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Physical Review A

Counterfactual distributed controlled-phase gate for quantum-dot spin qubits in double-sided optical microcavities

Qi Guo, Liu‐Yong Cheng, Li Chen, Hong‐Fu Wang, Shou Zhang

Published Oct 22, 2014• 24 citations
Publisher

Abstract

The existing distributed quantum gates required physical particles to be transmitted between two distant nodes in the quantum network. We here demonstrate the possibility to implement distributed quantum computation without transmitting any particles. We propose a scheme for a distributed controlled-phase gate between two distant quantum-dot electron-spin qubits in optical microcavities. The two quantum-dot-microcavity systems are linked by a nested Michelson-type interferometer. A single photon acting as ancillary resource is sent in the interferometer to complete the distributed controlled-phase gate, but it never enters the transmission channel between the two nodes. Moreover, we numerically analyze the effect of experimental imperfections and show that the present scheme can be implemented with high fidelity in the ideal asymptotic limit. The scheme provides further evidence of quantum counterfactuality and opens promising possibilities for distributed quantum computation.

DOI
10.1103/physreva.90.042327

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We demonstrate quantum information can be transferred between two distant participants without any physical particles travelling between them. The key procedure of the counterfactual scheme is to entangle two nonlocal qubits with each other without interaction, so the scheme can also be used to generate nonlocal entanglement counterfactually. We here illustrate the scheme by using flying photon qubits and stationary electron-spin qubits assisted by quantum dots inside double-sided optical microcavities. Unlike the typical teleportation, the present scheme does not require prior entanglement sharing or classical communication between the two distant participants.
To date, all schemes for entanglement distribution needed to send entangled particles or a separable mediating particle among distant participants. Here, we propose a counterfactual protocol for entanglement distribution against the traditional forms, that is, two distant particles can be entangled with no physical particles travel between the two remote participants. We also present an alternative scheme for realizing the counterfactual photonic entangled state distribution using Michelson-type interferometer and self-assembled GaAs/InAs quantum dot embedded in a optical microcavity. The numerical analysis about the effect of experimental imperfections on the performance of the scheme shows that the entanglement distribution may be implementable with high fidelity.
We present some deterministic schemes to construct universal quantum gates, that is, controlled- not, three-qubit Toffoli, and Fredkin gates, between flying photon qubits and stationary electron-spin qubits assisted by quantum dots inside double-sided optical microcavities. The control qubit of our gates is encoded on the polarization of the moving single photon and the target qubits are encoded on the confined electron spins in quantum dots inside optical microcavities. Our schemes for these universal quantum gates on a hybrid system have some advantages. First, all the gates are accomplished with a success probability of 100% in principle. Second, our schemes require no additional qubits. Third, the control qubits of the gates are easily manipulated and the target qubits are perfect for storage and processing. Fourth, the gates do not require that the transmission for the uncoupled cavity is balanceable with the reflectance for the coupled cavity, in order to get a high fidelity. Fifth, the devices for the three universal gates work in both the weak coupling and the strong coupling regimes, and they are feasible in experiment.
Exploiting the giant optical circular birefringence induced by the double-sided quantum-dot–cavity system, we construct a deterministic hybrid hyper-controlled-not (hyper-CNOT) gate, in which the spatial-mode and polarization states of a photon act as the two control qubits, whereas two stationary electron spins in quantum dots confined inside the optical microcavities serve as the two target qubits. In our scheme, the control qubits are easily manipulated with simple optical elements and the target qubits are suitable for storage and processing use. With our hybrid hyper-CNOT gates, we design a high-capacity direct transmission quantum communication network which requires neither the establishment of entanglement between remote locations nor the use of long-lived quantum memories. We discuss the feasibility and efficiency of our hybrid hyper-CNOT gate, concluding that it is feasible with current technology.
Counterfactual quantum communication allows distant users to share a message without any physical particles traveling through the quantum channel. Here, we present a counterfactual scheme to realize multiparty entanglement distribution. We first realize bipartite counterfactual entanglement distribution to create photon–electron entanglement by using interaction-free measurements and quantum dots. Then the atom–cavity systems are used to create many photon–photon entanglement pairs, which can connect photon–electron pairs with the help of an entanglement beam splitter. Consequently, the electron spins in distant optical microcavities are entangled by the coherent exchange of single flying photons. We also discuss the implementation issues to show that the presented scheme can be realized successfully in experiment. The numerical analysis about the efficiency of multiparty entanglement distribution indicates that there is a trade-off between the entanglement fidelity and the number of users.