Qubit

About: Qubit is a research topic. Over the lifetime, 29978 publications have been published within this topic receiving 723084 citations. The topic is also known as: quantum bit & qbit.

Self-verifying variational quantum simulation of lattice models

Abstract: Hybrid classical–quantum algorithms aim to variationally solve optimization problems using a feedback loop between a classical computer and a quantum co-processor, while benefiting from quantum resources. Here we present experiments that demonstrate self-verifying, hybrid, variational quantum simulation of lattice models in condensed matter and high-energy physics. In contrast to analogue quantum simulation, this approach forgoes the requirement of realizing the targeted Hamiltonian directly in the laboratory, thus enabling the study of a wide variety of previously intractable target models. We focus on the lattice Schwinger model, a gauge theory of one-dimensional quantum electrodynamics. Our quantum co-processor is a programmable, trapped-ion analogue quantum simulator with up to 20 qubits, capable of generating families of entangled trial states respecting the symmetries of the target Hamiltonian. We determine ground states, energy gaps and additionally, by measuring variances of the Schwinger Hamiltonian, we provide algorithmic errors for the energies, thus taking a step towards verifying quantum simulation. Quantum-classical variational techniques are combined with a programmable analogue quantum simulator based on a one-dimensional array of up to 20 trapped calcium ions to simulate the ground state of the lattice Schwinger model.

Deterministic delivery of remote entanglement on a quantum network.

Abstract: Large-scale quantum networks promise to enable secure communication, distributed quantum computing, enhanced sensing and fundamental tests of quantum mechanics through the distribution of entanglement across nodes1–7. Moving beyond current two-node networks8–13 requires the rate of entanglement generation between nodes to exceed the decoherence (loss) rate of the entanglement. If this criterion is met, intrinsically probabilistic entangling protocols can be used to provide deterministic remote entanglement at pre-specified times. Here we demonstrate this using diamond spin qubit nodes separated by two metres. We realize a fully heralded single-photon entanglement protocol that achieves entangling rates of up to 39 hertz, three orders of magnitude higher than previously demonstrated two-photon protocols on this platform14. At the same time, we suppress the decoherence rate of remote-entangled states to five hertz through dynamical decoupling. By combining these results with efficient charge-state control and mitigation of spectral diffusion, we deterministically deliver a fresh remote state with an average entanglement fidelity of more than 0.5 at every clock cycle of about 100 milliseconds without any pre- or post-selection. These results demonstrate a key building block for extended quantum networks and open the door to entanglement distribution across multiple remote nodes.

Symmetric multiparty-controlled teleportation of an arbitrary two-particle entanglement

Abstract: We present a way for symmetric multiparty-controlled teleportation of an arbitrary two-particle entangled state based on Bell-basis measurements by using two Greenberger-Horne-Zeilinger states, i.e., a sender transmits an arbitrary two-particle entangled state to a distant receiver, an arbitrary one of the n+1 agents, via the control of the others in a network. It will be shown that the outcomes in the cases that n is odd or is even are different in principle as the receiver has to perform a controlled-NOT operation on his particles for reconstructing the original arbitrary entangled state in addition to some local unitary operations in the former. Also we discuss the applications of this controlled teleporation for quantum secret sharing of classical and quantum information. As all the instances can be used to carry useful information, its efficiency for qubit approaches the maximal value.

Optically Active Quantum Dots in Monolayer WSe$_2$

Abstract: Semiconductor quantum dots have emerged as promising candidates for implementation of quantum information processing since they allow for a quantum interface between stationary spin qubits and propagating single photons. In the meanwhile, transition metal dichalcogenide (TMD) monolayers have moved to the forefront of solid-state research due to their unique band structure featuring a large band gap with degenerate valleys and non-zero Berry curvature. Here we report the observation of quantum dots in monolayer tungsten-diselenide with an energy that is 20 to 100 meV lower than that of two dimensional excitons. Photon antibunching in second-order photon correlations unequivocally demonstrates the zero-dimensional anharmonic nature of these quantum emitters. The strong anisotropic magnetic response of the spatially localized emission peaks strongly indicates that radiative recombination stems from localized excitons that inherit their electronic properties from the host TMD. The large $\sim$ 1 meV zero-field splitting shows that the quantum dots have singlet ground states and an anisotropic confinement most likely induced by impurities or defects in the host TMD. Electrical control in van der Waals heterostructures and robust spin-valley degree of freedom render TMD quantum dots promising for quantum information processing.

Optomechanical transducers for long-distance quantum communication.

Abstract: We describe a new scheme to interconvert stationary and photonic qubits which is based on indirect qubit-light interactions mediated by a mechanical resonator. This approach does not rely on the specific optical response of the qubit and thereby enables optical quantum interfaces for a wide range of solid state spin and charge based systems. We discuss the implementation of state transfer protocols between distant nodes of a quantum network and show that high transfer fidelities can be achieved under realistic experimental conditions.