focused light profile matches that of the emitter dipole emission. Even with realistic focusing geometries, our results could find applications in long-distance entanglement of spin qubits.     

Practical quantum cryptography

A quantum cryptography system based on a 4-basis protocol with geometrically uniform states is tested in a series of experiments. Quantum states of light transmitted through real fiber optic communication channels to a distance of 32 km in the presence of uncontrolled external actions are prepared, transformed, and measured. It is shown that the chosen algorithms of processing quantum information are adequate and can be used as foundations of practical devices in protected communication lines.₁     

Nonstationary-states quantum cryptography

A quantum cryptosystem is proposed in which a pair of nonstationary states differing in their times of preparation is used as the information carriers. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 742–745 (10 December 1997)     

Relativistic quantum cryptography

A new protocol of quantum key distribution is proposed to transmit keys through free space. Along with quantum-mechanical restrictions on the discernibility of nonorthogonal quantum states, the protocol uses additional restrictions imposed by special relativity theory. Unlike all existing quantum key distribution protocols, this protocol ensures key secrecy for a not strictly one-photon source of quantum states and an arbitrary length of a quantum communication channel.     

Mesoscopic quantum cryptography

Since a strictly single-photon source is not yet available, in quantum cryptography systems, one uses, as information quantum states, coherent radiation of a laser with an average number of photons of μ ≈ 0.1–0.5 in a pulse, attenuated to the quasi-single-photon level. The linear independence of a set of coherent quasi-single-photon information states leads to the possibility of unambiguous measurements that, in the presence of losses in the line, restrict the transmission range of secret keys. Starting from a certain value of critical loss (the length of the line), the eavesdropper knows the entire key, does not make errors, and is not detected—the distribution of secret keys becomes impossible. This problem is solved by introducing an additional reference state with an average number of photons of μ_cl ≈ 10³–10⁶, depending on the length of the communication line. It is shown that the use of a reference state does not allow the eavesdropper to carry out measurements with conclusive outcome while remaining undetected. A reference state guarantees detecting an eavesdropper in a channel with high losses. In this case, information states may contain a mesoscopic average number of photons in the range of μ _q ≈ 0.5–10². The protocol proposed is easy to implement technically, admits flexible adjustment of parameters to the length of the communication line, and is simple and transparent for proving the secrecy of keys.     

Classical noise-based cryptography similar to two-state quantum cryptography

A scheme of cryptographic key agreement via classical noise is introduced. The principle underlying its security is similar to that of the two-state quantum cryptosystem, but it has the advantage that signal amplification can be applied. Radio and optical implementations of the scheme are suggested.     

Spectral coherent-state quantum cryptography

A novel implementation of quantum-noise optical cryptography is proposed, which is based on a simplified architecture that allows long-haul, high-speed transmission in a fiber optical network. By using a single multiport encoder/decoder and 16 phase shifters, this new approach can provide the same confidentiality as other implementations of Yuen's encryption protocol, which use a larger number of phase or polarization coherent states. Data confidentiality and error probability for authorized and unauthorized receivers are carefully analyzed.     

Towards practical quantum cryptography

Quantum cryptography bases the security of quantum key exchange on the laws of quantum physics and is likely to become the first application employing quantum effects for communication. Here we present performance tests of a new design based on polarization encoding of attenuated, coherent light pulses. Our measurements show that this compact setup can achieve an effective key-bit rate in the kHz range with low alignment requirements and thus offers the tools for fast and user-friendly quantum key exchange. Received: 27 July 1999 / Revised version: 3 September 1999 / Published online: 10 November 1999     

Single nanoparticle quantum dynamics studied by the photon distribution function

The blinking fluorescence of a single nanoparticle excited by CW laser radiation was studied. In the case of a low time resolution setup, blinking fluorescence appears to be a continuous emission. Blinking can nevertheless be established if we measure the photon distribution function in such quasi-continuous fluorescence.     

Optimal dimensionality for quantum cryptography

A comparison of two protocols for generating a cryptographic key composed from d-valued symbols, one using a string of independent qubits and another utilizing d-level systems prepared in states belonging to d + 1 mutually unbiased bases, is performed. The protocol based on qubits is shown to be optimal for quantum cryptography, since it provides higher security and a higher key generation rate.     

Double entanglement and quantum cryptography

We propose a quantum transmission based on bi-photons, which are doubly-entangled both in polarisation and phase. This scheme finds a natural application in quantum cryptography, where we show that an eventual eavesdropper is bound to introduce a larger error on the quantum communication than for a single entangled bi-photon communication, when he steels the same information. Received 23 July 2001 / Received in final form 30 November 2001 Published online 24 September 2002     

Faraday-Michelson system for quantum cryptography

Quantum key distribution provides unconditional security for communication. Unfortunately, current experimental schemes are not suitable for long-distance fiber transmission because of phase drift or Rayleigh backscattering. In this Letter we present a unidirectional intrinsically stable scheme that is based on Michelson-Faraday interferometers, in which ordinary mirrors are replaced with 90 degree Faraday mirrors. With the scheme, a demonstration setup was built and excellent stability of interference fringe visibility was achieved over a fiber length of 175 km. Through a 125 km long commercial communication fiber cable between Beijing and Tianjin, the key exchange was performed with a quantum bit-error rate of less than 6%, which is to our knowledge the longest reported quantum key distribution experiment under field conditions.     

Limitations on practical quantum cryptography

We provide limits to practical quantum key distribution, taking into account channel losses, a realistic detection process, and imperfections in the "qubits" sent from the sender to the receiver. As we show, even quantum key distribution with perfect qubits might not be achievable over long distances when the other imperfections are taken into account. Furthermore, existing experimental schemes (based on weak pulses) currently do not offer unconditional security for the reported distances and signal strength. Finally we show that parametric down-conversion offers enhanced performance compared to its weak coherent pulse counterpart.     

Coherent-states quantum cryptography based on a quantum comparator

A quantum cryptosystem based on comparing an input signal from a communication channel with a reference state prepared at the receiving end is proposed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 736–741 (10 December 1997)     

Experimental quantum secret sharing and third-man quantum cryptography

Quantum secret sharing (QSS) and third-man quantum cryptography (TQC) are essential for advanced quantum communication; however, the low intensity and fragility of the multiphoton entanglement source in previous experiments have made their realization an extreme experimental challenge. Here, we develop and exploit an ultrastable high intensity source of four-photon entanglement to report an experimental realization of QSS and TQC. The technology developed in our experiment will be important for future multiparty quantum communication.     

Two-way and one-way quantum cryptography protocols

We present two quantum cryptography protocols. The first one generalizes the concept of the two-way deterministic protocol to work with qudits in prime d-dimensional system where d is odd. The second protocol makes use of the tomographically complete set construction for odd d-dimensional systems where d = p₁p₂ to modify the BB84 protocol to work with qudits of such systems. The securities of the two protocols are analyzed according to the intercept and resend attack.     

Error reconciliation for practical quantum cryptography

Error reconciliation is a necessary step for quantum key distribution process. In this paper, the correction ability of error reconciliation procedure which is called Winnow protocol is analyzed and estimated, moreover, it gives some expressions about it. The experiment results proved the capability of Winnow protocol are excellent. It is also indicates that the smallest N has the most correction efficiency where from the communication times between the legitimate parties and the data remaining efficiency consideration.     

Practical implementation of multilevel quantum cryptography

The physical principles of a quantum key distribution protocol using four-level optical systems are discussed. Quantum information is encoded into polarization states created by frequency-nondegenerate spontaneous parametric down-conversion in collinear geometry. In the scheme under analysis, the required nonorthogonal states are generated in a single nonlinear crystal. All states in the selected basis are measured deterministically. The results of initial experiments on transformation of the basis polarization states of a four-level optical system are discussed.