Electromagnetically induced transparency in superconducting quantum circuits : Effects of decoherence, tunneling and multi-level cross-talk

We explore theoretically electromagnetically-induced transparency (EIT) in a superconducting quantum circuit (SQC). The system is a persistent-current flux qubit biased in a $\Lambda$ configuration. Previously [Phys. Rev. Lett. 93, 087003 (2004)], we showed that an ideally-prepared EIT system provides a sensitive means to probe decoherence. Here, we extend this work by exploring the effects of imperfect dark-state preparation and specific decoherence mechanisms (population loss via tunneling, pure dephasing, and incoherent population exchange). We find an initial, rapid population loss from the $\Lambda$ system for an imperfectly prepared dark state. This is followed by a slower population loss due to both the detuning of the microwave fields from the EIT resonance and the existing decoherence mechanisms. We find analytic expressions for the slow loss rate, with coefficients that depend on the particular decoherence mechanisms, thereby providing a means to probe, identify, and quantify various sources of decoherence with EIT. We go beyond the rotating wave approximation to consider how strong microwave fields can induce additional off-resonant transitions in the SQC, and we show how these effects can be mitigated by compensation of the resulting AC Stark shifts

Test of Bell's Inequality using the Spin Filter Effect in Ferromagnetic Semiconductor Micro-structures

A theoretical proposal for testing Bell's inequality in mesoscopic systems is presented. We show that the entanglement of two electron spins can be detected in the spin filter effect in the mesoscopic semiconductor / ferromagnetic semiconductor / semiconductor junction. The current-current correlation function is calculated by use of the quantum scattering theory and we compare it with the local hidden variable theory. We also discuss the influence of an imperfect spin filter and derive the condition to see the violation of Bell's inequality experimentally.Comment: 6 pages, 4 figures, submitted to J. Phys. Soc. Jp

Electron Spins in Artificial Atoms and Molecules for Quantum Computing

Achieving control over the electron spin in quantum dots (artificial atoms) or real atoms promises access to new technologies in conventional and in quantum information processing. Here we review our proposal for quantum computing with spins of electrons confined to quantum dots. We discuss the basic requirements for implementing spin-qubits, and describe a complete set of quantum gates for single- and two-qubit operations. We show how a quantum dot attached to leads can be used for spin filtering and spin read-out, and as a spin-memory device. Finally, we focus on the experimental characterization of the quantum dot systems, and discuss transport properties of a double-dot and show how Kondo correlations can be used to measure the Heisenberg exchange interaction between the spins of two dots.Comment: 13 pages, 8 figures, Invited Review (Semiconductor Spintronics, Special Issue of SST

Dynamical Generation of Noiseless Quantum Subsystems

We present control schemes for open quantum systems that combine decoupling and universal control methods with coding procedures. By exploiting a general algebraic approach, we show how appropriate encodings of quantum states result in obtaining universal control over dynamically-generated noise-protected subsystems with limited control resources. In particular, we provide an efficient scheme for performing universal encoded quantum computation in a wide class of systems subjected to linear non-Markovian quantum noise and supporting Heisenberg-type internal Hamiltonians.Comment: 4 pages, no figures; REVTeX styl

Electrical current noise of a beam splitter as a test of spin-entanglement

We investigate the spin entanglement in the superconductor-quantum dot system proposed by Recher, Sukhorukov and Loss, coupling it to an electronic beam-splitter. The superconductor-quantum dot entangler and the beam-splitter are treated within a unified framework and the entanglement is detected via current correlations. The state emitted by the entangler is found to be a linear superposition of non-local spin-singlets at different energies, a spin-entangled two-particle wavepacket. Colliding the two electrons in the beam-splitter, the singlet spin-state gives rise to a bunching behavior, detectable via the current correlators. The amount of bunching depends on the relative positions of the single particle levels in the quantum dots and the scattering amplitudes of the beam-splitter. The singlet spin entanglement, insensitive to orbital dephasing but suppressed by spin dephasing, is conveniently quantified via the Fano factors. It is found that the entanglement-dependent contribution to the Fano factor is of the same magnitude as the non-entangled, making an experimental detection feasible. A detailed comparison between the current correlations of the non-local spin-singlet state and other states, possibly emitted by the entangler, is performed. This provides conditions for an unambiguous identification of the non-local singlet spin entanglement.Comment: 13 pages, 8 figures, section on quantification of entanglement adde

Effect of quantum fluctuations on structural phase transitions in SrTiO_3 and BaTiO_3

Using path-integral Monte Carol simulations and an ab initio effective Hamiltonian, we study the effects of quantum fluctuations on structural phase transitions in the cubic perovskite compounds SrTiO3 and BaTiO3. We find quantum fluctuations affect ferroelectric (FE) transitions more strongly than antiferrodistortive (AFD) ones, even though the effective mass of a single FE local mode is larger. For SrTiO3 we find that the quantum fluctuations suppress the FE transition completely, and reduce the AFD transition temperature from 130K to 110K. For BaTiO3, quantum fluctuations do not affect the order of the transition, but do reduce the transition temperature by 35-50 K. The implications of the calculations are discussed.Comment: Revtex (preprint style, 14 pages) + 2 postscript figures. A version in two-column article style with embedded figures is available at http://electron.rutgers.edu/~dhv/preprints/index.html#wz_qs

Quantum trajectories for the realistic measurement of a solid-state charge qubit

We present a new model for the continuous measurement of a coupled quantum dot charge qubit. We model the effects of a realistic measurement, namely adding noise to, and filtering, the current through the detector. This is achieved by embedding the detector in an equivalent circuit for measurement. Our aim is to describe the evolution of the qubit state conditioned on the macroscopic output of the external circuit. We achieve this by generalizing a recently developed quantum trajectory theory for realistic photodetectors [P. Warszawski, H. M. Wiseman and H. Mabuchi, Phys. Rev. A_65_ 023802 (2002)] to treat solid-state detectors. This yields stochastic equations whose (numerical) solutions are the ``realistic quantum trajectories'' of the conditioned qubit state. We derive our general theory in the context of a low transparency quantum point contact. Areas of application for our theory and its relation to previous work are discussed.Comment: 7 pages, 2 figures. Shorter, significantly modified, updated versio

Semiclassical theory of spin-polarized shot noise in mesoscopic diffusive conductors

We study fluctuations of spin-polarized currents in a three-terminal spin-valve system consisting of a diffusive normal metal wire connected by tunnel junctions to three ferromagnetic terminals. Based on a spin-dependent Boltzmann-Langevin equation, we develop a semiclassical theory of charge and spin currents and the correlations of the currents fluctuations. In the three terminal system, we show that current fluctuations are strongly affected by the spin-flip scattering in the normal metal and the spin polarizations of the terminals, which may point in different directions. We analyze the dependence of the shot noise and the cross-correlations on the spin-flip scattering rate in the full range of the spin polarizations and for different magnetic configurations. Our result demonstrate that noise measurements in multi-terminal devices allow to determine the spin-flip scattering rate by changing the polarizations of ferromagnetic terminals.Comment: 12 pages, 5 figure

Protecting Quantum Information Encoded in Decoherence Free States Against Exchange Errors

The exchange interaction between identical qubits in a quantum information processor gives rise to unitary two-qubit errors. It is shown here that decoherence free subspaces (DFSs) for collective decoherence undergo Pauli errors under exchange, which however do not take the decoherence free states outside of the DFS. In order to protect DFSs against these errors it is sufficient to employ a recently proposed concatenated DFS-quantum error correcting code scheme [D.A. Lidar, D. Bacon and K.B. Whaley, Phys. Rev. Lett. {\bf 82}, 4556 (1999)].Comment: 7 pages, no figures. Discussion in section V.A. significantly expanded. Several small changes. Two authors adde