Abstracts for CMI Meeting, June 2003

Artur Ekert
Centre for Quantum Computation, University of Cambridge
Tba

C. H. W. Barnes
Cavendish Laboratory, University of Cambridge
Progress in Surface Acoustic Wave Quantum Computation

Daniel Oi
Centre for Quantum Computation, University of Cambridge
Tba

Nilanjana Datta
Stats lab. University of Cambridge
Tba

Oliver Johnson
Stats lab. University of Cambridge
Lempel-Ziv compression of the canonical ensemble

 I will review the recent paper of Johnson and Suhov (math-ph/0305016),  extending previous work on data compression of the grand canonical  ensemble. In this new model, dependence is present through the fact that  the sum of the variable is fixed. I will show how probabilistic ideas such  as log-concavity, negative association and Newton's inequalities can be  applied to overcome this problem.

Seth Lloyd
Mechanical engineering, MIT
So You've Built a Quantum Computer, Now What Are You Going to Do with It?

F. Wong
Optical Communications Group, RLE, MIT
All photons, all entangled, all the time

Tim Havel
Dept. of Nuclear Eng. MIT
The Real Density Matrix

David  Mackay
Cavendish Laboratory, University of Cambridge
Sparse Graph Codes for Quantum Error-Correction (quant-ph/0304161)

We present sparse graph codes appropriate for use in quantum error-correction.

Quantum error-correcting codes based on sparse graphs are of interest for three reasons. First, the best codes currently known for classical channels are based on sparse graphs. Second, sparse graph codes keep the number of quantum interactions associated with the quantum error correction process small: a constant number per quantum bit, independent of the blocklength. Third, sparse graph codes often offer great flexibility with respect to blocklength and rate.

We believe some of the codes we present are unsurpassed by previously published quantum error-correcting codes.

Adrian Kent
Centre for Quantum Computation, University of Cambridge
Composability: Problems and Solutions

Combining individually secure quantum cryptographic sub-protocols does not necessarily produce a secure protocol. This talk uses a recently devised cheat sensitive bit commitment protocol to illustrate the problem and to show how it can be solved in this case.

Toshio Ohshima
Centre for Quantum Computation, University of Cambridge and Fujitsu Labs. Ltd
Electron Spin Quantum Computer using Quantum Dots. Concepts and Challenges

The quantum gate implementation scheme proposed by D. Loss utilizes the exchange interaction between electron spins in coupled quantum dots. We review the scheme and related experimental results. Then, we will point out its major difficulties and the possible solutions.

D.A. Williams
Hitachi Cambridge Laboratory, Hitachi Europe Ltd.
Silicon Devices for Quantum Information Processing.

I will describe some of our approaches to making silicon structures for quantum information processing, and how we aim to embody the quantum information (as qubits) in real silicon-based nanostructures. We will show recent experimental results, and discuss the various mechanisms which help and hinder the development of this field. There has been substantial recent progress, and we will give our view of the routes to making usable structures.

The qubits may be realized by the spin and/or charge states of individual electrons confined within artificially defined quantum dots. Manipulation of qubits, and control of qubit interactions, will be achieved with local electrodes. To define the quantum dots, we employ a trench isolation approach, which has significant advantages over other quantum dot fabrication methods. It allows the formation of Si and Si-Ge dot and electrometer structures that are close enough for strong interdot interactions and accurate measurement of the dot occupancies by an electrometer. It also allows the integration of quantum dot devices with standard CMOS circuitry, as has been demonstrated previously, and which is a significant advantage for the future use of this technology. Extra gates and other structures such as microcoils and SQUIDs may be added to the structures, which is not possible in entirely surface-gated heterostructures, in which the number of electrodes becomes prohibitive for non-trivial arrays. The use of a global control architecture also gives the option of further reducing the number of independent electrodes.



Pablo Arrighi/Cristophe Patricot
Computer Laboratory/DAMTP, Cambridge.
Information Gain versus Disturbance Tradeoff in the Conal Extension of the Bloch Sphere
We exploit an isomorphism from $\textrm{Herm}_d(\mathbf{C})$ into $\mathbf(R)^{d^2}$. This embedding gives us a convenient real vector representation of quantum states. Because these do not need to be normalized we find that they map into a subcone of a Minkowskian cone in $\mathbf(E)^{(d^2-1,1)}$, whose vertical cross-sections are nothing but generalized Bloch spheres. Pure states map into light-like vectors, unitary operations correspond to orthogonal transforms about the axis, and positive operations are represented by a subset of the real symmetric positive matrices. The latter can also be drawn in the cone - this allows us to visualize non trace-preserving quantum operations and their effects. In the case of a qubit, quantum operations turn out to be proportional to special orthochronous Lorentz transforms. We make use of this framework in order to quantify how much Shannon Information can be gained about an ensemble of two equiprobable non orthogonal pure quantum states against the amount of Disturbance this necessarily causes to the system. The answer to this particular instance of a very fundamental problem (how much does a measurement changes the object of observation?) was first obtained by Fuchs. But our method is simple and geometrical, and more likely to be applicable to variations of the scenario.
Dimitris Angelakis
Centre for Quantum Computation and St Catharine's College, University of Cambridge
Entangling atoms and photons in photonic crystals. Single photon switches and more.

Posters