Physics 437: Projects in Quantum Computing

Jonathan Baugh
BFG 2112 ext. 37491

My lab is aimed towards the realization of solid-state quantum devices for information processing and metrology. Electron and nuclear spins play a central role by representing quantum bits of information, or qubits. Our challenge is to engineer physical systems so that all the ingredients needed for computation - initialization, manipulation, readout - can be achieved with high fidelity and in a scalable fashion. The physical systems of interest are solid-state systems, such as nanoscale devices for confining single electrons (quantum dots) and ensemble spin systems for magnetic resonance quantum computing (e.g. spin-tailored single crystals).

Characterization of nanoscale devices
A project in this area will likely involve some work on improving the data acquisition system for low temperature electronic transport measurements, as well as analysis of data to characterize the energetics of quantum dot devices. Students will become familiar with LabView and MatLab software and with the main concepts of cryogenic low noise electronic measurements. Motivated students may also work on improving the home-built instrumentation for such measurements, such as construction of special filter circuitry or work on cryogenic signal amplification.

Nuclear and electron magnetic resonance approaches to quantum information
In this area, we exploit well-developed radio-frequency and microwave technologies to implement high fidelity quantum control in ensemble spin systems. Our aim is to use the high degree of polarization that can be achieved for electron spins to boost the polarization of the nuclear spins by several orders of magnitude, thereby improving the scalability of this approach. In addition, hyperfine coupled electron-nuclear spin systems can be jointly controlled so that electron spins mediate interactions between nuclear spins, greatly speeding up gate operations and allowing more efficient readout of quantum information. Relevant projects include:

1) Design and fabrication of microscale microwave couplers for electron spin resonance experiments in quantum dots and small ensemble NMR/ESR samples.

2) Dynamic nuclear polarization experiments for solid-state bulk NMR quantum computing.

Raymond Laflamme
BFG 2105, PHY 368 ext. 2430, 3252

Overview of the state of the art of single photon sources and detectors
Single photon sources and detectors are needed for quantum cryptography and in the recent suggestion to use linear optics for quantum computation.

The project consists of a critical evaluation of today's technologies to produce single photon sources and detectors.

A small theoretical project on investigating the effect of dominant error sources for linear optics for quantum computation might or not be included depnding on the progress.

Adrian Lupascu,
RAC 2112 ext. 35468

Research in the superconducting quantum devices lab addresses the physical properties of solid-state systems with superconductors, with an emphasis on quantum properties. This research has fundamental aspects (understanding decoherence in complex systems, fundamental limits on control and detection) as well as applications to quantum information and detectors.

Examples of available undergreaduate projects are given below.

Instrument control and data acquisition for experiments in quantum manipulation
The student working on this topic will design and implement control sequences for single and multiple superconducting quantum bits, as well as procedures for analysis of experimental data. The project involves learning the methods of quantum control with superconducting qubits, based on shaped microwave pulses. Implementation will involve programming of various instruments, as well as tests of the performance of pulse generation using eg oscilloscopes and spectrum/network analyzers.

Design and numerical simulation of superconducting quantum devices
Superconducting quantum devices can be modelled as electromagnetic circuits with non-linear components known as Josephson junctions. The Hamiltonian can be obtained using an equivalent circuit representation followed by canonical quantization. The project will involve the modelling a few types of circuits which are used as qubits as well as analytical and numerical calculation of important parameters, such as energy levels, transition matrix elements, and decoherence times.

Gregor Weihs
BFG 1101F, 1102A ext. 7485, 2107

Sources of entangled photon pairs
Photon pairs are said to be entangled when only their joint properties are defined but not their individual ones. They are often called twin photons and are used for quantum key distribution, teleportation and computation. The traditional sources of entangled photon pairs, such as parametric down-conversion in nonlinear optical crystals are very inefficient and suffer from dispersion and multi-pair emission.

We investigate two new types of sources of entangled photon pairs that should have distinctive advantages. A source based on nonlinear converison in a photonic crystal waveguide will allow group velocity matched conversion of an ultrafast laser pulse whereas single quantum dots may be able to funciton as deterministic sources of entangled photon pairs.

Experimental and theoretical/computational projects are available for both directions.

Examples:

• Single quantum dot spectroscopy in a He cryostat
• Photonic crystal waveguide design and fabrication
• Waveguide spectroscopy

Quantum Key Distribution
Classical cryptography suffers from the problem that the security of all known ways of distributing keys for secret communication relies on either a trusted courier or the alleged computational difficulty of a certain mathematical algorithm. Quantum cryptography provides a way of establishing identical, random and secret keys in two remote locations where the keys' security is guaranteed by the laws of physics. Single quanta are transmitted from one location to the other and any interception will show up as a disturbance that can be detected. Since no information is transmitted at this step, no interception can cause a leak of secret information. The established random keys will then encrypt any message with perfect security.

We are setting up a prototype quantum key distribution (QKD) system that will link the Institute for Quantum Information and the Perimeter Institute by sending entangled photon pairs via free-space optical telescope links.

Within the QKD project there are several experimental projects available either in the optical or in the data acquisition part of the prototype.