General QQQ Posters  

Posters submitted by QQQ members, for distribution on this site. Diffraction of Bose-Einstein Condensates Nottingham, Melbourne, Otago Recently, Bose-Einstein condensates (BECs) have been diffracted from surface based wire arrays. To make further progress with such atom optics devices, it is essential to understand the role of the atomic interactions when BECs are used in this context. These interactions often give rise to interesting physics but can also ruin attempts to coherently control condensates. Full poster download.

 QQQ Prize Winning Posters

QQQ sponsors prizes for posters that are presented at conferences whose subjects areas have significant overlap with those of the QQQ. The QQQ prize is awarded to the best student poster "most clearly communicating exciting new developments in the field of Quantum Information, Quantum Control and Quantum Optics". The past winners are presented on this page with links to their research groups. For information on future QQQ poster prizes see Events.

Light interactions in Rydberg ensembles

K.J. Weatherill , J.D. Pritchard , R.P. Abel , M.G. Bason , A.K. Mohapatra , C.S. Adams

Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK

Atoms in highly excited Rydberg states exhibit strong interactions over distance scales of a few microns. In our work we exploit the enhanced sensitivity of Rydberg states to control the propagation of light through an atomic ensemble. For example, if the atoms are prepared in a dark state corresponding to a superposition of ground and Rydberg states\footnote{AK Mohapatra et al. Phys. Rev. Lett. 98, 113003 (2007)} the medium acquires a giant electro-optic effect many orders of magnitude larger than other systems\footnote{AK Mohapatra et al.arXiv:0804.3273}. This giant electro-optic effect can be used to impose sidebands on light propagating through the ensemble as shown in Figure~1a). In ultra-cold ensembles we have observed Rydberg dark states with linewidths of less than 1 MHz (see Figure~1b) and have demonstrated the on-set of interactions effects as the Rydberg density is increased\footnote{KJ Weatherill et al. arXiv:0805.4327}. Our eventual goal is to exploit this giant non-linearity to control pulse propagation at the single photon level.

Figure 1: a) Power spectrum of light transmitted through the Rydberg dark state ensem- ble showing the generation of sidebands at the 2nd harmonic of an applied electric ?eld modulation with amplitude 3 V/cm and frequency ?m. Kerr coef?cients >10?6 m/V2 are measured. Inset: Frequency response of the dark state resonance determined from the sideband intensity. (b) Narrow EIT resonance corresponding to the n = 26d state in a cold Rb sample. Linewidths of ? 600 kHz are observed.

Full poster download

Endohedral Fullerenes and ODMR Spectroscopy for Endohedral Fullerenes

A Tiwari¹, A A R Watt¹, R A Taylor², K Porfyrakis A Ardavan² and G A D Briggs¹
1. QIPIRC, Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH.
2. Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PY

We propose endohedral fullerene based quantum computation using the endohedral spin state to encode quantum information. The ensemble readout of spin can be performed using electron spin resonance (ESR). This technique is senstive enough to reveal the spin activity but it is not suitable for single spin readout which is a prerequiste for most quantum algorithms.

Fullerenes are symmetric, nearly spherical hollow cages of carbon atoms. The space within the cage is large enough to accommodate one or more atoms or small clusters. The resultant structure is called an endohedral fullerene:

e.g. N@C₆₀, P@C₆₀, La@C₆₀, Sc₂@C₈₂ , Er₃N@C₈₀, ErSc₂ N@C₈₀ etc

Above: Experimental setup for PL and PLE of endohedral fullerenes

Rapid state purification of a superconducting charge qubit

E. J. Griffith¹, C. D. Hill¹, J. F. Ralph¹, K. Jacobs² and H. M. Wiseman³
1. The Department of Electrical Engineering and Electronics, The University of Liverpool. United Kingdom.
2. The Department of Physics and Astronomy, Louisiana State University. United States.
3. Centre for Quantum Computer Technology, Griffith University. Australia.

Purification of a qubit by using weak measurement can be a time consuming process when the measurement strength is significantly reduced.

However it has been previously discovered that it is possible to accelerate the purification process by using quantum feedback. Where the maximum purification rate is obtained by placing the Bloch vector on the plane orthogonal to the measurement axis, prior to each measurement.

Unfortunately, the nature of the superconducting charge qubit makes the task of holding the Bloch vector within this plane very difficult.

Therefore we propose a feedback protocol which yields near optimal performance, by using only one control field with simple π-pulses.

Left: Ideal feedback to the plane orthogonal to the measurement axis yields the fastest purification rate. However the perfect controls would be difficult to achieve in practice.

Local control of the quantum dynamics in multiple potential wells

Philipp Marquetand and Volker Engel.
Institut fur Physikalische Chemie, Am Hubland, 97074 Wurzburg, Germany.

The rotational motion of a molecular motor is investigated within the framework of local control theory. Within this approach, the laser fields are constructed from the system's dynamics at any instant of time. In this way, it is possible to give the field a clear physical interpretation.

Here, the aim is to initiate either a clockwise or a counter-clockwise rotation of a functional group in a molecule. We compare quantum and classical calculations, which exhibit the same basic features. This strengthens the point that local control is as close to intuition as possible.

Right: The molecule under investigation is (R)-2-chloro- 5-methylcyclopenta-2, 4-dienecarbaldehyde, which was first used by Fujimura et al. as a model system for triggered motor motion.

Dynamical instability in a nanomechanical superconducting single electron transistor

J. Imbers, D.A. Rodrigues and A.D. Armour.
School of Physics and Astronomy, University of Nottingham, Nottingham, UK.

When a superconducting single-electron transistor (SSET) is tuned to the vicinity of a transport resonance known as the Josephson quasiparticle resonance it acts as an exquisitely sensitive electrometer. Thus when a nanomechanical resonator is coupled electrostatically to the SSET as a movable gate the SSET can be used to monitor the position of the resonator with almost quantum-limited precision. However, Heisenberg's Uncertainty principle dictates that all measurements must disturb the object of measurement. For the SSET-resonator system the back-action of the SSET on the resonator depends very sensitively on how the SSET is operated: under some circumstances, the SSET acts on the resonator like a thermal bath with a surprisingly low effective temperature. In contrast, for a different choice of SSET operating point, the SSET can drive the resonator into an instability. In this poster we explore the nature of this instability. For more infomation see the preprint: http://xxx.soton.ac.uk/abs/cond-mat/0608166 Full poster download. Images: Wigner functions of resonator steady state with different ΔE, the energy gain of Cooper pairs: Top: ΔE = 0.1, system in thermal state; Middle: ΔE = -0.1, system in limit cycle; Bottom: ΔE = -0.16, system has bistability.