One of the long standing questions in quantum physics is the reason electrical conductivity varies so widely between materials, over 16 orders of magnitude, much more than can be explained by the availability of electrons. The accepted explanation lies in Anderson localisation , which relies on the quantum wave nature of the electrons, which carry electrical current.
Different paths from A to B in a random distribution of scatterers
For a wave to propagate from A to B in the figure by scattering off a random distribution of scatterers, the different paths have different lengths and therefore different phases. After summing over all the possible paths, these interfere destructively, and propagation is suppressed. On the other hand, the backward and forward going paths going from A to B and back have the same path lengths, and will interfere constructively. An insulator is an example of a system that creates such a random potential for electrons. The motion of electrons in an insulator cannot be tracked directly, but this phenomenon has been studied in a range of equivalent systems [2-5]. Recent experiments using ultra-cold atoms show Anderson localisation in a potential generated by laser speckle [6-9]. This exciting development is closely related to the quantum chaos work on the delta-kicked rotor system by both the P.I. [10-13] and the A.I. [14-17].
Critical remaining questions that we want to answer are:
- How do interactions influence Anderson localisation?
- How does the nature of the randomness of the potential influence Anderson localisation?
- What happens if the potential depends on time?
Answering these will significantly enhance our understanding of one of the most important properties of materials: their electrical conductivity. We propose to investigate the motion of ultra-cold atoms in a programmable quasi-random potential, which can be arbitrarily shaped. The atoms are so cold, that the wavelength of their quantum-mechanical wave function is much larger than the characteristic distance scale of the potential. We intend to use different potential length scales, as well as different random functions, thereby mapping out the transition from classical to quantum motion. We will confine the atoms to a wave guide to study the role of interactions. Furthermore, the atoms can be given a velocity with respect to the potential, which will allow us to observe a tell-tale signature of Anderson localisation, which is coherent back scattering .
We trap and cool rubidium atoms in a dipole trap, formed by the focus of a CO2 laser, to the point of Bose-Einstein Condensation (BEC) [19,20]. At this point, all atoms in the trap share the same quantum-mechanical wave function, and are at an equivalent temperature of 50 nanoKelvin above absolute zero. The atoms now are moving very slowly, and consequently have a large “deBroglie” wavelength, larger than the wavelength of light we use for the random potential.
After creating the BEC, the CO2 laser trap is turned off and replaced by the quasi-random potential, which is formed by a laser beam that has been shaped by a Spatial Light Modulator (SLM). The SLM consists of a two-dimensional array of 1024×768 microscopic mirrors, that can each be addressed separately, and dynamically. We then obtain an image of the atomic distribution after a propagation time. If Anderson localisation occurs, we should see a drastically reduced spread of the atoms after a time-of-flight, even when the maximum height of the potential is much less than the kinetic energy of the atoms.
The experiments described here build on the existing infrastructure at the University of Auckland, which has routinely made BECs since 2006, and was established by the P.I., and on a strong tradition in New Zealand in Quantum and Atom Optics research. It provides a great opportunity for graduate students to be involved in ground-breaking research.
- Absence of Diffusion in Certain Random Lattices, P. W. Anderson, Physical Review 109, 1492 (1958)
- Fifty years of Anderson localization, A. Lagendijk, B. van Tiggelen and D. Wiersma, Physics Today, pg 24, August 2009.
- D. S. Wiersma, P. Bartolini, A. Lagendijk, R. Righini, Nature 390, 671 (1997).
- H. Hu, A. Strybulevych, J. H. Page, S. E. Skipetrov, B. A. van Tiggelen, Nat. Phys. 4, 945 (2008).
- T. Schwartz, G. Bartal, S. Fishman, M. Segev, Nature 446, 52 (2007).
- Direct observation of Anderson localization of matter waves in a controlled disorder, Juliette Billy, Vincent Josse, Zhanchun Zuo, Alain Bernard, Ben Hambrecht, Pierre Lugan, David Clément, Laurent Sanchez-Palencia, Philippe Bouyer & Alain Aspect
Nature 453, 891-894 (12 June 2008).
- Anderson localization of a non-interacting Bose–Einstein condensate, Giacomo Roati, Chiara D’Errico, Leonardo Fallani, Marco Fattori, Chiara Fort, Matteo Zaccanti1,2, Giovanni Modugno, Michele Modugno and Massimo Inguscio, Nature 453, 895-898 (12 June 2008).
- Three-Dimensional Anderson Localization of Ultracold Matter, S. S. Kondov et al. Science 334, 66 (2011).
- Three-dimensional localization of ultracold atoms in an optical disordered potential,
F. Jendrzejewski, A. Bernard, K. Müller, P. Cheinet, V. Josse, M. Piraud, L. Pezzé, L. Sanchez-Palencia, A. Aspect & P. Bouyer, Nature Physics 8, 398–403 (2012).
- Quantum resonant effects in the delta kicked rotor revisited, A. Ullah, S.K. Ruddell, J-A. Currivan and M.D. Hoogerland, Euro. Phys. J. D 66, 315 (2012).
- Experimental observation of Loschmidt time reversal of a quantum chaotic system. A. Ullah and M. D. Hoogerland. Physical Review E 83, 046218 (2011)
- The initial velocity dependence of the quantum resonance in the delta-kicked rotor, J.A. Currivan, A. Ullah and M.D. Hoogerland, Europhysics Letters 85, 30005 (2009).
- Short-time energies in the atom optics kicked rotor, S.A. Wayper, W. Simpson and M.D. Hoogerland, Europhys. Lett. 79, 60006 (2007).
- The effect of amplitude noise on the quantum and diffusion resonances of the atom-optics kicked rotor, M. Sadgrove, T. Mullins, S. Parkins, and R. Leonhardt, Physica E 29, 369 (2005).
- Ballistic and localized transport for the atom optics kicked rotor in the limit of a vanishing kicking period, M. Sadgrove, S. Wimberger, S. Parkins, and R. Leonhardt, Phys. Rev. Lett. 94, 174103 (2005).
- Experimental verification of a one-parameter scaling law for the quantum and “classical” resonances of the atom-optics kicked rotor, S. Wimberger, M. Sadgrove, S. Parkins, and R. Leonhardt, Phys. Rev. A 71, 053404 (2005).
- Deviations from early-time quasilinear behavior for the atom-optics kicked rotor near the classical limit, M. Sadgrove, T. Mullins, S. Parkins, and R. Leonhardt, “ Phys. Rev. E 71, 027201 (2005).
- Coherent Backscattering of Ultracold Atoms F. Jendrzejewski, K. Müller, J. Richard, A. Date, T. Plisson, P. Bouyer, A. Aspect, and V. Josse, Physical Review Letters 109, 195302 (2012)
- Bose-Einstein Condensation in Dilute gases, C.J. Pethick and H. Smith, Cambridge (2001).
- A versatile all-optical Bose–Einstein condensates apparatus, Y. C. Wenas and M. D. Hoogerland, Rev. Sci. Instrum. 79, 053101 (2008).