Abstract
We experimentally demonstrate and characterize a four-qubit linear-optical quantum logic circuit. Our robust and versatile scheme exploits encoding of two qubits into polarization and path degrees of single photons and involves two crossed inherently stable interferometers. This approach allows us to design a complex quantum logic circuit that combines a genuine four-qubit C3Z gate and several two-qubit and single-qubit gates. The C3Z gate introduces a sign flip if and only if all four qubits are in the computational state |1〉. We verify high-fidelity performance of this central four-qubit gate using Hofmann bounds on quantum gate fidelity and Monte Carlo fidelity sampling. We also experimentally demonstrate that the quantum logic circuit can generate genuine multipartite entanglement and we certify the entanglement with the use of suitably tailored entanglement witnesses.
Highlights
We experimentally demonstrate and characterize a four-qubit linear-optical quantum logic circuit
Various elementary quantum gates for qubits encoded into states of single photons have been demonstrated[4,5,6,7,8,9,10,11,12,13,14,15,16], the optical quantum logic circuits have been miniaturized and integrated on a photonic chip[17,18,19,20], and alternative more efficient approaches to all-optical quantum computing such as utilization of photonic cluster states[21,22] have been developed
We exploit the polarization and path degrees of freedom to encode two qubits into a single photon and construct a two-photon four-qubit linear optical quantum logic circuit, which represents an important step beyond the previous implementations of two-4–11,13 and three-qubit[14,15,16] linear optical quantum gates
Summary
We experimentally demonstrate and characterize a four-qubit linear-optical quantum logic circuit. Our robust and versatile scheme exploits encoding of two qubits into polarization and path degrees of single photons and involves two crossed inherently stable interferometers This approach allows us to design a complex quantum logic circuit that combines a genuine four-qubit C3Z gate and several two-qubit and single-qubit gates. Important examples of the simultaneous exploitation of several degrees of freedom of single photons for encoding and processing quantum information include generation of hyper-entangled photon pairs[31], superdense quantum teleportation[32], design of certain linear optical quantum logic gates[14], and implementation of random quantum walks[33,34,35]. The two-qubit controlled-rotation gates applied to polarization and path qubits supported by the same photon can be implemented deterministically, while the core four-qubit C3Z gate is probabilistic, with theoretical success probability of 1
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