AtMol Research Seminar Archive
Index Current Seminars

Easter 2010

Date Speaker Institution
28th April Dr David Szwer University of Oxford

Trapped atomic ions are perhaps the most promising system for the implementation of a quantum computer, and also have important applications for atomic optical clocks and investigations of fundamental physics. In this talk I shall describe two recent experiments in the Oxford ion trap group, both involving the use of pulses to undo unwanted interactions with a trapped-ion qubit.

Dynamical decoupling is a development of the well-known spin-echo method. A sequence of pi-pulses is used to protect a qubit from noise that would otherwise lead to dephasing. This seminar will introduce the subject, including a particularly simple way to derive the optimal UDD sequence (originally discovered by Uhrig). I will describe an experiment that tests dynamical decoupling on a single trapped Calcium-43 ion. A 20-pulse UDD sequence was able to extend the coherence time by a factor of 65.

I shall also present a new way to store a qubit in Calcium-40, using Zeeman substates of a long-lived metastable level. We have recently used this to perform a quantum uncollapse experiment, in which the state collapse caused by a weakened measurement is undone by performing a second measurement. If (and only if) both measurements give a certain result, the original qubit state is recovered after the second measurement; otherwise the recovery attempt fails.

5th May This is Level 4 viva week. There will be no seminar.

Polynomial-Time Algorithm for Prime Factorization on a Quantum Computer
12th May Mr Adam West Journal Club Seminar

Most of us have referenced quantum computing in papers. This talk will be a brief excursion into that world. I will attempt to explain Shor’s algorithm in an hour.

Quantum analogue computation
19th May Dr Viv Kendon University of Leeds

The quantum version of analogue computation -- usually known as continuous variable quantum computing (CVQC) -- is relatively unexplored compared to digital quantum computation. We know that universal quantum computation is possible in an analogue setting [Lloyd+Braunstein PRL 82 1784 1999], with the same caveats as classical analogue computation, where the resources scale unfavourably with precision due to the lack of binary encoding of the data. Quantum simulation of quantum systems also does not binary encode the data [Brown et al, PRL 97 050504 2006]. In this talk I will give an introduction to both CVQC and qantum simulation, then explore the commonalities between them, and the implications this has for the development of both.

Improved constraints on non-Newtonian forces at 10 microns
26th May Miss Anna Marchant Journal Club Seminar

Gravity is the most mysterious of nature's four known forces. Because it is so weak, researchers have only been able to test Newton's inverse square law down to distances of about 0.1 mm in the last few years. Researchers at Stanford have now developed a tabletop experiment to rule out the existence of strong, gravitational-like forces at short length scales. Such forces, which could hint at additional space-time dimensions or weird new particles, would cause Newton's inverse square law of gravity to break down. By directly measuring the gravitational force on a micromechanical cantilever the Stanford research group have found no evidence for such effects down to a distance of about 10 microns. In this talk the current (and future) experimental setup will be discussed along the possibility of using ultracold atoms to carry out similar measurements.

2nd June Dr Thomas Pohl Max Planck Institute, Dresden


Tools for a quantum-optical world: Sources, circuits and detectors
9th June Dr Brian J. Smith University of Oxford

The ability to create, manipulate, and efficiently measure quantum states of light is of great importance to emerging quantum technologies and tests of quantum theory itself. Nonlinear optical approaches such as spontaneous parametric downconversion and four-wave mixing allow creation of a wide range of useful states - from highly-entangled to completely separable photon pairs as well as squeezed states of light. Guided-wave interferometers with thermo-optic phase control enable the coherent manipulation of non-classical mode entanglement, which is paramount for scalable linear optics quantum computing. Non-standard optical detection methods, such as photon counting or homodyne detection, are necessary to determine the quantum state of such non-classical sources and output states.

Here we present techniques for controlling the spatial-temporal mode structure of light produced by nonlinear processes. In particular, we show that it is possible to create heralded single photons in pure quantum states by engineering the mode structure for efficient conversion. Using these sources, a demonstration of phase-controlled photonic quantum circuits is presented. A phase-sensitive photon-counting detector based upon weak-field homodyne detection coupled with photon-counting detectors is introduced to characterize the few-photon states produced by these sources. This detector bridges the particle-like aspects of the PNR detector and the wave-like aspects of a standard homodyne detector.

Rochester Lecture: Antimatter: From Imagination to Application - and Back
Tuesday 15th June Professor Michael Charlton Swansea University

Please note the Rochester lecture has been rescheduled from 23rd June.

Antimatter was predicted and discovered in the 1930's. The positron, the antimatter counterpart to the electron, has since found numerous applications in material science, engineering and medicine making use of its annihilation with electrons

Recently, physicists have learnt how to create atoms of antihydrogen under controlled conditions in a vacuum. These experiments will be described as well as the motivation for undertaking them. This will involve one of nature's great conundrums: the absence of bulk antimatter in the current epoch of the Universe.

Experiments with ultracold fermions
16th June Dr Lucia Hackermueller University of Nottingham

Ultra cold fermions in optical lattices are a promising tool in order to simulate solid state physics, i.e. they represent an ideal and highly tunable realisation of the Hubbard model. In our system we study 40K atoms trapped in a blue detuned lattice in combination of a red-detuned dipole trap.

Tuning the scattering length via a Feshbach resonance we are able to prepare and detect metallic and Mott insulating states. When changing the interaction from non-interacting to strongly attractive, we measure a decrease of the cloud size for intermediate attraction and an unexpected re-increase in size for strong attractive interactions due to the finite entropy of the system. Moreover we are able to study the dynamical behaviour of fermionic mixtures in out-of-equilibrium situations. We prepare a band insulating system and by lowering the confinement suddenly release it into a homogenous lattice. We monitor the behaviour for various scattering lengths.

Additionally I will talk about the experiments we currently plan and set up at the University of Nottingham.

Robust mesoscopic quantum superpositions of strongly correlated atoms
Tuesday 22nd June Dr. David Hallwood Massey University

Large quantum superpositions carry great promise for enhancing precision measurements but can be extremely fragile, currently limiting the size of maximally entangled ``NOON'' states to 10 particles. The related mesoscopic superpositions of current states consisting of billions of Cooper pairs observed in superconducting rings have proven more robust but their microscopic nature is debated. Here we present a realistic microscopic model of ultra-cold atoms in a ring trap analogous to superconducting rings. Most importantly, we show how correlations in the strongly-interacting Tonks-Girardeau regime produce superposition states of mesoscopic flow that are robust against single-particle loss and excitation. This has allowed us to develop a scheme for performing precision measurements of rotation that has the same precision as a maximally entangled NOON state, while degrading as slowly as non-interacting atomic systems in the presence of atom loss.

Towards the functional imaging of thick (engineered) tissue with acousto-optics
30th June Dr Nick Parker University of Leeds

The artificial generation of human tissue is hampered by our inability to monitor the functional development of the tissue at both large depths and high resolution. Light is perhaps the most powerful modality for functional imaging and yet is so heavily scattered in tissue that imaging resolution falls off rapidly with distance. One promising technique to overcome this scattering problem is acousto-optical tomography whereby sound waves are exploited to spatially 'tag' the scattered light and thus enable optical imaging at the depths and resolution possible with acoustics. I will describe this hybrid technique and present our own progress towards applying it to engineered tissue.