Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U15: Focus Session: Open Quantum Systems and Decoherence |
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Sponsoring Units: GQI Chair: Sergio Boixo, University of New Mexico Room: Morial Convention Center 207 |
Thursday, March 13, 2008 8:00AM - 8:36AM |
U15.00001: Coherence and control of single electron spins in quantum dots Invited Speaker: Lieven Vandersypen Following our earlier work on single-shot read-out and relaxation of a single spin~in a quantum dot, we now demonstrate coherent control of a single spin (detection is done using a second spin in a neighbouring dot). First, we manipulate the spin using conventional magnetic resonance. Next, we show that we can also rotate the spin using electric fields instead of magnetic fields. In both cases, 90 rotations can be realized in about 50 ns or less. We use these control techniques to probe decoherence of an isolated electron spin. The spin dephases in about 30 ns, due to the hyperfine interaction with the uncontrolled nuclear spin bath in the host material of the dot. However, since the nuclear spin dynamics is very slow, this dephasing can be largely reversed using a spin-echo pulse. Echo decay times of about 0.5 us are obtained at 70 mT. In parallel, we have started work on quantum dots in graphene, which is expected to offer superior coherence times. As a first step, we have succeeded in opening a bandgap in bilayer graphene, necessary for electrostatic confinement of carriers. \newline \newline F.H.L. Koppens et al., Nature 446, 56 (2006). \newline K.C. Nowack et al., Science Express, 1 Nov 2007. \newline F.H.L. Koppens et al., arXiv:0711.0479. \newline J.B. Oostinga, Nature Mat., in press. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U15.00002: Non-Markovian thermalization for a few qubit system Pedro Manrique, Ferney Rodriguez Non-Markovian dynamics in the thermalization process of a single and coupled-qubit systems are analyzed by means on an effective master equation. Memory effects are included in a time dependent relaxation constant which is obtained from a proper bosonic bath spectral function. For different initial states, the population and qubit coherences are studied as a function of the qubit-bath coupling strenghts and bath temperature. Clear signatures of non-exponential decays for the qubit density matrix elements are found in a short-time regime corresponding to the back action of the qubit system on the bath dynamics. In the case of realistic two-qubit systems, such as quantum dots, entanglement oscillations should be observable. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U15.00003: Decoherence of coupled electron spins via nuclear spin dynamics in quantum dots Wen Yang, Ren-bao Liu Decoherence of coupled electron spins due to electron-nuclear hyperfine interaction in double quantum dots is a major issue of solid-state quantum computation. Using an interacting nuclear spin bath model, we show theoretically that the exchange interaction between the two electron spins renormalizes the pair- flip excitation energy in the bath and modifies the non- Markovian bath dynamics, which in turn changes the electron singlet-triplet (S-T) decoherence arising from electron-nuclear entanglement. As the energy renormalization varies with the Overhauser field mismatch between the quantum dots, the S-T decoherence depends on the sampling of nuclear spin states from an ensemble, leading to the transition from super-exponential decoherence in single-sample dynamics to power-law decay under ensemble average,[1] in contrast with the sample-independent super-exponential decoherence of a single electron spin in one dot. \newline \newline [1] W. Yang and R. B. Liu, arXiv:0707.2529v1. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U15.00004: Rabi oscillations decay from interaction with dynamical spin environments V.V. Dobrovitski, A.E. Feiguin Studying decoherence of spins/qubits interacting with a spin bath is important for quantum computation, high-precision metrology, coherent spintronics. Measurements of the Rabi oscillations decay provide much information about the decoherence dynamics and properties of the spin bath in single-spin quantum dots, dopant spins in a solid-state matrix, etc. [1] Also, for a static bath, application of a large Rabi field suppresses decoherence, changing fast exponential decay into slow power-law one. [1] However, internal dynamics of spin environment is important in such systems as NV centers in diamonds, magnetic molecules, and rare-earth dopant spins in solid state, but decay of Rabi oscillations for dynamical spin bath has been little studied. We present a detailed theoretical investigation of Rabi oscillations decay for a dynamic spin bath, demonstrating new unusual decay regimes useful for characterization of the bath and decoherence suppression. \newline [1] F. H. L. Koppens et al, Phys. Rev. Lett. 99, 106803 (2007); S. Bertaina et al, Nature Nano. 2, 39 (2007); V. V. Dobrovitski et al, quant-ph/0112053. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U15.00005: Environmentally-Induced Rabi Oscillations and Decoherence in Phase Qubits Kaushik Mitra, Carlos Sa de Melo, Christopher Lobb We study decoherence effects in a dc SQUID phase qubit caused by an isolation circuit with a resonant frequency. The coupling between the SQUID phase qubit and its environment is modeled via the Caldeira-Leggett formulation of quantum dissipation/coherence, where the spectral density of the environment is related to the admittance of the isolation circuit. When the frequency of the qubit is at least two times larger than the resonance frequency of the isolation circuit, we find that the decoherence time of the qubit is two orders of magnitude larger than the typical ohmic regime, where the frequency of the qubit is much smaller than the resonance frequency of the isolation circuit. Lastly, we show that when the qubit frequency is on resonance with the isolation circuit, an oscillatory non-Markovian decay emerges, as the dc SQUID phase qubit and its environment self-generate Rabi oscillations of characteristic time scales shorter than the decoherence time. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U15.00006: Boundary Conditions for Open Rotating Quantum Systems Arthur Davidson The customary boundary conditions for a 1 D rotational system (e.g. a rigid rotor on a surface) are continuity of the complex wave function and its gradient. These four boundary conditions are sufficient if the potential energy satisfies rotational symmetry, but fail for non-rotational potentials. However, classical systems with a rotational coordinate and non- rotational potential are easily solved if the gradient of the potential, the force, is rotational. A solution is thus needed for Schroedinger's equation with a rotational coordinate and force, but non- rotational potential. Such solutions emerge if the boundary conditions are modified, allowing a discontinuous phase in the wave function related to the discontinuous potential energy. It will be shown that the modified boundary conditions are continuity of three real quantities: the probability density, the gradient of the probability density, and the probability current density. Moreover, with these boundary conditions and non-rotational potential, energy can flow both ways between the system and its environment. The discontinuous wave functions obey the new boundary conditions, but nonetheless are not generally superposable. A subset of the discontinuous wave functions can be superposed, however, yielding the usual result for angular momentum states. The non-superposable wave functions offer an alternate interpretation of the Schroedinger's cat paradox. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U15.00007: Non-Markovian Open Quantum Systems Cesar Rodriguez-Rosario, E.C.G. Sudarshan A generalized non-markovian master equation is derived from the dynamical map of systems initially correlated with their environment. We study the connection between the initial correlations and the non-markovian memory effects. The significance of not-complete positive maps in order to obtain a consistent theory of non-markovian quantum dynamics is discussed. Previous specific instances of non-markovian master equations are examined in this framework. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U15.00008: Finite representations of continuum environments Michael Zwolak Understanding dissipative and decohering processes is fundamental to the study of non-equilibrium systems and quantum computing, and such processes can even induce quantum phase transitions. A typical construction is to have a system connected to a continuum environment, which acts as the source of dissipation or decoherence, or as a reservoir of particles. If the connection is strong or the environment has long-range correlations in time, the system dynamics are not easily separated from the dynamics of the environment. To study this situation numerically, one option is to simulate both the system and environment. This is a viable option so long as an efficient finite representation of the environment can be constructed. We will discuss a procedure to construct finite representations based on the concept of two-site recurrence and increasing smoothness. For solvable cases of non-interacting bosons, the procedure gives an exponential reduction in the number of discrete modes necessary to achieve some given accuracy in a real-time simulation. We will also discuss application of this procedure to interacting systems. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U15.00009: Induced decoherence and entanglement by an interacting spin chain Pochung Chen, Cheng-Yan Lai, Jo-Tzu Hung, Chung-Yu Mou We study the reduced dynamics of a single or two qubits coupled to an interacting spin chain using time-dependent density matrix renormalization group (TD-DMRG) technology. By using TD-DMRG we can go beyond the uniform coupling central spin model and evaluate nonperturbatively the reduced dynamics even when the coupling between qubits and the chain is non-uniform. Furthermore, the qubit-bath interaction can be of Ising, XY, or Heisenberg type. This allows us to go beyond pure dephasing model. For single qubit we use Loschmidt echo to gauge the decoherence and investigate how the short time decay parameter and large time behavior are linked to the phase of spin chain. We use concurrence to quantify the (dis)-entanglement process of two qubits due to spin chain. We show that one can induced entanglement for an initially disentangled pair of qubit. The competition between induced decoherence and entanglement is discussed. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U15.00010: Temperature Exchange in a System of Two Harmonic Oscillators Antonia Chimonidou, George Sudarshan We study the process by which quantum correlations are created when an interaction Hamiltonian is repeatedly applied to a system of two harmonic oscillators for some characteristic time interval, under what we call the ``interact-refresh-repeat'' scheme. We show that, for the case where the oscillator frequencies are equal, the initial Maxwell-Boltzmann distributions of the uncoupled parts evolve to a new Maxwell-Boltzmann distribution through a series of transient Maxwell-Boltzmann distributions, or quasi-stationary, non-equilibrium states. Further, we discuss why the equilibrium reached when the two oscillator frequencies are unequal, is not a thermal one. More specifically, we show that the ratio of the harmonic oscillator temperatures at the new equilibrium state is completely determined by the ratio of the inverse harmonic oscillator frequencies. We conclude that the selection rules imposed by the interaction Hamiltonian override the expected statistical mechanical effects. All the calculations are exact and the results are obtained through an iterative process, without using perturbation theory. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U15.00011: Impact of classical forces and decoherence in three-terminal Aharonov-Bohm devices Elia Strambini, Vincenzo Piazza, Fabio Beltram, Giorgio Biasiol, Lucia Sorba Multi-terminal Aharonov-Bohm (AB) rings are ideal building blocks for quantum networks (QNs) thanks to their ability to transform input states into controlled coherent superpositions of output states. We report on experiments performed on three-terminal GaAs/AlGaAs AB devices and compare our results with a scattering-matrix model of our device including Lorentz forces and decoherence. Our devices were studied as a function of external magnetic field ($B)$ and gate voltage ($V_{g})$ down to a T=350 mK. The total output current from two terminals while applying a small bias to the third lead was found to be symmetric with respect to $B$ with clear AB oscillations showing abrupt phase jumps between 0 and $\pi $ at different values of $V_{g}$, reminiscent of the phase-rigidity constraint due to Onsager-Casimir relations. Surprisingly, the individual outputs show an almost linear dependence of the oscillation phase on the external electric field. We emphasize that a simple scattering-matrix approach does not explain the observed behavior and show how to extend this model in order to fully describe the observed phenomena. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U15.00012: The Behavior of Electronic Interferometers in the Non-Linear Regime. Izhar Neder, Eran Ginossar We investigate theoretically the behavior of the current oscillations in an electronic Mach-Zehnder interferometer (MZI) as a function of its source bias. Recently, The MZI interference visibility showed an unexplained lobe pattern behavior with a peculiar phase rigidity. Moreover, the effect did not depend on the MZI paths difference. We argue that these effects have a fundamental reason. A simple invariance argument leads to an additional interaction term that must be added to the non-interacting Hamiltonian. It causes correlations inside each of the two MZI arm, resulting in the electrons affecting each other's phase. An approximate solution shows that the interference visibility has a lobe pattern with applied bias with a period proportional to the average path length (and independent of the paths difference), together with a phase rigidity. [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U15.00013: What is quantum about quantum trajectory equations? Jay Gambetta, Howard Wiseman, Steve Jones, Eric Cavalcanti Quantum trajectory equations are stochastic equations for the state of an open quantum system conditioned on a monitoring i.e. a continuous-in-time measurement of a bath to which it is coupled. They are closely related to classical stochastic equations for classical probability distributions called filtering equations (e.g. the Kalman filter). Given this close relation, the question arises: what is quantum about quantum trajectory equations? In this talk I suggest that the answer lies in the ability of an experimenter to choose different monitoring schemes. Moreover, I propose that there is an experimental way to distinguish between cases where this choice does demonstrate the quantum nature of the noise, and those where it does not. [Preview Abstract] |
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