FY2012 Annual Report
Quantum Systems Unit
Associate Professor Thomas Busch
Abstract
After finally relocating to Okinawa, the group has set up again and is finding its feet in the in the new and beautiful environment. Our focus currently remains the work on cold atomic gases and quantum information, but we are making first strides into areas related to biophysics and establishing local collaborations.
Even though they have been experimentally available for almost 20 years now, cold atomic gases are still finding applications as clean and highly controllable test systems in many areas of physics. Most prominent among these are quantum mechanics and solidstate physics and we are working on describing a number of systems and techniques to identify new applications and physics. These include strongly correlated systems, quantum walks, spinchains, newtrapping geometries, superfluid systems, double wells and others.
1. Staff
 Dr. Chandrashekar Madaiah, Researcher
 Dr. Steve Campbell, Researcher
 Dr. Ciaran Phelan, Researcher
 Dr. Jeremie Gillet, Researcher
 Thomas Fogarty, Student
 Nicola Lo Gullo, Student
 Tadhg Morgan, Student
 Tara Hennessy, Student
 Jonathan Richens, Research Intern
 Sawako Koki, Research Administrator
2. Collaborations
 Theme: Fourmode double well models for BoseEinstein condensates
 Type of collaboration: Joint research
 Researchers:
 M.A. Garcia March, University of Barcelona, Spain
 Fernando Sols, Universidad Complutense de Madrid, Spain
 Lincoln Carr, Colorado School of Mines, US
 Theme: Entanglement in Coupled Ion Systems
 Type of collaboration: Joint research
 Researchers:

Giovanna Morigi, Universität der Saarlandes, Germany

 Theme: Nonmarkovian master equations
 Type of collaboration: Joint research
 Researchers:
 Prof. Francesco Petruccione, University of Kwazulu Natal , South Africa
 Theme: Entanglement tests based on spin squeezing
 Type of collaboration: Joint research
 Researchers:
 Prof. Bryan Dalton, Swinburne University, Australia
 Theme: Multicomponent quantum gases in the strong correlation regime
 Type of collaboration: Joint Research
 Researchers:
 M.A. Garcia March, University of Barcelona, Spain
 Theme: Adiabatic techniques for quantum engineering
 Type of collaboration: Joint Research
 Researchers:
 J. Mompart, Universitat Autonoma de Barcelona, Spain
 V. Ahufinger, Universitat Autonoma de Barcelona, Spain
 R. Menchon, Universitat Autonoma de Barcelona, Spain
3. Activities and Findings
3.1 Decoherenceassisted energy transfer and quantum criticalities
As an example where an external environment can control a specified system. we have studied the dynamics of a twolevel quantum system interacting with an external XY spin chain in the presence of an external magnetic field. While the bath itself can enhance the transition probability from the lower level to the upper level of the system, we show that this noiseassisted phenomenon is also sensitive to a change of the quantum phase of the environment. The derivative of the transition probability displays a maximum in correspondence with the critical value of the applied field in the case of both isotropic and anisotropic chains.
The figure shows the transition probability as a function of external field strength for a increasing number of sites. The mean field limit is represented by the smooth black curve and the inset shows the maximum of the derivative appearing at the critical value.
3.2 Creating atomnumber states around tapered optical fibers
By introducing a tapered optical nanoﬁber into a Mott insulator state of ultracold atoms in an optical lattice potential, a mesoscopic atomic sample with a very well defined particle number can be created. This is possible due to the fact that the disturbance of the lattice geometry due to scattering effects on the fibre is small and we have calculated the best strategy for minimizing this effect. By transmitting offresonant light through the ﬁber one can then melt the lattice potential locally, so that only a welldefined number of atoms from the Mott state transfer into the lattice potential.
The figure shows an example in which the original lattice positions are identified, which are effected by the optical potential around the fibre such that the atoms are trapped in the fibretrap after the lattice is switched off. For this set of parameters a sample of 20 atoms is created.
3.3. Decoherence effects on a twodimensional quantum walk
Noise effects resulting in state ﬂipping and depolarization are known to be able to enhance transport in quantum walk systems, due to their destructive influence on the coherences build up during the walk. In this work we have compared the quantum correlations between the two spatial (xy) degrees of freedom using measurementinduced disturbance for several twodimensional discretetime quantum walks and found that the schemes using twostate coins are more robust against decoherence than the Grover walk, which uses a fourstate coin.
The figure shows the probability distribution of the Grover walk when subjected to a different stateﬂip noise level p = 0.1 (left hand side) and p = 0.9 (right hand side).
3.4. Effect of interparticle interaction in a free oscillation atomic interferometer
In this project we investigate the quantum states resulting from two interacting bosons repeatedly scattering off a beamsplitter in a free oscillation atom interferometer. In this process the interparticle scattering length and the beamsplitter probabilites can be used as control parameters, leading to the possible creation of a wide range of interesting states for quantum metrology, including the NOON state.
3.5 Entangling two defects via a surrounding crystal
As a model for entangling two particles through an environment we theoretically investigate how two impurity defects embedded in and interacting via a crystalline structure develop correlations as a function of time. For sufficiently cold chains and for a certain class of initial, separable states of the defects we show that entanglement can be created, which results from the interplay between localized modes , is independent of the chain size, and decays slowly with the distance between the impurities.
3.6. Quantum gas mixtures in different correlation regimes
Twocomponent ultracold bosonic gases, where one of the species is in the weakly interacting regime and the other is either weakly or strongly interacting, are of large interest in the area of quantum engineering. We are developing a description for the onedimensional limit, in which a strongly correlated component (a TonksGirardeau gas) is immersed in a BoseEinstein condensate, which is an example of a new class of quantum system involving a tunable, superfluid environment.
3.7. Spin squeezing and entanglement tests
In order to represent physical states the density operator for systems of identical particles must satisfy the symmetrisation principle (SP) and conform to superselection rules (SSR) that prohibit states that have coherences between differing total particle numbers. Here we consider the entanglement between two systems of modes, whose occupancies are given by the particle numbers, through spin squeezing inequalities and find that the presence of superselection rules requires existing inequalities to be modified. The work applies to systems with conserved particle number.
4. Publications
4.1 Journals
 G. L. Giorgi & Th. Busch, Decoherenceassisted energy transfer and quantum criticalities.
Physical Review A 86, 052112 (2012)  S. Campbell, Predominance of entanglement of formation over quantum discord under quantum channels.
Quantum Information Processing, 12, 26232636 (2013)  G. L. Giorgi & Th. Busch, Genuine correlations in finitesize spin systems.
International Journal of Modern Physics B 27, 1345034 (2013)  C. M. Chandrashekar & Th. Busch, Decoherence on a twodimensional quantum walk using four and twostate particle.
Journal of Physics A: Mathematical and Theoretical 46, 105306 (2013)
4.2 Books and other onetime publications
Nothing to report
4.3 Oral and Poster Presentations
 T. Morgan & Th. Busch, Radio Frequency Theory and Applications, Quantum Interfaces: Integrating Light, Atoms and SolidState Devices, Milton Keynes, UK (May 14, 2012) [Poster Presentation]
 L. O'Riordan, T. Morgan & Th. Busch, Simulations of 3D atomic dynamics using a GPU accelerated split operator method, Quantum Correlations Students Workshop, University of Notthingham, UK, (July 2, 2012) [Poster Presentation]
 N. Lo Gullo, G. M. Palma, M. Paternostro & Th. Busch, Coupling Matter Waves to Nanomechanical Oscillators, ICAP 2012: The 23rd International Conference on Atomic Physics, Paris, France (July 23, 2012) [Poster Presentation]
 T. Hennessy & Th. Busch, Preparing welldefined atomnumber states in the evanescent field of an optical nanofibre and using the fibre to resolve local atoms in an optical lattice, ICAP 2012: The 23rd International Conference on Atomic Physics, Paris, France (July 23, 2012) [Poster Presentation]
 S. Campbell, Propagation of nonclassical correlations across a quantum spin chain, 11th International Conference on Quantum Communication, Measurement and Computing, Vienna, Austria (Aug 3, 2012) [Poster Presentation]
 C. M. Chandrashekar & Th. Busch, Noiseenhanced quantum transport using quantum walks, Gordon Research Conference on Quantum Sciences, MA, USA (Aug 15, 2012) [Poster Presentation]
 C. F. Phelan, T. Hennessy & Th. Busch, Atomic motion in the evanescent field of tapered optical fibers, Photon12, Durham, UK (Sep 2, 2013) [Oral Presentation]
 J. Goold, T. Fogarty, N. Lo Gullo, M. Paternostro & Th. Busch, Quantum bit embedding in an ultracold Fermi sea, Quantum Africa, Durban, South Africa (Sep 3, 2012) [Oral Presentation]
 L. O'Riordan, T. Morgan & Th. Busch, Simulations of 3D atomic dynamics using a GPU accelerated split operator method, Quantum Atomic, Molecular, and Plasma Physics (QUAMP) Summer School, Queens University Belfast, UK, (Sep 11, 2012) [Poster Presentation]
 J. Gillet, M.A.GarciaMarch, Th. Busch, L.D. Carr, F. Sols, & I. Zapata, Tunnelling, selftrapping and manipulation of higher modes of a BEC in a double well, Quantum Technologies Conference III, Warsaw, Poland (Sep 14, 2012) [Oral Presentation]
 T. Fogarty, A. Kiely, S. Campbell & Th. Busch, Creating NOON states in a free oscillation atom interferometer, Quantum Technologies: taking concepts through to implementations London, UK (Dec 17, 2012) [Poster Presentation]
 T. Hennessy, C. F. Phelan & Th. Busch, Cold Atoms and Optical Nanofibres, Quantum Technologies: taking concepts through to implementations London, UK (Dec 17, 2012) [Poster Presentation]
5. Intellectual Property Rights and Other Specific Achievements
Nothing to report
6. Meetings and Events
6.1 Seminar
 Speaker: C. Madaiah
Title: Twostate quantum walk on different lattices and its applications
Date: Oct 1, 2012
Venue: Institute of Mathematical Sciences, Chennai, India
 Speaker: C. Madaiah
Title: Twostate quantum walk on different lattices and its applications
Date: October 3, 2012
Venue: Indian Institute of Sciences, Bangalore, India
 Speaker: C. Madaiah
Title: Quantum percolation of twostate particle and Anderson transition
Date: Feb 1, 2013
Place: Raman Research Institute, Bangalore, India
 Speaker: Madaiah, C.
Title: Localization induced storage and perfect retrieval of quantum state
Date: Feb 1, 2013
Venue: Electrical and Communication Engineering DepartmentIndian Institute of Sciences, Bangalore, India
 Speaker: Prof. Fabrice Debbasch, UPMC  University Pierre and Marie Curie, France
Title: Discrete time quantum walks in (1 + 1) dimensions: wave propagation and diffusive transport
Date: March 5, 2013
Venue: OIST
 Speaker: Dr. Ameneh Sheikhan, The National Institute for Theoretical Physics (NITheP), Stellenbosch, South Africa
Title: Fermi edge singularity in the semiinfinite one dimensional wire interacting with a quantum dot
Date: March 26, 2013
Venue: OIST
6.2 Workshop: Quantum Seminar
 Organizer: Thomas Busch
 Coorganizer: Sile Nic Chormaic, Nic Shannon
 Dates: May 710, 2012
 Place: OIST Campus
 Speakers:
 Prof. J. Miller, Tails of Genome Evolution, OIST
 Prof. T. Shintake, Quantum wave approaches nanoworld in biosystems, OIST
 Prof. E. De Schutter, A Sampling of Research in the Computational Neuroscience Unit, OIST
 Prof. D. Konstantinov, Quantum physics of electrons on helium, OIST
 Prof. B. Kuhn, In Vivo Imaging of the Brain at Cellular or Subcellular Resolution, OIST
 Prof. U. Skoglund, Using Molecular Electron Tomography to analyze macromolecules, OIST
 Prof. M. Wolf, Molecular structure from images – single particle cryoEM in a nutshell, OIST
 T. Fogarty, Strongly Correlated Quantum Gases, University College Cork
 M. Daly, Biological optical fibres, University College Cork
 Dr. C. F. Phelan, Optical Nanofibres, University College Cork
 Dr. O. Sikora, Quantum Monte Carlo Simulations of a Quantum Ice, OIST
 Dr. C. Madaiah, Quantum Walks, OIST
 A. Smerald, Optical nanofibres as atom probes, University of Bristol
 L. Russel, Optical nanofibres as atom probes, University College Cork
 A. Watkins, Microbubbles as sensors, University College Cork
 Dr. S. Campbell, PostMarkovian Master Equations, University College Cork
 J. O. Benton, How to make a photon in a quantum spin liquid, University of Bristol
 Dr. Y. Yang, Whispering gallery mode cavities, University College Cork
 Dr. J. Gillet, Quantum Entanglement, University College Cork
 Dr. L. Jaubert, Magnetic Monopoles in Spin Ice, OIST
 M. Frawley, Optical nanofibres for particle manipulation, University College Cork
 Dr. C. Cassidy, Nanoparticles by Design for Nanotechnology and Biomedical Applications, OIST
7. Others
7.1 Visitors
Jan 14  Mar 07 Richard Menchon (Universitat Autonoma de Barcelona, Barcelona, Spain)
Feb 03  Mar 02 Miguel Angel Garcia March (Universitat de Barcelona, Barcelona, Spain)
Mar 03  Mar 10 Fabrice Debbasch (University Pierre et Marie Curie, Paris, France)
Mar 10  Mar 21 Albert Benseny Cases (Universitat Autonoma de Barcelona, Spain)
Mar 10  Mar 25 John Goold (Oxford University, UK / University College Cork, Ireland)
Mar 10  Mar 30 Ameneh Sheikhan (National Institute for Theoretical Physics, Stellenbosch, South Africa)
Mar 12  Mar 16 Libby Heaney Centre for Quantum Technologies, National University of Singapore, Singapore)