Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F46: Continuous Measurements and Noncommuting ObservablesFocus

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Sponsoring Units: GQI Chair: Alexander Korotkov, University of California at Riverside Room: 393 
Tuesday, March 14, 2017 11:15AM  11:51AM 
F46.00001: Dynamics of simultaneously measured noncommuting observables Invited Speaker: Shay HacohenGourgy In quantum mechanics, measurement restores a classical notion of reality via collapse of the wavefunction, which yields a precisely defined outcome. On the other hand, the Heisenberg uncertainty principle dictates that incompatible observables, such as position and momentum, cannot both take on arbitrarily precise values. But how does a wavefunction evolve when two such quantities are probed simultaneously, and how does the uncertainty principle dynamically inhibit precise measurement outcomes? We present a novel detection scheme that allows control over the measurement operators of multiple readout channels of a superconducting qubit. We will show how the uncertainty principle governs the dynamics of the state by enforcing a lower bound on the measurementinduced disturbance, inhibiting wavefunction collapse and consequently leading to persistent diffusion. We will also present the ramifications of this scheme in the context of quantum control and metrology applications. [Preview Abstract] 
Tuesday, March 14, 2017 11:51AM  12:03PM 
F46.00002: Correlators in simultaneous measurement of noncommuting qubit observables Juan Atalaya, Shay HacohenGourgy, Leigh S. Martin, Irfan Siddiqi, Alexander N. Korotkov We consider simultaneous continuous measurement of noncommuting qubit observables and analyze multitime correlators $\langle i_{\kappa_1}(t_1)\cdots i_{\kappa_N}(t_N )\rangle$ for output signals $i_\kappa(t)$ from the detectors. Both informational ("spooky") and phase backactions from cQEDtype measurements with phasesensitive amplifiers are taken into account. We find an excellent agreement between analytical results and experimental data for twotime correlators of the output signals from simultaneous measurement of qubit observables $\sigma_x$ and $\sigma_\varphi = \sigma_x\cos\varphi + \sigma_y\sin\varphi$. The correlators can be used to extract small deviations of experimental parameters, e.g., phase backaction and residual Rabi frequency. The multitime correlators are important in analysis of BaconShor error correction/detection codes, operated with continuous measurements. [Preview Abstract] 
Tuesday, March 14, 2017 12:03PM  12:15PM 
F46.00003: Catching a quantum jump in midflight Z.K. Minev, S.O. Mundhada, E. ZalysGeller, S. Shankar, P. Rheinhold, L. Frunzio, R.J. Schoelkopf, M. Mirrahimi, M.H. Devoret Quantum jumps provide a fundamental manifestation of the interplay between coherent dynamics and strong continuous measurements. Interestingly, the modern theoretical vantage point of quantum trajectories (Carmichael, 1993) suggests that the jump is not instantaneous, but rather smooth, coherent, and under the right conditions may present a deterministic character. We revisit the original observation of quantum jumps in a Vtype, threelevel atom (Berquist, 1986; Sauter, 1986), in order to ``deterministically'' catch the jump in midflight. We have designed and operated a Vtype superconducting artificial atom with the 3 needed levels: G (for Ground), B (for Bright), and D (for Dark). The atom is coupled to a continuously monitored microwave mode that can distinguish B from the manifold formed by G and D, but without distinguishing G from D. We will present preliminary results showing how this experiment can be realized. [Preview Abstract] 
Tuesday, March 14, 2017 12:15PM  12:27PM 
F46.00004: Suppressing systematic errors in weak measurements Shengshi Pang, Jose Raul Gonzalez Alonso, Todd A. Brun, Andrew N. Jordan Noises are inevitable in real environments, and quantum systems are vulnerable to them, so it is vitally important to protect the quantum systems from noises in quantum information tasks and correct the errors in quantum systems caused by noises. In this talk, we focus on the systematic error of weakmeasurementbased quantum metrology under decoherence. We obtain the systematic error of maximum likelihood estimation in general to the firstorder approximation of a small deviation in the probability distribution, and investigate the robustness of both standard weak measurement and postselected weak measurement against systematic error. We find that, the systematic error of a weak measurement with the probe undergoing decoherence can be significantly reduced by postselecting the system with a large weak value. This indicates another advantage of weak value amplification in enhancing the performance of parameter estimation by weak measurements. The results are illustrated by an exact numerical simulation of decoherence arising from the coupling of the probe to a bosonic mode, and compared to the firstorder analytical results we obtain. [Preview Abstract] 
Tuesday, March 14, 2017 12:27PM  12:39PM 
F46.00005: Multiple Most Likely Paths in Diffusive Quantum Trajectories of PureState Qubits Philippe Lewalle, Areeya Chantasri, Andrew Jordan We examine mostlikely paths (MLPs) in the diffusive quantum trajectories for continuouslymonitored purestate qubits, obtained as extrema of a stochastic path integral. MLPs are expressed as solutions to a Hamiltonian dynamical system. By considering the evolution of the Lagrange Manifold in the MLP phase space, we locate multiplepath solutions (multipaths), mathematically analogous to optical caustics. We explicitly show how multipaths arise in two sample systems, including a qubit subject to Rabi drive and continuous monitoring of one observable, and a qubit subject to simultaneous measurement of two noncommuting observables[1]. The MLP phasespaces for these systems include multipaths generated by different winding numbers about the Bloch sphere, and multipaths within elliptic periodic islands. Experimental confirmation of multipaths in a continuously monitored fluorescing qubit system was recently found in collaboration with the Murch group at Washington University. This work is an important step towards understanding how to use our MLP formalism to predict the onset of dynamical instabilities in continuouslymonitored quantum systems. \newline $[1]$ S. HacohenGourgy, L. S. Martin, E. Flurin, V. V. Ramasesh, K. B. Whaley, I. Siddiqi. Nature \textbf{538}, 491494 (2016). [Preview Abstract] 
Tuesday, March 14, 2017 12:39PM  1:15PM 
F46.00006: Information and energy transfer via fluorescence in superconducting circuits Invited Speaker: Benjamin Huard Light emitted via fluorescence is associated with matter decaying in energy. This light can be viewed both as an energy carrier and as a probe that carries information about the state of its emitter. When this information is lost, the fragile quantum properties of the emitter are destroyed, resulting in decoherence. This talk will present a series of experiments that probe and use the information and the energy conveyed by fluorescence in a superconducting circuit. On the information side, we have realized an experiment that reconstructs the diffusive quantum trajectories of a superconducting qubit based on the heterodyne measurement of its fluorescence field. Going a step further, we have used that information to stabilize any state of the qubit by feedback. On the energy side, we were able to demonstrate directly how the emitted power by a qubit depends on its quantum state, hereby giving a textbook demonstration of the difference between spontaneous and stimulated emission. In the same lines, I will also present a new kind of router that is able to transfer power between two ports, and whose transfer direction depends solely on the phase of a quantum superposition. [Preview Abstract] 
Tuesday, March 14, 2017 1:15PM  1:27PM 
F46.00007: Resonance fluorescence trajectories in superconducting qubit Mahdi Naghiloo, Dian Tan, Patrick Harrington, Philippe Lewalle, Andrew Jordan, Kater Murch We employ phasesensitive amplification to perform homodyne detection of the resonance fluorescence from a driven superconducting artificial atom. Entanglement between the emitter and its fluorescence allows us to track the individual quantum state trajectories of the emitter. We analyze the ensemble properties of these trajectories by considering paths that connect specific initial and final states. By applying a stochastic path integral formalism, we calculate equations of motion for the most likely path between two quantum states and compare these predicted paths to experimental data. Drawing on the mathematical similarity between the action formalism of the most likely quantum paths and ray optics, we study the emergence of caustics in quantum trajectoriessituations where multiple extrema in the stochastic action occur. We observe such multiple most likely paths in experimental data and find these paths to be in reasonable quantitative agreement with theoretical calculations. [Preview Abstract] 
Tuesday, March 14, 2017 1:27PM  1:39PM 
F46.00008: Simultaneous continuous measurement of noncommuting observables and correlation in qubit trajectories Areeya Chantasri, Andrew Jordan We consider the continuous quantum measurement of two or more noncommuting observables of a single qubit. Examples are presented for the measurement of two observables which can be mapped to two measurement axes on the Bloch sphere; a special case being the measurement along the X and Z bases. The qubit dynamics is described by the stochastic master equations which include the effect of decoherence and measurement inefficiencies. We investigate the qubit trajectories, their most likely paths, and their correlation functions using the stochastic path integral formalism[1]. The correlation functions in qubit trajectories can be derived exactly for a special case and perturbatively for general cases. The theoretical predictions are compared with numerical simulations, as well as with trajectory data from the transmon superconducting qubit experiments[2]. [1] Phys. Rev. A {\bf 92}, 032125 (2015) [2] Nature, {\bf 538}, 491 (2016) [Preview Abstract] 
Tuesday, March 14, 2017 1:39PM  1:51PM 
F46.00009: Continuous Quantum Measurement of Noncommunting Observables Maicol Ochoa, Wolfgang Belzig, Abraham Nitzan Quantum measurement techniques can potentially provide feedback control over quantum systems. In this work we explore the possibility of obtaining simultaneous information on two noncommuting observables defining appropriate Kraus operators acting on a quantum system. We explore several options in the form of such operators and specialize to the case of simultaneous measurement of position and momentum observables. The result of weak and strong measurements are interpolated and continuousintime measurement of these observables is also described. This setting departs from previous approaches in which it does not make assumptions on the nature of the measuring devices and concentrates on the mathematical form and the properties of the Kraus operator. [Preview Abstract] 
Tuesday, March 14, 2017 1:51PM  2:03PM 
F46.00010: BaconShor code with continuous measurement of noncommuting operators Alexander N. Korotkov, Juan Atalaya, Mohammad Bahrami, Leonid P. Pryadko We analyze the fourqubit BaconShor code with simultaneous continuous measurement of noncommuting gauge operators. Error syndrome in this case is monitored via timeaveraged crosscorrelators of the output signals. We find the logical error rate and the termination rate for this quantum error detecting code for several models of decoherence. The code operation is comparable to that of the fourqubit BaconShor code using conventional projective measurements. An advantage of the continuousmeasurement implementation is the absence of time dependence in the code operation, with passive continuous monitoring of the error syndrome. [Preview Abstract] 
Tuesday, March 14, 2017 2:03PM  2:15PM 
F46.00011: Continuoustime quantum walks as an efficient multiqubit Toffoli gate Washma Anwar, Dmitry Solenov Nonlocal higherenergy auxiliary states have been successfully used to entangle pairs of qubits in different quantum computing systems. Typically a longerspan nonlocal state or sequential application of fewqubit entangling gates are needed to produce a nontrivial multiqubit gate. In many cases a single nonlocal state that spans over the entire system is difficult to use due to spectral crowding or impossible to have. At the same time, many multiqubit systems can naturally develop multiple nonlocal higherenergy states that span over few qubits each. We show that continuous time quantum walks can be used to address this problem by involving multiple such states to perform local and entangling operations concurrently and efficiently on many qubits. This introduces an alternative approach to multiqubit gate compression based on available physical resources. We will present analytical and numerical results that demonstrate performance of such gates, focusing on multiqubit Toffoli gate operations. [Preview Abstract] 
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