2017




WeiWei Zhang SEMINAR Time: 3pm4pm, Thursday, 7 December 2017 Abstract: The properties of topological systems have been the subject of intense interest in recent years, both for fundamental inves tigations in condensed matter physics and for their potential applications to faulttolerant quantum computation. A priority for experimentalists is verifying that a given implementation indeed supports topological phases. In this work [Phys. Rev. Lett. 119, 197401 (2017)], we show that continuoustime quantum walks of twocomponent particles governed by twodimensional spinorbit Hamiltonians can reveal the presence of topological order. The density profile in topologically nontrivial phases displays a characteristic peak in the vicinity of the origin that is absent in trivial phases. Likewise, a kink in the mean width of the particle distribution signals the presence of a quantum phase transition. The results are expected to have immediate application to systems of ultracold atoms [Science 354, 83 (2016)].


Clemens Mueller SEMINAR Time: 2pm3pm, Monday, 27 November 2017 Abstract: I will present our recent proposal for a fully passive, onchip microwave circulator based on a ring of superconducting tunnel junctions. We investigate two distinct physical realisations, based on either Josephson junctions (JJ) or quantum phase slip elements (QPS), with microwave ports coupled either capacitively (JJ) or inductively (QPS) to the ring structure. A constant bias applied to the center of the ring provides the symmetry breaking (effective) magnetic field, and no microwave or rf bias is required. We find that this design offers high isolation even when taking into account fabrication imperfections and environmentally induced bias perturbations and find a bandwidth in excess of 500 MHz for realistic device parameters.


Alexander Wood SEMINAR Time: 2pm3pm, Thursday, 23 November 2017 Abstract: Exploring Frame transformations are innocuous tricks that simultaneously simplify a problem and reveal the core underlying physics. For instance, we often enter the rotating frame on paper with cheerful abandon, confident that it's a way of simplifying magnetic resonance problems. But what happens when a qubit is physically rotated? Rotation leads to the emergence of "fictitious" magnetic pseudofields in the rotating frame, which nevertheless have real, measurable effects. The nitrogenvacancy (NV) centre in diamond is an ideal qubit to study rotation. However, there are considerable experimental challenges, such as how to realise preparation, control and readout of qubits rotating with a period comparable to the NV coherence time of a few hundred microseconds. In this talk, I discuss experiments demonstrating quantum state control and readout of single NV centres rotating at speeds in excess of 200,000 rpm. We study the appearance of magnetic pseudofields in an NVnuclear spin system rotating at up to 330,000 rpm. Not only can the NV spin detect these effective magnetic fields via the nuclear spins, but it is also a unique system where rotation can be used as a new method of quantum control in electronnuclear systems. We have also showed that rotation also offers better sensitivity to DC magnetic fields for NV centres by upconverting DC fields to AC fields in the rotating frame.


Benjamin Brown SEMINAR Time: 4pm5pm, Thursday, 23 November 2017 Abstract: Exploring the different features of a topological phase may allow us to find new ways of finding faulttolerant logical gates for quantum computing. Phases with a high degree of symmetry in the physics of their lowenergy excitations are particularly interesting in this respect. I will talk about the numerous symmetries of the color code, and the different topological defects and boundaries that emerge as a consequence. The defects we discuss can be exploited to achieve different computational protocols with the color code model.


Kamil Korzekwa SEMINAR Time: 3pm4pm, Thursday, 16 November 2017 Abstract: Thermodynamics is traditionally constrained to the study of macroscopic systems whose energy fluctuations are negligible compared to their average energy. Here, we push beyond this thermodynamic limit by developing a mathematical framework to rigorously address the problem of thermodynamic transformations of finitesize systems. More formally, we analyse state interconversion under thermal operations and between arbitrary energyincoherent states. We find precise relations between the optimal rate at which interconversion can take place and the desired infidelity of the final state when the system size is sufficiently large. These socalled secondorder asymptotics provide a bridge between the extreme cases of singleshot thermodynamics and the asymptotic limit of infinitely large systems. We illustrate the utility of our results with several examples. We first show how thermodynamic cycles are affected by irreversibility due to finitesize effects. We then provide a precise expression for the gap between the distillable work and work of formation that opens away from the thermodynamic limit. Finally, we explain how the performance of a heat engine gets affected when one of the heat baths it operates between is finite. We find that while perfect work cannot generally be extracted at Carnot efficiency, there are conditions under which these finitesize effects vanish.


Corey Jones SEMINAR Time: 3pm4pm, Thursday, 19 October 2017 Abstract: In this talk, we will describe the methods of GannonMorrison for computing the S and T matrices of the Drinfeld center of a fusion category from just its fusion ring.


Tomohiro Hashizume SEMINAR Time: 4pm5pm, Thursday, 28 September 2017 Abstract: The dynamical behavior of a spin 1/2 triangular lattice with long range interactions is studied numerically. It is an approximate model of an ion trap, which is experimentally realized. To simulate the system, a new algorithm for evolving infinite matrix product states is developed. Algorithm is first verified with an analytical model and a previously studied system. It is then used to simulate quenching on the ground states of the lattice.


Peter S. Turner, with Stasja Stanisic SEMINAR Time: 3pm4pm, Thursday, 28 September 2017 Abstract: Recent advances in scaling photonics for universal quantum computation, and the race to demonstrate quantum `supremacy' via analog computations that


Zhengfeng Ji SEMINAR Time: 11am12pm, Monday 25 September 2017 Abstract: The code space of a quantum error correcting code exhibits strong entanglement properties. Nonlocal games are important ways to reveal the nonlocal nature of entangled systems. These are two wellknown facts and, in this talk, we attempt to combine them in one topic by motivating and analyzing several natural definitions of nonlocal games for quantum error correcting codes. We will discuss the techniques for analyzing the rigidity properties of the games and introduce their applications in the study of quantum multiprover interactive proofs.


Prahald Warzawski SEMINAR Time: 34pm, Thursday 14 September 2017 Abstract: Optomechanical systems provide an attractive testbed for the creation and manipulation of nonclassical states of mechanical motion. A key experimental challenge is demonstrating that the desired quantum state has actually been prepared. We propose a new, realistic, experimental protocol for quantum state tomography of nonclassical states in optomechanical systems. Using a parametric drive, the procedure overcomes the challenges of weak optomechanical coupling and thermal noise to provide high efficiency homodyne measurement. Our analysis is based on the theoretical description of the generalised measurement that is performed when optomechanical position measurement competes with thermal noise and the parametric drive. The proposed experimental procedure is numerically simulated in realistic parameter regimes, which allows us to show that tomographic reconstruction of otherwise unverifiable nonclassical states is made possible.

Dominic Williamson SEMINAR Time: 34pm, Thursday 7 September 2017 Abstract: Tensor network descriptions of topologically ordered states possess hidden symmetries. I will describe the algebraic structure of these symmetries and how to extract the emergent topological order of a state from them. Time permitting, I will also explain how breaking these symmetries leads to extrinsic defects and anyon condensation.


Robin Harper SEMINAR Time: 34pm, Thursday 31 August 2017 Abstract: We are currently approaching the point where quantum systems with 15 or more qubits will be controllable with high levels of coherence over long timescales. One of the fundamental problems that has been identified is that, as the number of qubits increases to these levels, there is currently no clear way to use efficiently the information that can be obtained from such a system to make diagnostic inferences and to enable improvements in the underlying quantum gates. Even with systems of only a few bits the exponential scaling in resources required by techniques such as quantum tomography or gateset tomography will render these techniques impractical. Randomized benchmarking (RB) is a technique that will scale in a practical way with these increased system sizes. Although RB provides only a partial characterization of the quantum system, recent advances in the protocol and the interpretation of the results of such experiments confirm the information obtained as helpful in improving the control and verification of such processes. This thesis examines and extends the techniques of RB including practical analysis of systems affected by low frequency noise, extending techniques to allow the anisotropy of noise to be isolated, and showing how additional gates required for universal computation can be added to the protocol and thus benchmarked. Finally, it begins to explore the use of machine learning to aid in the ability to characterize, verify and validate noise in such systems, demonstrating by way of example how machine learning can be used to explore the edge between quantum nonlocality and realism.



Zachary Cristina SEMINAR Time: 45pm, Thursday 24 August 2017 Abstract: The ability to purify a system which begins in a mixed state is critical to many experimental implementations of quantum information protocols. Given a bipartite system comprising two coupled subsystems A and B, it is possible to purify system B by performing repeated projective measurements on system A interlaced with entangling unitaries, provided the desired measurement outcomes are obtained. In this talk, I will discuss two papers which explore this protocol: the first takes a quantitative look at the dynamics induced by these repeated measurements and the conditions required for subsystem B to converge to a pure state; and the second describes a more specific implementation of the protocol that can be used to purify baths of nuclear spins coupled to a central electronic spin.

Mitchell Hannah SEMINAR Time: 34pm, Thursday 17 August 2017 Abstract: Concatenation of two quantum error correcting codes with complementary sets of transversal gates can provide a means towards universal faulttolerant computation. We first show that it is generally preferable to choose the inner code with the higher pseudothreshold in order to achieve lower logical failure rates. We then explore the threshold properties of a wide range of concatenation schemes. Notably, we demonstrate that the concatenation of complementary sets of ReedMuller codes can increase the code capacity threshold under depolarizing noise when compared to extensions of previously proposed concatenation models. We also analyze the properties of logical errors under circuit level noise, showing that smaller codes perform better for all sampled physical error rates. Our work provides new insights into the performance of universal concatenated quantum codes for both code capacity and circuit level noise.


David Long SEMINAR Time: 4pm, Thursday 18 May 2017 Abstract: Understanding and classifying all phases of matter that can occur in systems of various dimensions is a broad goal of modern condensed matter physics. I will present some findings of a paper by Wang and Senthil (Phys. Rev. B 87, 235122, arXiv:1302.6234), where it is shown that some two dimensional symmetry protected topological (SPT) phases of matter can only occur at the surface of a three dimensional phase. This includes a construction of such a three dimensional system. The construction can be modified slightly to produce a timereversal symmetric realisation of the "three fermion Z2 liquid", a phase which in strictly two dimensions possesses chiral edge modes, and thus can never be timereversal symmetric.


Elija Perrier SEMINAR Time: 3pm Thursday 18 May 2017 Abstract: Practical challenges facing the realisation of nonabelian statistics via proposed Tjunction majoranabased architectures have spurred the development of a number of novel circuit designs of late. A recent proposal by Karzig et al. for the design quantum computers composed of qubits encoded in aggregates of four or more Majorana zero modes (realized at the ends of topological superconducting wire segments that are assembled into superconducting islands with significant charging energy) has generated much interest due to its prospective scalability and robustness. In this proposed architecture, quantum information can be manipulated according to a measurementonly protocol where braiding is implemented via anyonic teleportation. The protocol is facilitated by tunable couplings between Majorana zero modes and nearby semiconductor quantum dots. In this talk, I shall briefly recap the essential elements of majoranabased nanowire systems, discuss some of the problems with previous Tjunction protocols and outline how Karzig et al.'s proposal obviates a number of the difficulties faced by earlier designs.


Angela Karanjai SEMINAR Time: 3pm Thursday 18 May 2017 Abstract: This will be a talk about constraints on classical models that reproduce the qubit stabiliser subtheory. We show that the minimum number of classical bits required to specify the state of an nqubit system must scale as ~ (n^2)/2 in any model that does not contradict any predictions of the quantum stabilizer subtheory. The GottesmanKnill algorithm, is in fact, very close to this bound as it scales at ~ n(2n+1).


Christopher Chubb SEMINAR Time: 34pm Thursday 4 May 2017 Abstract: In this talk I will be considering the problem of transmitting classical data over a quantum channel. Specifically I will discuss the tradeoff between the amount of information that can be transmitted over a channel, and the error probability with which it can be decoded. This will not be a particularly technical talk, and I will focus more on known results than techniques. I will also briefly discuss some recent work of mine 1701.03114 (see also similar work 1701.03195), in which I develop a moderate deviation analysis of these tradeoffs.


Parth Girdhar SEMINAR Time: 2:30pm Thursday 13 April 2017 Abstract: Some may say quantum mechanics is the cherry on top of a creamy dessert called physics. Its predictive power has amazed us all. But the measurement problem is the Achilles heel of quantum mechanics and has been so since it was constructed in the early 20th century. Recently it has captivated Steven Weinberg, guru of the standard model and quantum field theory. In this talk I will go through some of Weinberg's research into modified quantum mechanics as a means to deal with the measurement problem. I will focus on a recent paper in which he proposes a method to test a nonunitary version of quantum mechanics with extreme precision.


Hakop Pashayan SEMINAR Time: 3:30pm, Thursday 23 March 2017 Abstract: We present a new algorithm for classical simulation of quantum circuits over the Clifford + T gate set. The runtime of the algorithm is polynomial in the number of qubits and the number of Clifford gates in the circuit but exponential in the number of T gates. The exponential scaling is sufficiently mild that the algorithm can be used in practice to simulate mediumsized quantum circuits dominated by Clifford gates. The first demonstrations of faulttolerant quantum circuits based on 2D topological codes are likely to be dominated by Clifford gates due to a high implementation cost associated with logical T gates. Thus our algorithm may serve as a verification tool for nearterm quantum computers which cannot in practice be simulated by other means. To demonstrate the power of the new method, we performed a classical simulation of a hidden shift quantum algorithm with 40 qubits, a few hundred Clifford gates, and nearly 50 T gates.



Event contact: Wicky West/ Fran Vega
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