AtMol Research Seminars
Seminar Archive   •   Next Term’s Seminars

Unless otherwise advertised, all talks are in room Ph30 at 4:00pm on Wednesdays.

Michaelmas Term 2012

Date Speaker Institution
Quantum Computation by Shaking Lattices — Journal Club Talk
10th Oct Mr. Tom Ogden Durham University

I'll talk about a scheme for quantum computation in optical lattices, proposed recently by Philipp-Immanuel Schneider and Alejandro Saenz at Humboldt-Universität zu Berlin. The idea is that qubits are encoded in the spatial wavefunction of the atoms, and operations are performed by shaking the lattice. I'll discuss the theoretical model and their numerical predictions of fidelity and gate times.

Highly polarized limit of the quasi-2D Fermi gas
17th Oct Dr. Jesper Levinsen University of Cambridge

An ultracold gas of fermionic atoms with short range interactions represents a unique playground for studying pairing and effective interactions in a strongly correlated system due to the fine control of dimensionality and interactions. In this talk, I will start by reviewing recent theoretical and experimental progress in the field. Focussing next on the highly polarized limit of the quasi-2D Fermi gas, I will discuss properties of the quasiparticles which a single impurity can form when immersed in a Fermi sea. I will show how the ground state transition of the attractive branch is shifted by the quasi-2D confinement and how this can be described quantitatively throughout the 2D to 3D crossover. I will also demonstrate how the fast decay of the repulsive branch precludes itinerant ferromagnetism in this system.

Superfluids and Superconductors with Spin Triplet Cooper pairing
24th Oct at 13.00 Prof. James Annett University of Bristol

In most known superconductors the Cooper pairs of electrons form into S=0 spin singlet pairs. However a much more rich set of phenomena can occur in the case of S=1 spin triplet pairing. Such a pairing state was first realized in superfluid helium-3 at 2.2mK. However there is now strong evidence for triplet superconductivity in a number of different materials. Perhaps the best evidence is in the material Sr2RuO4, which is superconducting below 1.5K. The pairing state in this system appears to be chiral, breaking time reversal symmetry. We discuss the intrinsic magnetism arising from this chiral state, and its relationship to the controversial spontaneous condensate angular momentum expected in superfluid helium-3. Another class of materials where triplet pairing may occur are crystals without a centre of inversion symmetry. Finally it is also possible to create spin triplet pairs in artificial multilayer systems which combine ferromagnetic and conventional singlet superconductors.

Optomechanics with levitated microspheres
31st Oct Dr. James Millen University College London

The field of optomechanics is the study of the interaction of light with the bulk mechanical motion of macroscopic objects. The goal is to use light to cool the centre-of-mass motion of an oscillator to the sub-phonon level, at which point the oscillator will be in a quantum state. Several groups have managed to cool micron sized objects to this c. o. m. motional quantum ground state [1-3], though quantum effects are difficult to observe due to extremely rapid decoherence mechanisms, mainly due to thermal excitation.

Recent theoretical work has suggested that by levitating the oscillator one can isolate it from sources of decoherence, prepare it in Gaussian state, drop it and produce a quantum superposition of a macroscopic object (containing ~1018 atoms) [4]. Our group has studied methods of cooling glass nano- and microspheres, using optical cavities [5-7] and whispering gallery mode resonances [8]. We are also using the levitated microsphere system to study non-equilibrium thermodynamics, a surprisingly fledgling field of research that has begun to experimentally verify deep relationships between energy and information [9], and the relevance of the thermodynamic laws on the microscale [10].

  • [1] O’Connell et al. ‘Quantum ground state and single-phonon control of a mechanical resonator’ Nature 464, 697-703 (2010)
  • [2] Teufel, J et al. ‘ Sideband cooling of micromechanical motion to the quantum ground state’ Nature 475, 359-363 (2011)
  • [3] Chan, J. et al. ‘Laser cooling of a nanomechanical oscillator into its quantum ground state’ Nature 478, 89-92 (2011)
  • [4] Romero-Isart, O et al. ‘Large Quantum Superpositions and Interference of Massive Nanometer-Sized Objects’ Physical Review Letters 107 020405 (2011)
  • [5] Barker, P. F. ‘Cavity cooling of an optically trapped nanoparticle’ Physical Review A 81 023826 (2010)
  • [6] Pender, G., Millen, J. et al. ‘Optomechanical cooling of levitated spheres with doubly resonant fields’ Physical Review A 85 021802 (2012)
  • [7] Monteiro, T. S., Millen, J. et al. ‘Dynamics of levitated nanospheres: towards the strong coupling regime’ (2012)
  • [8] Barker, P. ‘Doppler Cooling a Microsphere’ Physical Review Letters 105 073002 (2010)
  • [9] Bérut, A. et al. ‘Experimental verification of Landauer’s principle linking information and thermodynamics’ Nature 483 187-189 (2012)
  • [10] Blickle, V., & Bechinger, C. ‘Realization of a micrometre-sized stochastic heat engine’ Nature Physics 8 143-146 (2011)
Photonic Qubits, Qutrits and Ququads in Linear Optical Quantum Circuits
7th Nov Dr. Axel Kuhn University of Oxford

The ability of encoding arbitrary information in elementary quantum systems is the key to a novel approach to computing based on quantum mechanics. Particular attention has been paid toward the field of linear optics quantum computing (LOQC) which in principle is a scalable, albeit often restricted by the spontaneous nature of parametric down-conversion sources. Here, I will show that single photons emitted on demand from a single atom into an optical cavity can be used to get past those limits. With a coherence time greater than 500 ns, a subdivision of photons into d time bins of arbitrary amplitudes and phases has been achieved, which we use for encoding arbitrary qu-d-its in one single photon. The fidelity of the quantum state preparation is verified in time-resolved quantum-homodyne measurements, and the photons are used to operate elementary quantum gates in integrated photonic circuits.

Journal Club Talk — Optical Determination of Boltzmann’s Constant
14th Nov Mr. Mark Zentile Durham University

Boltzmann’s constant is our link between the macroscopic world of bulk objects and the microscopic world of atoms and molecules. In this journal club talk I will explain what Boltzmann’s constant is and why we want to determine it accurately. Recently, new experimental techniques have been developed to measure Boltzmann’s constant by measuring the Doppler broadening of line-shapes in laser absorption spectroscopy. I will explain these experimental techniques and also refined theoretical models for line-shapes which must be applied for these precision measurements.

Experiments with cold trapped Rydberg atoms and molecules
21st Nov Dr. Stephen Hogan University College London

The recent development of methods to manipulate the translational motion of atoms and molecules in Rydberg states, using inhomogeneous electric fields, has led to the realisation of Rydberg atom and molecule optics elements which include mirrors [1], lenses [2] and traps [3-5]. These devices have applications in (i) the development of hybrid approaches to quantum information processing involving Rydberg atoms and microwave circuits [6], (ii) the preparation of gas-phase molecular samples at temperatures below 1 K [7], for studies of slow decay processes and low-energy scattering, and (iii) the confinement and manipulation of anti-hydrogen atoms [8]. In this talk I will describe experiments with electrostatically trapped hydrogen atoms and hydrogen molecules, with an emphasis on the role of radiative processes in the decay of the trapped samples.

  • [1] E. Vliegen and F. Merkt, Phys. Rev. Lett., 97, 033002 (2006)
  • [2] E. Vliegen, P. Limacher and F. Merkt, Eur. Phys. J. D, 40, 73 (2006)
  • [3] E. Vliegen, S. D. Hogan, H. Schmutz and F. Merkt, Phys. Rev. A, 76, 023405 (2007)
  • [4] S. D. Hogan and F. Merkt, Phys. Rev. Lett., 100, 043001 (2008)
  • [5] S. D. Hogan, P. Allmendinger, H. Sassmannshausen, H. Schmutz and F. Merkt, Phys. Rev. Lett., 108, 063008 (2012)
  • [6] S. D. Hogan J. A. Agner, F. Merkt, T. Thiele, S. Filipp and A. Wallraff, Phys. Rev. Lett., 108, 063004 (2012)
  • [7] S. D. Hogan, Ch. Seiler and F. Merkt, Phys. Rev. Lett., 103, 123001 (2009)
  • [8] A. Kellerbauer et al., Nucl. Instr. and Meth. In Phys. Res. B ., 266, 351 (2008)
Interfacing cold atoms and superconductors
28th Nov Prof. Dr. Jozsef Fortagh University of Tuebingen

The goal of our experimental research is the realization of hybrid quantum systems based on cold atoms and solids. I will report on coherent manipulation of atoms near superconducting nano structures and will present data on the interaction between atoms and carbon nanotubes.

Stirring up entanglement: quantum metrology with rotating matter waves
5th Dec Dr. Jacob Dunningham University of Leeds

Quantum metrology makes use of entanglement to achieve measurement precisions beyond what could be achieved by conventional classical methods and is rapidly emerging as an exciting and feasible new technology. I will give a brief introduction to the field before focusing on the particular case of atomic Bose-Einstein condensates (BECs) trapped in rotating potentials. These are appealing because they are within reach of current experiments, provide a conceptually simple way of generating many-body entanglement, and have the prospect of leading to the development of ultra-precise gyroscopes. In order to employ such a system for metrology, it is important to understand the detailed form of the entangled states that can be created. I will present a study that goes beyond the Landau level (LLL) approximation. I will demonstrate that whilst the LLL can identify reasonably the critical frequency for a quantum phase transition and entangled state generation, it is vital to go beyond the LLL to identify the details of the state and quantify the quantum Fisher information (which bounds the accuracy of the phase measurement). We thus identify a new parameter regime for entangled state generation, amenable to experimental investigation.

Rochester Lecture. Place and time to be confirmed.
12th Dec Dr. Alain Aspect CNRS