Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session J33: Dynamical Decoupling and Bath Engineering for Quantum ControlFocus Live

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Sponsoring Units: DQI Chair: Archana Kamal, University of Massachusetts, Lowell 
Tuesday, March 16, 2021 3:00PM  3:12PM Live 
J33.00001: Experimental protection of quantum coherence by using a phasetunable image drive Bertaina Sylvain, Hervé Vezin, Hans de Raedt, Irinel Chiorescu The protection of spin coherence is an essential task in order to manipulate, store and read quantum information. It has been proposed to dynamically decouple (DD) qubits from their surroundings by applying a series of distinct pulses^{1}. For nitrogen vacancy centers, such protection was achieved by using concatenated DD up to the second order of dressing^{2}. We go beyond their specific case and demonstrate T2~T1 with a new pulse protocol, independent of qubit initial state, in a number of materials with different spin Hamiltonians and environments^{3}. The protocol uses two coherent microwave pulses: one drives the Rabi precession while a lowpower, circularly polarized (image) pulse continuously sustains the spin motion. The initial phase of the image drive allows tuning the spin dynamics by altering the Floquet modes. The technical implementation is simple and can be generalized to any type of qubit, such as superconducting circuits or spin systems. 
Tuesday, March 16, 2021 3:12PM  3:24PM Live 
J33.00002: Floquet engineering of quantum state control by exceptionalpoint proximity in a single dissipative qubit Maryam Abbasi, Mahdi Naghiloo, Weijian Chen, Yogesh N Joglekar, Kater Murch Open quantum systems interacting with an environment can be described by Lindblad density matrix equation that encodes their approach to a steady state. When the quantum trajectories of this decoherenceinducing dynamics are restricted to those with no quantum jumps, the resulting evolution is described by effective nonHermitian Hamiltonians. The presence of a special kind of degeneracy known as an exceptional point (EP) plays an important role in the unique topology of these nonHermitian systems. Such an EP occurs when both the eigenvalues and eigenstates of the system coalesce. We demonstrate realtime control over a single dissipative qubit described by an effective nonHermitian Hamiltonian. We reveal how quasistatic tuning of the system parameters results in coherent quantum gates enabled by the complex energy topology. Leaving the quasistatic limit, an effective Floquet Hamiltonian enables rapid quantum gates. Finally, we characterize the geometric phases accumulated from quantum state transport around the EP, revealing the chirality of the transport. Our work demonstrates a new method for control over quantum state, highlighting new facets of quantum bath engineering enabled through timeperiodic nonHermitian control. 
Tuesday, March 16, 2021 3:24PM  3:36PM Live 
J33.00003: Decoupling dipolar interactions in dense spin ensembles Linta Joseph, Benjamin Alford, Will Kaufman, Chandrasekhar Ramanathan Cleverly designed pulse sequences in NMR can decouple the strong dipolar interaction in dense solidstate spin systems. These decoupling sequences were originally designed to narrow spectral lines and improve resolution in NMR spectroscopy. The effective extension of coherence times using these sequences is also important for quantum control, simulation and sensing applications. In this work, we study the effectiveness of different dipolar decoupling sequences for a variety of uncorrelated singlespin and correlated multispin states in different samples. We explore the sensitivity of these sequences to different errors and examine potential error mitigation strategies. 
Tuesday, March 16, 2021 3:36PM  3:48PM Live 
J33.00004: Adiabatic Quantum Control of Dissipative State Preparation Emery Doucet, Archana Kamal Dissipative protocols for state preparation provide the ability to stabilize complex entangled states which are inherently robust to decoherence, though they typically exhibit a tradeoff between the preparation rate and the fidelity of the prepared state. In this talk I will describe a scheme for Bell state preparation employing only parametric qubitqubit and qubitresonator couplings, which afford a large degree of flexibility and control over the stabilized state. Both the amplitudes and phases of these couplings are tunable in situ, providing a natural avenue to implement timedependent control of the state preparation dynamics. Such control, here implemented through engineered dissipation, can circumvent the tradeoffs inherent in the scheme by first rapidly preparing a product state then adiabatically tuning to the desired Bell state. I will present analytical and numerical results which show that large speedups are possible with this approach even in the weakdissipation regime. To understand the achievable speedup, I will examine the adiabaticity criteria in this drivendissipative system and provide estimates of the resulting bounds on the state preparation time. 
Tuesday, March 16, 2021 3:48PM  4:00PM Live 
J33.00005: Timeoptimal Control of a Dissipative Qubit Chungwei Lin, Dries Sels A formalism based on Pontryagin's maximum principle is applied to determine the timeoptimal protocol that drives a general initial state to a target state by a Hamiltonian with limited control, i.e., there is a single control field with bounded amplitude. The coupling between the bath and the qubit is modeled by a Lindblad master equation. Dissipation typically drives the system to the maximally mixed state, consequently there generally exists an optimal evolution time beyond which the decoherence prevents the system from getting closer to the target state. For some specific dissipation channel, however, the optimal control can keep the system from the maximum entropy state for infinitely long. The conditions under which this specific situation arises are discussed in detail. The numerical procedure to construct the timeoptimal protocol is described. In particular, the formalism adopted here can efficiently evaluate the timedependent singular control which turns out to be crucial in controlling either an isolated or a dissipative qubit. 
Tuesday, March 16, 2021 4:00PM  4:12PM Live 
J33.00006: Accessing the role of quantum jumps on nonHermitian dynamics of a superconducting qubit Weijian Chen, Maryam Abbasi, Yogesh N Joglekar, Kater Murch We study the dynamics of a dissipative transmon superconducting qubit whose dissipation comes into two parts: a fast coherent nonunitary dissipation (energy loss) and a slow decoherence due to quantum jumps within the qubit. We observe that the coherence damping rate is enhanced near the exceptional point. Together with the effect of nonHermitian gain/loss, the decoherence also leads to breakdown of adiabatic evolution under the slowdriving limit. Our study shows the critical role of quantum jumps in generalizing the applications of classical nonHermitian systems to open quantum systems from sensing to control. 
Tuesday, March 16, 2021 4:12PM  4:24PM Live 
J33.00007: Robust Qudit Hamiltonian Engineering: Applications to NV Centers Hengyun Zhou, Nathaniel Leitao, Leigh Martin, Alexander Douglas, Iris Cong, Oksana Makarova, Matthew Tyler, Nishad Maskara, Joonhee Choi, Soonwon Choi, Mikhail Lukin While robust dynamical decoupling and Hamiltonian engineering techniques are welldeveloped and highly successful for qubit systems, extensions to qudits involving more than two levels are much less explored. However, the development of such techniques for qudit systems may enable the engineering of novel classes of Hamiltonians for manybody physics, or the development of quantum sensors with higher sensitivity. In this talk, we outline extensions of robust Hamiltonian engineering techniques to qudit systems. We develop general design techniques for pulse sequences that decouple spin1 dipolar interactions and disorder, while remaining robust against finite pulse duration imperfections, and show their applications in dynamical decoupling, quantum sensing, and the creation of exotic manybody states such as quantum manybody scars. While we illustrate our results in a dense ensemble of NV centers, our methodology and pulse sequences apply to any quantum information platform that obeys the rotating wave approximation, such as superconducting qubits, trapped atoms and quantum dots. 
Tuesday, March 16, 2021 4:24PM  5:00PM Live 
J33.00008: Geometric framework for noiseresistant quantum control Invited Speaker: Edwin Barnes Future technologies such as quantum computing, sensing and communication demand the ability to control microscopic quantum systems with unprecedented accuracy. This task is particularly daunting due to unwanted and unavoidable interactions with noisy environments that destroy quantum information through decoherence. I will present a new theoretical framework for deriving control waveforms that dynamically combat decoherence by driving qubits in such a way that noise effects destructively interfere and cancel out. This theory exploits a rich geometrical structure hidden within the timedependent Schrödinger equation in which quantum evolution is mapped to geometric curves. Control waveforms that suppress noise can be obtained by drawing closed curves and computing their curvatures. I will show how this can be done for single and multiqubit systems. 
Tuesday, March 16, 2021 5:00PM  5:12PM Live 
J33.00009: Ultrasubharmonic bifurcations in a driven nonlinear oscillator  Part 2/2: harmful and beneficial consequences in Josephson circuits Xu Xiao, Jayameenakshi Venkatraman, Yaxing Zhang, Michel Devoret Ultrasubharmonic (USH) bifurcation refers to a broad class of dynamical transitions wherein a driven nonlinear oscillator is resonantly excited to one of q equiprobable stable states by some p/qth USH of a drive, for naturals p and q. In circuit quantum electrodynamics (cQED), particular USH processes have been employed in quantum devices such as amplifiers based on bifurcation thresholds and qubits based on Schrodinger Catlike manifolds of degenerate drivendissipative states. While these instances are well documented, mapping out where in parameter space generic USH processes occur in cQED experiments is of increasing importance. This is especially relevant in cases where unexpected bifurcations plague the drivendissipative operations employed in quantum information processing. Moreover, controlled USH processes could lead to novel nonlinear interactions, like those involved in stabilizing higherdimensional cat states. In this talk, basing ourselves on Part 1, we will discuss different experimental manifestations of USH processes. Their comprehensive map in parameter space will be shown. Finally, we will discuss how to mitigate the undesired sideeffects of USH bifurcation and to harness it for quantum operations. 
Tuesday, March 16, 2021 5:12PM  5:24PM Live 
J33.00010: Ultrasubharmonic bifurcation in a driven nonlinear oscillator  Part 1/2: classical and quantum manifestations Jayameenakshi Venkatraman, Xu Xiao, Yaxing Zhang, Shoumik Chowdhury, Michel Devoret Ultrasubharmonic (USH) bifurcation refers to a broad class of dynamical transitions wherein a driven nonlinear oscillator is resonantly excited to one of q equiprobable stable states by some p/qth USH of a drive, for naturals p and q. Historically, this bifurcation class has attracted mostly theoretical interest due to the intricate topology of phase space trajectories and its connections to chaos. Recently, particular instances of USH processes have been revisited experimentally in Josephson circuits, specifically in the novel lowdissipation quantum regime, for the purpose of processing quantum information. However, the general physical structure of USH processes — their perturbative order and timeaveraged dynamics, remains unexamined, particularly for bridging the classical and quantum regimes. We show that the timeaveraged dissipative dynamics governing any USH process in the lowdissipation regime, whether classical or quantum, can always be represented as motion on an effective static Hamiltonian surface endowed with discrete qfold symmetry. Moreover, semiclassically, this socalled metapotential unifies two distinct quantum manifestations of USH bifurcation: multiphoton nonlinear resonances and protected multistability. 
Tuesday, March 16, 2021 5:24PM  5:36PM Live 
J33.00011: Highfidelity electron–nuclear spin entangling gates in NV centers via hybrid dynamical decoupling sequences Wenzheng Dong, Fernando. A. CalderonVargas, Sophia Economou The spinful nuclei around a controllable color center, such as the NV center in diamond, are promising candidates for a quantum memory. Experiments have demonstrated control of nuclear spins through CPMGlike dynamical decoupling of the NV in diamond and the divacancy in SiC. In this work, we show that the Uhrig decoupling sequence and a hybrid protocol we introduce improve these entangling gates in terms of fidelity, spin control range, and spin selectivity. We numerically show our method's efficacy on NV centers in diamond, but our results are general and applicable to other types of defects in solids. 
Tuesday, March 16, 2021 5:36PM  5:48PM Live 
J33.00012: Robust Qudit Hamiltonian Engineering: A General Theory Nathaniel Leitao, Hengyun Zhou, Leigh Martin, Iris Cong, Soonwon Choi, Mikhail Lukin Dynamical decoupling and Hamiltonian engineering lie at the cornerstone of modern quantum science and engineering. For qubits, conditions that determine whether a given pulse sequence transforms a native interaction into a desired form are straightforward to obtain due to the intimate connection between single qubit gates and rotations in three dimensions. In this talk, we extend this characterization to generic qudit systems with strongly anharmonic level structures. Utilizing results from group theory we show how the ClebschGordon coefficients of particular irreducible representations of SU(d) lead to simple analytic conditions for the cancellation of generic qudit interactions. We further use them to give a natural parameterization of engineerable Hamiltonians. Lastly, we analytically construct experimentally robust pulse sequences satisfying our cancellation conditions, leveraging the algebraic properties of the generalized Clifford group. Motivated by the prominence of modern qutrit quantum simulation and computation experiments, special attention will be given to this d=3 special case. These results offer an efficient, analytical design tool for modern quantum simulators, opening the door to exotic manybody physics lacking any analog in qubit systems. 
Tuesday, March 16, 2021 5:48PM  6:00PM On Demand 
J33.00013: Arbitrary quantum operation on a qudit Weizhou Cai, Jiaxiu Han, Ling Hu, Yuwei Ma, Xianghao Mu, Weiting Wang, Yuan Xu, ZIYUE HUA, Haiyan Wang, Yipu Song, JingNing Zhang, Changling zou, Luyan Sun Quantum technology requires full manipulation quantum systems, i.e. arbitrary quantum operations (AQuOs) for all possible physical process of an open quantum system. However, this goal demands great extra physical resources and hence is difficult to realize. In this talk, I will show our recent experiments about demonstrating a universal approach of AQuO on a superconducting photonic qudit with only a twolevel ancilla and a log_{2}dscale circuit depth for a ddimensional system. Specifically, I will show the AQuO can be applied in quantum trajectory simulations for quantum subspace stabilization and quantum Zeno dynamics, and incoherent manipulation of the qudit. Furthermore, rankd^{2} positive operatorvalued measures are also obtained, which outperform conventional tomography technique in practice. The demonstrated AQuOs in this talk for complete quantum control would play an important role in quantum information science. 
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