Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H39: New Frontiers in Quantum AlgorithmsFocus

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Sponsoring Units: DQI Chair: Itay Hen, University of Southern California Room: LACC 501B 
Tuesday, March 6, 2018 2:30PM  3:06PM 
H39.00001: Quantum information tools for simulating quantum field theories Invited Speaker: Stephen Jordan Quantum field theory lies at the heart of both condensedmatter and high energy physics. In some parameter regimes, extracting quantitiative predictions from quantum field theories, particularly regarding dynamical quantities such as scattering probabilities, appears to be intractable using conventional techniques. Here I will discuss new advances in quantum algorithms for simulating quantum field theories which achieve exponential speedup over known classical algorithms and polynomial speedup over prior quantum algorithms, and also describe new quantuminformationinspired classical algorithms for computing equaltime correlation functions. Prior knowledge of quantum field theory will not be assumed. This is joint work with Keith Lee, John Preskill, Hari Krovi, Troy Sewell, and Ali Moosavian. 
Tuesday, March 6, 2018 3:06PM  3:42PM 
H39.00002: Quantum speedups for semidefinite programming Invited Speaker: Fernando Brandao This abstract not available. 
Tuesday, March 6, 2018 3:42PM  3:54PM 
H39.00003: Nonperturbative evolution and measurement for quantum excited state preparation and error mitigation Jarrod McClean Quantum chemistry and other manybody simulations have unfolded as some of the most promising applications for early quantum devices. While significant work has gone into the preparation of manybody ground states and methods to study them, excited states are equally important for the description of dynamic processes such as charge transfer and light absorption. In this talk I introduce a new approach to generating excited states that are difficult to access accurately through previous perturbative approaches. These techniques are designed to be amenable for nearterm devices, and possible routes for error mitigation within this methodology will be discussed. 
Tuesday, March 6, 2018 3:54PM  4:06PM 
H39.00004: Efficiency Bounds on Quantum Search Algorithms in lowdimensional Geometries Stefan Boettcher, Shanshan Li, Tharso Fernandes, Renato Portugal We find a lower bound on the computational complexity of Grover's quantum search algorithms in lowdimensional networks using the renormalization group (RG). It reveals a competition between Grover's abstract algorithm, i.e., a rotation in Hilbert space, and quantum transport in an actual geometry. It can be characterized in terms of the quantum walk dimension d_{w}^{Q} and the spatial (fractal) dimension d_{f} or, alternatively, the spectral dimension of the network, d_{s}, even when translational invariance is broken.The analysis simultaneously determines the optimal time for a quantum measurement and the probability for successfully pinpointing the soughtafter element in the network. The RG further encompasses an optimization scheme devised by Tulsi that allows to tune this probability to certainty. Our RG considers entire families of problems to be studied, thereby establishing large universality classes for quantum search and how these can be broken, which we verify with extensive simulations. The methods we propose open the door to a systematic study of universality classes in computational complexity and how a class may be broken to modify and control search behavior. (https://arxiv.org/abs/1708.05339) 
Tuesday, March 6, 2018 4:06PM  4:18PM 
H39.00005: Quantum Circuit Learning Kosuke Mitarai, Keisuke Fujii, Masahiro Kitagawa, Makoto Negoro In recent years, machine learning has given some remarkable results, and the area called quantum machine learning (QML) is emerging fast. Google, IBM, and Intel are now demonstrating their quantum processors. However, a high depth quantum circuit used in most of suggested QML algorithms is still challenging in the near future. Classicalquantum hybrid algorithms with a low depth circuit, such as quantum variational eigensolver for quantum chemistry, are thought as candidates of applications of near term devices. Here we present a framework for classicalquantum hybrid machine learning with a low depth circuit, which we call quantum circuit learning (QCL). 
Tuesday, March 6, 2018 4:18PM  4:30PM 
H39.00006: Lowdepth circuit ansatz for preparing correlated fermionic states on a quantum computer PierreLuc DallaireDemers, Jhonathan Romero, Libor Veis, Alan AspuruGuzik Several quantum simulation methods are known to prepare a state on a quantum computer and measure the desired observables. The most resource economic procedure is the variational eigenvalue solver, which has traditionally employed unitary coupled cluster as the ansatz to approximate ground states of manybody fermionic Hamiltonians. A significant caveat of the method is that the initial state of the procedure is a single reference product state with no entanglement extracted from a classical HartreeFock calculation. In this work, we propose to improve the method by initializing the algorithm with a more general fermionic gaussian state. We show how this gaussian reference state can be prepared with a lineardepth quantum circuit. We also describe a lowdepth circuit ansatz that can accurately prepare the ground state of correlated fermionic systems. This extends the range of applicability of the variational eigenvalue solver to systems with strong pairing correlations such as superconductors, atomic nuclei and topological materials. 
Tuesday, March 6, 2018 4:30PM  4:42PM 
H39.00007: KeldyshETH quantum computation algorithm Khadijeh Najafi, James Freericks, Jeffrey cohn, Forest Yang

Tuesday, March 6, 2018 4:42PM  4:54PM 
H39.00008: Constant Time Adiabatic Preparation of the Laughlin State Sonika Johri, Zlatko Papic The Laughlin wavefunction describes the quantum Hall state at filling factor 1/3 and is an example of a strongly correlated topological phase with anyonic excitations. Such systems are of importance in quantum computing because they are protected by a gap to excited states and thus are robust to local noise. We show that the Laughlin state can be adiabatically connected to a product state by tuning a geometric degree of freedom of the system [1]. Furthermore the gap along this pathway is a constant even in the thermodynamic limit indicating that the state can be prepared in constant time for arbitrary system size up to subpolynomial factors. In particular, we design and optimize a digital quantum circuit that can be used to prepare the Laughlin state for 6 electrons on as few as 16 qubits, which is within the range of nearterm quantum hardware [2]. Techniques like this will be essential for digital quantum computers to aid in the understanding and application of manybody topologically ordered states. 
Tuesday, March 6, 2018 4:54PM  5:06PM 
H39.00009: From nonstoquastic to stoquastic Hamiltonians Milad Marvian Mashhad, Itay Hen, Daniel Lidar Local stoquastic Hamiltonians are important both in practice and theory. Motivated by the problem of simulability by quantum Monte Carlo algorithms, we propose a definition of stoquasticity that emphasizes computational complexity. In its simplest form, we allow preprocessing on the description of the input Hamiltonian using polynomial classical computation to find and then apply the transformation that converts the Hamiltonian into a stoquastic Hamiltonian. We provide several examples and results motivating this definition. 
Tuesday, March 6, 2018 5:06PM  5:18PM 
H39.00010: Quantum walks of two interacting particles in a classical environment Ilaria Siloi, Claudia Benedetti, Enrico Piccinini, Jyrki Piilo, Sabrina Maniscalco, Matteo Paris, Paolo Bordone Quantum walks (QWs) describe the propagation of a quantum particle over a discrete lattice with equal tunnelling probability between adjacent sites. Twoparticle QWs are paradigmatic systems to study the interplay between particle indistinguishability and particle interaction. Here we address the decoherent dynamics of two interacting and indistinguishable particles over a chain with random, timedependent, tunneling amplitudes. In particular, the hopping amplitudes have been modeled as independent stochastic processes in the form of nonGaussian random telegraphic noise. Upon tuning the ratio between the time scale of the noise and the walkers’ coupling strength, we may explore very different dynamical regimes. In particular, we show that noise with fastdecaying autocorrelation function may lead to a faster propagation with respect to the noiseless case. On the other hand, in the slow noise regime, the system displays a dynamical Andersonlike localization. Finally, we study the nonMarkovian character of the dynamical map in order to relate specific features of the interacting QWs dynamics to the presence of memory effect. [1] Phys. Rev. A 95 (2), 022106 (2017). [2] Comp. Phys. Comm. 215, 235 (2017). 
Tuesday, March 6, 2018 5:18PM  5:30PM 
H39.00011: The Cluster Generalization of the Truncated Wigner Approximation for interacting spin lattices Jonathan Wurtz, Anatoli Polkovnikov This talk will detail a generalization of the Truncated Wigner Approximation (TWA) through systematic inclusion of entanglement degrees of freedom in a semiclassical phase space. Linear quantum dynamics is replaced by classical nonlinear dynamics, where difficulty is polynomial instead of exponential in system size. Observables, entanglement and quantum correlations are encoded in the phase space via nonlinear evolution. We demonstrate the method on manybody systems of interacting spins, reproducing essential features in both the Many Body Localized and Thermalizing regimes. 
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