Eyob A. Sete

Correlated spontaneous emission on the Danube

We consider collective spontaneous emission from an ensemble of N identical two-level atoms prepared by absorption of a single photon–a.k.a. single photon Dicke superradiance. We discuss dynamical properties of superradiance for small (R ≪ λ) and large (R ≫ λ) atomic cloud. Moreover, we address the effects of virtual processes on collective decay rate and Lamb shift. It turns out that virtual processes lead to relatively small yet interesting effects on the time evolution of a rapidly decaying state. However, such processes substantially modify the dynamics of trapped states by bringing in new channels of decay.

Light-to-matter entanglement transfer in optomechanics

We analyze a scheme to entangle the movable mirrors of two spatially separated nanoresonators via a broadband squeezed light. We show that it is possible to transfer the Einstein–Podolsky–Rosen-type continuous-variable entanglement from the squeezed light to the mechanical motion of the movable mirrors. An optimal entanglement transfer is achieved when the nanoresonators are tuned at resonance with the vibrational frequencies of the movable mirrors and when strong optomechanical coupling is attained. Stationary entanglement of the states of the movable mirrors as strong as that of the input squeezed light can be obtained for sufficiently large optomechanical cooperativity, achievable in currently available optomechanical systems. The scheme can be used to implement long-distance quantum-state transfer provided that the squeezed light interacts with the nanoresonators.

Strong squeezing and robust entanglement in cavity electromechanics

(Received 29 October 2013; published 29 January 2014)We investigate nonlinear effects in an electromechanical system consisting of a superconducting charge qubitcoupled to a transmission line resonator and a nanomechanical oscillator, which in turn is coupled to anothertransmission line resonator. The nonlinearities induced by the superconducting qubit and the optomechanicalcoupling play an important role in creating optomechanical entanglement as well as the squeezing of thetransmitted microwave field. We show that strong squeezing of the microwave field and robust optomechanicalentanglementcanbeachievedinthepresenceofmoderatethermaldecoherenceofthemechanicalmode.Wealsodiscusstheeffectofthecouplingofthesuperconducting qubittothenanomechanical oscillator onthebistabilitybehavior of the mean photon number.DOI: 10.1103/PhysRevA.89.013841 PACS number(s): 42

Using quantum coherence to generate gain in the XUV and X-ray: gain-swept superradiance and lasing without inversion

It was shown some time ago that when the excitation of an ensemble of two-level atoms is swept in the direction of lasing, so that atoms are prepared in the excited state just as the radiation from previously excited atoms reaches them, the resulting laser amplifier is “highly anomalous” and yields superradiant emission without population inversion. We here show that transient gain in a three-level system has common features with Dicke superradiance and can yield strong extreme ultraviolet lasing in, for example, He atoms (at 58 nm) or He-like ions such as B3+ (at 6.1 nm).

Interaction of a quantum well with squeezed light: Quantum-statistical properties

We investigate the quantum statistical properties of the light emitted by a quantum well interacting with squeezed light from a degenerate subthreshold optical parametric oscillator. We obtain analytical solutions for the pertinent quantum Langevin equations in the strong-coupling and low-excitation regimes. Using these solutions we calculate the intensity spectrum, autocorrelation function, and quadrature squeezing for the fluorescent light. We show that the fluorescent light exhibits bunching and quadrature squeezing. We also show that the squeezed light leads to narrowing of the width of the spectrum of the fluorescent light.

A functional architecture for scalable quantum computing

Quantum computing devices based on superconducting quantum circuits have rapidly developed in the last few years. The building blocks-superconducting qubits, quantum-limited amplifiers, and two-qubit gates-have been demonstrated by several groups. Small prototype quantum processor systems have been implemented with performance adequate to demonstrate quantum chemistry simulations, optimization algorithms, and enable experimental tests of quantum error correction schemes. A major bottleneck in the effort to develop larger systems is the need for a scalable functional architecture that combines all the core building blocks in a single, scalable technology. We describe such a functional architecture, based on a planar lattice of transmon and fluxonium qubits, parametric amplifiers, and a novel fast DC controlled two-qubit gate.

Quantum theory of a bandpass Purcell filter for qubit readout

© 2015 American Physical Society. ©2015 American Physical Society. The measurement fidelity of superconducting transmon and Xmon qubits is partially limited by the qubit energy relaxation through the resonator into the transmission line, which is also known as the Purcell effect. One way to suppress this energy relaxation is to employ a filter which impedes microwave propagation at the qubit frequency. We present semiclassical and quantum analyses for the bandpass Purcell filter realized by E. Jeffrey et al. [Phys. Rev. Lett. 112, 190504 (2014)PRLTAO0031-900710.1103/PhysRevLett.112.190504]. For typical experimental parameters, the bandpass filter suppresses the qubit relaxation rate by up to two orders of magnitude while maintaining the same measurement rate. We also show that in the presence of a microwave drive the qubit relaxation rate further decreases with increasing drive strength.

Purcell effect with microwave drive: Suppression of qubit relaxation rate

We analyze the Purcell relaxation rate of a superconducting qubit coupled to a resonator, which is coupled to a transmission line and pumped by an external microwave drive. Considering the typical regime of the qubit measurement, we focus on the case when the qubit frequency is significantly detuned from the resonator frequency. Surprisingly, the Purcell rate decreases when the strength of the microwave drive is increased. This suppression becomes significant in the nonlinear regime. In the presence of the microwave drive, the loss of photons to the transmission line also causes excitation of the qubit; however, the excitation rate is typically much smaller than the relaxation rate. Our analysis also applies to a more general case of a two-level quantum system coupled to a cavity.

Catch-Disperse-Release Readout for Superconducting Qubits

We analyze a single-shot readout for superconducting qubits via the controlled catch, dispersion, and release of a microwave field. A tunable coupler is used to decouple the microwave resonator from the transmission line during the dispersive qubit-resonator interaction, thus circumventing damping from the Purcell effect. We show that, if the qubit frequency tuning is sufficiently adiabatic, a fast high-fidelity qubit readout is possible, even in the strongly nonlinear dispersive regime. Interestingly, the Jaynes-Cummings nonlinearity leads to the quadrature squeezing of the resonator field below the standard quantum limit, resulting in a significant decrease of the measurement error.

Quantum interference in timed Dicke basis and its effect on bipartite entanglement

We analyze the effect of the position-dependent excitation phase on the properties of entanglement between two qubits formed in atomic systems. We show that the excitation phase induces a vacuum-mediated quantum interference in the system that affects the dynamical behavior of entanglement between the qubits. It is also found that the quantum interference leads to a coherent population transfer between the symmetric and antisymmetric states that can considerably modify the dynamics of two-qubit entanglement and can even prevent finite-time disentanglement (sudden death) under certain conditions.