We discuss the unconditional security of a quantum key distribution protocol in which bit values are encoded in the phase of a weak coherent-state pulse relative to a strong reference pulse, which is essentially the one proposed by Bennett in 1992 (the B92 scheme). In the BB84 protocol with a perfect single photon source, the key rate decreases linearly with the transmission T of the channel. If we simply replace this source with a weak coherent-state pulse, the key rate drops more rapidly (as O(η²)) since the presence of multiple photons favors the eavesdropper, Eve. The B92 protocol, if modified to be encoded on the polarization of a single photon, also shows O(η²) dependence. This comes from Eve's option of sending the vacuum state, which is always registered as an inconclusive result and never causes bit errors. In the original B92 scheme with phase coding, Eve has no such option--replacing the weak pulse by the vacuum cannot avoid errors completely, thanks to the strong reference pulse. This should make the scheme stronger against the loss in the channel even coherent-state pulses from conventional lasers are used. We have obtained an unconditional security proof allowing the presence of small bit errors, and confirmed the above observation in a rigorous way. The key rate is found to drop linearly with the transmission η, and it is lower than the BB84 with a perfect single-photon source only by a constant.

Practical quantum key distribution can be understood as a two-step procedure: in a first step two parties exchange quantum mechanical signals and perform measurements on them, in a second step auxiliary classical communication protocols are performed over an authenticated public channel to transform the data of the first step into an information-theoretic secure key. In this article we address the question of necessary conditions on the correlated (classical) data of the first step such that there can be a successful second step at all. As it turns out, a necessary condition is that these data, together with the knowledge about the physical set-up of sender and receiver, allow to establish a proof of effective entanglement between the two parties. We then demonstrate methods to systematically search for such a proof in basic settings, involving the 2-, 4-, and 6-state protocols.

One of our major target is quantum key distribution (QKD,) which is closest to the practical use. Though QKD can be performed only with current technology, there still remain many thing to be improved. One of the most important devices that determine the system performance is photon detectors. It limits the transmission distance in optical fiber communication wavelength. A photon detector combining the two avalanche photon diodes (APD) has been demonstrated for qubit discrimination in 1550 nm. Spikes accompanied with the signals in gated-mode were canceled by balanced output from the two APDs. The balanced APD photon detector shows very low dark count rate (2x10-7) counts/pulse) with 10 % detection efficiency. We have also developed a highly stable interferometer on a planer-lightwave-circuit (PLC.) We have achieved single-photon interference over 150 km using time-division interferometers for quantum cryptography, which were composed of the two integrated-optic asymmetric Mach-Zehnder interferometers, and the balanced gated-mode photon detectors. The observed visibility was more than 80 % after 150-km transmission, which refers to a quantum bit error rate of less than 10 %. We will also describe temperature insensible QKD system and high speed (100 bps) key transmission over 40 km fiber.

We will briefly introduce our progress on quantum teleportation and its applications, especially on creation and detection of tripartite entanglement and making a quantum teleportation network.

We present experimental homodyne tomography of an optical quantum bit represented by a single photon split into two optical modes. The reconstructed four-dimensional density matrix extends over the entire Hilbert space and thus reveals, for the first time, complete information about the dual-rail optical qubit as a state of the electromagnetic field. The experimental data violate the Bell inequality albeit with a loophole similar to the detection loophole in photon counting experiments. We also show that a homodyne measurement on just one of the modes leads to remote preparation of a single-rail optical qubit in the other mode.

We review on-going progress in the development of fiber-based telecom-band entanglement sources. Two different schemes (a Sagnac-loop scheme and a counter-propagating scheme) for generating polarization entanglement are reviewed and the pros and cons of each are summarized. A new scheme, called the double-loop scheme is proposed, which is theoretically shown to be capable of combining the benefits and avoiding the pitfalls of each previous scheme.

Quantum process tomography is often cited as providing all the information that can be known about a given quantum process. In this paper we have shown that even if two processes appear identical under process tomography, it is possible to distinguish them using an interferometric setup. Using this setup, it is possible to gain more information about a process than just tomography provides.

We investigate the possibility of implementing a given measurement using linear optics and continuous measurement. In particular, we revisit the so-called Dolinar receiver, a quasi-physical model attaining the minimum error discrimination of binary coherent states, to give an alternative derivation for the optimal discrimination scheme. Our approach is rather simple and can be applied to various kinds of measurements such as projection measurements in the regime of photonic-qubit signals.

We show that it is in principle possible to teleport a superposed coherent state with the success probability and fidelity of nearly 100% using only linear optical means and an atom-filed interaction. A successful completion of the teleportation requires, however, photocounters that can distinguish between odd and even numbers of photons.

The photon subtraction operation is an effective method to enhance an entanglement of the two-mode squeezed vacuum state. We show that it can be well approximated to the two-mode squeezed state with an enhanced squeezing although it is basically a mixed and non-Gaussian state. Such property can be directly observed by the current technology. We also study the continuous variable quantum dense coding with the photon subtracted two-mode squeezed state. It is shown that, in particular squeezing region, its error probability performance can be greatly improved from the original two-mode squeezed state. Since quantum dense coding is the protocol to transmit classical information, our analysis provides an alternative operational meaning of the entanglement involved in the on-off photon subtracted state, compared to the previous results, such as teleportation or the nonlocality, that could help to quantify the entanglement of mixed and non-Gaussian states.

The power of quantum communication channel arises from additively of Holevo information, which certainly depends on the fact that the Hilbert space grows exponentially. The additively will be be shown to be achieved by stochastically maximizing a Hilbert space. In the conjecture we introduce operator statistics which is a natural extension of functions of operator, and employing it we discuss that the information is additive in probability.

We present a new type of authentication scheme for quantum message based on algebraic coding theory and quantum computation operations between different quantum registers. The results are that if the pre-coding generator matrix in SN-S code is public, the quantum scheme is a public-key data integrity scheme; if it is secret, the quantum scheme is a hybrid data origin authentication scheme. The advantage of this scheme is that the public and secret keys are merely some classical data.

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 U_com or U_anti. 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 consider the entanglement transformations of the protocols that convert quantum qubit system to continuous variable system and convert it back. Here the quantum D/A conversion protocol maps finite-dimensional input states onto continuous variable states while the quantum A/D conversion protocol maps two mode squeezed state onto entangled state in 2x2 dimensions. Quantum carrier is investigated when pure qubit state interacts with coherent state.

Single photon counting module (SPCM) has been widely used in quantum information processing (QIP) to investigate the novel quantum-mechanical phenomena. We analysis the effect of SPCMs on photon statistics of light fields by mean of Hanbury Brown and Twiss (HBT) configuration. It shows that the measured second-order degree of coherence g⁽²⁾ and Mandel Q factor for different states are strongly corrected. The total efficiency and background are taken into account. The agreement between experiment based on the coherent as well as thermal fields and the theory is quite good.

We examine the gain property of a probe field interacting with two different three-level Lambda-type atomic systems with an open loop or a close loop. In the atomic system with an open loop, there exists quantum interference resulting from spontaneous emissions to two near-degenerate lower levels from a common upper level, and the weak probe field can be greatly amplified due to the spontaneously generated coherence. Moreover, the probe gain becomes sensitive to the relative phase between the probe and coupling fields, so we can realize phase control of the probe gain in principle. In the atomic system with a close loop, we use a microwave field to couple the two well-spaced lower levels so that quantum coherence similar to SGC can be generated. In this atomic system, we also can achieve the phase-sensitive probe gain due to the quantum interference between two different absorption channels for the probe field. Note, only in the case of three-photon resonance, this close-loop atomic system can reach a steady state. While in the case of three-photon off-resonance, the probe gain without inversion always oscillates periodically versus time, thus no steady-state probe gain can be achieved.

We study a four-level atomic system with a double-Λ configuration interacting with two probe and two control fields. The linear susceptibilities of the atomic medium show not only the dual electromagnetically induced transparency (EIT) effect, but also an amplification mechanism. The numerical simulations verify that one signal can be amplified in both the transmission and the storage processes, provided two synchronous signals are prepared as input.

Two theoretical models of single-photon acquisition probability based on the fundamental-mode Gaussian beam are established for free-space quantum key distribution and are compared in theory and simulation. The parameters that influence the single-photon acquisition probability are the transmitter's tracking-pointing error, the far-field divergence angle, the link distance between the transmitter and receiver and the receiver's antenna aperture. The single-photon acquisition probability is analyzed in the numerical simulation and its orders of magnitude are given for the ground-to-ground, satellite-to-ground and satellite-to-satellite links. The results of the theoretical analysis and the numerical simulation show that it is feasible to acquire single-photon for the satellite-to-ground and satellite-to-satellite quantum key distribution.

In this paper, we simulate the quantum channel with a binary symmetric channel and a binary erasure channel, a series channel and a Markovian chain channel in classical information theory, then calculate respectively the mutual information between the signal's deliverer, the legal receiver and the eavesdropper, the bit error rate during propagating the signals with the theory about the quantum measurement channel and the quantum information theory. For B92 protocol, a simple quantum cryptography distribution scheme, we study the bound and the property of mutual information obtained by the legal receiver and the eavesdropper, seek the relationship between the bit error rates and the eavesdropper's way, in two cases of the opaque eavesdropping and the translucent eavesdropping. A new criteria for checking the eavesdropper and ensuring the legal correspondent is estimated. Furthermore, the comparison in bit error rates caused respectively in two different measuring ways indicates that POVM is better than the standard measurement by the way of orthogonal projecting for reducing the bit error rate and increasing effective communication.

We suggest here a two-point eavesdropping strategy aimed at a two nonorthogonal states protocol of quantum key distribution over a fiber-optic channel. When the single-photon sources and detectors of Alice, Bob and the two Eve are all ideal, the two-point attack can break the two nonorthogonal states protocol if the distance between Alice and Bob is longer than 30 km. However, Bennett's original multi-photon protocol is secure against both two-point and beam splitting attacks, though the protocol is realized with a weak pulsed source.

The width of an electromagnetically induced transparency (EIT) resonance is studied theoretically for an ideal three-level system in a L-type configuration. The optical Bloch equations are solved and the power broadening behavior of the EIT resonance is studied as a function of both couple and probe laser intensities over a broad range. It is shown that at relative low couple laser intensity there is a quadratic dependence of the EIT width on the couple laser Rabi frequency and at large couple laser intensity the EIT resonance evolve into the well known dynamic Stark splitting and its width has a linear dependence on the couple Rabi frequency. The dependence of the EIT width on the probe laser intensity shows different characteristics with the dependence on the couple laser intensity. Furthermore, the probe laser causes saturation and this is also discussed in this paper.

In this paper we present a theoretical study of the effect of a microwave field on an EIT feature. The EIT feature is associated with the well-known three-level Λ type configuration where a pump and probe laser field couples two separate optical transitions. In addition to these two laser fields, there is a microwave field which drives one of the two lower levels of the Λ type three-level system to another hyperfine level. The EIT feature is studied as a function of microwave field frequency and intensity. Our results show that the presence of a microwave field can dramatically modify the EIT feature. When microwave is resonant with the hyperfine transition, the EIT feature can be split into two EIT features. When it is off resonant with the hyperfine transition, it causes a frequency shift of the EIT feature, reminiscent of the well-known light shift effect.

We in this paper calculate the relative entropy between the bipartite Gaussian states. By evaluating the minimization of the relative entropy of an entangled Gaussian state with respect to the separable Gaussian states, we obtain the Gaussian relative entropy of entanglement of the state. The bipartite operations of squeezing and rotations on the separable states are also addressed. We prove that for bipartite symmetric Gaussian states the separable states we sought will be symmetric Gaussian states, and we get quite simple expressions of Gaussian relative entropy of entanglement for such states.

Multipartite entanglement is quite difficult to deal with. In this paper, I will consider a particular kind of tripartite Gaussian states, which can be produced as the initial pure tripartite states undergo different noise for each party. I will make use of the relative entropy of entanglement to evaluate the entanglement properties of the states based on the fact that the separability of tripartite Gaussian states is well known. In addition, the procedure of calculating the relative entropy between Gaussian states have developed recently. All these make the evaluating the relative entropy of entanglement of tripartite Gaussian states possible.

Two essential features of free-space quantum key distribution system are its timing pulses and frequency tracking devices. We make use of these two features to design a new type of quantum key distribution system with synchronously delayed classical signals. This scheme is much more secure since its synchronously randomizing of frequency and arriving time of quantum signals makes Eves have almost no opportunity to catch the quantum signals in time. Besides, this scheme can improve the intrinsic photon utilization efficient from 50% to 100%. Because the scheme breaks the symmetry between Bob and Eve before open discussion, it extends the concept of quantum key distribution and increases our choice of protocol bases.

We present a quantum public-key cryptography protocol for quantum message transmission. The private key of this protocol includes three classical matrices: a generator matrix of a Goppa code, an invertible matrix and a permutation matrix. The public key is product of these three matrices. The encryption and decryption algorithms are merely quantum computations related with the transformations between bases of the quantum registers. The security of this protocol is based on the hypothesis that there is no effective algorithm of NP-complete problem.

In this paper, we propose a new scheme to generate a dark hollow-beam, which includes dark hollow-beam optical pipe and hollow-beam optical dipole trap, through using a system composed of a four steps phase plate and a spherical lens. This kind of light beam can be used to focus, guide and trap the cold atom. We also calculate the intensity distributing and characteristic parameters of a dark hollow-beam and the optical dipole potential which manipulate the cold atoms of 85Rb. At last, we analyze and demonstrate the experimental feasibility of the scheme.

Chaotic synchronization in injection semiconductor lasers with optical feedback applied to optical secure communications is presented. The two chaotic systems can be synchronized with a gradually process to gradually steady in full space. Synchronous transient response to a pulse of injecting light is studied. Influences of the white noise on the synchronous error, and the synchronous characteristic with optical feedback are studied in detail. Chaos masking with a sinusoidal frequency of 6GHz signal and chaotic injecting shift keying (CISK) in the system are numerically simulated in optical secure communications. The both systems show a good ability of robust security and anti-uncover.

Photons source generated by light pulses tapping in a fiber optic rinvg resonator is presented. Light pulses from OTDR were launched into a single mode fiber optic, which was a form of a ring resonator. All fiber optic components were connected and used to realize the practical in quantum information via fiber optic. Light pulses were trapped in the ring resonator in a period of time, i.e. memor time, before the S/N ratio of the detected signal was not valid. Results obtained have shown that number of pulse/phton trains obtained depend on a fiber optic ring resonator length, input pulse width and gain. In applications, the signal amplifier using inline connection with fiber laser could be used to amaintain the required S/N. The attenuation part could be employed into the system to produce a single photon pulse during the circulation in fiber optic ring resonator. The photon visibility in term of polarization extinction ratio of 10 dB is noted.

Kak's Quantum Key Distribution (QKD) protocol provides not only the distribution but also the integrity of secret key simultaneously in quantum channel; consequently the additional exchange of information, used to check whether an eavesdropper exists, is unnecessary. In this comment, we will point out the failure of Kak's protocol and show that Kak's protocol is no longer provided with the joint distribution and integration that the author declares in [1].

Quantum-correlated twin beams were successfully generated from an optical parametric oscillator (OPO) pumped by a frequency-doubled diode laser. The 50-mW diode laser output at 1080 nm was first converted to 22.5-mW green light, and then was used to pump an OPO to yield signal and idler beams with a total power of 5 mW. Noise reduction of 4.3 dB below the shot-noise level was observed in the intensity-difference spectrum. The setup opens a simple way to a compact and low-cost source of diverse nonclassical states of light.

We demonstrate the first experimental realization of the entangled photon pair generation via biexciton-resonant hyper parametric scattering in CuCl crystal. Polarization correlation measurements and quantum tomographic analysis shows that the generated photon pairs have high degrees of polarization entanglement.