May 2014 ColLoQuI

Please note that this event has now taken place

The 6th meeting of ColLoQuI (Colleges of London Quantum Information) will take place at UCL at 2:00pm on Tuesday May 13th 2014 in room 508 in the Roberts Building (5th floor) at UCL (see bottom of page for map).

The event gives researchers, and PhD students in particular, a chance to present their work to the wider London community. In the past, a typical event has taken an afternoon with two longer talks and four shorter ones followed by a trip to the pub.

So we can keep track of numbers, please register your interest by filling in the form here.

Schedule:

  • 14:00 Start
  • 14:05 Fernando Brandão
  • 14:50 Ben Brown
  • 15:15 Seto Balian
  • 15:40 Break
  • 16:10 Bobby Antonio
  • 16:35 Abolfazl Bayat
  • 17:00 Terry Rudolph
  • 17:45 Go to the pub

Abstracts:

Fernando Brandão
Is Quantum Darwinism Generic?

The transition from quantum to classical is a fascinating aspect of nature. How do we observe a classical reality, considering that the fundamental laws of physics are quantum mechanical? In the talk I'll show that under very general conditions, observers who access a quantum system indirectly, by measuring part of its environment, have access only to classical information about the system. Moreover the information is objective, with different observers having access only to information about the measurement of a common observable on the quantum system. I'll thus argue that this emergence of classicality by observing parts of the environment, known as quantum Darwinism and studied before in a variety of particular situations, is a general feature of quantum dynamics. As it will be outlined, the result follows by further developing recent information-theoretic tools for analysing the distribution of quantum information and the phenomenon of entanglement monogamy. The findings show the usefulness of quantum information theory to address problems in other areas of physics.

Based on joint work with Marco Piani and Pawel Horodecki

Ben Brown
Studying Topological Defects using Entanglement Entropy

It is widely recognised that scalable quantum computation will require fault-tolerant protocols to deal with unwanted interactions with environmental degrees of freedom that will otherwise disturb the quantum phenomena we hope to exploit. A remarkable proposal for fault-tolerant quantum computation, topological quantum computation, encodes quantum information in robust non-local degrees of freedom of particles known as 'anyons'. More remarkably still, the information encoded over these special particles can be manipulated fault-tolerantly by braiding their world lines into non-trivial topologies. This mathematically elegant proposal is not just a theoretical pleasantry, but it is believed that anyonic particles can be realised as low energy excitations of topologically ordered lattice models. While this promises an exciting route towards an age of quantum technology, anyons are yet to be detected, and their realisation remains a challenging feat of engineering. Alternatively, it has recently been identified that defects in topologically ordered lattice models will also demonstrate some features that are analogous to anyonic particles. This presents an exciting new avenue towards quantum fault-tolerance. In this talk I will discuss topological quantum computation, using topological defects. We also examine the tools we can use to study such objects.

Seto Balian
‪Quantum-bath-driven decoherence of mixed spin systems‬

‪The decoherence of mixed electron-nuclear spin qubits is a topic of great current importance, but understanding is still lacking: While important decoherence mechanisms for spin qubits arise from quantum spin bath environments with slow decay of correlations, the only analytical framework for explaining observed sharp variations of decoherence times with magnetic field is based on the suppression of classical noise. Here we obtain a general expression for decoherence times of the central spin system which exposes significant differences between quantum-bath decoherence and decoherence by classical field noise. We perform measurements of decoherence times of bismuth donors in natural silicon using both electron spin resonance (ESR) and nuclear magnetic resonance (NMR) transitions, and in both cases find excellent agreement with our theory across a wide parameter range. The universality of our expression is also tested by quantitative comparisons with previous measurements of decoherence around “optimal working points” or “clock transitions” where decoherence is strongly suppressed. We further validate our results by comparison to cluster expansion simulations.‬

‪Bobby Antonio
Adiabatic graph-state quantum computation

Measurement-based quantum computation (MBQC) and adiabatic quantum computation (AQC) are two very different computational methods. The computation in MBQC is driven by adaptive measurements executed in a particular order on a large entangled state. In contrast in AQC the system starts in the ground state of a Hamiltonian which is slowly changed such that the final ground state encodes the answer to the problem. Following the approach of Bacon and Flammia, we show that any measurement-based quantum computation on a graph state with gflow can be converted into an adiabatic computation, which we call adiabatic graph-state quantum computation (AGQC). We then investigate how properties of AGQC relate to the properties of MBQC, such as computational depth. We identify a trade-off that can be made between the number of adiabatic steps and the norm of the time derivative of the Hamiltonian, in analogy to the trade-off between the number of measurements and classical post-processing seen in MBQC. Finally the effects of performing AGQC with orderings that differ from standard MBQC are investigated.

Abolfazl Bayat
An order parameter for impurity systems at quantum criticality

We address the issue of quantum criticality in quantum impurity systems. In contrast to ordinary bulk quantum phase transitions, the notion of a conventional order parameter which exhibits scaling is notably missing at an impurity quantum critical point. We here explore the possibility to use the Schmidt gap, which is an observable obtained from the entanglement spectrum, as an order parameter. A case study of the two-impurity Kondo model confirms that the Schmidt gap faithfully captures the scaling behavior by correctly predicting the critical exponent of the dynamically generated length scale at the quantum critical point.

Location: