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We propose a scheme to realize quantum networking of superconducting qubits based on the optomechanical interface. The superconducting qubits interact with the microwave photons, which then couple to the optical photons through the optomechanical interface. The interface generates a quantum link between superconducting qubits and optical flying qubits with tunable pulse shapes and carrier frequencies, enabling transmission of quantum information to other superconducting or atomic qubits. We show that the scheme works under realistic experimental conditions and it also provides a way for fast initialization of the superconducting qubits under 1 K instead of an operation temperature of 20 mK. [ABSTRACT FROM AUTHOR]

We investigate the non-Markovian dynamics of quantum correlation between two initially entangled nitrogen-vacancy centers (NVCs) embedded in photonic-crystal cavities (PCCs). We find that a finite quantum correlation is preserved even asymptotically when the transition frequency of the NVC is within the band gap of the FCC, which is quantitatively different from the result of approaching zero under the Bom-Markovian approximation. In addition, once the transition frequency of NVC is far beyond the band gap of the PCC, the quantum correlation initially prepared in NVC will be fully transferred to the reservoirs in the long-time limit. Our result reveals that the interplay between the non-Markovian effect of the structured reservoirs and the existence of emitter-field bound state plays an essential role in such quantum correlation preservation. This feature may open new perspectives for devising active decoherence-immune solid-state optical devices. [ABSTRACT FROM AUTHOR]

We investigate the dissipative dynamics of a hybrid system consisting of two inductively coupled superconducting flux qubits (FQs), each of which couples magnetically with a nitrogen-vacancy-center spin ensemble (NVE). The system displays a series of damped oscillations under various experimental situations, where we obtain analytically the time-dependent populations associated with arbitrary values of the FQ-FQ inductively coupling strength g and the FQ-NVE magnetically coupling strength G in the one-excitation case. Our results show that a reliable high-fidelity quantum state transfer between the two distant NVEs can be realized by modulating the parameters g and G. Furthermore, we also present a potentially practical idea to entangle the two separate NVEs, which is within reach due to the currently available technology. [ABSTRACT FROM AUTHOR]

We present a scheme to engineer a two-mode squeezed state of effective bosonic modes realized by collective excitations of two distant nitrogen-vacancy-center ensembles (NVEs) coupled to separated transmission line resonators (TLRs), which are interconnected by a current-biased Josephson-junction superconducting qubit. By making use of the engineered NVE-TLR magnetic coupling with Raman transition between the ground sublevéis of the NVEs, we may manipulate the artificial reservoir by tuning the external driving fields. The TLR decay induces an artificial reservoir, which can drive the system to the desired entangled squeezed states. Our idea provides a scalable way to a NVE-based continuous-variable quantum-information processing, which is close to being achievable with currently available technology. [ABSTRACT FROM AUTHOR]

We investigate dynamics of a laser-driven and dissipative system consisting of two nitrogen-vacancy (N-V) centers embedded in two spatially separated single-mode nanocavities in a planar photonic crystal (PC). Spontaneous emission from the excited states of the N-V centers can be effectively suppressed by virtue of the Raman transition in the dispersive regime. The system displays a series of damped oscillations under various experimental situations, where we solve the time-dependent Schr6dinger equation analytically for arbitrary values of the hopping and PC-N- V coupling strengths. In particular, our results indicate that some special values should be taken for the hopping strength if we hope to have high-fidelity quantum state transfer between the two distant N-V centers. We have also analyzed the relevant entanglement dynamics in the presence of decoherence. The experimental feasibility and challenge are justified using currently available technology. [ABSTRACT FROM AUTHOR]

We propose a potentially practical scheme for creating entanglement between two distant nitrogen-vacancy-center (N-V) spin ensembles using a current-biased Josephson junction superconducting qubit and transmission line resonators. The idea provides a scalable way to an N-V ensemble-based quantum network, which is close to the reach with currently available technology. [ABSTRACT FROM AUTHOR]

Polarized light microscopy using path-entangled N-photon states (i.e., the N00N states) has been demonstrated to surpass the shot-noise limit at very low light illumination. However, the microscopy images suffer from divergence of phase sensitivity, which inevitably reduces the image quality. Here we show that due to experimental imperfections, such a singularity also takes place in the microscopy that uses twin-Fock states of light for illumination. We propose two schemes to completely eliminate this singularity: (i) locking the phase shift sensed by the beams at the optimal working point using a spatially dependent offset phase; (ii) a combination of two binary-outcome photon counting measurements, one with a fixed offset phase and the other without any offset phase. Our observations remain valid for any kind of binary-outcome measurement and may open the way for quantum-enhanced microscopy with high N photon states [ABSTRACT FROM AUTHOR]

The quantum speed limit (QSL) time for open system characterizes the most efficient response of the system to the environmental influences. Previous results showed that the non-Markovianity governs the quantum speedup. Via studying the dynamics of a dissipative two-level system, we reveal that the non-Markovian effect is only the dynamical way of the quantum speedup, while the formation of the system-environment bound states is the essential reason for the quantum speedup. Our attribution of the quantum speedup to the energy-spectrum character can supply another vital path for experiments when the quantum speedup shows up without any dynamical calculations. The potential experimental observation of our quantum speedup mechanism in the circuit QED system is discussed. Our results may be of both theoretical and experimental interest in exploring the ultimate QSL in realistic environments, and may open new perspectives for devising active quantum speedup devices. [ABSTRACT FROM AUTHOR]

Optimal measurement scheme with an efficient data processing is important in quantum-enhanced interferometry. Here we prove that for a general binary-outcome measurement, the simplest data processing based on inverting the average signal can saturate the Cramér-Rao bound. This idea is illustrated by binary-outcome homodyne detection, even-odd photon counting (i.e., parity detection), and zero-nonzero photon counting that have achieved super-resolved interferometric fringe and shot-noise limited sensitivity in coherent-light Mach-Zehnder interferometer. The roles of phase diffusion are investigated in these binary-outcome measurements. We find that the diffusion degrades the fringe resolution and the achievable phase sensitivity. Our analytical results confirm that the zero-nonzero counting can produce a slightly better sensitivity than that of the parity detection, as demonstrated in a recent experiment. [ABSTRACT FROM AUTHOR]

Memory (non-Markovian) effect is found to be able to accelerate quantum evolution [S. Deffner and E. Lutz, Phys. Rev. Lett. 111, 010402 (2013)]. In this work, for an atom in a structured reservoir, we show that the mechanism for the speedup is not only related to non-Markovianity but also to the population of excited states under a given driving time. In other words, it is the competition between non-Markovianity and population of excited states that ultimately determines the acceleration of quantum evolution in memory environment. A potential experimental realization for verifying the above phenomena is discussed by using a nitrogen-vacancy (N-V) center embedded in a planar photonic crystal cavity (PCC) under the current experimental conditions. [ABSTRACT FROM AUTHOR]

We investigate the performance of entangled coherent states for quantum-enhanced phase estimation. An exact analytical expression of quantum Fisher information is derived to show the role of photon losses on the ultimate phase sensitivity. We find a transition of the sensitivity from the Heisenberg scaling to the classical scaling due to quantum decoherence of the photon state. This quantum-classical transition is uniquely determined by the number of photons being lost, instead of the number of incident photons or the photon loss rate alone. Our results also reveal that a crossover of the sensitivity between the entangled coherent state and the NOON state can occur even for very small photon loss rate. [ABSTRACT FROM AUTHOR]

Squeezed-state interferometry plays an important role in quantum-enhanced optical phase estimation, as it allows the estimation precision to be improved up to the Heisenberg limit by using ideal photon-number-resolving detectors at the output ports. Here we show that for each individual N-photon component of the phase-matched coherent ⊗ squeezed vacuum input state, the classical Fisher information always saturates the quantum Fisher information. Moreover, the total Fisher information is the sum of the contributions from each individual N-photon component, where the largest N is limited by the finite number resolution of available photon counters. Based on this observation, we provide an approximate analytical formula that quantifies the amount of lost information due to the finite photon number resolution; e.g., given the mean photon number ¯ n in the input state, over 96% of the Heisenberg limit can be achieved with the number resolution larger than 5n. [ABSTRACT FROM AUTHOR]

We extend the non-Hermitian one-dimensional quantum walk model [Phys. Rev. Lett. 102, 065703 (2009)] by taking the dephasing effect into account. We prove that the feature of topological transition does not change even when dephasing between the sites within units is present. The potential experimental observation of our theoretical results in the circuit QED system consisting of superconducting qubit coupled to a superconducting resonator mode is discussed and numerically simulated. The results clearly show a topological transition in quantum walk and display the robustness of such a system to the decay and dephasing of qubits. We also discuss how to extend this model to higher dimension in the circuit QED system. [ABSTRACT FROM AUTHOR]

We consider a mechanical resonator made of diamond, which contains a nitrogen-vacancy center (NV center) locating at the end of the oscillator. A second-order magnetic gradient is applied and induces coupling between mechanical modes and the NV center. By applying a proper external magnetic field, the energy difference between NV center electron spin levels can be tuned to match the energy difference between two mechanical modes a and b. A laser is used for continuously initializing the NV center electron spin. The mode a with lower frequency is driven by a thermal bath. We find that the temperature of the mode b is significantly cooled when the heating bath temperature is increased. We discuss the conditions that quantum regime cooling requires and confirm the results by numerical simulation. Finally we give the intuitive physical explanation of this unusual effect. [ABSTRACT FROM AUTHOR]