April 16, 2014

Toronto Quantum Information Seminars QUINF 2007-08
held at the Fields Institute

The Toronto Quantum Information Seminar - QUINF - is held roughly every two weeks to discuss ongoing work and ideas about quantum computation, cryptography, teleportation, et cetera. We hope to bring together interested parties from a variety of different backgrounds, including math, computer science, physics, chemistry, and engineering, to share ideas as well as open questions.

Organizing Committee:Daniel James, Aephraim Steinberg, Paul Brumer or Hoi-Kwong Lo. (Physics, University of Toronto)

Talks are held Fridays at 11 am unless otherwise indicated

11:10am to 12:10pm

Bei-Lok Hu, University of Maryland and Perimeter Institute for Theoretical Physicss
Non-Markovian Entanglement Dynamics of Two Qubits Interacting Through a Quantum Field

Two necessary requirements on the physical conditions of a system suitable for quantum information processing are the sustenance of a sufficient degree of quantum coherence and the preservation of quantum entanglement. Interaction of a quantum system with its environment has a tendency to diminish or destroy its quantum coherence and entanglement. Our research has focused on these two issues. Working with simple systems but with more probing analysis, we aim to provide results in regimes physically relevant but often glossed over in textbook treatments. In particular we focus on the non-Markovian (processes involving memory) regimes which correspond usually to short time, low temperature conditions or for strongly coupled or correlated systems. These are also the conditions more conducive to quantum information processing. In this talk I present results [1] from studies
of the non-equilibrium dynamics of a pair of qubits made of two-level atoms at a finite distance apart and interacting with one common electromagnetic field but not directly with each other. The case of two
qubits each interacting with its own field has been studied by Yu and Eberly [2] earlier who reported the appearance of 'sudden death' of quantum entanglement in time. With two qubits in the same field, where
the field mediates the qubits through induced interaction, the behavior is much more complex [3]. This is also a more commonly encountered situation such as in the construction of quantum gates. We obtain
analytic expressions for the dependence of quantum entanglement on time and on the spatial separation between the two qubits, the latter is an effect which has never been obtained, or even conjured, in entanglement
studies. Our investigation also brings out a new perspective on some basic issues such as nonlocality in quantum entanglement understood in the EPR sense. We assert that the quantum mechanical interpretation of
entanglement is incomplete because the causal propagation of the interceding field is left out completely in prior studies. Inclusion of this key element from relativistic quantum field theory considerations may paint alter our view and understanding of these basic issues at the foundation of quantum mechanics.

[1] C. Anastopoulos, S. Shresta and B. L. Hu, Quantum Entanglement under
Non-Markovian Dynamics of Two Qubits Interacting with a Common
Electromagnetic Field, under consideration by Phys. Rev. A

[2] T. Yu and J. H. Eberly, Phys. Rev. Lett. 93, 140404 (2004).

[3] Z. Ficek and R. Tanas, Phys. Rev. A 74, 024304 (2006).

Fri, 23-May-08

Stephen Bartlett, University of Sydney
Identifying phases of matter that are universal for quantum computation

A recent breakthrough in quantum computing has been the realization that quantum computation can proceed solely through single-qubit measurements on an appropriate quantum state - for example, the ground state of an interacting many-body system. It would be unfortunate, however, if the usefulness of a ground state for quantum computation was critically dependent on the details of the system's Hamiltonian; a much more powerful result would be the existence of a robust ordered phase which is characterized by the ability to perform measurement-based quantum computation (MBQC). To identify such phases, we propose to use nonlocal correlation functions that quantify the fidelity of quantum gates performed between distant qubits. We investigate a simple spin-lattice system based on the cluster-state model for MBQC, and demonstrate that it possesses a zero temperature phase transition between a disordered phase and an ordered "cluster phase" in which it is possible to perform a universal set of quantum gates.

Fri, 9-May-08
Katya Babourina, University of Queensland
Quantum noise in a nano mechanical Duffing resonator

We determine the small signal gain and noise response of an amplifier based on the nonlinear response of a quantum nanomechanical resonator. The resonator is biased in the nonlinear regime by a strong harmonic bias force and we determine the response to a small additional driving signal detuned with respect to the bias force.
Fri, 25-Apr-08
Jeremy O'Brien, Centre for Quantum Photonics, University of Bristol
Quantum information science with photons on a chip

Quantum information science has shown that quantum mechanical effects can dramatically improve performance for certain tasks in communication, computation and measurement. Of the various physical systems being pursued, single particles of light – photons – have been widely used in quantum communication, quantum metrology, and quantum lithography settings. Low noise (or decoherence) also makes photons attractive quantum bits (or qubits), and they have emerged as a leading approach to quantum information processing [1,2].

In addition to single photon sources and detectors, photonic quantum technologies require sophisticated optical circuits involving high-visibility classical and quantum interference with photons. While a number of photonic quantum circuits have been realized for quantum metrology [3], quantum lithography, and quantum logic gates [4]. These demonstrations have relied on large-scale (bulk) optical elements bolted to large optical tables, thereby making them inherently unscalable.

Quantum technologies based on photons will likely require an integrated optics architecture for improved performance, miniaturization and scalability. We demonstrate high-fidelity silica-on-silicon integrated optical realizations of key quantum photonic circuits, including two-photon quantum interference with a visibility of 94.8(5)%; a controlled-NOT gate with an average logical basis fidelity of 94.3(2)%; and a path entangled state of two photons, relevant to quantum metrology, with fidelity >92% [5].
The monolithic nature of these devices means that the correct phase can be stably realized in what would otherwise be an unstable interferometer, greatly simplifying the task of implementing sophisticated photonic quantum circuits. We fabricated 100's of devices on a single wafer and find that performance across the devices is robust, repeatable and well understood. We have also demonstrated an all optical fibre CNOT gate [6].

These results show that it is possible to directly “write” sophisticated photonic quantum circuits onto a silicon chip, which will be of benefit to future quantum technologies based on photons, as well as the fundamental science of quantum optics.

[1] E Knill, R Laflamme, G J Milburn, Nature 409, 46 (2001)

[2] J L O’Brien, Science 318 1567 (2007)

[3] T Nagata, R Okamoto, J L O'Brien, K Sasaki, S Takeuchi Science 316, 726 (2007)

[4] J L O'Brien, G J Pryde, A G White, T C Ralph, D Branning, Nature 426, 264 (2003)

[5] A Politi, M J Cryan, J G Rarity, S Yu, J L O’Brien Science to appear (2008) / arXiv:0802.0136

[6] A S Clark, J Fulconis, J G Rarity, W J Wadsworth, J L O’Brien Nature Physics under review / arxiv/0802.1676

Fri, 4-Apr-08

Asoka Biswas, Dept. of Chemistry and CQIQC, University of Toronto
Overlapping resonance in the control of decoherence: N spins coupled to a bosonic bath
Coherent control of quantum systems rely upon the presence of coherence, loss of which ("decoherence") results in marked decrease in controllability. This issue is of significant interest also in the subject of quantum computation. There are several techniques either to avoid or to eliminate decoherence. Those techniques demand either certain symmetry in the system Hamiltonian or severe technical challenges in implementing them. In this talk, I will discuss a more general approach which can combat the above two issues. This approach is quite fundamental and relies on quantum interferences between overlapping resonances.

We demonstrate this technique by considering a system comprising spin-half particles interacting with a bosonic thermal bath. In presence of overlapping resonances, decoherence of the spin-system can be minimized by choosing in an optimal way an initial superposition of the spin states. We show the results for an available spin-boson system, namely, Cooper-pair qubits interacting with a nano-mechanical oscillator.

Fri, 28-Mar-08
Qin Wang, KTH- Royal institute of technology, Sweden
Experimently demonstration on decoy-state QKD with heralded single photon source
We have experimentally demonstrated a decoy-state quantum key distribution scheme (QKD) with a heralded single-photon source based on parametric down-conversion. We used a one-way BB84 protocol with a four states and one-detector phase-coding scheme, which is immune to recently proposed time-shift attacks, photon-number splitting attacks, and can also be proven to be secure against Trojan horse attacks and any other standard individual or coherent attacks. In principle, the setup can tolerate the highest losses or it can give the highest secure key generation rate under fixed losses compared with other practical schemes. This makes it a quite promising candidate for future quantum key distribution systems.
Fri, 14-Mar-08
Nicolas Godbout, École Polytechnique de Montréal
Optical fibre technology for quantum communication and quantum information processing
An overview of relevant available optical fibre technologies for quantum information is presented. Examples of applications in quantum cryptography networks and quantum information processing are given. A scheme for few qubit processing using the cluster-state model of quantum processing is introduced.
Friday, 7-March-2008
11:10am to 12:10pm
Place: BA3004, Bahen Centre
Shohini Ghose, Wilfrid Laurier University
Studies of chaos, entanglement and decoherence in a quantum kicked top using cold atoms
The quantum kicked top has become a standard paradigm for theoretical studies of quantum chaos in spin systems. We describe the first experimental realization of a quantum kicked top using cold Cesium atoms interacting with laser and magnetic fields. The kicked top Hamiltonian can be accurately implemented using the nonlinear AC Stark shift and a pulsed magnetic field. Preparation of arbitrary initial states from a fiducial state can be achieved using Stark shifts and magnetic fields,
Measurement of the complete spin density matrix is performed via Faraday rotation of a probe laser. A variety of interesting phenomena can be observed such as dynamical tunneling, rapid spreading of the wave function in the chaotic phase space, signatures of chaos in the evolution of nuclear-spin entanglement and robustness of the dynamics in the presence of decoherence. These dynamics can be understood by examining the Floquet eigenstates of the system.

Fri, 29-Feb-08
**revised location
Room 230 Fields.
Robin Williams, Institute for Microstructural Sciences, National Research Council
Scalable Routes to Entangled Photon Pair Sources –Gated InAs/InP Quantum Dots in Photonic Crystal Microcavities
Entangled photon pairs (EPP) can be produced through the biexciton (XX) – exciton (X) radiative decay cascade in semiconductor quantum dots (QD) [1-3]. In existing devices, the requirement to enforce degeneracy of the intermediate excitonic states, whose degeneracy is lifted by the anisotropic exchange splitting (AES) [2-5], has led to remedies that include the application of large external magnetic fields [2], or the materials engineering of individual dots [3, 4]. Such schemes are impractical if large arrays of integrated EPP sources are to be constructed for quantum information applications.

In the work presented here we propose a scheme for EPP generation that does not require the removal of the AES. By application of a lateral electric field to an individual, pre-positioned InAs quantum dot on a patterned InP substrate, we engineer the quantum dot to introduce Hidden Symmetry within the s-shell. In such circumstances the biexciton binding energy vanishes, “which path” information for the XX-X radiative cascade is not available through a photon energy measurement and polarization entanglement is produced even if AES is still present. Photoluminescence measurements as a function of applied lateral electric field will be presented for individual InAs/InP quantum dots emitting close to l=1300nm. The nucleation sites of these dots can be controlled with nanometer precision using an in-situ, ‘nanotemplate deposition’ technique [6], so that it is possible to build control structures, such as electrostatic gates, around individual QDs. Such a capability is a pre-requisite if arrays of such dots are to be employed for quantum information applications. Our measurements demonstrate the removal of the biexciton binding energy, a reduction of the AES, quenching of the neutral exciton emission and the appearance of new, normally forbidden transitions involving an s-shell electron and p-shell hole. Full Configuration-Interaction calculations will be presented that explain how the biexciton binding energy can be removed through manipulation of the electron-hole Coulomb interaction and consequent introduction of Hidden Symmetry.

Friday, 22-Feb-08
Joseph Emerson, Institute for Quantum Computing, University of Waterloo
Negativity and contextuality as criteria for classicallity in discrete phase-space and other quasi-probability representations of quantum theory
In recent years several quasi-probability representations of finite dimensional quantum mechanics have been proposed as analogs of the phase space representation of continuous quantum systems. I will describe a formalism based on the theory of frames which allows us to characterize the set of possible quasi-probability representations for finite dimensional quantum systems that satisfy two reasonable conditions. This formalism leads to a direct proof that any such representation (that reproduces the quantum statistics) is non-classical in the sense that either the states or the measurements must be modeled by negative valued functions. This condition turns out to be equivalent to a proof of contextuality. This formalism may lead to a new method for assessing the degree of non-classicality of a given quantum information task or process.
Friday, 15-Feb-08
Yoritoshi Adachi, Department of Materials Engineering Science, Osaka University
Efficient quantum key distribution with parametric down-conversion source
Quantum key distribution (QKD) allows two parties to share an unconditional secret key. The first QKD protocol has been proposed by Bennett and Brassard in 1984, which is called BB84. The practical BB84 is vulnerable against photon-number splitting (PNS) attacks, however, it is shown that this problem can be solved by utilizing information from a built-in decoy state. In order to prepare a decoy state, it seems to need an additional complexity to the experimental setup, such as the random amplitude modulation for weak coherent pulse scheme. Here, we propose an efficient QKD protocol based on photon-pair generation from parametric down-conversion, which uses the only different post-processing of the classical data from the conventional protocol. Assuming the use of practical detectors, we analyze the unconditional security of the new scheme, and show that it improves the secure key generation rate by several orders of magnitude at long distances.
Friday, 01-Feb-08
Note : The venue of Xingxing's talk has been changed to MP307 due to the closure of the Fields Institute.

It will start now, 11:10am!

Xingxing Xing, Dept. of Physics and CQIQC, University of Toronto
Towards the atom-photon interface in Quantum information: An ultrabright entangled photon source
Non-classical light sources are important technologies for quantum information because light is robust to decoherence from the environment. The flip side to this advantage, however, is the difficulty in controlling the quantum state of light. The atom-photon interface serves as a promising solution for making photons "talk to
each other". In this talk, I will discuss some ideas in designing and implementing a suitable light source for light-matter interfaces and report progress made on a recent research trip to ICFO in Barcelona where we built an ultrabright, narrowband (~85000 pairs/mW) entangled photon source suitable for exciting atomic (Rb) transitions.

Ultimately this system will allow us to use atoms to mediate interactions between photons and provide the means to implement quantum light state storage, controlled quantum gates, quantum non-demolition measurements etc.

This project is in collaboration with Morgan Mitchell's group in ICFO-Institut de Ciències Fotòniques, Spain, and supported by the CIPI TEN programme and OCE.

Friday, 30-Nov-07
Jonathan Oppenheim, University of Cambridge
Intrinsic decoherence and the destruction of information
Friday, 23-Nov-07

Rolando Somma, Perimeter Institute for Theoretical Physics
Quantum Computing the Physical World

If a large quantum computer (QC) existed today, what type of physical problems could we simulate on it more efficiently than conventional computer? In this talk, I argue that a QC could solve some relevant physical "questions" more efficiently than its classical counterpart. To show this, I will use tools borrowed from quantum metrology and quantum phase estimation and show how they can be implemented to obtain quantum speed-ups. I will begin by focusing on the quantum simulation of quantum systems and, time permitting, I will also describe possible ways to simulate classical systems.

Friday, 09-Nov-07

Kirill Shtengel, University of California, Riverside
Non-Abelian Anyon Interferometry

Topologically-ordered states supporting excitations with non-Abelian braiding statistics are expected to occur at several observed fractional quantum Hall plateaux. I will begin by presenting a proposal for interferometric experiments designed to detect such non-Abelian quasiparticle statistics -- one of the hallmark characteristics of the Moore-Read and Read-Rezayi states, which are likely candidates for the observed fractional quantum Hall plateaux at nu=5/2 and 12/5 respectively. Aside from their potential utility for experimental verification of non-Abelian anyonic statistics, such interferometric experiments appear to provide the most promising route for qubit read out in a topological quantum computation. With these potential applications in mind, I will also address interferometric measurements of states having superpositions of anyonic charges and discuss their measurement collapse behavior.

Friday, 26-Oct-07

Frédéric Dupuis, University of Montreal
Quantum entropic security and approximate quantum encryption"

An approximate quantum encryption scheme uses a private classical key to encrypt a quantum state while leaking only a very small amount of information to the adversary. Previous work has shown that while we need 2n bits of key to encrypt n qubits exactly, we can get away with only n bits in the approximate case, provided that we know that the state to be encrypted is not entangled with something that the adversary already has in his possession. In this talk, I will show a generalisation of this result: approximate quantum encryption requires roughly n-t bits of key, where t is a lower bound on the quantum conditional min-entropy of the state to be encrypted given the
adversary's prior knowledge. I will show that this result follows naturally from a quantum version of entropic security and indistinguishability. This is joint work with Simon-Pierre Desrosiers.

Friday, 7-Sep-2007
Marcos Curty
Dept. of Electronic Engineering and Communications, University of Zaragoza (Spain)

One-way and Two-way Classical Post-Processing Quantum Key Distribution
We investigate one-way and two-way quantum key distribution (QKD) protocols. Our analysis is based on a simple precondition for secure QKD in each case. In particular, the legitimate users need to prove that there exists no quantum state having a symmetric extension (in the case of one-way QKD), or that there exists no separable state (two-way QKD) that is compatible with the available measurements results. We show that both criteria can be formulated as a convex optimization problem known as a semidefinite program, which can be efficiently solved. Moreover, we prove that the solution to the dual optimization corresponds to the evaluation of an optimal witness operator that belongs to the minimal verification set of them for the given one-way (or two-way) QKD protocol. A positive expectation value of this optimal witness operator states that no secret key can be distilled from the available measurements results. We apply such analysis to several well-known QKD protocols and obtain ultimate upper bounds on the maximal rate and distance that can be achieved with these schemes.