Theoretical Study On Counterfactual Quantum Information Processing

Quantum information science, combining quantum theory and information science,has been developing rapidly and many significant progresses have been achieved in recent years. On one hand, through applying the basic principle of quantum mechanics to the information science, some incredible information processing tasks in the classical framework, such as quantum teleportation, exponential acceleration of quantum parallel computation and so on, can be achieved, which may bring about an information revolution. On the other hand, the development of the quantum information can also provide more reliable evidences for the test of the basic principle of quantum mechanics. The manipulation and measurement techniques for single quantum system in quantum information science make it possible to implement some previous thought experiments of quantum mechanics in quantum optics laboratory. Therefore, the researches of quantum information can enrich the contents of quantum mechanics, which in turn promotes the development of quantum mechanics. This thesis, focusing on the above two aspects, firstly designs relevant quantum information protocols based on counterfactual quantum effects, and then tries to explore the physical mechanism behind these schemes.We first review the principle of Rydberg dipole blockade, and then propose a scheme for counterfactual quantum information transfer based on the dipole interaction between a single Rydberg atom and a mesoscopic atom ensemble. Here, the counterfactuality indicates that an unknown qubit can be transmitted without transmitting any physical particles between the sender and the receiver. The present scheme can counterfactually teleported an unknown quantum state of the single atom to the spatial mode degree of freedom of the photon in a distant place. Qubit is the basic unit of quantum information, and the transmission of qubit, e.g. quantum teleportation, usually requires prior entanglement sharing and classical communication between the two distant participants, which means the physical particles should be transmitted. Therefore, the present scheme is very different from the existing ones and demonstrates quantum information can be transferred without particle transmission. The theoretical calculation and numerical analysis show that the scheme can be accomplished with perfect fidelity in the ideal asymptotic limit and without contradicting any existing physical law.The counterfactual distributed controlled-phase(CPHASE) gate is proposed based on quantum-dot-microcavity system. Universal quantum computation can be implemented by single-qubit gates and two-qubit controlled-NOT gate or CPHASE gate, so the CPHASE gate always occupies important position in the research of quantum computation. With the increase of the quantum computation, it is necessary to construct largescale quantum computation network. The computation tasks between the distant quantum nodes in the network are performed through distributed quantum computation. The existing distributed quantum gate is usually achieved by two ways. One method to realize the nonlocal interaction is that remote qubits interact respectively with one flying qubit of an entangled pair; the other is that a mediating particle is transmitted in the quantum channel between the two nodes and interacted with the two separate qubits successively. The present scheme implements the CPHASE gate between two distant quantum dot electronspin qubits assisted with an ancillary photon, but the photon never enters the transmission channel between the two nodes. The scheme enables the counterfactual universal distributed quantum computation.Inspired by the counterfactual quantum communication, the tripartite chained nested Mach-Zehnder type interferometer is constructed for counterfactual entanglement distribution, which is the essential prerequisite for many quantum information tasks and is always the research focus. To date, all schemes for entanglement distribution needed to send entangled particles or a separable mediating particle among distant participants.The present scheme shows two distant particles can be entangled with each other by the nested Mach-Zehnder type interferometer, and no physical particles are required to travel between the two remote participants. Then a realizable implementation scheme for counterfactual photonic entangled state distribution is given by using Michelson-type interferometer and self-assembled Ga As/In As quantum dot in a double-side optical microcavity.The numerical analysis about the effect of experimental imperfections on the performance of the scheme shows that the present entanglement distribution scheme can be effectively implemented under the current experimental conditions.There is much debate about the physical mechanism behind the counterfactual quantum information processing. Recently, the focus of passionate debate returns to the fundamental question in quantum mechanics, i.e., what is photon? For this problem, Wheeler proposed the famous thought experiment, delayed-choice experiment. With the development of the quantum technology, the thought experiment not only can be performedin laboratory, but also can be extended to the quantum version which makes it possible to observe the photon’s wave behavior and particle behavior simultaneously. Here the implementation scheme for quantum delayed-choice experiment based on linear optical elements is designed. By choosing different detecting devices, one can selectively observe the photon’s particle behavior, wave behavior, wave-particle mixed behavior, and wave-particle superposition behavior. Especially, we compare the wave-particle superposition behavior and the wave-particle mixture behavior in detail, and find the quantum interference effect between wave and particle behavior.