Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session E28: Quantum Control of Open and Tracked Quantum SystemsFocus

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Sponsoring Units: DQI Chair: Shruti Puri, Yale Univ Room: BCEC 161 
Tuesday, March 5, 2019 8:00AM  8:12AM 
E28.00001: Quantum dynamics and fluctuating Hamiltonians: controlling manybody decoherence Aurelia Chenu, Avadh Saxena, Adolfo Del Campo Beating decoherence and dissipation is a core problem to develop quantum computation and quantum technologies. While tailoring the environmentsystem coupling has been proposed as a solution, techniques to engineer the required manybody decoherence in the laboratory remain to be developed. 
Tuesday, March 5, 2019 8:12AM  8:24AM 
E28.00002: Gradientbased optimal control of open systems using quantum trajectories and automatic differentiation Mohamed Abdelhafez, David Schuster, Jens Koch We present a gradientbased optimalcontrol technique for open quantum systems that utilizes quantum trajectories. Using trajectories allows for optimizing open systems with less computational cost than the regular density matrix approaches. In addition, we propose an improved sampling algorithm to minimize the required number of trajectories needed per optimization iteration. Together with employing stochastic gradient descent techniques, this reduces the complexity of optimizing realistic open quantum systems. Our optimizer harnesses automatic differentiation to provide flexibility in optimization and to suit the different constraints and diverse parameter regimes of reallife experiments. The optimizer is utilized in a variety of applications to demonstrate how the use of quantum trajectories significantly reduces the computation complexity while achieving a multitude of simultaneous optimization targets. Demonstrated targets include high state transfer fidelities despite dissipation, and maximizing the readout fidelities of a qubit while maintaining the quantum nondemolition nature of the measurement and allowing for subsequent fast resonator reset. 
Tuesday, March 5, 2019 8:24AM  8:36AM 
E28.00003: Dissipative selfinterference and robustness of continuous errorcorrection to miscalibration Victor Albert, Kyungjoo Noh, Florentin Reiter We derive an effective equation of motion within the steadystate subspace of a large family of Markovian open systems (i.e., Lindbladians) subject to perturbations of their Hamiltonians and systembath couplings [1]. We derive a set of conditions under which competing dissipative processes destructively interfere, producing no dissipation within the steadystate subspace. Due to the mildness of the conditions, such destructive interference turns out to be much more generic than expected. For quantum errorcorrection, these effects imply that continuously errorcorrecting Lindbladians are robust to calibration errors, including miscalibrations consisting of operators undetectable by the code. A similar interference is present in more general systems if one implements a particular Hamiltonian drive, resulting in a coherent cancellation of dissipation. On the opposite extreme, instead of suppressing dissipation, we provide a simple implementation of universal Lindbladian simulation. 
Tuesday, March 5, 2019 8:36AM  8:48AM 
E28.00004: A Quantum Law of Requisite Variety Davide Girolami The Law of Requisite Variety was the first attempt to quantify the ability of a controller to shield informationprocessing systems against error sources. It was then rediscovered in classical control theory, complexity science and computational mechanics. I here extend it to the quantum domain, establishing informationtheoretic limits to the controllability of open quantum systems in terms of the resources available to the controller, quantum coherence and correlations. I also introduce a measure of controllability to capture the influence exerted on a system by a controlling device. I verify the result by implementing a control protocol in the IBM 5qubit chip ``ibmqx4''. 
Tuesday, March 5, 2019 8:48AM  9:00AM 
E28.00005: Conditions allowing error correction in driven qubits Robert Throckmorton We consider a qubit that is driven along its logical z axis, with noise along the z axis in the driving field Ω proportional to some function f(Ω), as well as noise along the logical x axis. We establish that whether or not errors due to both types of noise can be canceled out, even approximately, depends on the explicit functional form of f(Ω) by considering a power law form, f(Ω) ∝ Ω^{k}. In particular, we show that such cancellation is impossible for k = 0, 1, or any even integer. However, any other odd integer value of k besides 1 does permit cancellation; in fact, we show that both types of errors can be corrected with a sequence of four square pulses of equal duration. We provide sets of parameters that correct for errors for various rotations and evaluate the error, measured by the infidelity, for the corrected rotations versus the naive rotations. We also consider a train of four trapezoidal pulses, which take into account the fact that there will be, in real experimental systems, a finite rise time, again providing parameters for errorcorrected rotations that employ such pulse sequences. Our dynamical decoupling error correction scheme works for any qubit platform as long as the errors are quasistatic. 
Tuesday, March 5, 2019 9:00AM  9:12AM 
E28.00006: Parametricallymediated Dissipative Entanglement Generation Emery Doucet, Florentin Reiter, Leonardo M Ranzani, Raymond Simmonds, Jose Aumentado, Archana Kamal Dissipative state preparation provides a powerful alternative to traditional gatebased state preparation protocols. Rather than generating the desired state through a sequence of unitary operations, dissipative protocols engineer alwayson interactions with the environment such that the system autonomously converges to the desired state. Such an approach allows improved robustness to initialization errors and decoherence. Nonetheless, dissipative state preparation methods realized thus far have the limitation that the preparation time and error are anticorrelated, meaning that accurate preparation requires long stabilization times. In this talk, I will present a novel scheme which makes use of parametric qubitqubit and qubitresonator interactions to avoid this issue. This scheme allows highfidelity preparation of arbitrary maximallyentangled twoqubit states with stabilization times in the few hundred nanosecond range. Uniquely, it also enables continuous insitu control of the target state in a specified parity manifold. I will discuss the robustness of the scheme to experimental imperfections and qubit decoherence, and an implementation which is readily achievable with current circuitQED technology. 
Tuesday, March 5, 2019 9:12AM  9:48AM 
E28.00007: To catch and reverse a quantum jump midflight Invited Speaker: Zlatko Minev Quantum physics differs fundamentally from classical physics in that the measurement of a quantity cannot always give certain results, even in the ideal case where both the preparation and the measurement of the system is perfect. This idea is epitomized in the phenomenon of quantum jumps, first hypothesized by Bohr in his description of the radiation emitted by an excited hydrogen atom, and now routinely observed in the laboratory on a single quantum entity. Quantum jumps are fundamentally random: the time at which they occur cannot be predicted. However, modern measurement theory stipulates that it is possible to obtain an advance warning signalling the imminent occurence of jump, before its full completion. Consequently, it is possible to reverse the jump if it is initiated by a coherent drive. We have successfully caught and reversed jumps by implementing the indirect QND measurement of a superconducting artificial atom that undergoes a transition from its ground state G to a dark state D. This is achieved by monitoring the occupancy of an auxiliary bright level B coupled to G through a Rabi drive. Our experimental results, in agreement with the predictions of quantum trajectory theory with essentially no adjustable parameters, provide new ground for the exploration of realtime intervention techniques in the control of quantum systems, such as early detection of error syndromes for computation and sensing. More generally, our results provide support to the point of view that a single system under continuous, efficient observation is characterized by a timedependent wavefunction inferred from the record of previous measurement outcomes, and whose meaning is that of an objective, generalized degree of freedom. 
Tuesday, March 5, 2019 9:48AM  10:00AM 
E28.00008: Adaptive RotatingWave Approximation for Driven Open Quantum Systems Brian Baker, Andy C. Y. Li, Nicholas Irons, Nathan D Earnest, Jens Koch In this talk, I will present a numerical method to approximate the longtime asymptotic solution ρ_{∞}(t) to the Lindblad master equation for an open quantum system under the influence of an external drive. The proposed scheme uses perturbation theory to rank individual drive terms according to their dynamical relevance, and adaptively determines an effective Hamiltonian. In the constructed rotating frame, ρ_{∞} is approximated by a timeindependent, nonequilibrium steadystate. This steadystate can be computed with much better numerical efficiency than asymptotic longtime evolution of the system in the lab frame. I will illustrate the use of this method by simulating recent transmission measurements of the heavyfluxonium device, for which ordinary timedependent simulations are severely challenging due to the presence of metastable states with lifetimes of the order of milliseconds. 
Tuesday, March 5, 2019 10:00AM  10:12AM 
E28.00009: Exploring topological features of a dissipative qubit near an exceptional point Maryam Abbasi, Mahdi Naghiloo, Yogesh N Joglekar, Kater Murch We study the behavior of a single dissipative qubit which is described by a nonHermitian Hamiltonian that has effective spacetime reflection (PT) symmetry. We use quantum state tomography to observe the PTsymmetry breaking transition as we cross the exceptional point by varying the coupling strength between the two quantum states. By changing the detuning of the coupling, we probe the topological features associated with encircling the exceptional point. In addition, we introduce a timemodulated coupling strength where the resulting Floquet Hamiltonian provides a richer phase diagram where the PT broken phase can occur for dissipations levels that are much smaller than the PT symmetry threshold in the static case. Here we observe multiple transitions breaking and restoring the PTsymmetric phase. 
Tuesday, March 5, 2019 10:12AM  10:24AM 
E28.00010: Experimental repetitive quantum channel simulation Weizhou Cai, Hu Ling, Xianghao Mu, Yuwei Ma, Yuan Xu, Haiyan Wang, Yipu Song, ChangLing Zou, Luyan Sun Universal control of quantum systems is a major goal to be achieved for quantum information processing, which demands thorough understanding of fundamental quantum mechanics and promises applications of quantum technologies. So far, most studies concentrate on ideally isolated quantum systems governed by unitary evolutions, while practical quantum systems are open and described by quantum channels due to their inevitable coupling to environment. Here, we experimentally simulate arbitrary quantum channels for an open quantum system, i.e. a single photonic qubit in a superconducting quantum circuit. The arbitrary channel simulation is achieved with minimum resource of only one ancilla qubit and measurementbased adaptive control. By repetitively implementing the quantum channel simulation, we realize an arbitrary Liouvillian for a continuous evolution of an open quantum system for the first time. Our experiment provides not only a testbed for understanding quantum noise and decoherence, but also a powerful tool for full control of practical open quantum systems. 
Tuesday, March 5, 2019 10:24AM  10:36AM 
E28.00011: Measurementinduced phase transition in the dynamics of entanglement Brian Skinner, Jonathan Ruhman, Adam Nahum We study the dynamics of quantum entanglement in a manybody system that undergoes unitary evolution punctuated by projective measurements. We show that when these measurements occur randomly with a finite rate per degree of freedom, there is a critical measurement rate that separates "entangling" and "disentangling" phases. The entangling phase is characterized by a linear growth of the entanglement entropy with time that leads to volumelaw entanglement, while in the disentangling phase the entanglement entropy takes a constant, arealaw value. We demonstrate this dynamical transition using numerics and a mapping to classical percolation that becomes exact in certain limits. 
Tuesday, March 5, 2019 10:36AM  10:48AM 
E28.00012: Controling the dynamics across a quantum phase transition Adolfo Del Campo, Luis Pedro GarciaPintos, Diego Tielas, Fernando GómezRuiz When a quantum phase transition is crossed in finite time, critical slowing down leads to the breakdown of adiabatic dynamics and the formation of topological defects. The average density of defects scales with the quench rate following a universal powerlaw predicted by the KibbleZurek mechanism (KZM). The scaling theory of phase transitions can however be used to determine the full counting statistics of topological defects, beyond the KZM. Knowledge of the distribution of topological defects provides new insights into the breakdown of adiabaticity. 
Tuesday, March 5, 2019 10:48AM  11:00AM 
E28.00013: Quantum Tracking Control of Molecular Rotor Orientation is Singularityfree Alicia Magann, TakSan Ho, Herschel A Rabitz Quantum tracking control aims to identify applied fields to steer particular observable expectation values along desired paths in time. The fields can be identified by inverting the underlying dynamical equations for the observables. However, fields found in this manner are often plagued by undesirable singularities. In this talk I will consider a planar molecular rotor and derive singularityfree expressions for the fields that steer the expectation value of the rotor’s orientation along desired trajectories in time. Simulations will be presented that utilize two orthogonal control fields to drive the orientation of the rotor along a series of designated tracks. 
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