Contact Info

  • Room 4004
  • School of Physics, SNH
  • The University of Sydney
  • Camperdown, NSW 2006
  • Australia
  • (map)

Research Projects

Each of the research projects below highlights a particular aspect of quantum information theory that my research group and I focus on.

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    Topologically Ordered Quantum Phases

    What are the possible phases of matter in the quantum world, and can we exploit them for for quantum computing?

    Coming soon...

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    Quantum Error Correction

    Quantum computers will be prone to errors; here's how to prevent them from ruining your computation.

    Coming soon...

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    Quantum Tomography & Compressed Sensing

    How can you debug a quantum computer if even looking at it destroys the fragile superpositions that power the device?

    Coming soon...

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    Tensor Networks

    Efficient methods for simulating complex many-body quantum systems.

    Coming soon...

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    Measurement-based Quantum Computing

    With a little entanglement as a catalyst, even just single-system measurements can actuate a quantum computation.

    Coming soon...

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    Quantum Metrology

    The study of the most precise measurements achievable in an uncertain quantum world.

    Coming soon...

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Explaining quantum correlations through evolution of causal models

Robin Harper, Robert J. Chapman, Christopher Ferrie, Christopher Granade, Richard Kueng, Daniel Naoumenko, STF, Alberto Peruzzo
submitted
arXiv:1608.03281

Abstract

We propose a framework for the systematic and quantitative generalization of Bell's theorem using causal networks. We first consider the multi-objective optimization problem of matching observed data while minimizing the causal effect of nonlocal variables and prove an inequality for the optimal region that both strengthens and generalizes Bell's theorem. To solve the optimization problem (rather than simply bound it), we develop a novel genetic algorithm treating as individuals causal networks. By applying our algorithm to a photonic Bell experiment, we demonstrate the trade-off between the quantitative relaxation of one or more local causality assumptions and the ability of data to match quantum correlations.

Estimating the fidelity of $T$ gates using standard interleaved randomized benchmarking

Robin Harper and STF
submitted
arXiv:1608.02943

Abstract

Randomized benchmarking (RB) is an important protocol for robustly characterizing the error rates of quantum gates. The technique is typically applied to the Clifford gates since they form a group that satisfies a convenient technical condition of forming a unitary 2-design, in addition to having a tight connection to fault-tolerant quantum computing and an efficient classical simulation. In order to achieve universal quantum computing one must add at least one additional gate such as the $T$ gate (also known as the $\pi$/8 gate). Here we propose and analyze a simple variation of the standard interleaved RB protocol that can accurately estimate the average fidelity of the $T$ gate while retaining the many advantages of a unitary 2-design and the fidelity guarantees that such a design delivers, as well as the efficient classical simulation property of the Clifford group. Our work complements prior methods that have succeeded in estimating $T$ gate fidelities, but only by relaxing the 2-design constraint and using a more complicated data analysis.

Approximate symmetries of Hamiltonians

Christopher T. Chubb and STF
submitted
arXiv:1608.02600

Abstract

We explore the relationship between approximate symmetries of a gapped Hamiltonian and the structure of its ground space. We start by showing that approximate symmetry operators---unitary operators whose commutators with the Hamiltonian have norms that are sufficiently small---which possess certain mutual commutation relations can be restricted to the ground space with low distortion. We generalize the Stone-von Neumann theorem to matrices that approximately satisfy the canonical (Heisenberg-Weyl-type) commutation relations, and use this to show that approximate symmetry operators can certify the degeneracy of the ground space even though they only approximately form a group. Importantly, the notions of "approximate" and "small" are all independent of the dimension of the ambient Hilbert space, and depend only on the degeneracy in the ground space. Our analysis additionally holds for any gapped band of sufficiently small width in the excited spectrum of the Hamiltonian, and we discuss applications of these ideas to topological quantum phases of matter and topological quantum error correcting codes. Finally, in our analysis we also provide an exponential improvement upon bounds concerning the existence of shared approximate eigenvectors of approximately commuting operators which may be of independent interest.

Experimental quantum compressed sensing for a seven-qubit system

C. Riofrio, D. Gross, STF, T. Monz, D. Nigg, R. Blatt, and J. Eisert
submitted
arXiv:1608.02263

Abstract

Well-controlled quantum devices with their increasing system size face a new roadblock hindering further development of quantum technologies: The effort of quantum tomography-the characterization of processes and states within a quantum device-scales unfavorably to the point that state-of-the-art systems can no longer be treated. Quantum compressed sensing mitigates this problem by reconstructing the state from an incomplete set of observables. In this work, we present an experimental implementation of compressed tomography of a seven qubit system---the largest-scale realization to date---and we introduce new numerical methods in order to scale the reconstruction to this dimension. Originally, compressed sensing has been advocated for density matrices with few non-zero eigenvalues. Here, we argue that the low-rank estimates provided by compressed sensing can be appropriate even in the general case. The reason is that statistical noise often allows only for the leading eigenvectors to be reliably reconstructed: We find that the remaining eigenvectors behave in a way consistent with a random matrix model that carries no information about the true state. We report a reconstruction of quantum states from a topological color code of seven qubits, prepared in a trapped ion architecture, based on tomographically incomplete data involving 127 Pauli basis measurement settings only, repeated 100 times each.

Practical Adaptive Quantum Tomography

Christopher Granade, Christopher Ferrie and STF
Submitted
arXiv:1605.05039

Abstract

We introduce a fast and accurate heuristic for adaptive tomography that addresses many of the limitations of prior methods. Previous approaches were either too computationally intensive or tailored to handle special cases such as single qubits or pure states. By contrast, our approach combines the efficiency of online optimization with generally applicable and well-motivated data-processing techniques. We numerically demonstrate these advantages in several scenarios including mixed states, higher-dimensional systems, and restricted measurements.

Generating Ray Class Fields of Real Quadratic Fields via Complex Equiangular Lines

Marcus Appleby, STF, Gary McConnell and Jon Yard
Submitted
arXiv:1604.06098

Abstract

Let $K$ be a real quadratic field. For certain $K$ with sufficiently small discriminant we produce explicit unit generators for specific ray class fields of $K$ using a numerical method that arose in the study of complete sets of equiangular lines in $\CC^d$ (known in quantum information as symmetric informationally complete measurements or SICs). The construction in low dimensions suggests a general recipe for producing unit generators in infinite towers of ray class fields above arbitrary $K$ and we summarise this in a conjecture. Such explicit generators are notoriously difficult to find, so this recipe may be of some interest.

In a forthcoming paper we shall publish promising results of numerical comparisons between the logarithms of these canonical units and the values of $L$-functions associated to the extensions, following the programme laid out in the Stark Conjectures.

Detecting Topological Order with Ribbon Operators

J. C. Bridgeman, STF and D. Poulin
Phys. Rev. B 94, 205123 (2016)
arXiv:1603.02275

Abstract

We introduce a numerical method for identifying topological order in two-dimensional models based on one-dimensional bulk operators. The idea is to identify approximate symmetries supported on thin strips through the bulk that behave as string operators associated to an anyon model. We can express these ribbon operators in matrix product form and define a cost function that allows us to efficiently optimize over this ansatz class. We test this method on spin models with abelian topological order by finding ribbon operators for $\ZZ_d$ quantum double models with local fields and Ising-like terms. In addition, we identify ribbons in the abelian phase of Kitaev's honeycomb model which serve as the logical operators of the encoded qubit for the quantum error-correcting code. We further identify the topologically encoded qubit in the quantum compass model, and show that despite this qubit, the model does not support topological order. Finally, we discuss how the method supports generalizations for detecting nonabelian topological order.

Comparing Experiments to the Fault-Tolerance Threshold

R. Kueng, D. M. Long, A. C. Doherty and STF
Phys. Rev. Lett. 117 170502 (2016)
arXiv:1510.05653

Abstract

Achieving error rates that meet or exceed the fault-tolerance threshold is a central goal for quantum computing experiments, and measuring these error rates using randomized benchmarking is now routine. However, direct comparison between measured error rates and thresholds is complicated by the fact that benchmarking estimates average error rates while thresholds reflect worst-case behavior when a gate is used as part of a large computation. These two measures of error can differ by orders of magnitude in the regime of interest. Here we facilitate comparison between the experimentally accessible average error rates and the worst-case quantities that arise in current threshold theorems by deriving relations between the two for a variety of physical noise sources. Our results indicate that it is coherent errors that lead to an enormous mismatch between average and worst case, and we quantify how well these errors must be controlled to ensure fair comparison between average error probabilities and fault-tolerance thresholds.

Symmetry-respecting real-space renormalization for the quantum Ashkin-Teller model

A. O'Brien, S. D. Bartlett, A. C. Doherty and STF
Phys. Rev. E 92 042163 (2015)
arXiv:1507.00038

Abstract

We use a simple real-space renormalization group approach to investigate the critical behavior of the quantum Ashkin-Teller model, a one-dimensional quantum spin chain possessing a line of criticality along which critical exponents vary continuously. This approach, which is based on exploiting the on-site symmetry of the model, has been shown to be surprisingly accurate for predicting some aspects of the critical behavior of the Ising model. Our investigation explores this approach in more generality, in a model where the critical behavior has a richer structure but which reduces to the simpler Ising case at a special point. We demonstrate that the correlation length critical exponent as predicted from this real-space renormalization group approach is in broad agreement with the corresponding results from conformal field theory along the line of criticality. Near the Ising special point, the error in the estimated critical exponent from this simple method is comparable to that of numerically-intensive simulations based on much more sophisticated methods, although the accuracy decreases away from the decoupled Ising model point.

Classical Simulation of Quantum Error Correction in a Fibonacci Anyon Code

S. Burton, C. G. Brell and STF
Submitted
arXiv:1506.03815

Abstract

Classically simulating the dynamics of anyonic excitations in two-dimensional quantum systems is likely intractable in general because such dynamics are sufficient to implement universal quantum computation. However, processes of interest for the study of quantum error correction in anyon systems are typically drawn from a restricted class that displays significant structure over a wide range of system parameters. We exploit this structure to classically simulate, and thereby demonstrate the success of, an error-correction protocol for a quantum memory based on the universal Fibonacci anyon model. We numerically simulate a phenomenological model of the system and noise processes on lattice sizes of up to 128x128 sites, and find a lower bound on the error-correction threshold of approximately 12.5%, which is comparable to those previously known for abelian and (non-universal) nonabelian anyon models.

The Effect of Noise Correlations on Randomized Benchmarking

H. Ball, T. M. Stace, STF, and M. J. Biercuk
Phys. Rev. A 93, 022303 (2016)
arXiv:1504.05307

Abstract

Randomized benchmarking is an important statistical tool for characterizing the performance of high-fidelity quantum gates. Here we study randomized benchmarking in the presence of quasi-static (i.e. low-frequency) errors. We contrast the resulting fidelity with that under the conventional assumption of Markovian errors. We treat these limiting cases analytically by mapping the averaging process to a random walk in "Pauli space". In the quasi-static case, we find a broad, highly skewed distribution of fidelities, while Markovian errors produce a narrow, approximately Gaussian distribution of fidelities. We use the filter-transfer function formalism to reveal the underlying reason for these differences in terms of effective coherent averaging of correlated errors in certain random sequences. Large skew in the distribution towards high-fidelity outcomes -- consistent with existing experimental data -- highlights potential finite-sampling pitfalls when deploying randomized benchmarking. Moreover, these results demonstrate general challenges in extracting useful single-gate fidelities from randomized benchmarking measurements with correlated errors.

Error Compensation of Single-Qubit Gates in a Surface Electrode Ion Trap Using Composite Pulses

E. Mount, C. Kabytayev, S. Crain, R. Harper, S.-Y. Baek, G. Vrijsen, STF, K. R. Brown, P. Maunz, J. Kim
Phys. Rev. A 92, 060301(R) (2015)
arXiv:1504.01440

Abstract

The trapped atomic ion qubits feature desirable properties for use in a quantum computer such as long coherence times, high qubit measurement fidelity, and universal logic gates. The quality of quantum logic gate operations on trapped ion qubits has been limited by the stability of the control fields at the ion location used to implement the gate operations. For this reason, the logic gates utilizing microwave fields have shown gate fidelities several orders of magnitude better than those using laser fields. Here, we demonstrate low-error single-qubit gates performed using stimulated Raman transitions on an ion qubit trapped in a microfabricated chip trap. Gate errors are measured using a randomized benchmarking protocol, where amplitude error in the control beam is compensated using various pulse sequence techniques. Using B2 compensation, we demonstrate single-qubit gates with an average error per randomized Clifford group gate of 3.6(3) × 10−4. We also show that compact palindromic pulse compensation sequences (PDn) compensate for amplitude errors as designed.

Estimating the Coherence of Noise

Joel Wallman, Christopher Granade, Robin Harper, and STF
New J. Phys. 17 113020 (2015)
arXiv:1503.07865

Abstract

Noise mechanisms in quantum systems can be broadly characterized as either coherent (i.e., unitary) or incoherent. For a given fixed average error rate, coherent noise mechanisms will generally lead to a larger worst-case error than incoherent noise. We show that the coherence of a noise source can be quantified by the unitarity, which we relate to the average change in purity averaged over input pure states. We then show that the unitarity can be efficiently estimated using a protocol based on randomized benchmarking that is efficient and robust to state-preparation and measurement errors.

Computing the Degenerate Ground Space of Gapped Spin Chains in Polynomial Time

Christopher T. Chubb and STF
Chicago J. Theor. Comp. Sci., 2016 (9), 1-35 (2016)
arXiv:1502.06967

Abstract

Given a gapped Hamiltonian of a spin chain, we give a polynomial-time algorithm for finding the degenerate ground space projector. The output is an orthonormal set of matrix product states that approximate the true ground space projector up to an inverse polynomial error in any Schatten norm, with a runtime exponential in the degeneracy. Our algorithm is an extension of the recent algorithm of Landau, Vazirani, and Vidick for the nondegenerate case, and it includes the recent improvements due to Huang. The main new idea is to incorporate the local distinguishability of ground states on the half-chain to ensure that the algorithm returns a complete set of global ground states.

Non-exponential Fidelity Decay in Randomized Benchmarking with Low-Frequency Noise

M. A. Fogarty, M. Veldhorst, R. Harper, H. Yang, S. D. Bartlett, STF, and A. S. Dzurak
Phys. Rev. A 92, 022326 (2015)
arXiv:1502.05119

Abstract

We show that non-exponential fidelity decays in randomized benchmarking experiments on quantum dot qubits are consistent with numerical simulations that incorporate low-frequency noise. By expanding standard randomized benchmarking analysis to this experimental regime, we find that such non-exponential decays are better modeled by multiple exponential decay rates, leading to an instantaneous control fidelity for isotopically-purified-silicon MOS quantum dot qubits which can be as high as 99.9% when low-frequency noise conditions and system calibrations are favorable. These advances in qubit characterization and validation methods underpin the considerable prospects for silicon as a qubit platform for fault-tolerant quantum computation.

Sparse Quantum Codes from Quantum Circuits

Dave Bacon, STF, Aram W. Harrow and Jonathan Shi
STOC 2015
arXiv:1411.3334

Abstract

We describe a general method for turning quantum circuits into sparse quantum subsystem codes. Using this prescription, we can map an arbitrary stabilizer code into a new subsystem code with the same distance and number of encoded qubits but where all the generators have constant weight, at the cost of adding some ancilla qubits. With an additional overhead of ancilla qubits, the new code can also be made spatially local.

Applying our construction to certain concatenated stabilizer codes yields families of subsystem codes with constant-weight generators and with minimum distance $d = n^{1-\varepsilon}$, where $\varepsilon = O(1/\sqrt{\log n})$. For spatially local codes in $D$ dimensions we nearly saturate a bound due to Bravyi and Terhal and achieve $d = n^{1-\varepsilon-1/D}$. Previously the best code distance achievable with constant-weight generators in any dimension, due to Freedman, Meyer and Luo, was $O(\sqrt{n\log n})$ for a stabilizer code.

Adiabatic Topological Quantum Computation

Chris Cesare, Andrew J. Landahl, Dave Bacon, STF, and Alice Neels
Phys. Rev. A 92, 012336 (2015)
arXiv:1406.2690

Abstract

Topological quantum computing promises error-resistant quantum computation without active error correction. However, there is a worry that during the process of executing quantum gates by braiding anyons around each other, extra anyonic excitations will be created that will disorder the encoded quantum information. Here we explore this question in detail by studying adiabatic code deformations on Hamiltonians based on topological codes, notably Kitaev's surface codes and the more recently discovered color codes. We develop protocols that enable universal quantum computing by adiabatic evolution in a way that keeps the energy gap of the system constant with respect to the computation size and introduces only simple local Hamiltonian interactions. This allows one to perform holonomic quantum computing with these topological quantum computing systems. The tools we develop allow one to go beyond numerical simulations and understand these processes analytically.

Randomized Benchmarking with Confidence

Joel J. Wallman and STF
New J. Phys 14, 103032 (2014)
arXiv:1404.6025

Abstract

Randomized benchmarking is a promising tool for characterizing the noise in experimental implementations of quantum systems. In this paper, we prove that the estimates produced by randomized benchmarking (both standard and interleaved) for arbitrary Markovian noise sources are remarkably precise by showing that the variance due to sampling random gate sequences is small. We discuss how to choose experimental parameters, in particular the number and lengths of random sequences, in order to characterize average gate errors with rigorous confidence bounds. We also show that randomized benchmarking can be used to reliably characterize time-dependent Markovian noise (e.g., when noise is due to a magnetic field with fluctuating strength). Moreover, we identify a necessary property for time-dependent noise that is violated by some sources of non-Markovian noise, which provides a test for non-Markovianity.

Local PT symmetry violates the no-signaling principle

Yi-Chan Lee, Min-Hsiu Hsieh, STF, and Ray-Kuang Lee
Phys. Rev. Lett. 112, 130404 (2014)
arXiv:1312.3395

Abstract

Bender et al. have developed PT-symmetric quantum theory as an extension of quantum theory to non-Hermitian Hamiltonians. We show that when this model has a local PT symmetry acting on composite systems it violates the non-signaling principle of relativity. Since the case of global PT symmetry is known to reduce to standard quantum mechanics, this shows that the PT-symmetric theory is either a trivial extension or likely false as a fundamental theory.

Thermalization, Error-Correction, and Memory Lifetime for Ising Anyon Systems

Courtney G. Brell, Simon Burton, Guillaume Dauphinais, STF, and David Poulin
Phys. Rev. X 4, 031058 (2014)
arXiv:1311.0019

Abstract

We consider two-dimensional lattice models that support Ising anyonic excitations and are coupled to a thermal bath. We propose a phenomenological model for the resulting short-time dynamics that includes pair-creation, hopping, braiding, and fusion of anyons. By explicitly constructing topological quantum error-correcting codes for this class of system, we use our thermalization model to estimate the lifetime of the quantum information stored in the encoded spaces. To decode and correct errors in these codes, we adapt several existing topological decoders to the non-Abelian setting. We perform large-scale numerical simulations of these two-dimensional Ising anyon systems and find that the thresholds of these models range between 13% to 25%. To our knowledge, these are the first numerical threshold estimates for quantum codes without explicit additive structure.

Programmable quantum simulation by dynamic Hamiltonian engineering

David L. Hayes, STF, and Michael J. Biercuk
New J. Phys. 16 083027 (2014).
arXiv:1309.6736

Abstract

Quantum simulation is a promising near term application for mesoscale quantum information processors, with the potential to solve computationally intractable problems at the scale of just a few dozen interacting quantum systems. Recent experiments in a range of technical platforms have demonstrated the basic functionality of quantum simulation applied to quantum magnetism, quantum phase transitions, and relativistic quantum mechanics. In all cases, the underlying hardware platforms restrict the achievable inter-particle interaction, forming a serious constraint on the ability to realize a versatile, programmable, quantum simulator. In this work, we address this problem by developing novel sequences of unitary operations that engineer desired effective Hamiltonians in the time-domain. The result is a hybrid programmable analog simulator permitting a broad class of interacting spin-lattice models to be generated starting only with an arbitrary native inter-particle interaction and single-qubit addressing. Building on previous work proving that universal simulation is possible using both entangling gates and single-qubit unitaries, we show how determining the relevant hardware "program" of unitary pulses to implement an arbitrary spin Hamiltonian on such a simulator can be formulated as a linear program that runs in polynomial time and scales efficiently in hardware resources. Our analysis extends from circuit model quantum information to adiabatic quantum evolutions, where our approach allows for the creation of non-native ground state solutions to a computation.

Adiabatic Quantum Transistors

Dave Bacon, STF, and Gregory M. Crosswhite
Phys. Rev. X 3, 021015 (2013)
arXiv:1207.2769

Abstract

We describe a many-body quantum system which can be made to quantum compute by the adiabatic application of a large applied field to the system. Prior to the application of the field quantum information is localized on one boundary of the device, and after the application of the field this information has propagated to the other side of the device with a quantum circuit applied to the information. The applied circuit depends on the many-body Hamiltonian of the material, and the computation takes place in a degenerate ground space with symmetry-protected topological order. Such adiabatic quantum transistors are universal adiabatic quantum computing devices which have the added benefit of being modular. Here we describe this model, provide arguments for why it is an efficient model of quantum computing, and examine these many-body systems in the presence of a noisy environment.

Counterexamples to Kalai's Conjecture C

STF and Aram W. Harrow
Quant. Inf. & Comp. 13 No.1-2, pp.1-8 (2013)
arXiv:1204.3404

Abstract

We provide two simple counterexamples to Kalai's Conjecture C and discuss our perspective on the implications for the prospect of large-scale fault-tolerant quantum computation.

Quantum Tomography via Compressed Sensing: Error Bounds, Sample Complexity, and Efficient Estimators

STF, David Gross, Yi-Kai Liu, and Jens Eisert
New J. Phys. 14, 095022 (2012)
arXiv:1205.2300

Abstract

Intuitively, if a density operator has small rank, then it should be easier to estimate from experimental data, since in this case only a few eigenvectors need to be learned. We prove two complementary results that confirm this intuition. First, we show that a low-rank density matrix can be estimated using fewer copies of the state, i.e., the sample complexity of tomography decreases with the rank. Second, we show that unknown low-rank states can be reconstructed from an incomplete set of measurements, using techniques from compressed sensing and matrix completion. These techniques use simple Pauli measurements, and their output can be certified without making any assumptions about the unknown state.

We give a new theoretical analysis of compressed tomography, based on the restricted isometry property (RIP) for low-rank matrices. Using these tools, we obtain near-optimal error bounds, for the realistic situation where the data contains noise due to finite statistics, and the density matrix is full-rank with decaying eigenvalues. We also obtain upper-bounds on the sample complexity of compressed tomography, and almost-matching lower bounds on the sample complexity of any procedure using adaptive sequences of Pauli measurements.

Using numerical simulations, we compare the performance of two compressed sensing estimators -- the matrix Dantzig selector and the matrix Lasso -- with standard maximum-likelihood estimation (MLE). We find that, given comparable experimental resources, the compressed sensing estimators consistently produce higher-fidelity state reconstructions than MLE. In addition, the use of an incomplete set of measurements leads to faster classical processing with no loss of accuracy.

Finally, we show how to certify the accuracy of a low rank estimate using direct fidelity estimation and we describe a method for compressed quantum process tomography that works for processes with small Kraus rank, and requires only Pauli eigenstate preparations and Pauli measurements.

Computational Difficulty of Computing the Density of States

Brielin Brown, STF, and Norbert Schuch
Phys. Rev. Lett. 107, 040501 (2011)
arXiv:1010.3060

Abstract

We study the computational difficulty of computing the ground state degeneracy and the density of states for local Hamiltonians. We show that the difficulty of both problems is exactly captured by a class which we call #BQP, which is the counting version of the quantum complexity class QMA. We show that #BQP is not harder than its classical counting counterpart #P, which in turn implies that computing the ground state degeneracy or the density of states for classical Hamiltonians is just as hard as it is for quantum Hamiltonians.

Toric codes and quantum doubles from two-body Hamiltonians

Courtney G. Brell, STF, Stephen D. Bartlett, Andrew C. Doherty
New J. Phys. 13, 053039 (2011)
arXiv:1011.1942

Abstract

We present a procedure to obtain the Hamiltonians of the toric code and Kitaev quantum double models as the low-energy limits of entirely two-body Hamiltonians. Our construction makes use of a new type of perturbation gadget based on error-detecting subsystem codes. The procedure is motivated by a PEPS description of the target models, and reproduces the target models' behavior using only couplings which are natural in terms of the original Hamiltonians. This allows our construction to exactly capture the symmetries of the target models.

Direct Fidelity Estimation from Few Pauli Measurements

STF and Yi-Kai Liu
Phys. Rev. Lett. 106, 230501 (2011)
arXiv:1104.4695

Abstract

We describe a simple method for certifying that an experimental device prepares a desired quantum state $\rho$. Our method is applicable to any pure state $\rho$, and it provides an estimate of the fidelity between $\rho$ and the actual (arbitrary) state in the lab, up to a constant additive error. The method requires measuring only a constant number of Pauli expectation values, selected at random according to an importance-weighting rule. Our method is faster than full tomography by a factor of $d$, the dimension of the state space, and extends easily and naturally to quantum channels.

Graphical calculus for Gaussian pure states

Nicolas C. Menicucci, STF, and Peter van Loock
Phys. Rev. A 83, 042335 (2011)
arXiv:1007.0725

Abstract

We provide a unified graphical calculus for all Gaussian pure states, including graph transformation rules for all local and semi-local Gaussian unitary operations, as well as local quadrature measurements. We then use this graphical calculus to analyze continuous-variable (CV) cluster states, the essential resource for one-way quantum computing with CV systems. Current graphical approaches to CV cluster states are only valid in the unphysical limit of infinite squeezing, and the associated graph transformation rules only apply when the initial and final states are of this form. Our formalism applies to all Gaussian pure states and subsumes these rules in a natural way. In addition, the term "CV graph state" currently has several inequivalent definitions in use. Using this formalism we provide a single unifying definition that encompasses all of them. We provide many examples of how the formalism may be used in the context of CV cluster states: defining the "closest" CV cluster state to a given Gaussian pure state and quantifying the error in the approximation due to finite squeezing; analyzing the optimality of certain methods of generating CV cluster states; drawing connections between this new graphical formalism and bosonic Hamiltonians with Gaussian ground states, including those useful for CV one-way quantum computing; and deriving a graphical measure of bipartite entanglement for certain classes of CV cluster states. We mention other possible applications of this formalism and conclude with a brief note on fault tolerance in CV one-way quantum computing.

The Lie Algebraic Significance of Symmetric Informationally Complete Measurements

D. M. Appleby, STF,and Christopher A. Fuchs
J. Math. Phys. 52, 022202 (2011)
arXiv:1001.0004

Abstract

Examples of symmetric informationally complete positive operator valued measures (SIC-POVMs) have been constructed in every dimension less than or equal to 67. However, it remains an open question whether they exist in all finite dimensions. A SIC-POVM is usually thought of as a highly symmetric structure in quantum state space. However, its elements can equally well be regarded as a basis for the Lie algebra $\mathrm{gl}(d,\CC)$. In this paper we examine the resulting structure constants, which are calculated from the traces of the triple products of the SIC-POVM elements and which, it turns out, characterize the SIC-POVM up to unitary equivalence. We show that the structure constants have numerous remarkable properties. In particular we show that the existence of a SIC-POVM in dimension d is equivalent to the existence of a certain structure in the adjoint representation of $\mathrm{gl}(d,\CC)$. We hope that transforming the problem in this way, from a question about quantum state space to a question about Lie algebras, may help to make the existence problem tractable.

Efficient quantum state tomography

Marcus Cramer, Martin B. Plenio, STF, David Gross, Stephen D. Bartlett, Rolando Somma, Olivier Landon-Cardinal, Yi-Kai Liu, and David Poulin
Nat. Commun., 1 (9) 149, (2010)
arXiv:1101.4366

Abstract

Quantum state tomography, the ability to deduce the state of a quantum system from measured data, is the gold standard for verification and benchmarking of quantum devices. It has been realized in systems with few components, but for larger systems it becomes infeasible because the number of quantum measurements and the amount of computation required to process them grows exponentially in the system size. Here we show that we can do exponentially better than direct state tomography for a wide range of quantum states, in particular those that are well approximated by a matrix product state ansatz. We present two schemes for tomography in 1-D quantum systems and touch on generalizations. One scheme requires unitary operations on a constant number of subsystems, while the other requires only local measurements together with more elaborate post-processing. Both schemes rely only on a linear number of experimental operations and classical postprocessing that is polynomial in the system size. A further strength of the methods is that the accuracy of the reconstructed states can be rigorously certified without any a priori assumptions.

Quantum state tomography via compressed sensing

David Gross, Yi-Kai Liu, STF, Stephen Becker, Jens Eisert
Phys. Rev. Lett. 105, 150401 (2010)
arXiv:0909.3304

Abstract

We establish methods for quantum state tomography based on compressed sensing. These methods are specialized for quantum states that are fairly pure, and they offer a significant performance improvement on large quantum systems. In particular, they are able to reconstruct an unknown density matrix of dimension $d$ and rank $r$ using $O(rd \log^2 d)$ measurement settings, compared to standard methods that require $d^2$ settings. Our methods have several features that make them amenable to experimental implementation: they require only simple Pauli measurements, use fast convex optimization, are stable against noise, and can be applied to states that are only approximately low-rank. The acquired data can be used to certify that the state is indeed close to pure, so no a priori assumptions are needed. We present both theoretical bounds and numerical simulations.

Adiabatic Cluster State Quantum Computing

Dave Bacon and STF
Phys. Rev. A 82, 030303(R) (2010)
arXiv:0912.2098

Abstract

Models of quantum computation are important because they change the physical requirements for achieving universal quantum computation (QC). For example, one-way QC requires the preparation of an entangled "cluster" state followed by adaptive measurement on this state, a set of requirements which is different from the standard quantum circuit model. Here we introduce a model based on one-way QC but without measurements (except for the final readout), instead using adiabatic deformation of a Hamiltonian whose initial ground state is the cluster state. This opens the possibility to use the copious results from one-way QC to build more feasible adiabatic schemes.

Random unitary maps for quantum state reconstruction

Seth T. Merkel, Carlos A. Riofrio, STF, and Ivan H. Deutsch
Phys. Rev. A 81, 032126 (2010)
arXiv:0912.2101

Abstract

We study the possibility of performing quantum state reconstruction from a measurement record that is obtained as a sequence of expectation values of a Hermitian operator evolving under repeated application of a single random unitary map, $U_0$. We show that while this single-parameter orbit in operator space is not informationally complete, it can be used to yield surprisingly high-fidelity reconstruction. For a $d$-dimensional Hilbert space with the initial observable in $\mathrm{su}(d)$, the measurement record lacks information about a matrix subspace of dimension $\gt d-2$ out of the total dimension $d^2-1$. We determine the conditions on $U_0$ such that the bound is saturated, and show they are achieved by almost all pseudorandom unitary matrices. When we further impose the constraint that the physical density matrix must be positive, we obtain even higher fidelity than that predicted from the missing subspace. With prior knowledge that the state is pure, the reconstruction will be perfect (in the limit of vanishing noise) and for arbitrary mixed states, the fidelity is over 0.96, even for small $d$, and reaching $F \gt 0.99$ for $d \gt 9$. We also study the implementation of this protocol based on the relationship between random matrices and quantum chaos. We show that the Floquet operator of the quantum kicked top provides a means of generating the required type of measurement record, with implications on the relationship between quantum chaos and information gain.

Topological Entanglement Rényi Entropy and Reduced Density Matrix Structure

STF, Alioscia Hamma, Taylor L. Hughes, and Xiao-Gang Wen
Phys. Rev. Lett. 103, 261601 (2009)
arXiv:0909.3305

Abstract

We generalize the topological entanglement entropy to a family of topological Rényi entropies parametrized by a parameter alpha, in an attempt to find new invariants for distinguishing topologically ordered phases. We show that, surprisingly, all topological Rényi entropies are the same, independent of alpha for all non-chiral topological phases. This independence shows that topologically ordered ground-state wavefunctions have reduced density matrices with a certain simple structure, and no additional universal information can be extracted from the entanglement spectrum.

Adiabatic Gate Teleportation

Dave Bacon and STF
Phys. Rev. Lett. 103, 120504 (2009)
arXiv:0905.0901

Abstract

The difficulty in producing precisely timed and controlled quantum gates is a significant source of error in many physical implementations of quantum computers. Here we introduce a simple universal primitive, adiabatic gate teleportation, which is robust to timing errors and many control errors and maintains a constant energy gap throughout the computation above a degenerate ground state space. Notably this construction allows for geometric robustness based upon the control of two independent qubit interactions. Further, our piecewise adiabatic evolution easily relates to the quantum circuit model, enabling the use of standard methods from fault-tolerance theory for establishing thresholds.

The Optical Frequency Comb as a One-Way Quantum Computer

STF, Nicolas C. Menicucci, and Olivier Pfister
J. Phys. B 42, 114009 (2009)
arXiv:0811.2799

Abstract

In the one-way model of quantum computing, quantum algorithms are implemented using only measurements on an entangled initial state. Much of the hard work is done up-front when creating this universal resource, known as a cluster state, on which the measurements are made. Here we detail a new proposal for a scalable method of creating cluster states using only a single multimode optical parametric oscillator (OPO). The method generates a continuous-variable cluster state that is universal for quantum computation and encoded in the quadratures of the optical frequency comb of the OPO. This work expands on the presentation in Phys. Rev. Lett. 101, 130501 (2008).

Most quantum states are too entangled to be useful as computational resources

D. Gross, STF, and J. Eisert
Phys. Rev. Lett. 102, 190501 (2009)
arXiv:0810.4331

Abstract

It is often argued that entanglement is at the root of the speedup for quantum compared to classical computation, and that one needs a sufficient amount of entanglement for this speedup to be manifest. In measurement-based quantum computing (MBQC), the need for a highly entangled initial state is particularly obvious. Defying this intuition, we show that quantum states can be too entangled to be useful for the purpose of computation. We prove that this phenomenon occurs for a dramatic majority of all states: the fraction of useful n-qubit pure states is less than $\exp(-n^2)$. Computational universality is hence a rare property in quantum states. This work highlights a new aspect of the question concerning the role entanglement plays for quantum computational speed-ups. The statements remain true if one allows for certain forms of post-selection and also cover the notion of CQ-universality. We identify scale-invariant states resulting from a MERA construction as likely candidates for physically relevant states subject to this effect.

Weighing matrices and optical quantum computing

STF and Simone Severini
J. Phys. A: Math. Theor. 42, 065302 (2009)
arXiv:0808.2057

Abstract

Quantum computation in the one-way model requires the preparation of certain resource states known as cluster states. We describe how the construction of continuous-variable cluster states for optical quantum computing relate to the existence of certain families of matrices. The relevant matrices are known as weighing matrices, with a few additional constraints. We prove some results regarding the structure of these matrices, and their associated graphs.

Quantum Metrology: Dynamics vs. Entanglement

Sergio Boixo, Animesh Datta, Matthew J. Davis, STF, Anil Shaji, and Carlton M. Caves
Phys. Rev. Lett. 101, 040403 (2008)
arXiv:0805.2180

Abstract

A parameter whose coupling to a quantum probe of n constituents includes all two-body interactions between the constituents can be measured with an uncertainty that scales as $1/n^{3/2}$, even when the constituents are initially unentangled. We devise a protocol that achieves the $1/n^{3/2}$ scaling without generating any entanglement among the constituents, and we suggest that the protocol might be implemented in a two-component Bose-Einstein condensate.

One-Way Quantum Computing in the Optical Frequency Comb

Nicolas C. Menicucci, STF, and Olivier Pfister
Phys. Rev. Lett. 101, 130501 (2008)
arXiv:0804.4468

Abstract

One-way quantum computing allows any quantum algorithm to be implemented easily using just measurements. The difficult part is creating the universal resource, a cluster state, on which the measurements are made. We propose a radically new approach: a scalable method that uses a single, multimode optical parametric oscillator (OPO). The method is very efficient and generates a continuous-variable cluster state, universal for quantum computation, with quantum information encoded in the quadratures of the optical frequency comb of the OPO.

Entangling the optical frequency comb: simultaneous generation of multiple 2x2 and 2x3 continuous-variable cluster states in a single optical parametric oscillator

Hussain Zaidi, Nicolas C. Menicucci, STF, Russell Bloomer, Matthew Pysher, and Olivier Pfister
Laser Physics 18(5), 659 (2008)
arXiv:0710.4980

Abstract

We report on our research effort to generate large-scale multipartite optical-mode entanglement using as few physical resources as possible. We have previously shown that cluster- and GHZ-type $N$-partite continuous-variable entanglement can be obtained in an optical resonator that contains a suitably designed second-order nonlinear optical medium, pumped by at most $O(N^2)$ fields. In this paper, we show that the frequency comb of such a resonator can be entangled into an arbitrary number of independent $2\times 2$ and $2\times 3$ continuous-variable cluster states by a single optical parametric oscillator pumped by just a few optical modes.

Quantum-limited metrology with product states

Sergio Boixo, Animesh Datta, STF, Anil Shaji, Emilio Bagan, and Carlton M. Caves
Phys. Rev. A 77, 012317 (2007)
arXiv:0710.0285

Abstract

We study the performance of initial product states of $n$-body systems in generalized quantum metrology protocols that involve estimating an unknown coupling constant in a nonlinear $k$-body ($k \ll n$) Hamiltonian. We obtain the theoretical lower bound on the uncertainty in the estimate of the parameter. For arbitrary initial states, the lower bound scales as $1/n^k$, and for initial product states, it scales as $1/n^{k-1/2}$. We show that the latter scaling can be achieved using simple, separable measurements. We analyze in detail the case of a quadratic Hamiltonian ($k = 2$), implementable with Bose-Einstein condensates. We formulate a simple model, based on the evolution of angular-momentum coherent states, which explains the $O(n^{-3/2})$ scaling for $k = 2$; the model shows that the entanglement generated by the quadratic Hamiltonian does not play a role in the enhanced sensitivity scaling. We show that phase decoherence does not affect the $O(n^{-3/2})$ sensitivity scaling for initial product states.

Phase transition of computational power in the resource states for one-way quantum computation

Daniel E. Browne, Matthew B. Elliott, STF, Seth T. Merkel, Akimasa Miyake, and Anthony J. Short
New J. Phys. 10, 023010 (2008)
arXiv:0709.1729

Abstract

We study how heralded qubit losses during the preparation of a two-dimensional cluster state, a universal resource state for one-way quantum computation, affect its computational power. Above the percolation threshold we present a polynomial-time algorithm that concentrates a universal cluster state, using resources that scale optimally in the size of the original lattice. On the other hand, below the percolation threshold, we show that single qubit measurements on the faulty lattice can be efficiently simulated classically. We observe a phase transition at the threshold when the amount of entanglement in the faulty lattice directly relevant to the computational power changes exponentially.

Ultracompact Generation of Continuous-Variable Cluster States

Nicolas C. Menicucci, STF, Hussain Zaidi, and Olivier Pfister
Phys. Rev. A 76, 010302(R) (2007)
arXiv:quant-ph/0703096

Abstract

We propose an experimental scheme that has the potential for large-scale realization of continuous-variable (CV) cluster states for universal quantum computation. We do this by mapping CV cluster-state graphs onto two-mode squeezing graphs, which can be engineered into a single optical parametric oscillator (OPO). The desired CV cluster state is produced directly from a joint squeezing operation on the vacuum using a multi-frequency pump beam. This method has potential for ultracompact experimental implementation. As an illustration, we detail an experimental proposal for creating a four-mode square CV cluster state with a single OPO.

Constrained bounds on measures of entanglement

Animesh Datta, STF, Anil Shaji, and Carlton M. Caves
Phys. Rev. A 75, 062117 (2007)
arXiv:quant-ph/0612049

Abstract

Entanglement measures constructed from two positive, but not completely positive maps on density operators are used as constraints in placing bounds on the entanglement of formation, the tangle, and the concurrence of $4 \times N$ mixed states. The maps are the partial transpose map and the $\Phi$-map introduced by Breuer [H.-P. Breuer, Phys. Rev. Lett. 97, 080501 (2006)]. The norm-based entanglement measures constructed from these two maps, called negativity and $\Phi$-negativity, respectively, lead to two sets of bounds on the entanglement of formation, the tangle, and the concurrence. We compare these bounds and identify the sets of $4 \times N$ density operators for which the bounds from one constraint are better than the bounds from the other. In the process, we present a new derivation of the already known bound on the concurrence based on the negativity. We compute new bounds on the three measures of entanglement using both the constraints simultaneously. We demonstrate how such doubly constrained bounds can be constructed. We discuss extensions of our results to bipartite states of higher dimensions and with more than two constraints.

Generalized Limits for Single-Parameter Quantum Estimation

Sergio Boixo, Steven T. Flammia, Carlton M. Caves, and J. M. Geremia
Phys. Rev. Lett. 98, 090401 (2007)
arXiv:quant-ph/0609179

Abstract

We develop generalized bounds for quantum single-parameter estimation problems for which the coupling to the parameter is described by intrinsic multi-system interactions. For a Hamiltonian with $k$-system parameter-sensitive terms, the quantum limit scales as $1/N^k$ where $N$ is the number of systems. These quantum limits remain valid when the Hamiltonian is augmented by any parameter independent interaction among the systems and when adaptive measurements via parameter-independent coupling to ancillas are allowed.

On SIC-POVMs in Prime Dimensions

STF
J. Phys. A: Math. Gen. 39, 13483 (2006)
arXiv:quant-ph/0605050

Abstract

The generalized Pauli group and its normalizer, the Clifford group, have a rich mathematical structure which is relevant to the problem of constructing symmetric informationally complete POVMs (SIC-POVMs). To date, almost every known SIC-POVM fiducial vector is an eigenstate of a "canonical" unitary in the Clifford group. I show that every canonical unitary in prime dimensions $p \gt 3$ lies in the same conjugacy class of the Clifford group and give a class representative for all such dimensions. It follows that if even one such SIC-POVM fiducial vector is an eigenvector of such a unitary, then all of them are (for a given such dimension). I also conjecture that in all dimensions $d$, the number of conjugacy classes is bounded above by 3 and depends only on $d \bmod 9$, and I support this claim with computer computations in all dimensions $\lt 48$.

Entanglement and the Power of One Qubit

Animesh Datta, STF, and Carlton M. Caves
Phys. Rev. A 72, 042316 (2005)
arXiv:quant-ph/0505213

Abstract

The "Power of One Qubit" refers to a computational model that has access to only one pure bit of quantum information, along with $n$ qubits in the totally mixed state. This model, though not as powerful as a pure-state quantum computer, is capable of performing some computational tasks exponentially faster than any known classical algorithm. One such task is to estimate with fixed accuracy the normalized trace of a unitary operator that can be implemented efficiently in a quantum circuit. We show that circuits of this type generally lead to entangled states, and we investigate the amount of entanglement possible in such circuits, as measured by the multiplicative negativity. We show that the multiplicative negativity is bounded by a constant, independent of $n$, for all bipartite divisions of the $n+1$ qubits, and so becomes, when $n$ is large, a vanishingly small fraction of the maximum possible multiplicative negativity for roughly equal divisions. This suggests that the global nature of entanglement is a more important resource for quantum computation than the magnitude of the entanglement.

Minimal Informationally Complete Measurements for Pure States

STF, Andrew Silberfarb, and Carlton M. Caves
Foundations of Physics 35 (12), 1985 (2005)
arXiv:quant-ph/0404137

Abstract

We consider measurements, described by a positive-operator-valued measure (POVM), whose outcome probabilities determine an arbitrary pure state of a $D$-dimensional quantum system. We call such a measurement a pure-state informationally complete (PSI-complete) POVM. We show that a measurement with $2D-1$ outcomes cannot be PSI-complete, and then we construct a POVM with $2D$ outcomes that suffices, thus showing that a minimal PSI-complete POVM has $2D$ outcomes. We also consider PSI-complete POVMs that have only rank-one POVM elements and construct an example with $3D-2$ outcomes, which is a generalization of the tetrahedral measurement for a qubit. The question of the minimal number of elements in a rank-one PSI-complete POVM is left open.

Currrent Teaching

  • 2016

    Statistical Mechanics, A Computational Approach

    Course website.

    Semester-long undergraduate course on statistical mechanics focusing on computational aspects such as Monte Carlo methods as well as modern techniques such as tensor networks.

Former Courses

  • 2013-15

    Statistical Mechanics, A Computational Approach

    Semester-long undergraduate course on statistical mechanics focusing on computational aspects such as Monte Carlo methods as well as modern techniques such as tensor networks.

Postdocs and Senior Staff

  • image

    Dr. Marcus Appleby

    Research Scientist

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    Dr. Chris Ferrie

    Postdoctoral Researcher

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    Dr. Chris Granade

    Postdoctoral Researcher

Students

  • image

    Simon Burton

    PhD student, co-supervised with Andrew Doherty.

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    Christopher Chubb

    PhD student.

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    Robin Harper

    PhD student.

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    Alan Robertson

    Honours Student.

My Former Students

  • 2014

    Ishraq Uddin

    Honours student. Currently doing a PhD in mathematical biology at the University of Sydney.

  • 2011

    Adrian Chapman

    Undergraduate Summer Student at Caltech. Currently a graduate student at University of New Mexico.

  • 2010

    Brielin Brown

    Undergraduate Summer Student at Perimeter Institute. Currently a graduate student at UC Berkeley.

  • 2008

    Sreraman Muralidharan

    Undergraduate Summer Student at Perimeter Institute. Currently a graduate student at Yale.

Slides

Coming soon...

Code

Coming soon...

Miscellaneous

Some old notes

About the group

The Sydney quantum information group has very broad interests, and students are encouraged to learn as much as possible from their peers and postdocs as well as their supervisor. Collaboration is strongly supported, and we strive to find a congenial atmosphere for exploring creative ideas in groups as well as individually. The projects in the group range from highly mathematical to more experimentally focused theory projects.

The group is located in the heart of Sydney, one of the most iconic and beautiful cities in the world, a few minutes walk from the central business district and just a few kilometers by bus from the world-class beaches. We are also located within a short distance from other quantum information groups at the University of Technology-Sydney, Macquarie University, and the University of New South Wales.

Sydney skyline at night

Career Overview and Highlights

  • Press
    Popular and broad-scientific press coverage

    My work has been covered in popular press, including Nature, Science, Physics Viewpoint (twice), PhysOrg, American Scientist, Science News, and Physics World.

  • Papers
    High-impact publications

    30+ published research articles in peer-reviewed journals, including 10 papers in Physical Review Letters.

  • Citations
    Recent citation record and h-index

    More than 2000+ citations in the last 5 years, and an h-index of 23, increasing more than +2 per year on average (source: Google Scholar).

  • Grants
    Competitive grant funding

    Chief Investigator on an ARO grant worth US$1.6M through 2019, and co-Principal Investigator on two other grants with ARO and IARPA. Holder of an ARC Future Fellowship for the next four years (until 2018).

Academic Employment

  • 2016 present

    Associate Professor

    University of Sydney, School of Physics

  • 2015 2012

    Senior Lecturer

    University of Sydney, School of Physics

  • 2012 2011

    Principal Research Scientist

    University of Washington, Department of Computer Science

  • 2011 2010

    Postdoctoral Researcher

    Caltech, Institute for Quantum Information

  • 2010 2007

    Postdoctoral Researcher

    Perimeter Institute for Theoretical Physics, Quantum Information group

Education & Training

  • Ph.D. 2007

    Ph.D. in Physics

    University of New Mexico

  • B.S.2002

    Physics & Mathematics

    Pennsylvania State University

For the full pdf version of my CV, click here.