In order to create polarization based entanglement networks of W4 state, we propose an optical setup, which uses only four horizontally polarized photons as resource which implies no entanglement requirement as a resource.This setup can generate target state deterministically, by operating several quantum optical gates, which can be realized with current photonics technology.The setup we propose is composed of one Not, two Hadamard, ve Controlled Not (CNot) and one Tooli gate.
We propose an optical scheme to build an entangled network composed of W state based on polarization encoded qubits (photons). This new setup consists of 2 cNOT gates, 4 V gates, 2 Hadamard gates and basic optical tools such as polarizing beamsplitters (PBSs) and path couplers (PCs). V gate is a specially-designed tool acting as a two-qubit gate which is composed of a cNOT gate, 3 PBSs and a PC. By using this gate, one benefits from the temporarily generated optical degree of freedom, which is the spatial mode of a photon in the proposed scheme. Using an extra degree of freedom allows us to perform more capable processing for W-state creation protocols. We use four photons as input, which means we do not need entanglement as a resource. Also, we show that the proposed scheme can be implemented by operating the quantum optical gates which can be realized via current photonics technology. Additionally, we provide discussions of quantum contextuality and non-locality for W states.
We present a scheme to produce an entangled four-photon W state by using linear optical elements. The symmetrical setup of linear optical elements consists of four beam splitters, four polarization beam splitters, and four mirrors. A photon Einstein-Podolsky-Rosen pair and two single photons are required as the input modes. The projection on the W state can be made by a four-photon coincidence measurement. Further, we show that by means of a horizontally oriented polarizer in front of one detector, the W state of three photons can be generated.
We propose a scheme for optical realization of deterministic entanglement concentration of polarized photons. To overcome the difficulty due to the lack of sufficiently strong interactions between photons, teleportation is employed to transfer the polarization states of two photons onto the path and polarization states of a third photon, which is made possible by the recent experimental realization of the deterministic and complete Bell state measurement. Then the required positive operator-valued measurement and further operations can be implemented deterministically by using a linear optical setup. All these are within the reach of current technology.
We present a general scheme for a family of linear-optical quantum control gates, including controlled-not and controlled-swap gates for two or three qubits. Our approach utilizes polarization-path-entangled pairs of photons and encodes qubits in mixed degrees of freedom with the control qubit specifically occupying the polarization degree of freedom. By exploiting multiple degrees of freedom and initial state entanglement of the two photons, the proposed control gates do not require any ancilla photons or measurement-induced nonlinearities. Since our gates are purely linear and we implicitly use nonlinearities in a standard manner to create entanglement via parametric nonlinear process, our work demonstrates that a need to have nonlinearities in the photonic gates can be shifted to the state preparation stage; the cost of such shift is that the construction of a certain class of single-qubit operations, such as Hadamard, needs to be probabilistic. In particular, we focus on a deterministic linear-optical quantum Fredkin (controlled-swap) gate and perform a full characterization of the gate performance with a high fidelity typically well above
99 under realistic conditions. The proposed control gates rely on simple linear-optical elements and polarization-entangled photon pairs readily generated from ubiquitous sources, making the gates experimentally feasible with current technologies.
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.