Non-Markovian entanglement transfer to distant atoms in a coupled superconducting resonator

We investigate the non-Markovian effects on the entanglement transfer to the distant non-interacting atom qubits,which are embedded in a coupled superconducting resonator. The master equation governing the dynamics of the system is derived by the non-Markovian quantum state diffusion(NMQSD) method. Based on the solution, we show that the memory effect of the environment can lead to higher entanglement revival and make the entanglement last for a longer time. That is to say, the non-Markovian environment can enhance the entanglement transfer. It is also found that the maximum entanglement transferred to distant atoms can be modified by appropriately selecting the frequency of the modulated intercavity coupling. Moreover, with the initial anti-correlated state, the entanglement between the cavity fields can be almost completely transferred to the separated atoms. Lastly, we show that the memory effect has a significant impact on the generation of entanglement from the initial non-entangled states.     

Non-Markovian random unitary qubit dynamics

We compare two approaches to non-Markovian quantum evolution: one based on the concept of divisible maps and the other one based on distinguishability of quantum states. The former concept is fully characterized in terms of local generator whereas it is in general not true for the latter one. A simple example of random unitary dynamics of a qubit shows the intricate difference between those approaches. Moreover, in this case both approaches are fully characterized in terms of local decoherence rates.     

Implementing multi-qubit entanglement of two-level systems inside a superconducting phase qubit

The interaction between a superconducting phase qubit and the two-level systems located inside the Josephson tunnel barrier is described by the XY model, which is naturally used to implement the i-SWAP gate. With this gate, we propose a scheme to efficiently generate multi-qubit entangled states of such two-level systems, including multipartite W state and cluster states. In particular, it is found that, with the help of the phase qubit, the entanglement witness can be used to efficiently detect the produced multi-qubit entangled states.     

Quantum entanglement generation on magnons assisted with microwave cavities coupled to a superconducting qubit

We present protocols to generate quantum entanglement on nonlocal magnons in hybrid systems composed of yttrium iron garnet (YIG) spheres, microwave cavities and a superconducting (SC) qubit. In the schemes, the YIGs are coupled to respective microwave cavities in resonant way, and the SC qubit is placed at the center of the cavities, which interacts with the cavities simultaneously. By exchanging the virtual photon, the cavities can indirectly interact in the far-detuning regime. Detailed protocols are presented to establish entanglement for two, three and arbitrary N magnons with reasonable fidelities.     

Non-Markovian effects on entanglement dynamics in lossy cavities

We consider the non-markovian entanglement dynamics of two independent atoms, each off-resonantly coupling with a zero temperature reservoir. For two types of initial entanglement, corresponding to spin correlated and anti-correlated Bell-like states |φ〉 and |ψ〉, we study the dependence of the two-qubit entanglement transfer on both the off-resonant interactions and the cavity pseudomode decay. The speed of the entanglement transfer is related to the choice of the atom detuning from the cavity pseudomode, the cavity pseudomode decay and the relative coupling. Furthermore, compared to the ideal cavities, entanglement transfer and sudden death can not be prevented using off-resonant interactions in the lossy cavities, and the conservation rules are always satisfied in the off-resonant and lossy system.     

Information entropy and entanglement of a superconducting qubit coupled to a cavity field with its spontaneous decay

The quantum entanglement between superconducting qubit and cavity field is described quantitatively in the presence of spontaneous decay. Depending on how how a system is quantum correlated with its environment, the entanglement dynamics between the qubit and cavity is evaluated and investigated during the dissipative process. The motivation based on recent experiments wherein the Cooper box can be used to probe the decay of the resonator superposition state due to environmental decoherence, we theoretically investigate the dynamics of entanglement measured by the negativity. Wehrl entropy and Wehrl phase distribution of a superconducting qubit coupled to a cavity field induced by a superconducting qubit-damping reservoir governed by a master equation.     

Quantum entanglement and control in a capacitively coupled charge qubit circuit

The macroscopic quantum entanglement in capacitively coupled SQUID (superconducting quantum interference device)-based charge qubits is investigated theoretically. The entanglement characteristic is discussed by employing the quantum Rabi oscillations and the concurrence. An interesting conclusion is obtained, i.e., the magnetic fluxes Ф_x1 and Ф_x2 through the superconducting loops can adjust the entanglement degree between the qubits.     

Non-Markovian dynamics of a qubit in a reservoir: different solutions of non-Markovian master equation

We present a non-Markovian master equation for a qubit interacting with a general reservoir, which is derived according to the Nakajima-Zwanzig and the time convolutionless projection operator technique. The non-Markovian solutions and Markovian solution of dynamical decay of a qubit are compared. The results indicate the validity of non-Markovian approach in different coupling regimes and also show that the Markovian master equation may not precisely describe the dynamics of an open quantum system in some situation. The non-Markovian solutions may be effective for many qubits independently interacting with the heated reservoirs.     

Hybrid quantum circuit with a superconducting qubit coupled to a spin ensemble

We report the experimental realization of a hybrid quantum circuit combining a superconducting qubit and an ensemble of electronic spins. The qubit, of the transmon type, is coherently coupled to the spin ensemble consisting of nitrogen-vacancy centers in a diamond crystal via a frequency-tunable superconducting resonator acting as a quantum bus. Using this circuit, we prepare a superposition of the qubit states that we store into collective excitations of the spin ensemble and retrieve back into the qubit later on. These results constitute a proof of concept of spin-ensemble based quantum memory for superconducting qubits.     

Quantum jumps in Landau-Zener transitions in the dissipative dynamics of a superconducting qubit

The effect of noise on the populations of the levels of a qubit in individual experimental implementations has been studied by the quantum trajectory method. A transition to the average dynamics obtained by means of multiple measurements of the state of the qubit is analyzed. The developed method is applied to investigate the effect of noise on the interference pattern appearing in the amplitude spectroscopy of the qubit in a strong variable field owing to Landau-Zener transitions. The effect of the number of repeated measurements and the fluctuation of the phase of a pump pulse on the formation of the response of the qubit to the external field has been analyzed. This makes it possible to interpret recent experiments in terms of individual implementations and averaged dynamics.     

Effective dynamics for two-atom entanglement and quantum information processing by coupled cavity QED systems

We propose a scheme for deterministic generation of entanglement embodied by two L\Lambda -type atoms distributed in two coupled cavities. We study such a system in the dispersive atom-field interactions, where the dynamics of the system operates through the virtual population of both the atomic excited states and the photonic states in the cavities (plus the fiber). We verify the validity of the dynamics, and moreover, study the influences of the decoherence due to the spontaneous emission and photon leakage. We also apply the dynamics for realizing quantum state transfer and quantum phase gates.     

Induced entanglement revival and entanglement protection in a two qubit system

A system of two initially entangled qubits interacting with a bosonic environment is considered. The interaction induces a loss of the initial entanglement of the two qubits, and for specific initial states it causes entanglement sudden death. An investigation of the modifications on the entanglement dynamics by a single pulse control field, performed in the two qubit system, shows that the control field can not only protect entangled states against sudden death but also induce a revival of entanglement in the two qubit system.     

Locking entanglement with a single qubit

We study the loss of entanglement of a bipartite state subjected to discarding or measurement of one qubit. Examining behavior of different entanglement measures, we find that entanglement of formation, entanglement cost, logarithmic negativity, and one-way distillable entanglement are lockable measures in that they can decrease arbitrarily after measuring one qubit. We prove that any convex and asymptotically noncontinuous measure is lockable. As a consequence, all the convex-roof measures can be locked. The relative entropy of entanglement is shown to be a nonlockable measure.     

The entanglement dynamics of two coupled qubits in different environment

In this Letter, the entanglement dynamics of two interacting qubits in a common bath and in two independent baths, at finite and zero temperature are investigated. Entanglement sudden death (ESD) and entanglement sudden birth (ESB) are observed when the two qubits are embedded in two independent baths at finite temperature. At zero temperature, the entanglement of the two qubits may evolve to a steady state with non-zero value when the two qubits are embedded in a common bath, the entanglement sudden birth does not occur when the qubits are embedded in two independent baths.     

Engineering correlation and entanglement dynamics in spin systems

We show that correlation and entanglement dynamics of spin systems can be precisely controlled and engineered using only a small number of external physical control parameters. We first point out that the correlation dynamics of such systems can be understood in terms of spin-wave propagation, giving a simple physical explanation of the behavior seen in a number of recent works. We then extend this picture to more realistic translationally invariant systems prepared in product states. Since spin waves propagate according to a system's dispersion relation which typically depends on external physical parameters, this insight provides a convenient way to understand how dynamics can be controlled. We demonstrate these ideas in a simple example system, showing that correlations can be made to propagate in well-defined packets whose speed can be engineered in advance, controlled during the evolution, or even stopped altogether.     

Coupling bosonic modes with a qubit: entanglement dynamics at zero and finite temperatures

We consider a system of two iso-spectral bosonic modes coupled with a single two-level systems i.e., a qubit. The dynamics is described by a mode-symmetric two-modes Jaynes–Cummings Hamiltonian. The entanglement, induced between the two bosonic modes, is analyzed and quantified by negativity. We computed the time evolution of negativity starting from an initial thermal state of the bosonic sector for both zero and finite temperature. We also studied the entangling power of the interaction as a function of mode-qubit detuning and its resilience against temperature increase. Finally a two-qubit gate based on bosonic virtual subsystem is discussed.     

Decoherence dynamics of a charge qubit coupled to the noise bath

By virtue of the canonical quantization method, we present a quantization scheme for a charge qubit based on the superconducting quantum interference device (SQUID), taking the self-inductance of the loop into account. Under reasonable short-time approximation, we study the effect of decoherence in the ohmic case by employing the response function and the norm. It is confirmed that the decoherence time, which depends on the parameters of the circuit components, the coupling strength, and the temperature, can be as low as several picoseconds, so there is enough time to record the information.     

General monogamy inequality for bipartite qubit entanglement

We consider multipartite states of qubits and prove that their bipartite quantum entanglement, as quantified by the concurrence, satisfies a monogamy inequality conjectured by Coffman, Kundu, and Wootters. We relate this monogamy inequality to the concept of frustration of correlations in quantum spin systems.     

The superconducting transition in coupled linear chain systems

The superconducting transition and the critical fluctuations of a model system that can pass continuously from one-dimension to three-dimensions are investigated. The transition is brought about by a variable coupling between families of linear chains. First, a Bose lattice-gas is considered, and the Bose-Einstein transition temperature T_BE is calculated as a function of the coupling strength. Second, the temperature T_oz at which the fluctuations in the gap parameter equal the average gap parameter is calculated as a function of the coupling, and is found to behave in a similar way to T_BE. Both these temperatures go continuously to zero as the system becomes one-dimensional while T_c calculated in mean-field theory does not vanish in this limit.It is found that for coupling parameters believed to be characteristic of some superconductors possessing the A-15 crystal structure, such as Nb₃Sn, the system is essentially three-dimensional (3D) as far as superconducting properties are concerned; but critical fluctuations may be somewhat enhanced, in particular when the electronic density of states is not very large.