arXiv subjects

D. G. Cory

Publications and source records attributed to D. G. Cory.

At least 55 records · Page 3Linked to original sources

arXiv2012

Robust Online Hamiltonian Learning

In this work we combine two distinct machine learning methodologies, sequential Monte Carlo and Bayesian experimental design, and apply them to the problem of inferring the dynamical parameters of a quantum system. We design the algorithm with practicality in mind by including parameters that control trade-offs between the requirements on computational and experimental resources. The algorithm can be implemented online (during experimental data collection), avoiding the need for storage and post-processing. Most importantly, our algorithm is capable of learning Hamiltonian parameters even when the parameters change from experiment-to-experiment, and also when additional noise processes are present and unknown. The algorithm also numerically estimates the Cramer-Rao lower bound, certifying its own performance.

quant-ph↗

arXiv2012

Radical-free dynamic nuclear polarization using electronic defects in silicon

Direct dynamic nuclear polarization of 1H nuclei in frozen water and water-ethanol mixtures is demonstrated using silicon nanoparticles as the polarizing agent. Electron spins at dangling-bond sites near the silicon surface are identified as the source of the nuclear hyperpolarization. This novel polarization method open new avenues for the fabrication of surface engineered nanostructures to create high nuclear-spin polarized solutions without introducing contaminating radicals, and for the study of molecules adsorbed onto surfaces.

cond-mat.mes-hall↗

arXiv2012

How to best sample a periodic probability distribution, or on the accuracy of Hamiltonian finding strategies

Projective measurements of a single two-level quantum mechanical system (a qubit) evolving under a time-independent Hamiltonian produce a probability distribution that is periodic in the evolution time. The period of this distribution is an important parameter in the Hamiltonian. Here, we explore how to design experiments so as to minimize error in the estimation of this parameter. While it has been shown that useful results may be obtained by minimizing the risk incurred by each experiment, such an approach is computationally intractable in general. Here, we motivate and derive heuristic strategies for experiment design that enjoy the same exponential scaling as fully optimized strategies. We then discuss generalizations to the case of finite relaxation times, T_2 < \infty.

quant-ph↗

arXiv2011

Adaptive Hamiltonian Estimation Using Bayesian Experimental Design

Using Bayesian experimental design techniques, we have shown that for a single two-level quantum mechanical system under strong (projective) measurement, the dynamical parameters of a model Hamiltonian can be estimated with exponentially improved accuracy over offline estimation strategies. To achieve this, we derive an adaptive protocol which finds the optimal experiments based on previous observations. We show that the risk associated with this algorithm is close to the global optimum, given a uniform prior. Additionally, we show that sampling at the Nyquist rate is not optimal.

quant-ph↗

arXiv2010

Extending Quantum Coherence in Diamond

We experimentally demonstrate over two orders of magnitude increase in the coherence time of nitrogen vacancy centres in diamond by implementing decoupling techniques. We show that equal pulse spacing decoupling performs just as well as non-periodic Uhrig decoupling and has the additional benefit that it allows us to take advantage of "revivals" in the echo (due to the coherent nature of the bath) to explore the longest coherence times. At short times, we can extend the coherence of particular quantum states out from T_2*=2.7 us out to an effective T_2 > 340 us. For preserving arbitrary states we show the experimental importance of using pulse sequences, that through judicious choice of the phase of the pulses, compensate the imperfections of individual pulses for all input states. At longer times we use these compensated sequences to enhance the echo revivals and show a coherence time of over 1.6 ms in ultra-pure natural abundance 13C diamond.

quant-ph↗

arXiv2009

Testing contextuality on quantum ensembles with one clean qubit

We present a protocol to evaluate the expectation value of the correlations of measurement outcomes for ensembles of quantum systems, and use it to experimentally demonstrate--under an assumption of fair sampling--the violation of an inequality that is satisfied by any non-contextual hidden-variables (NCHV) theory. The experiment is performed on an ensemble of molecular nuclear spins in the solid state, using established Nuclear Magnetic Resonance (NMR) techniques for quantum information processing (QIP).

quant-ph↗

arXiv2009

Error characterization in Quantum Information Processing: a protocol for analyzing spatial correlations and its experimental implementation

We present a protocol for error characterization and its experimental implementation with 4 qubits in liquid state NMR. The method is designed to retrieve information about spatial correlations and scales as $O(n^w)$, where $w$ is the maximum number of qubits that have non-negligible interaction. We discuss the practical aspects regarding accuracy and implementation.

quant-ph↗

arXiv2009

Hyperpolarized Long-T1 Silicon Nanoparticles for Magnetic Resonance Imaging

Silicon nanoparticles are experimentally investigated as a potential hyperpolarized, targetable MRI imaging agent. Nuclear T_1 times at room temperature for a variety of Si nanoparticles are found to be remarkably long (10^2 to 10^4 s) - roughly consistent with predictions of a core-shell diffusion model - allowing them to be transported, administered and imaged on practical time scales without significant loss of polarization. We also report surface functionalization of Si nanoparticles, comparable to approaches used in other biologically targeted nanoparticle systems.

cond-mat.mtrl-sci↗

arXiv2008

Dynamic Nuclear Polarization in Silicon Microparticles

We report record high Si-29 spin polarization obtained using dynamic nuclear polarization in microcrystalline silicon powder. Unpaired electrons in this silicon powder are due to dangling bonds in the amorphous region of this intrinsically heterogeneous sample. Si-29 nuclei in the amorphous region become polarized by forced electron-nuclear spin flips driven by off-resonant microwave radiation while nuclei in the crystalline region are polarized by spin diffusion across crystalline boundaries. Hyperpolarized silicon microparticles have long T1 relaxation times and could be used as tracers for magnetic resonance imaging.

cond-mat.mes-hall↗

arXiv2007

Quantum information processing using nuclear and electron magnetic resonance: review and prospects

This paper describes recent progress using nuclear magnetic resonance (NMR) as a platform for implementing quantum information processing (QIP) tasks. The basic ideas of NMR QIP are detailed, examining the successes and limitations of liquid and solid state experiments. Finally, a future direction for implementing quantum processors is suggested,utilizing both nuclear and electron spin degrees of freedom.

quant-ph↗

arXiv2006

Experimental Implementation of Logical Bell State Encoding

Liquid phase NMR is a general purpose test-bed for developing methods of coherent control relevant to quantum information processing. Here we extend these studies to the coherent control of logical qubits and in particular to the unitary gates necessary to create entanglement between logical qubits. We report an experimental implementation of a conditional logical gate between two logical qubits that are each in decoherence free subspaces that protect the quantum information from fully correlated dephasing.

quant-ph↗

arXiv2006

Subsystem Pseudo-pure States

A critical step in experimental quantum information processing (QIP) is to implement control of quantum systems protected against decoherence via informational encodings, such as quantum error correcting codes, noiseless subsystems and decoherence free subspaces. These encodings lead to the promise of fault tolerant QIP, but they come at the expense of resource overheads. Part of the challenge in studying control over multiple logical qubits, is that QIP test-beds have not had sufficient resources to analyze encodings beyond the simplest ones. The most relevant resources are the number of available qubits and the cost to initialize and control them. Here we demonstrate an encoding of logical information that permits the control over multiple logical qubits without full initialization, an issue that is particularly challenging in liquid state NMR. The method of subsystem pseudo-pure state will allow the study of decoherence control schemes on up to 6 logical qubits using liquid state NMR implementations.

quant-ph↗

arXiv2006

Efficient error characterization in Quantum Information Processing

We describe how to use the fidelity decay as a tool to characterize the errors affecting a quantum information processor through a noise generator $G_τ$. For weak noise, the initial decay rate of the fidelity proves to be a simple way to measure the magnitude of the different terms in $G_τ$. When the generator has only terms associated with few-body couplings, our proposal is scalable. We present the explicit protocol for estimating the magnitude of the noise generators when the noise consists of only one and two-body terms, and describe a method for measuring the parameters of more general noise models. The protocol focuses on obtaining the magnitude with which these terms affect the system during a time step of length $τ$; measurement of this information has critical implications for assesing the scalability of fault-tolerant quantum computation in any physical setup.

quant-ph↗

arXiv2006

Principles of Control for Decoherence-Free Subsystems

Decoherence-Free Subsystems (DFS) are a powerful means of protecting quantum information against noise with known symmetry properties. Although Hamiltonians theoretically exist that can implement a universal set of logic gates on DFS encoded qubits without ever leaving the protected subsystem, the natural Hamiltonians that are available in specific implementations do not necessarily have this property. Here we describe some of the principles that can be used in such cases to operate on encoded qubits without losing the protection offered by the DFS. In particular, we show how dynamical decoupling can be used to control decoherence during the unavoidable excursions outside of the DFS. By means of cumulant expansions, we show how the fidelity of quantum gates implemented by this method on a simple two-physical-qubit DFS depends on the correlation time of the noise responsible for decoherence. We further show by means of numerical simulations how our previously introduced "strongly modulating pulses" for NMR quantum information processing can permit high-fidelity operations on multiple DFS encoded qubits in practice, provided that the rate at which the system can be modulated is fast compared to the correlation time of the noise. The principles thereby illustrated are expected to be broadly applicable to many implementations of quantum information processors based on DFS encoded qubits.

quant-ph↗

arXiv2006

Benchmarking quantum control methods on a 12-qubit system

In this letter, we present an experimental benchmark of operational control methods in quantum information processors extended up to 12 qubits. We implement universal control of this large Hilbert space using two complementary approaches and discuss their accuracy and scalability. Despite decoherence, we were able to reach a 12-coherence state (or 12-qubits pseudo-pure cat state), and decode it into an 11 qubit plus one qutrit labeled observable pseudo-pure state using liquid state nuclear magnetic resonance quantum information processors.

quant-ph↗

arXiv2005

Hydrodynamic approach to coherent nuclear spin transport

We develop a linear response formalism for nuclear spin diffusion in a dipolar coupled solid. The theory applies to the high-temperature, long-wavelength regime studied in the recent experiments of Boutis et al. [Phys. Rev. Lett. 92, 137201 (2004)], which provided direct measurement of interspin energy diffusion in such a system. A systematic expansion of Kubo's formula in the flip-flop term of the Hamiltonian is used to calculate the diffusion coefficients. We show that this approach is equivalent to the method of Lowe and Gade [Phys. Rev. 156, 817 (1967)] and Kaplan [Phys. Rev. B 2, 4578 (1970)], but has several calculational and conceptual advantages. Although the lowest orders in this expansion agree with the experimental results for magnetization diffusion, this is not the case for energy diffusion. Possible reasons for this disparity are suggested.

cond-mat.mes-hall↗

arXiv2004

Read out of a Nuclear Spin Qubit

We propose a detector to read out the state of a single nuclear spin, with potential application in future scalable NMR quantum computers. It is based on a ``spin valve'' between bulk nuclear spin systems that is highly sensitive to the state of the measured spin. We suggest a concrete realization of that detector in a Silicon lattice. Transport of spin through the proposed spin valve is analogous to that of charge through an electronic nanostructure, but exhibits distinctive new features.

quant-ph↗

arXiv2004

Xenon NMR Measurements of Permeability and Tortuosity in Reservoir Rocks

In this work we present measurements of permeability, effective porosity and tortuosity on a variety of rock samples using NMR/MRI of thermal and laser-polarized gas. Permeability and effective porosity are measured simultaneously using MRI to monitor the inflow of laser-polarized xenon into the rock core. Tortuosity is determined from measurements of the time-dependent diffusion coefficient using thermal xenon in sealed samples. The initial results from a limited number of rocks indicate inverse correlations between tortuosity and both effective porosity and permeability. Further studies to widen the number of types of rocks studied may eventually aid in explaining the poorly understood connection between permeability and tortuosity of rock cores.

cond-mat.mtrl-sci↗