We review recent theoretical progress in finding ways to do quantum processing with linear optics, non-classical input states and conditional measurements. We focus on a dual rail photonic scheme and a single rail coherent state scheme.

The development of key devices and systems in quantum information technology, such as entangled particle sources, quantum gates and quantum cryptographic systems, requires a reliable and well-established method for characterizing how well the devices or systems work. We report our recent work on experimental characterization of pulsed entangled photonic states and photonic quantum channels, using the methods of state and process tomography. By using state tomography, we could reliably evaluate the states generated from a two-photon source under development and develop a highly entangled pulsed photon source. We are also devoted to characterization of single-qubit and two-qubit photonic quantum channels. Characterization of typical single-qubit decoherence channels has been demonstrated using process tomography. Characterization of two-qubit channels, such as classically correlated channels and quantum mechanically correlated channels is under investigation. These characterization techniques for quantum states and quantum processes will be useful for developing photonic quantum devices and for improving their performances.

Quantum cryptography bases the security of key exchange on the laws of quantum physics and will become the first application of quantum information methods. Here we present the design of novel hardware components which enabled the demonstration of secure key exchange over a 23.4 km free-space link.

This paper presents novel quantum key distribution (QKD) schemes that use differential phases of sequential pulses as an information carrier. Alice sends a photon in a linear superposition state of three temporal slots, in which each amplitude is phase-modulated by {0,π}. Bob measures the phase difference of an asymmetric interferometer, and tells Alice time-instance at which the photon was counted. From this time information and her modulation data, Alice knows which detector counted the photon in Bob s site. Then, the two parties create a secret key. The scheme is suitable for fiber transmission systems and offers key creation efficiency higher than conventional phase-encoding QKD. The above scheme utilizes fully non-orthogonal four states. Differential phase shift QKD utilizing two non-orthogonal states is also presented. Alice sends a coherent pulse train with less than one photon per pulse, which is phase modulated by {0,π} for each pulse, and Bob measures the pulse train by a one-bit delay circuit. The system has a simple configuration such that Alice has no interferometer and sends no intense reference pulse, unlike to conventional QKD scheme using two non-orthogonal states, and also has an advantage of high key creation efficiency.

Quantum key distribution (QKD) is stepping out of the lab. We present a commercial fibre-optic QKD-prototype based on faint laser pulses and the results of field tests. Faint laser, single photon and entangled-photon based systems are compared with respect to possible bit rates, detector noise and security and their possible implementation in a commercial apparatus.

Multiphoton entanglement is the basis of many quantum communication schemes, quantum cryptographic protocols, and fundamental tests of quantum theory. Spontaneous parametric down-conversion is the most effective source for polarization entangled photon pairs. Here we show, that a entangled 4-photon state can be directly created by parametric down-conversion. This state exhibit perfect quantum correlations and a high robustness of entanglement against photon loss. We have used this state for four-particle test of local realistic theories. Therefore this state can be used for new types of quantum communication. We also report on possibilities for the experimentally realization of a 3-photon entangled state, the so called W-state, and discuss some of its properties.

An efficient quantum key distribution (QKD) protocol has been developed. Results show that our new QKD protocol enhances the efficiency of B92 protocol, a two-state protocol invented by Bennet [1], from 25% to 28.6%.

We find that the photon system in the black body filled in its interior by a Kerr-nonlinear crystal is in a squeezed thermal radiation state, in which the photon system possesses a new kind of quasiparticles - nonpolaritons. The existence of nonpolaritons should be manifested by observing the predicted temperature dependence of the velocity and squeezing of nonpolaritons. The propagation velocity and noise properties of nonpolaritons constitute potentially novel and significant effects in solid-state quantum optics and hence these effects should have extensive application to fields of science and technology.

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.

We investigate electromagnetically induced transparency and slow group velocity of light in ultracold Bose gas with two Roman pulses structure. The light speed and properties of Electromagnetically induced transparency can be changed by controlling the atomic interaction. Atomic interaction can be as a knob to control the optical properties of atomic media. This can be realized in experiment by using Feshbach resonance technique.

Only one Mach-Zehnder interferometer is used in the proposed quantum bit distribution system where the two arms composing the Mach-Zehnder interferometer have been used as the transmission line. The loss of the photons due to absorption will decrease and the wavelets composing the photon are expected to transmit longer distance. The optical cable which contains many fiber belts which contain many fibers in parallel that is possible to make a Mach-Zehnder interferometer with its two arms as long as possible. This transmission system uses only two couplers and physically ensures transmission security.

Linearly polarized photon can be encoded by phase modulation of half wavelets composing the photon to form rotating photon that is circularly polarized clockwise or counterclockwise. Quantum key distribution can be performed by the rotating photons. Theoretical analysis indicates that this new scheme of quantum key distribution system uses less optical components and therefore less insert loses. The rotating photon is insensitive to birefringence. The experimental procedure is also described.

Laser chaotic system is used to complete private communication experiment in this paper. In the process of private communication, both chaotic laser beam and electric chaos signal which contain private signals are transferred to the place where the signals is received. Two same non-linear electric circuits with same response behavior are used in the experiment setup to change frequencies and waves of laser chaotic signal and electrical chaotic signal at the same time. Even if both laser chaotic signal and electric signal are lost, transferred private signals are still safe.

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.

We present a repeatable BB84 protocol for a hybrid quantum key distribution system based on the dual-velocity protocol and an error-correction scheme. This protocol is the first one that is immune to photon absorption, suitable for single-photon transmission and does not need any entanglement between photons.

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%.

A scheme of teleportation of an arbitrary two-particle state is presented when two pairs of entangled particles are used as quantum channels. After the Bell state measurements are operated by the sender, the original state with deterministic probability can be reconstructed by the receiver when a corresponding unitary transformation is followed.

It is shown that each of two copies of the non-orthogonal states randomly selected from a certain set can be probabilistically deleted by a general unitary-reduction operation if and only if the states are linearly independent. A bound on the best possible deleting efficiencies is derived.

Response of Bragg acousto-optic bistable system to external rectangular wave signals has been studied in this paper. When there is not external signals, the system is set up into stable state, and external signals are input into the system, response waves are observed on the bottoms of rectangular waves of external signals. As bifurcation parameters are changed, bifurcation and chaos are observed in the response waves of the system.

We present a scheme to realize the quantum logic operation without an auxiliary level or the limitation on the Lamb-Dicke parameter. The scheme uses two pairs of laser beams that are perpendicular to each other, both of the beams are tuned to the carrier. The internal state of the ion acts as the target qubit and the Fock state of the ion as the control qubit.

Electromagnetically induced transparency (EIT) is observed in a three-level multi-V-type system in cesium vapor at room temperature. The absorption and dispersion properties are measured under the condition that the coupling frequency is exactly resonance with transition 6S_1/2(F=4) to 6P_3/2(F'=3) while the probe frequency is scanned across the transitions from the 6S_1/2(F=4) to all structures of the 6P_3/2. The effects of the intensity of the pumping beam on EIT signal are investigated. The simple theory model was presented and the results of the experiment agree with the simple theoretical analysis.

The entanglement of formation of bipartite quantum Gaussian state is investigated by means of local operation, which maps continuous variable state to a bipartite qubits system. A two parameters quantum Guassian state is introduced, the concurrence of its mapped qubits state is calculated to characterize the entanglement of the original state.

Quantum Cryptolographic Key Distribution technology breaks the limitation of safety and efficiency in classic method, it provides a certifiable safety based on its physical property, and solves the famous problem of Catch22 in privacy communication. But it is impossible to apply the technology to the network at present because of various limitations of the condition. A protocol simulating quantum distribution technology under present network condition is presented, which applies the chaotic pseudo random series in the distribution protocol. A method which can generate series initial values that are independent on the operator is also given.

We present a new kind of quantum cryptography protocol based on Shamir's three-pass protocol of classical cryptography, which allows the transmission of qubits directly and secretly via the aid of an unjammable classical channel. In this protocol we implement the encryption and decryption transformations via rotations on the Poincare sphere of the photons polarization parameters. The key technique is that Bob's encryption rotation must be commutative with Alice s decryption rotation; this means that the axes of these two rotations must be parallel. We also present a security analysis of the protocol under a man-in-the-middle attack.