Thursday, March 5
Lazaridis QNC 0101
Schedule
Abstracts
Hassan Alam, Physics and Astronomy
An Off-Lattice Sanchez-Lacombe Related Equation of State Extensible to Polymeric Foams*
We have extended an off-lattice model of the Sanchez-Lacombe equation of state for pure polymeric systems by including internal degrees of freedom to polymer chains. The extension allows us to predict glass transition temperatures of pure polymeric systems at different pressures. To find unknown parameters of the model only two experimental values are required, namely, the glass transition temperature of the pure polymer at atmospheric pressure and the change in isobaric heat capacity of the pure polymer across the glass transition. The model is also successful in predicting the behavior of isobaric heat capacities of polymer melts as a function of temperature. The model shows that the Gibbs-DiMarzio criterion for the glass transition is invalid for the Sanchez-Lacombe equation of state. Thus, we proposed a new criterion for glass transition temperature calculations. Predictions of the model are compared with a lattice-based version of the Sanchez-Lacombe equation of state for several pure polymeric systems. The proposed model can be extended to multicomponent polymeric systems and thus to retrograde vitrification in polymeric foams.
*Natural Sciences and Engineering Research Council of Canada
Tom Beardsley, Physics and Astronomy
Phase diagram for diblock copolymer melts from Langevin field-theoretic simulations
T M Beardsley and M W Matsen
Field-theoretic simulations (FTS) provide fluctuation corrections to self-consistent field theory (SCFT) by simulating its field-theoretic Hamiltonian rather than applying the saddle-point approximation. Here, we apply Langevin FTS (L-FTS) to AB diblock copolymer melts, where the composition field fluctuates via Langevin dynamics but the saddle-point approximation is still applied to the pressure field that enforces incompressibility. The method is demonstrated to be one or two orders of magnitude faster than previous Monte Carlo simulations (MC-FTS), permitting the rapid formation of spontaneously ordered configurations and the accurate determination of their order-disorder transitions. The results are used to construct a phase diagram for diblock copolymer melts of high invariant polymerization index, N = 104.
Jérémy Béjanin, Institute for Quantum Computing
Resonant Coupling Parameter Estimation with Superconducting Qubits
Today's quantum computers comprise tens of qubits, which are interconnected either directly or indirectly via various types of coupling elements. In order to calibrate and operate such systems it is necessary to have a complete knowledge of all parameters in the Hamiltonian describing the system. In this article, we demonstrate a method to efficiently learn the parameters of resonant interactions for quantum computers consisting of frequency-tunable superconducting qubits. Such interactions include, for example, those to other qubits, resonators, two-level state defects, or other unwanted modes. Our method is based on a significantly improved swap spectroscopy calibration and consists of an offline data collection algorithm, followed by an online Bayesian learning algorithm. The purpose of the offline algorithm is to detect and roughly estimate resonant interactions from a state of zero knowledge. It produces a square root reduction in the number of measurements. The online algorithm subsequently refines the estimate of the parameters to comparable accuracy as the standard method, but in constant time. The experimental implementation of our technique shortens the qubit calibration time by an order of magnitude. We believe the method investigated will improve present medium-scale superconducting quantum computers and will also scale up to larger systems. Finally, the two algorithms presented can readily be adopted by communities working on different physical implementations of quantum computing architectures.
Jamal Busnaina, Institute for Quantum Computing
Demonstration of programmable quantum simulations of lattice models using a superconducting parametric cavity
There has been a growing interest in realizing quantum simulators for important physical systems where perturbative methods are ineffective. The scalability and flexibility of circuit quantum electrodynamics (cQED) make it a promising platform for implementing various types of simulators, including lattice models of strongly-coupled field theories. With this in mind, we use a multimode superconducting parametric cavity to create programmable lattices of bosonic modes by parametrically pumping at mode-difference frequencies. The choice of pump frequencies allows to change the graph of the lattice in situ. Further, the resulting hopping terms induced between the modes can be made complex by controlling the relative phases of the parametric drives. This enables us to study a wide variety of interesting lattice models. For instance, controlling the total loop phase in closed plaquettes allows us to simulate the motion of particles in a static gauge field, including producing nonreciprocal transport. The system can also realize models with topological features such as the bosonic Creutz ladder. In this talk, we present experimental results on a variety of different small lattice models.
Mohamed Hibat-Allah, Physics and Astronomy
Recurrent Neural Network Wavefunctions
A core technology that has emerged from the artificial intelligence revolution is the recurrent neural network (RNN). Its unique sequence-based architecture provides a tractable likelihood estimate with stable training paradigms, a combination that has precipitated many spectacular advances in natural language processing and neural machine translation. This architecture also makes a good candidate for a variational wavefunction, where the RNN parameters are tuned to learn the approximate ground state of a quantum Hamiltonian. In this paper, we demonstrate the ability of RNNs to represent several many-body wavefunctions, optimizing the variational parameters using a stochastic approach. Among other attractive features of these variational wavefunctions, their autoregressive nature allows for the efficient calculation of physical estimators by providing perfectly uncorrelated samples. We demonstrate the effectiveness of RNN wavefunctions by calculating ground state energies, correlation functions, and entanglement entropies for several quantum spin models of interest to condensed matter physicists in one and two spatial dimensions.
Jimmy Hung, Institute for Quantum Computing
Superconducting Parametric Cavities as an “Optical” Quantum Computation Platform
Quantum information may be encoded into systems of discrete variables (DV) or continuous variables (CV). CV quantum computation has typically been studied at optical frequencies using linear quantum optics to realize Gaussian operations. To achieve universal computation, however, non-Gaussian resources such as the photon number measurements or the cubic phase state are necessary. In superconducting circuits, DV quantum computation is dominant. Here, we propose and study the superconducting parametric cavity for optical quantum computation using microwave photons. At optical frequencies, the qumodes are often separated spatial modes. Here we use the orthogonal frequency modes of the cavity. Gaussian operations between the modes are achieved via standard parametric interactions. In addition, the recent realization of three-photon spontaneous parametric downconversion in this system provides access to both a non-Gaussian gate and resource state, which provides a path to universality. We will present preliminary results towards the development of the parametric cavity for optical quantum computation starting with demonstrations of simple algorithms. One such algorithm is a quantum machine learning algorithm called Quantum Kitchen Sinks.
Wen Jin, Physics and Astronomy
Interplay between magnetic exchange, multipolar interactions and virtual crystal field fluctuations in non-Kramers pyrochlore magnets
Wen Jin (University of Waterloo)
Michel J P Gingras (University of Waterloo)
Hallas Alannah (University of British Columbia)
Jonathan Gaudet (Johns Hopkins University)
Bruce D. Gaulin (McMaster University)
It has been shown that multipolar degrees of freedom can play a vital role in causing exotic phases in pyrochlore magnets, such as octupoles in Kramers Nd3+ or Ce3+ ions and quadrupoles in non-Kramers Pr3+ and Tb3+ ions. Terbium-based pyrochlores are peculiar for hosting virtual crystal field excitation (VCFE) due to the small energy separation between the two low-lying crystal electric field doublets. Here we consider a two-doublets system with magnetic bilinear exchange and electric quadrupole-quadrupole interaction. Via a mean field approach, we show that the proposed model exhibits complex dipolar and quadrupolar phases. Different dipolar order parameters coexist due to VCFE, which also leads to a “parasitic” ferroquadrupolar order accompanying the dominant antiferroquadrupolar order. We find for a set of model parameters locating the system near the dipolar/quadrupolar phase boundary that, upon cooling, such system may undergo a two-step thermal transition into the ultimate low-temperature dipolar phase with an intermediated quadrupolar ordered state. We also propose a range of acceptable parameters for the Tb2Ge2O7 pyrochlore that allows us to reproduce some of the main inelastic neutron scattering features.
Junan Lin, Institute for Quantum Computing
Independent State and Measurement Characterization on Quantum Computers
Correctly characterizing state preparation and measurement (SPAM) processes is a necessary step towards building reliable quantum computers. In this work, we derive a simple experimental procedure to separately estimate the SPAM error strengths on a QPU. After discussing principles behind the experimental design, we present the protocol along with an asymptotic bound for the uncertainty in the estimated parameters in terms of quantities that can be estimated independently of SPAM processes. We test this protocol on a publicly available 5-qubit quantum processor and discuss the applicability on near-term devices.
Nizar Messaoudi, Institute for Quantum Computing
Quantum Enhanced Noise Radar
Quantum Illumination (QI) promises improvement in the sensitivity of target detection technologies. The approach takes advantage of strong correlations that can be created in electromagnetic beams using quantum processes, through a form of entanglement. Notably, QI has proven to be very robust to the presence of noise and loss, suggesting that it may have practical applications. We have made a proof-of-principle demonstration of a novel QI protocol: quantum-enhanced noise radar (QENR). In QENR, we use a parametric amplifier to produce a two-mode squeezed (TMS) state, which exhibits continuous-variable entanglement between signal and idler beams. This state is the input to the radar system. Compared to existing proposals for QI, our protocol does not require joint measurement of the signal and idler. This greatly enhances the practicality of the system by eliminating the need for a quantum memory to store the idler. We compare the performance of a TMS source to an ideal classical source that saturates the classical bound for correlation, finding a quantum enhancement approaching a factor of 10. One of the main challenges to making QENR practical is bringing the quantum microwaves out of the cryostat. We will discuss progress towards overcoming this challenge.
Adam Raegen, Physics and Astronomy
Anisotropy of polymer stable glass thin films
Adam Raegen, Junjie Yin, Qi Zhou, James A Forrest
In the thirteen years since their introduction, ultrastable molecular glasses have become a topic of great interest. Thin films prepared by vapour deposition at substrate temperatures slightly below their glass transition temperature can exhibit increased densities and stabilities similar to those expected for glassy films cooled from liquid before being aged hundreds or thousands of years. However, these ultrastable glasses may not truly present the same state as aged glasses. In many ultrastable glasses, the samples display anisotropy, unless steps are taken to make films only in a narrow range of production parameters such as substrate temperature and deposition rate. Simulations have suggested (Lin et. al. J. Chem. Phys. 140, 204504 (2014)) that evaporated oligomeric samples may provide a way to avoid this anisotropy and produce truly isotropic stable glass. In this case, the length of oligomeric samples can greatly influence anisotropy. Using our recently demonstrated technique for making polymer stable glasses, we present an experimental study of extremely monodisperse, ultrastable polymeric/oligomeric thin films. We use spectroscopic ellipsometry to measure birefringence caused by anisotropy in such samples as a function of the stability (fictive temperature) and the oligomeric size.
Vadiraj Ananthapadmanabha Rao, Insitute for Quantum Computing
Realizing giant artificial atoms in superconducting waveguide QED
In most studies of light-matter interaction, the atoms, either natural or artificial, are approximated as featureless dipoles, since the atomic dimension is much smaller than the wavelength of light. However, a new regime in waveguide QED, first proposed by Kockum et al, can realize a “giant” artificial atom by coupling to light at multiple points along a waveguide. As a result, the atom interacts with itself, resulting in a range of phenomena including non-Markovian dynamics and frequency-dependent coupling. The same proposal also discussed possibilities to extend this architecture to multiple giant atoms with interesting new physics. Motivated by this, we experimentally investigate circuits with one and two giant transmon qubits which are coupled to propagating microwaves at multiple points separated by wavelength-scale distances. For one qubit circuit, we demonstrate that we can enhance or suppress the relaxation rate of the 1-2 transition relative to the 0-1 transition by more than an order of magnitude. Using this capability, we show that we can engineer the giant transmon into an effective lambda system, including demonstrating EIT. We will also present preliminary measurements of a circuit with two giant qubits coupled in a braided geometry.
Allison Sachs, Institute for Quantum Computing
Entanglement Harvesting with Modified Dispersion Relations
We study the effects of modified dispersion relations on the entanglement harvesting protocol using the Unrun-Dewit Model in 2+1 and 3+1 dimensions. We find entanglement harvesting to be robust under those modified dispersion relations that are found in surface wave and Bose Einstein condensate analog gravity systems. These results support the assertion that the entanglement harvesting phenomenon should be seen in analog gravity experiments that are quantum in nature.
Junjie Yin, Physics and Astronomy
Stable polystyrene glass films through PVD and UV radiation
Stable glasses have proved to be an extraordinary type of material with enhanced density and exceptional kinetic stability compared to normal glasses. So far stable glasses prepared through physical vapor deposition (PVD) have been generally limited to organic small molecules such as indomethacin. We have recently employed PVD to make stable oligomeric glasses. In this study, we extend our capacity to make high molecular weight polystyrene stable glass by using oligomeric stable glass as a starting point. We do this by crosslinking the stable oligomeric glasses by dehydrogenation reaction under ultraviolet (UV) radiation. Depending on the degree of crosslinking, the resulting stable glass can have significantly higher molecular weight and kinetic stability. The stable glass properties of the samples are characterized by ellipsometry.