We represent an optical scheme using cross-Kerr nonlinearities (XKNLs) and quantum dot (QD) within a single-sided optical cavity (QD-cavity system) to generate three-photon entangled W state containing entanglement against loss of one photon of them. To generate W state (three-photon) with robust entanglement against loss of one photon, we utilize effects of optical nonlinearities in XKNLs (as quantum controlled operations) and QD-cavity system (as a parity operation) with linearly optical devices. In our scheme, the nonlinear (XKNL) gate consists of weak XKNLs, quantum bus beams, and photon-number-resolving measurement to realize controlled-unitary gate between two photons while another nonlinear (QD) gate employs interactions of photons and an electron of QD confined within a single-sided optical cavity for implementation of parity gate. Subsequently, for the efficiency and experimental feasibility of our scheme generating W state, we analyze the immunity of the controlled-unitary gate using XKNLs against decoherence effect and reliable performance of parity gate using QD-cavity system.
To date, all work concerning the construction of quantum logic gates, an essential part of quantum computing, has focused on operating in one degree of freedom (DOF) for quantum systems. Here, we investigate the possibility of achieving scalable photonic quantum computing based on two DOFs for quantum systems. We construct a deterministic hyper-controlled-not (hyper-CNOT) gate operating in both the spatial mode and polarization DOFs for a photon pair simultaneously, using the giant optical Faraday rotation induced by a single-electron spin in a quantum dot inside a one-side optical microcavity as a result of cavity quantum electrodynamics. With this hyper-CNOT gate and linear optical elements, two-photon four-qubit cluster entangled states can be prepared and analyzed, which give an application to manipulate more information with less resources. We analyze the experimental feasibility of this hyper-CNOT gate and show that it can be implemented with current technology.
Spin systems such as silicon and nitrogen-vacancy centers in diamond, quantum dots, and quantum dot molecules coupled to optical cavities appear to be key elements for creating quantum networks by not only constituting the nodes of the network but also assisting the creation of photonic networks. Here we study the deterministic preparation of arbitrary-size
W states with spin systems. We present an efficient operation on three qubits, with two being the logical qubits and one being the ancillary qubit, where no interaction between the logical qubits is required. The proposed operation can create a
W-type Einstein-Podolsky-Rosen (EPR) pair from two separable qubits and expand that EPR pair or an arbitrary-size
W state by one, creating a
W-like state. Taking this operation as the fundamental building block, we show how to create a large-scale
W state out of separable qubits or double the size of a
W state. Based on this operation and focusing on nitrogen-vacancy centers in diamond as an exemplary spin system, we propose a setup for preparing
W states of circularly polarized photons, assisted by a single-spin qubit, where no photon-photon interactions are required. Next, we propose a setup for preparing
W states of spin qubits of spatially separated systems, assisted by a single photon. We also analyze the effects of possible imperfections in implementing the gates on the fidelity of the generated
W states. In our setups, neither postmeasurement nor postprocessing the states of spin or photonic qubit is required. Our setups can be implemented with current technology, and we anticipate that they will contribute to quantum science and technologies.
Abstract The construction of a near‐deterministic photonic hyperparallel quantum Fredkin (hyper‐Fredkin) gate is investigated for a three‐photon system with the optical property of a diamond nitrogen vacancy center embedded in an optical cavity (cavity‐NV center system). This hyper‐Fredkin gate can be used to perform double Fredkin gate operations on both the polarization and spatial‐mode degrees of freedom (DOFs) of a three‐photon system with a near‐unit success probability, compared with those on the double three‐photon systems in one DOF. In this proposal, the hybrid quantum logic gate operations are the key elements of the hyper‐Fredkin gate, and only two cavity‐NV center systems are required. Moreover, the possibility of constructing a high‐fidelity and high‐efficiency hyper‐Fredkin gate in the experimental environment of a cavity‐NV center system is discussed, which may be used to implement high‐fidelity photonic computational tasks in two DOFs with a high efficiency.
Universal quantum logic gates are important elements for a quantum computer. In contrast to previous constructions on one degree of freedom (DOF) of quantum systems, we investigate the possibility of parallel quantum computations dependent on two DOFs of photon systems. We construct deterministic hyper-controlled-not (hyper-CNOT) gates operating on the spatial-mode and the polarization DOFs of two-photon or one-photon systems by exploring the giant optical circular birefringence induced by quantum-dot spins in one-sided optical microcavities. These hyper-CNOT gates show that the quantum states of two DOFs can be viewed as independent qubits without requiring auxiliary DOFs in theory. This result can reduce the quantum resources by half for quantum applications with large qubit systems, such as the quantum Shor algorithm.
Entangled state analysis is one of the basic techniques in quantum information processing. However, it is impossible to fully distinguish the three-photon Greenberger–Horne–Zeilinger (GHZ) states with only linear optical elements. Here, we propose a deterministic scheme to complete three-photon spatial-polarization hyperentangled GHZ-state analysis (HGSA) by using the giant optical Faraday rotation induced by an excess electron spin in a quantum dot inside a one-sided optical microcavity as a result of cavity quantum electrodynamics. It is divided into two steps. The first step is used to distinguish the eight three-photon GHZ states in the spatial-mode degree of freedom (DOF) without destroying the polarization states. The second step is used to distinguish the eight GHZ states in the polarization DOF. This scheme can be generalized to N-photon spatial-polarization HGSA and has useful applications in quantum communication protocols.