Talks

Heavy-hole spin states have been proposed as a robust qubit candidate. Nevertheless, the coupling of the hole spins to nuclei in the surrounding medium likely limits hole-spin coherence and has, until very recently, been overlooked. We describe the spin decoherence of a heavy-hole in a semiconductor quantum dot, subject to spin echo pulses. We do so both analytically and numerically for an experimentally realistic number (10^4) of nuclear spins. Including the (previously neglected) nuclear Zeeman term in the Hamiltonian, we observe novel effects uniquely characterizing the decoherence mechanisms under study. In particular, we find a nontrivial dependence of the decay on the applied magnetic field, as well as novel predictions for motional narrowing and envelope modulation, which could significantly extend the hole-spin memory time in near-future experiments.

Quantum Key Distribution is a form of public-key cryptography where the security comes from the unique properties of quantum mechanical systems: entanglement and the no-cloning theorem, rather than computational complexity. With increased adoption of fibre optic networks, it may be possible to implement QKD in parallel with classical data traffic. Many research projects have demonstrated QKD over fibre optic networks at the same wavelengths as existing network traffic. These projects require sophisticated noise cancellation due to wave mixing between quantum and classical signals, as well as having to use complex non-silicon based photodiodes. Our research uses lower wavelengths for QKD over active telecom fibres to avoid these problems. Entangled lower-wavelength photons are combined with telecom wavelength laser signals carrying a large amount of traffic, and passed through single mode telecom fibres. We show that data bandwidth usage has a negligible effect on the quantum bit error rate (QBER) and visibility for distances up to 6km. We find key rates of 61 bits per second with QBER rates of 10% at 6km. This research demonstrates the simplicity and applicability of QKD to existing fibre optic infrastructure in corporate, government, and academic campuses.

Development of quantum computing promises, among other things, improvement of scientific computation performance. Indeed, a computer exploiting the proprieties of quantum mechanics would allow for computation power exponentially greater than a classic computer.We develop double lateral quantum dots with micro-magnets to control spin orientation of electrostatically confined electrons. In this talk, an introduction to the mechanisms used in the spin control will be given. Then, methods used to characterize the micro-magnets will be described. Finally, we will present the results obtained with Hall effect devices for the micro-magnets.

A quantum computer is a computer fabricated using quantum bits (qubits) that uses the quantum properties of matter (entanglement, superposition of states, etc.). Such a computer would allow certain calculations to be done exponentially more quickly than with a classical computer. An electron in a quantum box constitutes a perfect two-level system and can thus be used as a qubit. In my talk, I will give an introduction to lateral quantum dots, their fabrication process and how they can be used as qubits.

Quantum entanglement is a valuable resource in the field of quantum information science and allows one to accomplish many information processing tasks. In quantum transformations an entangled state A can be converted to another state B through local operations assisted by classical communication (LOCC). It has also been demonstrated that there exist entangled states A, B, C such that state A cannot be converted to a state B, but A otimes C can be converted to B otimes C by LOCC, where C is a suitably chosen entangled state acting as the catalyst. This is known as entanglement assisted LOCC or eLOCC. I will show that for certain A and B it is possible to obtain an extra entangled state R, called the residue entangled state in an eLOCC transformation. That is to say A otimes C can be converted to B otimes R otimes C even though A cannot be converted to B by LOCC. I will discuss the necessary and sufficient conditions for such a transformation to occur.

Thermodynamics is, at heart, a probabilistic theory about the state of physical systems. Traditionally, however, our knowledge of systems is modelled implicitly: for instance, it is often assumed that we only have access to a few macroscopic parameters, like the temperature, energy, or volume of a gas, and that all states satisfying those parameters are equally likely. Another example is Maxwell's demon, an apparent violation of the second law: a demon operates the trapdoor between two boxes filled with a gas at the same temperature. He lets fast particles fly to the right box, cooling the left container and heating the right one at no work cost. The paradox comes from ignoring the demon's memory, a system where he stores his information about the speed of the particles, which has finite capacity. Eventually, he will have to erase his memory, an irreversible operation that costs him work. Classical and quantum information theory have given us tools to model knowledge explicitly: we use them to analyse the security of cryptographic protocols, or how much information can be sent through a noisy channel, for example. In this talk, I will explore what happens when we apply information-theoretical tools to thermodynamics. In particular, I will discuss the implications of having quantum information about a physical system, with the example of erasure of information.

We can prove that for certain problems, quantum computers do better than classical computers. I will introduce the query complexity framework, which lets us compare classical and quantum computers, and then describe a problem where quantum computers do better than classical. The problem I will discuss is evaluating boolean trees with a promise on the input.

By exploiting the properties of quantum mechanical systems, two parties can achieve cryptographically secure communication in a manner not possible in a purely classical world, through the process of quantum key distribution. In this talk, I will briefly introduce the field of cryptography and explain one of the most fundamental applications of quantum mechanics to cryptography.

Our understanding of the physical world at the most fundamental level is based on two theories: quantum theory and general relativity. They are impressively successful but only when each is considered on its own. In situations where both play a role, we are reduced to puzzles and absurdity. Hence the search for a quantum theory of gravity, the currently missing theory that will work sensibly in exactly these situations. To the great frustration of researchers in this field, candidate quantum theories of gravity tend to produce more puzzles instead of answers. We shall take a tour of some of the problems, focusing on the role of spacetime and causality. We will consider the possibility that spacetime did not always exist but is instead emergent and explore how one can create a spacetime from a world with no notion of "here" and "there".

I present a proposal, originally motivated by a result in graph theory: the entropy function of a density matrix naturally associated to a simple undirected graph, is maximized, among all graphs with a fixed number of links and nodes, by regular graphs.I recover this result starting from the Hamiltonian operator of a non-relativistic quantum particle interacting with the loop-quantized gravitational field and setting elementary area and volume eigenvalues to a fixed value. This operator provides a spectral characterization of the physical geometry, and can be interpreted as a state describing the spectral information about the geometry available when geometry is measured by its physical interactionwith matter. It is then tempting to interpret the associated entropy function as a genuine physical entropy: I discuss the difficulties of this interpretation and I present a possible viable definition of quantum-gravitational entropy.