Du Jiang-Feng

Realization of the Fredkin Gate by Three Transition Pulses in a Nuclear Magnetic Resonance Quantum Information Processor

We have experimentally realized the Fredkin gate with only three transition pulses in a solution of alanine. It appears that no experimental realization of the Fredkin gate with fewer pulses has been reported yet. In addition, the simple structure of our scheme makes it easy to implement in experiments.

Multi-player and Multi-choice Quantum Game

We investigate a multi-player and multi-choice quantum game. We start from two-player and two-choice game and the result is better than its classical version. Then we extend it to N-player and N-choice cases. In the quantum domain, we provide a strategy with which players can always avoid the worst outcome. Also, by changing the value of the parameter of the initial state, the probabilities for players to obtain the best payoff will be much higher that in its classical version.We investigate a multi-player and multi-choice quantum game. We start from a two-player and two-choice game, and the result is better than its classical version. Then we extend this to N-player and N-choice cases. In the quantum domain, we provide a strategy with which players can always avoid the worst outcome. Also, by changing the value of the parameter of the initial state, the probabilities for players to obtain the best pay-off will be much higher than in its classical version.

Preparing Pseudo-Pure States in a Quadrupolar Spin System Using Optimal Control

Pseudo-pure state (PPS) preparation is crucial in nuclear magnetic resonance quantum computation. There have been some methods in spin-1/2 systems and a few attempts in quadrupolar spin systems. As optimal control via gradient ascent pulses engineering (GRAPE) has been widely used in quantum information science, we apply this technique to PPS preparation in quadrupolar spin systems. This approach shows an effective and fast quantum control method for both the state preparation and the realization of quantum gates in quadrupolar systems.

Quantum precision metrology based on single-spin magnetic resonance

The ability of penetrating deeply into single quantum system can help reveal the unique information of the quantum individuals used to be averaged by the ensemble statistics in traditional macroscopic measurement. The nitrogen-vacancy (NV) center in diamond, which has an ultralong coherence time and can be readout and manipulated through ODMR (optically detected magnetic resonance) techniques under room temperature, has been an important platform achieving quantum precision measurement and quantum information processing. Particularly in magnetometry, the researches on single spin imaging are in full swing. In this review we retrospect the work about quantum precision metrology based on the NV center, and introduce the progress in sensing magnetic field, electric field, mechanical system and temperature. We also discuss the possible developing directions in this area.

Entanglement Dynamics of Arbitrary Two-Qubit Pure States under Amplitude and Phase Damping Channels

Analytic results are presented for the entanglement evolution for arbitrary two-qubit pure state under amplitude damping and particularly phase damping channel. The disentanglement time and the relationship between it and the form of initial state are given explicitly. The lower bounds of disentanglement time are obtained and shown to be the monotone functions of initial concurrence.

Realization of the Three-Qubit Toffoli Gate in Molecules *

We present the experimental realization of this gate with a solution of chlorostyrene molecules. Our method does not depend heavily on the two-qubit controlled operation, which used to serve as the basic quantum operation in quantum computing. At present, we use transition operator that can be applied to all qubits in one operation. It appears that no experimental realization has yet been reported up to now regarding the implementation of quantum Toffoli gate using transition pulse on three-qubit nuclear magnetic resonance quantum computers. In addition, our method is experimentally convenient to be extended to more qubits.

Thermally Controllable Break Junctions with High Bandwidths and High Integrabilities

Break junctions are important in generating nanosensors and single molecular devices. The mechanically controllable break junction is the most widely used method for a break junction due to its simplicity and stability. However, the bandwidths of traditional devices are limited to about a few hertz. Moreover, when using traditional methods it is hard to allow independent control of more than one junction. Here we propose on-chip thermally controllable break junctions to overcome these challenges. This is verified by using finite element analysis. Adopting microelectromechanical systems produces features of high bandwidth and independent controllability to this new break junction system. The proposed method will have a wide range of applications on on-chip high speed independent controllable and highly integrated single molecule devices.

Liquid Nuclear Magnetic Resonance Implementation of Quantum Computation in Subspace

Based on the logical labelling method, we prepare an effective pure state in a subsystem of a three spin system via liquid nuclear magnetic resonance technique. Then with this subspace effective pure state we implement the Deutsch-Jozsa algorithm. The tomography for the subspace effective pure state and the corresponding spectrum of the output for the Deutsch-Jozsa algorithm in a subsysytem of a nuclear spin system and demonstrated a subspace quantum computation.

Experimentally Obtaining the Likeness of Two Unknown Qubits on a Nuclear-Magnetic-Resonance Quantum Information Processor

We study the discrimination of quantum states from the other way around, i.e. the likeness of two quantum states. The fidelity is used to describe the likeness of two quantum states. Then we present a scheme to obtain the fidelity of two unknown qubits directly from the integral area of the spectra of the assistant qubit (spin) on a nuclear-magnetic-resonance quantum information processor. Finally, we demonstrate the scheme on a three-qubit quantum information processor. The experimental data are consistent with the theoretical expectation with an average error of 0.05, which confirms the scheme.Recently quantum states discrimination has been frequently studied. In this paper we study them from the other way round, the likeness of two quantum states. The fidelity is used to describe the likeness of two quantum states. Then we presented a scheme to obtain the fidelity of two unknown qubits directly from the integral area of the spectra of the assistant qubit(spin) on an NMR Quantum Information Processor. Finally we demonstrated the scheme on a three-qubit quantum information processor. The experimental data are consistent with the theoretical expectation with an average error of 0.05, which confirms the scheme.

Geometric Phase for Mixed States

The geometric phase of mixed states with non-degenerate eigenvalues is investigated. A general formula of geometric phase for mixed state under unitary evolution is given. In particular, we also furnish an expression of Hamiltonians for equivalent evolutions, by which one can understand what kind of evolutional operator U(t) (or Hamiltonian) is related to zero instantaneous dynamic phase. Moreover, the geometric phase and related Hamiltonians in the spin-half case are provided as an explicit example.