2004 IRISH SCIENTIST YEAR BOOK

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Cork Institute of Technology
Ruth Carey, Mohamed Boustimi & Síle Nic Chormaic
Cold atoms and microlasers – tools for information and communication technologies

Ruth Carey examining microstructures at CIT

The importance of cold atom research in the field of information and communication technology is indisputable. As a short-term goal, researchers are continuously striving towards obtaining colder atomic sources for use in atomic clocks – the time standard that defines the second. Atomic clocks are used in global positioning systems in all walks of life, from package delivery to a specified address to air navigational systems, all of which rely on the fundamental definition of the time unit. Another area of interest is that of atom lithography, where cold atom sources rather than light are used to etch submicron lines on chips, and progress is advancing at a rapid pace. Light masks can be extended beyond the features of optical lithography, and more recent advances show that the wavelength of light no longer determines the minimum feature separation 1 . The principles behind optical trapping and storing of atoms can also be directly transferred to another extremely diverse application – that of optical tweezers – whereby biological samples are trapped and manipulated using the light force acting on them. It is possible to precisely and non-destructively position small structures inside living cells using this technique. As a more long-term goal, cold atom research has become one of the forerunners in the search for a suitable scheme for the implementation of quantum computation as a means of exploring quantum information processing.

One may ask: why the interest in developing quantum strategies for information processing? Simply put, the real world is quantum mechanical in nature. Information is stored in the state of a physical system, and computation is performed using physical interactions. Therefore, the fundamental basis behind information theory must be founded in quantum mechanics. Quantum information can be viewed as information stored in the quantum state of a physical system, and exhibits many properties that differ from classical information. In particular, a quantum computer would have the capabilities of performing certain types of tasks far more efficiently than a classical computer. In the shorter term, there is a wealth of information to be gathered in the field of cold atoms that will make direct contributions to specific aspects of quantum information technology.

The Quantum Optics Group in the Department of Applied Physics and Instrumentation in CIT is creating a source of cold, neutral atoms using a technique known as magneto-optical trapping, whereby a cloud of atoms is trapped using a combination of lasers and magnetic fields. As one of the more obvious short-term goals, we are aiming at a thorough investigation of the interaction of cold atoms with surfaces, where the surface in question could form part of a quantum logic gate, be that the surface of a microcavity laser or an atom chip. In particular, the atom interaction with a microcavity is of interest for demonstrating basic quantum logic gates. Each two-level atom can be treated as a qubit for quantum logic operations, and entanglement may be obtainable via photon transmission. By improving the positional and parameter control of the cloud of cold atoms using standard control strategies, it should be possible to enhance the efficiency of photon exchange between the atom and the cavity.


Schematic representation of the whispering gallery mode in a microsphere laser

The microcavity lasers themselves are also being studied for applications in telecommunications. Such lasers are of interest due to a combination of laser miniaturisation and an increase in the cavity quality (Q) factor resulting in a near loss-less cavity. The research within the Group is concentrating on spherical microcavities made from special glass such as ZBLALiP doped with erbium, and with a diameter of about 80 micron. These are known to have a very high Q-factor. Whispering gallery modes within the sphere are excited by matching the angular momentum and frequency of the pump with that of one of the modes. This results in light propagation around the equator and laser emissions through an evanescent wave. Lasing thresholds for these lasers are extremely low, with excitation achievable for pump powers as low as 200 nW in silicate 2 . Coupling the emitted light into neighbouring microspheres is also being investigated to see the effect on signal transmission.

This material is based upon works supported by the Science Foundation Ireland under Foundation Grant No. 02/IN.1/I28.


References

1. Thywissen J. H. and Prentiss M., arxiv.org/abs/physics/0209084 (2002).

2. Sandoghdar V. et al, Phys. Rev, A 54, R1777 (1996).

Contact: Dr Síle Nic Chormaic, Quantum Optics Group, Department of Applied Physics and Instrumentation,
Cork Institute of Technology, Bishopstown, Cork;
Tel: +353-21-432- 6300; E-mail: [email protected] ;Web: www.physics.cit.ie/Sile/Quantum_Optics.html