Entangled Coherent States Research Articles

Controllable preparation of two-mode entangled coherent states in circuit QED

Although the multi-level structure of superconducting qubits may result in calculation errors, it can be rationally used to effectively improve the speed of gate operations. Utilizing a current-biased Josephson junction (λ-type rf-SQUID) as a tunable coupler for superconducting transmission line resonators (TLRs), under the large detuning condition, we demonstrate the controllable generation of entangled coherent states in circuit quantum electrodynamics (circuit QED). The coupling between the TLRs and the qubit can be effectively regulated by an external bias current or coupling capacitor. Further investigations indicate that the maximum entangled state can be obtained through measuring the excited state of the superconducting qubits. Then, the influence of the TLR decay on the prepared entangled states is analyzed.

Entangled coherent states for quantum information processing

Entangled coherent states (ECSs) with relative phase equal to the phase shift between two coherent states are constructed. We study the degree of entanglement and the nonclassical features exhibited by the so-constructed states keeping in view their role in quantum information processing (QIP).

Entanglement concentration for W-type entangled coherent states

An entangled coherent state (ECS) is one type of entanglement, which is widely discussed in the application of quantum information processing (QIP). In this paper, we propose an entanglement concentration protocol (ECP) to distill the maximally entangled W-type ECS from the partially entangled W-type ECS. In the ECP, we adopt the balanced beam splitter (BS) to make the parity check measurement. Our ECP is quite different from the conventional ECPs. After performing the ECP, not only can we obtain the maximally entangled ECS with some success probability, but also we can increase the amplitude of the coherent state. Therefore, it is especially useful in long-distance quantum communication, if the photon loss is considered.

Generation of multipartite entangled coherent states via a superconducting charge qubit

In this paper, we propose a method to generate multipartite entangled coherent states (MECS) using a multi-mode superconducting cavity containing an SQUID-based charge qubit. After the projective measurement of simple charge states, MECS representing nonlocal electromagnetic fields of cavity modes have been generated. Furthermore, we investigate the nonlocal properties of the generated MECS by constructing the new concurrence-like entanglement based on the Bell–Klyshko argument.

A Continuous Variable Quantum Key Distribution Protocol Based on Entanglement Swapping of Quasi-Bell Entangled Coherent States

A continuous variable quantum key distribution protocol with entanglement swapping of quasi-Bell entangled coherent states is proposed. As the preliminary step, a sender shares quasi-Bell entangled coherent states with a receiver. After their measurements to distinguish the cases of a zero response, a nonzero even-photon response and an odd-photon response, two legitimate participants fulfill the task of key distribution. The correlations resulting from entanglement swapping of quasi-Bell entangled coherent states and the order rearrangement of transmitted states provide the possibility to protect secret key distribution. In the ideal channel, the success probability increases with the amplitude of the coherent state, and approaches unity when the amplitude of the coherent state is larger than two. However, in the loss channel, the decoherence will introduce error in the generated key, and the error rate increases with the amplitude of the coherent state. When the amplitude of the coherent state is smaller than 0.2 (or so), the error rate approaches zero, although the success probability is less than 0.5 in this case.

Attaining subclassical metrology in lossy systems with entangled coherent states

Quantum mechanics allows entanglement enhanced measurements to be performed, but loss remains an obstacle in constructing realistic quantum metrology schemes. However, recent work has revealed that entangled coherent states (ECSs) have the potential to perform robust subclassical measurements [J. Joo et al., Phys. Rev. Lett. 107, 083601 (2011)]. Up to now no read-out scheme has been devised that exploits this robust nature of ECSs, but we present here an experimentally accessible method of achieving precision close to the theoretical bound, even with loss. We show substantial improvements over unentangled classical states and highly entangled NOON states for a wide range of loss values, elevating quantum metrology to a realizable technology in the near future.

Deterministic Generation of Macroscopic Entangled Coherent States for Distant Atomic Ensembles in Coupled Cavities

We proposes an efficient scheme for deterministic generation of entangled coherent states (ECSs) of effective bosonic modes realized by collective excitations of two separate atomic ensembles in coupled cavities. Our protocol is fast compare to the convectional dispersive interaction induced operation in a cavity-assisted system. In addition, the implemented evolution is independent on the cavity field states with the assistance of strong classical fields, and thus not sensitive to the thermal states of the cavities. Our analysis shows that the protocol is robust against the main decoherence sources and unavoidable parameter variations. Moreover, current experimental design provides us the potential of extending this deterministic scheme to multimode case.

Scheme for generating cluster-type entangled squeezed vacuum states via cross-Kerr nonlinearity

We propose a scheme to generate cluster-type entangled squeezed vacuum states via cross-Kerr nonlinearity. Our scheme involves a single-photon source, balanced beam-splitters, mirrors, the Kerr medium and single-photon detectors. It is found that the generation of cluster-type entangled squeezed vacuum states is easier than the generation of cluster-type entangled coherent states. We also discuss the effect of an imprecise nonlinear phase shift between two modes on the fidelity and the experimental feasibility of the proposed scheme.

Generation of macroscopic entangled coherent states with large Josephson junctions

Abstract We propose a simple and experimental architecture to generate macroscopic entanglement in a solid system which consists of two large Josephson junctions and a flux qubit. Through quantum measuring of flux qubit, entangled coherent states of two large Josephson junctions are obtained. The concurrence of entangled coherent states can be accommodated by adjusted systematic parameters. We also give a brief discussion on the experimental feasibility of this proposal.

Quantum entanglement of an entangled coherent state: Role of particle losses

We analyze entanglement properties of entangled coherent state (ECS), |α, 0〉1,2 + |0, α〉1,2, with and without photon losses. By separating the coherent state into , we derive exact results of the logarithmic negativity EN, which quantifies the degree of entanglement between the two bosonic modes. Without particle losses, EN = 1 for the N00N state; while for the ECS, EN increases from 0 to 1 as |α|2 → ∞. In the presence of photon losses, we find that the ECS with large enough photon number is more robust than that of the N00N state. An optimal ECS is obtained by maximizing EN with respect to |α|2.

Practical Entanglement Concentration for Entangled Coherent States

Entangled coherent state (ECS) is quite important in current quantum communication and quantum computation. In this paper, we discuss two practical entanglement concentration protocols (ECPs) to recover the maximally ECSs from the partially ECSs. In the first ECP, the parties do not need to know the initial coefficients of the partially ECSs. In the second ECP, they should know the initial coefficients. Both protocols are based on the linear optics which make them feasible in current experiment. Especially, in the second ECP, they only require one pair of initial partially ECSs, which makes this ECP optimal. Both ECPs may be useful in current quantum information processing.

Quantum Fisher Information of Entangled Coherent States in a Lossy Mach—Zehnder Interferometer

We give an analytical result for the quantum Fisher information of entangled coherent states in a lossy Mach—Zehnder interferometer recently proposed by Joo et al., [Phys. Rev. Lett. 107 (2011) 083601]. For small loss of photons, we find that the entangled coherent state can surpass the Heisenberg limit. Furthermore, the formalism developed here is applicable to the study of phase sensitivity of multipartite entangled coherent states.

Enhancing entanglement of entangled coherent states by single-mode coherent superposition of photon subtraction and addition

We introduce a new entangled quantum state generated by applying single-mode coherent superposition of photon subtraction and addition (a†cosθ+asinθ)m to the entangled coherent state |Ψ±α,0〉, and then investigate the entanglement properties affected by coherent superposition operation. It is shown that this operation can be applied to enhance the entanglement of the state |Ψ+α,0〉. In addition, the effects of the coherent operation is better to improve the entanglement than that of the creation operation (a†m) for |Ψ+α,0〉 in a small-amplitude regime and for |Ψ−α,0〉 in any regime.

Quantum phase estimation for nonlinear phase shifts with entangled spin coherent states of two modes

Recently, we presented (Berrada K, Abdel-Khalek S and Raymond Ooi C H 2012 Phys. Rev. A 86 033823) an improved phase estimation scheme employing entangled spin coherent states (ESCSs) using the Holstein–Primakoff realization of angular momentum algebra. Here, we study the nonlinear phase enhancement from a generalized nonlinearity on ESCSs under perfect and lossy conditions with the same mean photon number and nonlinearity order. The results show that an increase in the spin number gives the smallest variance in the phase parameter in comparison to N00N states for different orders of nonlinearity. Finally, we study the physical properties of the input optical field and explore a connection between this quantity and the output state phase uncertainty. In particular, we show that the Mandel parameter may be used as an indicator of the phase estimation behavior in this interferometric setting.

Mesoscopic entangled coherent states implemented with a circuit quantum electrodynamics system

We show a scheme to generate entangled coherent states in a circuit quantum electrodynamics system, which consists of a nanomechanical resonator, a superconducting Cooper-pair box (CPB), and a superconducting transmission line resonator. In the system, the CPB plays the role of a nonlinear medium and can be conveniently controlled by a gate voltage including direct-current and alternating-current components. The scheme provides a powerful tool for preparing the multipartite mesoscopic entangled coherent states.

Polarization and entanglement of photon-added coherent states

Polarization of light has been used extensively in quantum information processing, and quantum entanglement is essential to many areas of research, including quantum computing. Here we investigate the degree of polarization and the entanglement of a family of quantum states known as photon-added entangled coherent states. Such states could serve as means of entanglement distribution and quantum key distribution. Using the quantum Stokes parameters and the $Q$ function, we demonstrate that, in general, the degree of polarization of two two-mode photon-added coherent states increases significantly with the number of added photons. And using the concurrence, we show that the amount of entanglement in this kind of superposition presents a behavior that is dependent on whether or not the number of added photons in each mode is the same.

Non-classicality versus channel capacity for a superposition of entangled coherent states

The entropy and Mandel function as entanglement predictable of multipartite entangled coherent states are studied. The possibility of using these states as quantum channel to perform quantum teleportation is investigated. Quantum teleportation is achieved by using both even and odd entangled coherent states in the presence of environmental noise. The effect of the field’s parameters are investigated on the fidelity of the teleported state.

Generating entanglement between microwave photons and qubits in multiple cavities coupled by a superconducting qutrit

We discuss how to generate entangled coherent states of four \textrm{microwave} resonators \textrm{(a.k.a. cavities)} coupled by a superconducting qubit. We also show \textrm{that} a GHZ state of four superconducting qubits embedded in four different resonators \textrm{can be created with this scheme}. In principle, \textrm{the proposed method} can be extended to create an entangled coherent state of $n$ resonators and to prepare a Greenberger-Horne-Zeilinger (GHZ) state of $n$ qubits distributed over $n$ cavities in a quantum network. In addition, it is noted that four resonators coupled by a coupler qubit may be used as a basic circuit block to build a two-dimensional quantum network, which is useful for scalable quantum information processing.

New Parameterized Coherent-Entangled State Representation and Its Applications

Based on EPR’s idea of quantum entanglement, new parameterized coherent-entangled states |α,p〉 μ,ν are introduced in two-mode Fock space, which can be implemented by using a beam splitter. Possible superpositions of |α,p〉 μ,ν result in some new entangled states. The asymmetric ket-bra integration from |α,p〉 μ,ν leads to a new one- and two-mode combinatorial squeezing operator with stronger squeezing, which clearly shows the intrinsic relationship between squeezing mechanism and quantum entanglement.

Genuine entanglement among coherent excitonic states of three quantum dots located individually in separated coupled QED cavities

New scheme for generating genuine three-partite entanglement among three quantum dots (QDs) is proposed. The QDs are trapped in an one-dimensional (1D) array of three equidistance single-mode coupled cavities. Photon hopping is considered to be responsible for coupling between the cavities. The effective dynamics of the system leads to generate genuine three-partite entangled coherent excitonic states in QDs. The entanglement of these states, after encoding as three-qubit system, can be detected by entanglement witnesses (EWs) based on GHZ-states. It is shown that the generated entangled states can be arbitrarily very close to the GHZ-states.