Abstracts QUBIT symposium
Tuesday 18th of March


09:00


Plenary talk by Israel Prize Laureate Mordechai (Moti) Segev
Photonic Quantum Simulations

09:45


Structurebased superresolution quantum state tomography
We show that prior information, for example – that a quantum state is sparse in a known mathematical basis, enables algorithmic reconstruction of an initial threephoton state from twofold coincidence measurements in a single experimental setup, thereby achieving quantum superresolution. We further extend this concept of sparsitybased quantum superresolution to cases where the sparsity basis is not known apriori, and demonstrate the recovery of the general three photon state from twofold coincidence measurements. This approach enables a general reduction both in the number of measurements needed for state recovery and in their complexity, allowing, for example, multi photon state recovery without photon numberresolving detectors.

10:00


Two photon spontaneous emission as a possible source for photonic qubits

11:00


Quantum optics with semiconductor quantum dots

11:30


The dark exciton: a superior solid state spin qubit

11:45


Quantum transport in nanowires: developing platforms for semiconductorbased QIP
Lowbandgap semiconductor nanowires such as InAs and InSb have recently gained attention as materials for realizing topological states and spinorbit qubits. Critical to understanding these experiments and designing nanowirebased quantum devices is a deep understanding of the radially confined modes and the shape of the radial potential that determines these states. Two recent experiments performed by my group help to illuminate the radial subband structure in InAs nanowires in two very different contexts. One is a study of the magnetic field and gate dependence of shortchannel (quasiballistic) fieldeffect devices that shows how magnetoconductance provides a ‘fingerprint’ of the radial subband structure. The second is a study of proximity effect superconductivity in fieldeffect device with Nb contacts. A puzzling interference pattern in the critical current versus axial magnetic field is observed, which we explain by a model of quantum interference between the paths of electrons occupying subbands characterized by different angular momentum quantum numbers.

14:15


Fractal behavior of polaritons in a quasiperiodic potential : a quantum simulator of topological systems
I will present recent results obtained both theoretically and experimentally on fractal spectral properties of a polariton gas in a Fibonacci quasiperiodic potential. The observed spectrum is accurately reproduced from a theoretical model that we shall present. We have observed for the first time logperiodic oscillations and the opening of minigaps following the gap labeling theorem. These results illustrate the potential of cavity polaritons as a quantum simulator in complex topological geometries.

14:45


Lightmatter interactions in photoniccrystal nanocavities: applications and potential for scalability
I will introduce our recent experiments with single, selfassembled InAs quantum dots embedded in GaAs photoniccrystal nanocavities. The small size of the photoniccrystal cavities and their compatibility with semiconductor fabrication processes make the coupled cavitydot platform an excellent candidate for scaling up to more complex systems needed for practical devices. At the same time, the coupled cavitydot system enables interactions between single photons, which we have used to demonstrate highspeed alloptical switching at ultralow powers and generation of nonclassical light.
I will also describe our latest work exploring the use of a single quantum emitter coupled to a cavity as a photon number filter, extending the concept of photon blockade from single photons to twophoton Fock state generation by coupling the probe laser to the second manifold of the JaynesCummings ladder via a twophoton transition. This approach can be further generalized to create third and higherorder photon states inside the cavity through multiphoton transitions to the corresponding manifold. We report the probing of these multiphoton transitions into the higher manifolds of the JaynesCummings ladder of a strongly coupled quantum dot–photonic crystal nanocavity system by measuring the thirdorder autocorrelation function of a probe laser transmitted through such a system. We observe bunching in the thirdorder autocorrelation function for transmitted photons when the probe laser is resonant with the third manifold and antibunching when the probe laser is tuned away from resonance.
Lastly, I will discuss our first steps toward a network of interacting nonlinear cavities, focusing on our demonstration of strong coupling of a single quantum dot to a pair of proximitycoupled cavities.

15:15


Quantum complexity theory and the BoseHubbard model
This talk will be split in two parts. The first will give a brief introduction to quantum complexity theory, which will allow a review of recent results classifying the complexity of several problems related to the BoseHubbard Hamiltonian. The second part of the talk will focus on the graduate student experience at the University of Waterloo, with a focus on graduate studies at the IQC.

Wednesday 19th of March


09:00


The NitrogenVacancy qubit in Diamond
Quantum computers and other potential quantum applications require the presence of Qubits. These must be realized in the laboratory in a controlled way.
The most promising qubit to date is the negatively charged NitrogenVacancy (NV) defect in diamond.
Here we will overview the quantum properties of this defect showing why it is most suitable for the realization of qubits.
We’ll describe how it is realized in the laboratory by the use of ion implantation, stressing still existing difficulties.
The photons emitted by the qubit must be controlled and manipulated. This is best done in Photonic Crystal structures realized in the NV center containing diamond.
Ways of realizing such structures in diamond and their properties will be reviewed.

09:30


TBA

10:00


Superconducting flux quantum bits: control and quantum sensing
Superconducting artificial atoms provide a new testbed for the study of quantum control, decoherence, and lightmatter interaction and have applications in quantum information processing and quantum sensing.
I will discuss our recent work on superconducting artificial atoms of the flux type, also called flux quantum bits (qubits) in the context of quantum information processing. We study flux qubits coupled to superconducting coplanar waveguide resonators. I will present our results on measurements of coherence and on implementation of single and two qubit control.
In the second part of the talk, I will present experiments in which we employ the high sensitivity of flux qubits to magnetic fields to design a magnetometer with unprecedented sensitivity over the frequency range from tens of kHz to tens of MHz

11:00


The use of braid operators for implementing entangled large nqubits states (n>2)
Braid theories are applied to quantum computation processes, where to each crossing in the Braid diagram a unitary YangBaxter operator R is associated, representing either a Braiding matrix or a universal quantum gate. By operating with Braid operators on the computational basis of nQUBITS states, orthonormal entangled states are obtained, referred here as general Bell states. The 3QUBITS Bell states are explicitly developed and the present methods are generalized to any nQUBITS system. The quantum properties of the general Bell states are analyzed and these properties are related to concurrence.

11:30


Dissipative encoding and state preparation for topological order
We study the suitability of dissipative (nonunitary) processes for (a) encoding logical information into a topologically ordered ground space and (b) preparing an (arbitrary) topologically ordered state. We give a construction achieving (a) in time O(L) for the L×Ltoric code by evolution under a geometrically local, timeindependent Liouvillean. We show that this scaling is optimal: even the easier problem (b) takes at least Ω(L) time when allowing arbitrary (possibly timedependent) dissipative evolution. For more general topological codes, we obtain similar lower bounds on the required time for (a) and (b). These bounds involve the code distance and the dimensionality of the lattice. The proof involves LiebRobinson bounds, recent cleaninglemmatype arguments for topological codes, as well as uncertainty relations between complementary observables. By allowing general localitypreserving evolutions (including, e.g., circuits of CPTPMs), our work extends earlier work characterizing unitary state preparation.
This is joint work with John Dengis and Fernando Pastawski.

12:00


Engineering new platforms for topological quantum computing
Quantum computation requires controlled engineering of quantum states to perform tasks that go beyond those possible with classical computers. Topological quantum computation aims to achieve this task by using nonAbelian quantum phases of matter. Such phases allow for quantum information to be stored and manipulated in a nonlocal manner, which protects it from imperfections in the implemented protocols and from interactions with the environment. Despite progress in the field, experimentally controlling and manipulating nonAbelian quantum Hall states still proves to be a highly challenging task. We consider the possibility to engineer new nonAbelian systems by interfacing simpler, Abelian components. We show that this route leads to zero modes exhibiting a new type of nonAbelian statistics. Furthermore, judicially coupling these zero modes leads to new topological phases of matter, which can harbor excitations with computationally universal braid statistics 
12:30


Flux braiding and anyons in Pauli hamiltonian
Aharonov and Casher showed that Pauli Hamiltonians in two dimensions have gapless zero modes. We study the adiabatic evolution of these modes under the slow motion of N fluxons with non integer fluxes.
The holonomies associated with closed paths need not be topological in general. We find however that if all the fluxons are subcritical, (i.e.<1), and the number of zero modes is D=N1, then fluxon braiding is
topological. In the special case that the fluxons carry identical fluxes they can be interpreted as non abelian anyons that satisfy the Burau representations of the braid group.

14:15


Plenary talk by IQC director Raymond Laflamme
Experimental quantum error correction
To realize the potential of quantum information processors requires the ability to overcome the imprecision and imperfection inherent in physical systems. Quantum error correction (QEC) has provided a solution, showing that errors can be corrected with a reasonable amount of resources as long as their rate is sufficiently small. Implementing QEC protocols remains one of the most important challenges in QIP. In the experimental arena, the quest to build quantum processors that could outperform their classical counterparts has led to many blueprint proposals for potential devices based on NMR, electron spin resonance, ion traps, atom traps, optics, superconducting devices and nitrogenvacancy centres, among others. Many have demonstrated not only the possibility of controlling quantum bits, but also the ability to do so in practice, showing the progression of quantum information science from the blackboard to the laboratory. My presentation will give an overview of some of the recent results in quantum information science on the way to implement quantum error correction. I will show how noise can be characterize efficiently when our goal is to find suitable quantum error correcting codes. I will show demonstrations of control to implement some quantum error correcting codes and finally how can noise be extracted through algorithmic cooling. I will comments on some challenges that need to be solved and a path towards implementing many round of quantum error correction.

15:00


Modern electron spin resonance for quantum computing
There are many suggestions for quantum computer architectures that make use of electron spins as a physical system for quantum bits (qubit). Electron Spin Resonance (ESR) is a wellestablished technique in physics and chemistry, which provides information about unpaired electron spins in a variety of paramagnetic materials. Thus, it may seem natural to make use of ESR a tool in the field of quantum computing (QC). However, conventional ESR has very bad spin sensitivity, making it unsuitable for tacking most of the challenges associated with QC, which often require single qubit (spin) manipulating and reading capability. In this talk we will show how modern techniques and methodologies that were recently developed in the field of ESR, such us ultrahigh sensitive resonators and ultrahigh imaging resolution can be used in a possible novel architecture for a spinbased quantum computer.

15:30


Measuring weak values with operational constraints
Weak values are an interesting and useful characteristic of quantum systems with past (pre selected) and future (post selected) boundary conditions. In theory they can be observed directly by in a relatively simple setup of `weak measurements'. In reality weak values may be hard or even impossible to measure due to operational constraints. For example the absence of projective measurements in ensemble quantum processors poses an obstacle in the postselection stage. I will present a method for sidestepping this obstacle and present experimental results of the first weak measurement in NMR. The experiment involves 3 qubits but can be extended to larger systems of up to 12 qubits using conventional methods. Moreover the method for implementing the post selection stage can be useful for more general purposes.
Joint work with Dawei Lu, Jun Li, Hang Li and Raymond Laflamme,
arXiv:1311.5890

16:45


Maximally entangled states
Every maximally entangled state (MES) is shown to be a product state expressed in (suitably chosen) collective coordinates. Such product state may be viewed as defining an origin for a "phase space" like d ^2 array representing d^2 orthonormal MES. (Straight) Lines in this "phase space" is shown to effect reversal of the Schmidt diagonalization: it gives particles product state in terms of d terms of MES.

17:15


Limitations of algorithmic cooling
Heatbath algorithmic cooling (AC) of spins is a powerful spincooling approach that (ideally) cools exponentially better than cooling by reversible entropy manipulations. I will discuss two limitations of AC.
For nonideal AC, we studied the impact of realistic relaxation times of spins on the achievable cooling. We derived, by simulations, the attainable cooling levels for given ratios of relaxation times. We expect this analysis of the limitations to be valuable for the planning of future experiments. For ideal and optimal AC, I will discuss some bounds on the number of required steps (based on entropy considerations). These bounds present important consequences of using AC for improving signaltonoise ratio in magnetic resonance spectroscopy.

17:30


Experimental heat bath cooling and algorithmic cooling in liquid state NMR
Heatbath algorithmic cooling (AC) utilizes thermalization to purify a subset of qubits, such that the total Shannon entropy of the qubit system is reduced. AC was originally developed for scalable quantum computers, and was initially demonstrated in solidstate nuclear magnetic resonance (NMR), where rapid removal of entropy was achieved via spin diffusion. Alternatively, AC may be applied for quantum simulation, as recently demonstrated by cooling quantum gases near absolute zero, and by distilling states in quantum optics.
We utilized gradient ascent pulse engineering (GRAPE), an optimal control algorithm, to apply AC in liquid state NMR, where entropy removal is hindered by the relatively small differences in relaxation times among the spins. Various cooling algorithms were applied onto the three qubits of 13Clabeled trichloroethylene, cooling the system beyond Shannon's entropy bound. For example, in one experiment, one of the two carbon qubits was cooled by a factor of 4.61 ± 0.02 after seven cycles of AC, beyond the initial information content of the spin system.

17:45


A state separator

18:00


Alice Bob and Eve in quantum land
Quantum key distribution (QKD), in contrast to `classical' key distribution, is in principle informationtheoretic secure.
I will briefly present several recent results. On the one hand I'll present a QKD protocol in which one of the parties (Alice, or Bob) is classical, and I will argue that it is still secure.
On the other hand, I'll present a novel attack against practical QKD, called the reversedspace attack, and a special case of that attack – the `fixed apparatus' attack. I'll show that QKD implementations in which the receiver's apparatus is fixed (namely, does not depend on a choice of basis at each qubit transmission) are totally insecure against a strong eavesdropper that has a onetime
(single) access to the receiver's equipment.
