Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X28: Quantum Annealing: TheoryFocus

Hide Abstracts 
Sponsoring Units: DQI Chair: Tameem Albash, Univ of Southern California Room: LACC 405 
Friday, March 9, 2018 8:00AM  8:36AM 
X28.00001: Error suppression for Hamiltonianbased quantum computation Invited Speaker: Milad Marvian Mashhad In this talk, I will present recent results on using quantum codes for error suppression. In particular, I present general conditions for quantum error suppression for Hamiltonianbased quantum computation using subsystem codes. This involves encoding the Hamiltonian performing the computation using an error detecting subsystem code and the addition of a penalty term that commutes with the encoded Hamiltonian. I illustrate the power of subsystembased error suppression with several examples of twolocal constructions for protection against local errors, which circumvent an earlier nogo theorem about twolocal commuting Hamiltonians. I also discuss the generalization of the quantum error suppression results of Jordan, Farhi, and Shor to arbitrary Markovian dynamics. In this setting, we show that it is possible to suppress the initial decay out of the encoded ground state with an energy penalty strength that grows only logarithmically in the system size, at a fixed temperature. 
Friday, March 9, 2018 8:36AM  8:48AM 
X28.00002: Reducing the adiabatic error via boundary cancellation method Lorenzo Campos Venuti The adiabatic theorem for open systems is at the core of realistic implementations of adiabatic quantum computing. I will show that the adiabatic error can be reduced by engineering the system's Hamiltonian alone irrespective of the bath. Surprisingly there is an asymmetry with the analogous result for unitary evolution. The origin of such asymmetry must be traced back, ultimately, to the arrow of time which is inherent in a Lindbladian description of a system. 
Friday, March 9, 2018 8:48AM  9:00AM 
X28.00003: When quantum relaxation benefits quantum annealing: a case study of ferromagnetic chains with alternating sectors Anurag Mishra, Daniel Lidar, Tameem Albash We study the behavior of a commercial quantum annealing processor on the problem of a ferromagnetic chain with alternating strength sectors. This is a problem for which, at constant runtime, the closedsystem adiabatic quantum algorithm is known to exhibit exponentially decreasing success probability in the sector size. We find that the behavior of the quantum annealing processor departs significantly from the prediction of closedsystem behavior, with the success probability rising for sufficiently large sector sizes. Rather than correlating with the size of quantum minimum gap, the success probability exhibits a strong correlation with the number of singlefermion states calculated at the minimum gap with an energy below the processor temperature. We demonstrate that this behavior is consistent with a quantum opensystem description of the process, in which the excitation rate out of the ground states depends on the number of available excitations weighted by the ratio of the excitation energy to the temperature. 
Friday, March 9, 2018 9:00AM  9:12AM 
X28.00004: Probing phases of matter with advanced control of a quantum annealing processor Andrew King Much of the research performed on DWave superconducting processors has focused on the annealing of randomly generated spinglass instances of various constructions. Now, new doors are open that allow us to more effectively study systems of interest with nonzero transverse field. The most recent generation of processor offers new features, including the ability to reverse anneal from a userdefined classical state and to quickly quench the anneal to its terminus faster than annealing allows. We show how these features can be used to study the interplay between low dimensionality, quantum fluctuations and thermal effects in a geometrically frustrated lattice, with potential applications to materials simulation. 
Friday, March 9, 2018 9:12AM  9:24AM 
X28.00005: Engineering quantum phase transitions via spatiotemporal gapped Hamiltonians Masoud Mohseni, Johan Strumpfer, Marek Rams We introduce a general framework for controlling quantum critical phenomena. We engineer quantum phase transitions via constructing spatiotemporal inhomogeneous driving Hamiltonians. We show that nonequilibrium evolution of disordered quantum systems can be manipulated leading to new dynamical critical exponent and correlation length scales that are fundamentally different from corresponding parameters when such complex systems are driven homogeneously. In particular, we construct a class of causallyinduced nonadiabatic quantum annealing transitions for lowdimensional spinglass systems leading to substantial suppression of topological defects beyond standard KibbleZurek predications. Using DMRG techniques for such quantum Hamiltonian systems, we demonstrate that our approach could outperform standard (homogeneous) adiabatic quantum computing and simulated annealing by several order of magnitudes in residual energy. 
Friday, March 9, 2018 9:24AM  9:36AM 
X28.00006: Mechanism of quantum speedup in novel population transfer protocol for binary optimization problems Kostyantyn Kechedzhi, Vadim Smelyanskiy, Sergei Isakov, Sergio Boixo, Boris Altshuler We consider a novel quantum population transfer protocol to solve binary optimization problems that exploits quantum manybody dynamics in the delocalized regime. Hard optimization problems are characterized by energy landscape with a large number of local minima separated by large Hamming distances which scale with the problem size. This landscape gives rise to an interesting computational primitive: given an initial bitstring, we are to produce other bitstrings within certain narrow range of energies around the initial state. We consider a specific model we call "impurity band": a system of n qubits in a transverse field, where a number of bitstrings $M<< 2^n$ selected at random are assigned random energies distributed in a narrow window of width $W<<1$ around the mean energy $–n$. We demonstrate the existence of the manybody delocalized regime in this model when the spectrum of the model splits into manybody minibands, and a typical eigenstate wave function is a superposition of peaks centered at a large number of local minima. The typical width of the minibands in energy determines the efficiency of the population transfer protocol. We demonstrate theoretically that the population transfer protocol achieves Grover type speedup in the unstructured impurity band model. 
Friday, March 9, 2018 9:36AM  9:48AM 
X28.00007: Improving the Adiabaticity of Adiabatic Quantum Computers with Unknown Energy Spectrum Lin Tian Adiabatic quantum computing brings powerful insights into the realization of manybody ground states and the solution to optimization problems. For an adiabatic quantum algorithm to be successful, the adiabatic criterion needs to be satisfied. However, as the energy gap between the quantum ground state and the excited states decreases with the number of qubits, the adiabaticity of the quantum evolution can be seriously violated. Previous approaches to improve the adiabaticity require either a priori knowledge of the energy spectrum or the construction of impractical manybody interactions. Here we present an approach that can significantly improve the adiabaticity of an adiabatic quantum computer without the knowledge of the energy spectrum, where an ancilla provides nonlinear control over the adiabatic quantum computer. We show that in the vicinity of the bifurcation points in this nonlinear system, the dynamics of the adiabatic quantum computer can be slowed down and the adiabaticity can be greatly enhanced. We illustrate this approach with a quantum twolevelsystem, an exactly1 3SAT instance on six qubits, and a transverse field Ising model. 
Friday, March 9, 2018 9:48AM  10:00AM 
X28.00008: EvolutionTime Dependence in NearAdiabatic Quantum Evolutions Lucas Brady, Wim van Dam We expand upon the standard quantum adiabatic theorem, examining the timedependence of quantum evolution in the nearadiabatic limit. We examine a Hamiltonian that evolves along some fixed trajectory from H0 to H1 in a total evolutiontime T, and our goal is to determine how the final state of the system depends on T. If the system is initially started in a nondegenerate ground state, the adiabatic theorem says that in the limit of large T, the system will stay in the ground state. We examine the nearadiabatic limit where the system evolves slowly enough that most but not all of the final state is in the ground state, and we find that the probability of leaving the ground state oscillates in T with a frequency determined by the integral of the spectral gap along the trajectory of the Hamiltonian, so long as the gap is big. If the gap becomes exceedingly small, the final probability is the sum of oscillatory behavior determined by the integrals of the gap before and after the small gap. We confirm these analytic predictions with numerical evidence from barrier tunneling problems in the context of quantum adiabatic optimization. 
Friday, March 9, 2018 10:00AM  10:12AM 
X28.00009: Quantum Enhancement via ManyBody Localization Evgeny Mozgunov In 2009, Altshuler, Krovi, Roland raised an objection to the adiabatic quantum annealing program, noting that a phenomenon of ManyBody Localization occurs at the end of the protocol, resulting in exponentially small gaps. I revisit their estimates, and discuss how although these gaps generally render the closedsystem protocol impractical, an enhancement over classical algorithms is possible for computational tasks that do not require reaching the end of adiabatic path. 
Friday, March 9, 2018 10:12AM  10:24AM 
X28.00010: Quantum Solitons: Control and Spectral Broadening Giulia Marcucci, Claudio Conti, Simone Montangero, Tommaso Calarco Quantum soliton (QS) evolution and highly nonclassical supercontinuum generation (SG) are noteworthy challenges in nonlinear quantum optics, which can be addressed by quantum control (QC) current techniques, as chopped random basis optimization (CRAB). 
Friday, March 9, 2018 10:24AM  10:36AM 
X28.00011: Quantum Dynamics in Rugged Energy Landscapes Christopher Baldwin, Christopher Laumann We study the ability of quantum dynamics to navigate rugged energy landscapes, for example as exist in many optimization problems in computer science. In particular, we consider the performance of tunneling in "energymatching" problems, where one solution to an optimization problem is already given and used as a starting point to find others. Clustering in the solution space and the existence of energy barriers make this problem difficult for classical algorithms. Focusing on a specific family of toy models in the presence of a transverse magnetic field, we identify three sharp phases of the quantum dynamics. At small fields, the system cannot tunnel between solutions (and energy matching fails). At intermediate fields, the system does tunnel between solutions (energy matching succeeds). At high fields, the system is excited into states that are not solutions (energy matching fails). We discuss the relation to certain simple classical algorithms. 
Follow Us 
Engage
Become an APS Member 
My APS
Renew Membership 
Information for 
About APSThe American Physical Society (APS) is a nonprofit membership organization working to advance the knowledge of physics. 
© 2024 American Physical Society
 All rights reserved  Terms of Use
 Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 207403844
(301) 2093200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 5914000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 200452001
(202) 6628700