André Xuereb
Royal Commission for the Exhibition of 1851 Research Fellow,
Visiting Researcher at Queen's University Belfast, and
Affiliate Lecturer at the University of Malta

School of Mathematics and Physics, David Bates Building,
Queen's University Belfast,
Belfast BT7 1NN,
United Kingdom

Department of Physics, Faculty of Science
University of Malta,
Msida MSD2080,
       Date of birth: 2nd January, 1985
Tel.: +44 79 4672 3826, +356 7909 2662
E-mail: andre (at)



Affiliation: I am currently a Research Fellow of the Royal Commission for the Exhibition of 1851, as well as a visiting researcher working in the QTeQ group at Queen's University Belfast and an affiliate lecturer in the Department of Physics at the University of Malta.

Previous affiliations: From February to October 2011, I was a research assistant in Klemens Hammerer's group in Hannover. Officially, I was part of the Institute for Theoretical Physics at the Leibniz University, Hannover, and the QUEST (Centre for Quantum Engineering and Space-Time Research) excellence cluster in Hannover.
From September 2007 to March 2011 I was a doctoral student in Tim Freegarde's Quantum Control group at the University of Southampton. My PhD thesis, "Optical Cooling Using the Dipole Force", is available in book form, published by Springer Theses; see, e.g., Amazon.

Research interests:

Curriculum vitae: Available in PDF and Xetex formats.



Peer-reviewed publications:

Published journal articles

1. André Xuereb, Claudiu Genes, Guido Pupillo, Mauro Paternostro, and Aurélien Dantan; Reconfigurable long-range phonon dynamics in optomechanical arrays; Phys. Rev. Lett. 112, 133604 (2014); arXiv:1312.5303. [+] Abstract: We investigate periodic optomechanical arrays as reconfigurable platforms for engineering the coupling between multiple mechanical and electromagnetic modes and for exploring many-body phonon dynamics. Exploiting structural resonances in the coupling between light fields and collective motional modes of the array, we show that tunable effective long-range interactions between mechanical modes can be achieved. This paves the way towards the implementation of controlled phononic walks and heat transfer on densely-connected graphs as well as the coherent transfer of excitations between distant elements of optomechanical arrays.
2. André Xuereb, Hendrik Ulbricht, and Mauro Paternostro; Optomechanical interface for probing matter-wave coherence; Sci. Rep. 3, 3378 (2013); arXiv:1308.3576. [+] Abstract: We combine matter-wave interferometry and cavity optomechanics to propose a coherent matter–light interface based on mechanical motion at the quantum level. We demonstrate a mechanism that is able to transfer non-classical features imprinted on the state of a matter-wave system to an optomechanical device, transducing them into distinctive interference fringes. This provides a reliable tool for the inference of quantum coherence in the particle beam. Moreover, we discuss how our system allows for intriguing perspectives, paving the way to the construction of a device for the encoding of quantum information in matter-wave systems. Our proposal, which highlights previously unforeseen possibilities for the synergistic exploitation of these two experimental platforms, is explicitly based on existing technology, available and widely used in current cutting-edge experiments.
3. André Xuereb, Claudiu Genes, and Aurélien Dantan; Collectively-enhanced optomechanical coupling in periodic arrays of scatterers; Phys. Rev. A 88, 053803 (2013); arXiv:1304.4574. [+] Abstract: We investigate the optomechanical properties of a periodic array of identical scatterers placed inside an optical cavity and extend the results of [A. Xuereb, C. Genes, and A. Dantan, Phys. Rev. Lett. 109, 223601 (2012)]. We show that operating at the points where the array is transmissive results in linear optomechanical coupling strengths between the cavity field and collective motional modes of the array that may be several orders of magnitude larger than is possible with an equivalent reflective ensemble. We describe and interpret these effects in detail and investigate the nature of the scaling laws of the coupling strengths for the different transmissive points in various regimes.
4. Claudiu Genes, André Xuereb, Guido Pupillo, and Aurélien Dantan; Enhanced optomechanical readout using optical coalescence; Phys. Rev. A 88, 033855 (2013); arXiv:1304.1715. [+] Abstract: We present a scheme to strongly enhance the readout sensitivity of the squared displacement of a mobile scatterer placed in a Fabry-Pérot cavity. We investigate the largely unexplored regime of cavity electrodynamics in which a highly reflective element positioned between the end mirrors of a symmetric Fabry-Pérot resonator strongly modifies the cavity response function, such that two longitudinal modes with different spatial parity are brought close to frequency degeneracy and interfere in the cavity output field. In the case of a movable middle reflector we show that the interference in this generic "optical coalescence" phenomenon gives rise to an enhanced frequency shift of the peaks of the cavity transmission that can be exploited in optomechanics.
5. André Xuereb, and Mauro Paternostro; Selectable linear or quadratic coupling in an optomechanical system; Phys. Rev. A 87, 023830 (2013); arXiv:1212.0641. [+] Abstract: There has been much interest recently in the analysis of optomechanical systems incorporating dielectric nano- or microspheres inside a cavity field. We analyse here the situation when one of the mirrors of the cavity itself is also allowed to move. We reveal that the interplay between the two oscillators yields a cross-coupling that results in, e.g., appreciable cooling and squeezing of the motion of the sphere, despite its nominal quadratic coupling. We also discuss a simple modification that would allow this cross-coupling to be removed at will, thereby yielding a purely quadratic coupling for the sphere.
6. Tim Freegarde, James Bateman, André Xuereb, and Peter Horak; Mirror-mediated cooling: A paradigm for particle cooling via the retarded dipole force; Annual Review of Cold Atoms and Molecules 1, 353 (2013); arXiv:0904.3059. [+] Abstract: Cooling forces result from the retarded dipole interaction between an illuminated particle and its reflection. For a one-dimensional example, we find cooling times of milliseconds and limiting temperatures in the millikelvin range. The force, which may be considered the prototype for cavity-mediated cooling, may be enhanced by plasmon and geometric resonances at the mirror.
7. André Xuereb, Claudiu Genes, and Aurélien Dantan; Strong coupling and long-range collective interactions in optomechanical arrays; Phys. Rev. Lett. 109, 223601 (2012); arXiv:1202.6210. [+] Abstract: We study cavity optomechanics with an array of equidistant scatterers in a Fabry–Pérot resonator. Our treatment departs from the traditional reflective optomechanics regime with single membranes and introduces a novel transmissive regime that allows better coupling of light to the collective motional dynamics of the array. We show that the array can behave as a single, highly transmissive, 'superscatterer' possessing a specific collective motional mode which is strongly coupled to the cavity field with an optomechanical coupling strength that scales as N3/2 with the number of scatterers N. We also show that as opposed to the usual reflective optomechanics, where the coupling saturates as the polarisability of each scatterer increases (i.e., as the reflectivity tends to unity), a scaling of the coupling strength as the square of the polarisability arises. For the case of small polarisability, e.g., atoms in an optical lattice, our treatment also uncovers a new regime where the same N3/2 scaling is expected.
8. André Xuereb, and Peter Domokos; Dynamical scattering models in optomechanics: going beyond the 'coupled cavities' model; New J. Phys. 14, 095027 (2012); arXiv:1204.5301. [+] Abstract: Recently [A. Xuereb, et al., Phys. Rev. Lett. 105, 013602 (2010)], we calculated the radiation field and the optical forces acting on a moving object inside a general one-dimensional configuration of immobile optical elements. In this article we analyse the forces acting on a semi-transparent mirror in the 'membrane-in-the-middle' configuration and compare the results obtained from solving scattering model to those from the coupled cavities model that is often used in cavity optomechanical system. We highlight the departure of this model from the more exact scattering theory when the reflectivity of the moving element drops below about 50%.
9. André Xuereb, Koji Usami, Andreas Naesby, Eugene S. Polzik, and Klemens Hammerer; Exciton-mediated photothermal cooling in GaAs membranes; New J. Phys. 14, 085024 (2012); arXiv:1205.6726. [+] Abstract: Cooling of the mechanical motion of a GaAs nano-membrane using the photothermal effect mediated by excitons was recently demonstrated by some of us [K. Usami, et al., Nature Phys. 8, 168 (2012)] and provides a clear example of the use of thermal forces to cool down mechanical motion. Here, we report on a single-free-parameter theoretical model to explain the results of this experiment which matches the experimental data remarkably well.
10. André Xuereb, Marco Barbieri, and Mauro Paternostro; Multipartite optomechanical entanglement from competing nonlinearities; Phys. Rev. A 86, 013809 (2012); arXiv:1204.5870. [+] Abstract: We investigate the nature of the three-mode interaction inside an optomechanically-active microtoroid with a sizeable χ(2) coefficient. Experimental techniques are quickly advancing to the point where structures with the necessary properties can be made, and we argue that these provide a natural setting in which to observe rich dynamics leading, for instance, to genuine tripartite steady-state entanglement. We also show that this approach lends itself to a full characterisation of the three-mode state of the system.
11. André Xuereb, Roman Schnabel, and Klemens Hammerer; Dissipative Optomechanics in a Michelson–Sagnac Interferometer; Phys. Rev. Lett. 107, 213604 (2011); arXiv:1107.4908. [+] Abstract: Dissipative optomechanics studies the coupling of the motion of an optical element to the decay rate of a cavity. We propose and theoretically explore a realization of this system in the optical domain, using a combined Michelson–Sagnac interferometer, which enables a strong and tunable dissipative coupling. Quantum interference in such a setup results in the suppression of the lower motional sideband, leading to strongly enhanced cooling in the non-sideband-resolved regime. With state-of-the-art parameters, ground-state cooling and low-power quantum-limited position transduction are both possible. The possibility of a strong and tunable dissipative coupling opens up a new route towards observation of fundamental optomechanical effects such as ponderomotive squeezing or nonlinear dynamics. Beyond optomechanics, the method suggested here can be readily transferred to other setups involving such systems as nonlinear media, atomic ensembles, or single atoms..
12. André Xuereb, Peter Domokos, Peter Horak, and Tim Freegarde; Cavity cooling of atoms: Within and without a cavity; Eur. Phys. J. D 65, 273 (2011); arXiv:1101.2739. [+] Abstract: We compare the efficiencies of two optical cooling schemes, where a single particle is either inside or outside an optical cavity, under experimentally-realisable conditions. We evaluate the cooling forces using the general solution of a transfer matrix method for a moving scatterer inside a general one-dimensional system composed of immobile optical elements. Assuming the same atomic saturation parameter, we find that the two cooling schemes provide cooling forces and equilibrium temperatures of comparable magnitude.
13. André Xuereb, Peter Horak, and Tim Freegarde; Amplified optomechanics in a unidirectional ring cavity; J. Mod. Opt. 58, 1342 (2011); arXiv:1101.0130. [+] Abstract: We investigate optomechanical forces on a nearly lossless scatterer, such as an atom pumped far off-resonance or amicromirror, inside an optical ring cavity. Our model introduces two additional features to the cavity: an isolator is used to prevent circulation and resonant enhancement of the pump laser field and thus to avoid saturation of or damage to the scatterer, and an optical amplifier is used to enhance the effective Q-factor of the counterpropagating mode and thus to increase the velocity-dependent forces by amplifying the back-scattered light. We calculate friction forces, momentum diffusion, and steady-state temperatures to demonstrate the advantages of the proposed setup.
14. André Xuereb, Peter Domokos, Peter Horak, and Tim Freegarde; Scattering theory of multilevel atoms interacting with arbitrary radiation fields; Phys. Scr. T140, 014010 (2010); arXiv:0910.0802. [+] Abstract: We present a generic transfer matrix approach for the description of the interaction of atoms possessing multiple ground state and excited state sublevels with light fields. This model allows us to treat multi-level atoms as classical scatterers in light fields modified by, in principle, arbitrarily complex optical components such as mirrors, resonators, dispersive or dichroic elements, or filters. We verify our formalism for two prototypical sub-Doppler cooling mechanisms and show that it agrees with the standard literature.
15. Peter Horak, André Xuereb, and Tim Freegarde; Optical cooling of atoms in microtraps by time–delayed reflection; J. Comput. Theor. Nanosci. 7, 1747 (2010); arXiv:0911.4805. [+] Abstract: We present a theoretical analysis of a novel scheme for optical cooling of particles that does not in principle require a closed optical transition. A tightly confined laser beam interacting with a trapped particle experiences a phase shift, which upon reflection from a mirror or resonant microstructure produces a time-delayed optical potential for the particle. This leads to a nonconservative force and friction. A quantum model of the system is presented and analyzed in the semiclassical limit.
16. James Bateman, Richard Murray, Matthew Himsworth, Hamid Ohadi, André Xuereb, and Tim Freegarde; Hänsch–Couillaud locking of Mach–Zehnder interferometer for carrier removal from a phase-modulated optical spectrum; J. Opt. Soc. Am. B 27, 1530 (2010); arXiv:0911.1695. [+] Abstract: We describe and analyse the operation and stabilization of a Mach–Zehnder interferometer, which separates the carrier and the first-order sidebands of a phase-modulated laser field, and which is locked using the Hänsch–Couillaud method. In addition to the necessary attenuation, our interferometer introduces, via total internal reflection, a significant polarization-dependent phase delay. We employ a general treatment to describe an interferometer with an object which affects the field along one path, and we examine how this phase delay affects the error signal. We discuss the requirements necessary to ensure the lock point remains unchanged when phase modulation is introduced, and we demonstrate and characterize this locking experimentally. Finally, we suggest an extension to this locking strategy using heterodyne detection.
17. André Xuereb, Tim Freegarde, Peter Horak, and Peter Domokos; Optomechanical cooling with generalized interferometers; Phys. Rev. Lett. 105, 013602 (2010); arXiv:1002.0463. [+] Abstract: The fields in multiple-pass interferometers, such as the Fabry--P'erot cavity, exhibit great sensitivity not only to the presence but also to the motion of any scattering object within the optical path. We consider the general case of an interferometer comprising an arbitrary configuration of generic 'beam splitters' and calculate the velocity-dependent radiation field and the light force exerted on a moving scatterer. We find that a simple configuration, in which the scatterer interacts with an optical resonator from which it is spatially separated, can also significantly enhance the optomechanical friction.
18. James Bateman, André Xuereb, and Tim Freegarde; Stimulated Raman transitions via multiple atomic levels; Phys. Rev. A 81, 043808 (2010); arXiv:0908.2389. [+] Abstract: We consider the stimulated Raman transition between two long-lived states via multiple intermediate states, such as between hyperfine ground states in the alkali metal atoms. We present a concise treatment of the general, multilevel, off-resonant case, and we show how the lightshift emerges naturally in this approach. We illustrate our results by application to alkali metal atoms and we make specific reference to cesium. We comment on some artifacts, due solely to the geometrical overlap of states, which are relevant to existing experiments.
19. André Xuereb, Mathias Groth, Karl Krieger, Otto Asunta, Taina Kurki-Suonio, Jari Likonen, David P Coster, ASDEX Upgrade Team; DIVIMP-B2-EIRENE modelling of 13C migration and deposition in ASDEX Upgrade L-mode plasmas; J. Nucl. Mater. 396, 2–3, 228 (2010). [+] Abstract: Carbon transport and migration were studied experimentally and numerically in a high-density, low-confinement mode plasma in the ASDEX Upgrade tokamak. On the last day of plasma operation of the 2004–2005 experimental campaign, 13CH4 was injected into the vacuum vessel from the low field side midplane. A poloidal set of tiles was subsequently removed and analysed for 13C deposition. In this work the measured deposition profile is interpreted using the impurity transport code DIVIMP. The simulated poloidal distribution of 13C deviates significantly from the measured profile. The simulations indicate that 13C is promptly deposited at the wall in the vicinity of the injection port, and is transported to the low field side divertor plate predominately via the scrape-off layer. The B2-EIRENE plasma solution produce stagnant plasma flow in the main scrape-off layer, in contrast to measurements in ASDEX Upgrade and other tokamaks. This is the likely cause of the discrepancy between the measured and the calculated poloidal distribution of the 13C deposition. Key model parameters of DIVIMP were varied to determine their effect on the simulated deposition profile.
20. Hamid Ohadi, Matthew Himsworth, André Xuereb, and Tim Freegarde; Magneto-optical trapping and background-free imaging for atoms near nanostructured surfaces; Opt. Express 17, 25, 23003 (2009); arXiv:0910.5003. [+] Abstract: We demonstrate a combined magneto-optical trap and imaging system that is suitable for the investigation of cold atoms near surfaces. In particular, we are able to trap atoms close to optically scattering surfaces and to image them with an excellent signal-to-noise ratio. We also demonstrate a simple magneto-optical atom cloud launching method. We anticipate that this system will be useful for a range of experimental studies of novel atom-surface interactions and atom trap miniaturization.
21. André Xuereb, Peter Horak, and Tim Freegarde; Atom cooling using the dipole force of a single retroreflected laser beam; Phys. Rev. A 80, 013836 (2009); arXiv:0903.2945. [+] Abstract: We present a mechanism for cooling atoms by a laser beam reflected from a single mirror. The cooling relies on the dipole force and thus in principle applies to arbitrary refractive particles including atoms, molecules, or dielectric spheres. Friction and equilibrium temperatures are derived by an analytic perturbative approach. Finally, semiclassical Monte-Carlo simulations are performed to validate the analytic results.
22. André Xuereb, Peter Domokos, János Asbóth, Peter Horak, and Tim Freegarde; Scattering theory of heating and cooling in optomechanical systems; Phys. Rev. A 79, 053810 (2009); arXiv:0903.3132. [+] Abstract: We present a one-dimensional scattering theory which enables us to describe a wealth of effects arising from the coupling of the motional degree of freedom of scatterers to the electromagnetic field. Multiple scattering to all orders is taken into account. The theory is applied to describe the scheme of a Fabry-Perot resonator with one of its mirrors moving. The friction force, as well as the diffusion, acting on the moving mirror is derived. In the limit of a small reflection coefficient, the same model provides for the description of the mechanical effect of light on an atom moving in front of a mirror.

Conference proceedings

23. Peter Domokos, André Xuereb, Peter Horak, and Tim Freegarde; Efficient optomechanical cooling in one-dimensional interferometers; presented at Proc. SPIE 7951, 79510B (2011) (presented SPIE Photonics West 2011; invited contribution). [+] Abstract: We present a scattering model which enables us to describe the mechanical force, including the velocity dependent component, exerted by light on polarizable massive objects in a general one-dimensional optical system. We show that the light field in an interferometer can be very sensitive to the velocity of a moving scatterer. We construct a new efficient cooling scheme, `external cavity cooling', in which the scatterer, that can be an atom or a moving micromirror, is spatially separated from the cavity.

Books and chapters

24. André Xuereb; Optical Cooling Using the Dipole Force (Springer Theses); ISBN: 978-3-642-29714-4.