Masashi Ban

Optimum measurements for discrimination among symmetric quantum states and parameter estimation

An optimum quantum measurement that minimizes the average probability of error is considered for symmetric quantum states. The positive operator-valued measure (POM) which satisfies the necessary and sufficient condition for the minimization of the average probability of error is derived by using the quantum Bayes strategy. It is also shown that the mutual information obtained in the optimum quantum measurement becomes extremum. Furthermore, an optimum quantum measurement for parameter estimation is found for symmetric quantum states by applying the maximum-likelihood estimation. The optimum POM for the parameter estimation has the same structure as that for the quantum state discrimination.

Photon-echo technique for reducing the decoherence of a quantum bit

Abstract A useful method for reducing the decoherence of a quantum bit (qubit) is proposed, which is based on the photon-echo phenomenon in coherent optical transient. The decoherence of the qubit is caused by the dephasing coupling with a bosonic thermal reservoir. In the method, successive coherent π pulses are applied with very short pulse widths to reduce the decoherence. It is found that the decoherence of the qubit disappears at a certain time which can be controlled by appropriately choosing the pulse separation time and the number π pulses.

Experimental demonstration of quantum teleportation of a squeezed state

Quantum teleportation of a squeezed state is demonstrated experimentally. Due to some inevitable losses in experiments, a squeezed vacuum necessarily becomes a mixed state which is no longer a minimum uncertainty state. We establish an operational method of evaluation for quantum teleportation of such a state using fidelity and discuss the classical limit for the state. The measured fidelity for the input state is 0.85{+-}0.05, which is higher than the classical case of 0.73{+-}0.04. We also verify that the teleportation process operates properly for the nonclassical state input and its squeezed variance is certainly transferred through the process. We observe the smaller variance of the teleported squeezed state than that for the vacuum state input.

Decoherence of continuous variable quantum information in non-Markovian channels

Decoherence of quantum information carried by single-mode and two-mode Gaussian states is investigated under the influence of non-Markovian quantum channels, where correlation times of reservoir variables are assumed to take finite values. Degradation of purity, distinguishability and non-classicality of single-mode Gaussian states and relaxation of entanglement of two-mode Gaussian states are examined. The results show the importance of the non-Markovian effect of a thermal reservoir. Furthermore, continuous variable quantum teleportation is considered when a sender and a receiver share a two-mode Gaussian state through non-Markovian quantum channels.

Optimal parameter estimation of a depolarizing channel

We investigate strategies for estimating a depolarizing channel for a finite dimensional system. Our analysis addresses the double optimization problem of selecting the best input probe state and the measurement strategy that minimizes the Bayes cost of a quadratic function. In the qubit case, we derive the Bayes optimal strategy for any finite number of input probe particles when bipartite entanglement can be formed in the probe particles.

Photon statistics of conditional output states of lossless beam splitter

Abstract When an observation is carried out at one of the two output ports of a lossless beam splitter, photon statistics of a conditional output state in the unobserved output port are investigated for several input states. Observations at the output port are carried out by means of photon counting, heterodyne detection and homodyne detection. The dependence of the average value of the photon number and the Q factor of the conditional output state on the measurement outcome are investigated in detail. The statistical distance between the input and conditional output states is also considered. Furthermore, it is shown that the relationship between the input and conditional output states of the beam splitter is characterized as a conditional isometric process.

Quantum dense coding of continuous variables in a noisy quantum channel

Quantum dense coding of continuous variables via a two-mode squeezed vacuum state is considered for a noisy quantum channel. Classical information is encoded in the two quadrature components of one mode of a two-mode squeezed vacuum state and the encoded mode is sent through a noisy quantum channel which is assumed to be linearly dissipative. Decoding at the receiver side is performed by simultaneous measurement of two quadrature components of the two-mode state. By calculating the error probability of a binary communication system, we examine whether the dense coding is effective or not when a quantum channel is noisy.

Decoherence of quantum information in the non-Markovian qubit channel

Decoherence of quantum information of qubits is investigated under the influence of the non-Markovian quantum channel, where the correlation time of reservoir variables takes a finite value. Degradation of purity, distinguishability and entanglement of qubit states are evaluated. It is found that the non-Markov effect makes the coherence time of quantum information longer than that obtained for the Markovian quantum channel. Furthermore, the quantum teleportation and quantum dense coding of qubits are considered under the influence of non-Markov channels. The fidelity between teleported and original states and the Holevo capacity are obtained.

Decoherence of entanglement in the Bloch channel

Decoherence of entanglement of qubits is investigated which is caused by the phenomenological quantum channel (the Bloch channel) equivalent to the Bloch equations. It is shown how the decoherence of entanglement depends on the longitudinal and transverse relaxation times and the equilibrium value of the qubit. The quantum dense coding system under the influence of the Bloch channel is also investigated. The Shannon mutual information obtained by the Bell measurement and the Holevo capacity is calculated. Furthermore, the microscopic system-reservoir model which yields the Bloch equations is considered. The result shows that the temperature of the thermal reservoir significantly affects the decoherence of entanglement.

The maximum mutual information without coding for binary quantum-state signals

The maximum mutual information without coding is investigated for binary quantum-state signals. The necessary condition for detection operators to represent the information optimal detection is satisfied for M-ary symmetric quantum-state signals by the detection operators derived from the quantum minimax strategy while its purpose is a minimization of the error probability. The true maximum mutual information is numerically analysed for the binary case.