It was realized more than a decade ago that the computation based on quantum mechanics leads to completely different constraints on information processing than those of the computation based on the laws of classical physics. However, the interest in the quantum information processing has dramatically increased just since 1994, when the first algorithms for quantum computing were developed and quantum error-correction codes were invented, which would enable quantum computers to operate despite some decoherence present in real physical systems. By now, various physical systems have been proposed as experimental embodiments of quantum computers. These include cold ion traps, nuclear magnetic resonance (NMR) systems, all-optical logic gates, Josephson junctions, and semiconductor nanostructures. Successful experimental demonstrations of one and two quantum bits of information (qubit) operations were reported for trapped ions systems and with up to seven qubits for NMR systems.
The most developed technique using PURE quantum-mechanical states with long coherence time is ion trapping for which demonstrations of elementary quantum logic gate on a single ion, generation of nonclassical states of motion, and deterministic entanglement of two ions have been already reported.
However, ion traps have their own drawbacks-anomalous heating in kHz region, difficulty to apply resolved sideband cooling to many ions, and others, which make it difficult to scale a few-ions-experiment to hundreds and thousands of ions (qubits) necessary to perform real-scale quantum calculations. Therefore many groups all over the world search for physical systems which could experimentally suit for the purpose of quantum information storage and quantum computing.
The proposal is focused on both experimental and theoretical investigation of quantum information that can be stored and manipulated in trapped neutral atoms as an alternative to the well-known proposals for the ion-trap quantum computers. In close analogy with trapped ions, trapped neutral atoms can safely store quantum information in stable hyperfine states and ensure readout via resonance fluorescence. It was experimentally proved that atoms stored in a far-detuned dipole trap have the coherence times for hyperfine states of many seconds. Trapped neutral atoms are much more robust against electromagnetic disturbances. Optical dipole traps are free from many technical imperfections associated with ion-trap potentials; optical potentials are flexible and rapidly switchable. Last, but not the least advantage for quantum computing could be that the number of used qubits can be easily scaled. It is possible to simultaneously trap and cool into the ground vibrational state many thousands of atoms.
Having the advantages of neutral atoms traps at hand, two extreme cases will be studied-two isolated atoms and an ensemble of many (up to 109) atoms. With the former one can get a deeper insight into the fundamentals of quantum information manipulation in the simplest possible system. With the latter one can take advantage of the recent breakthroughs in quantum manipulations with many-photon and many-atom systems (as has been proposed theoretically and recently demonstrated experimentally, the quantum state of an optical field can be transferred onto the state of an ensemble of trapped atoms).
Specifically, the project has the following objectives:
To study theoretically and develop new experimental methods for creation and characterization of stable entangled states of atoms (cavity quantum electrodynamics and interaction with squeezed light).
To investigate the impact of quantum decoherence on deterioration of quantum information and to search for mechanisms of relaxation suppression.
To identify the most suitable approaches to quantification of quantum information in experiments.
The project falls into several tasks in an effort to encompass the bulk of theoretical issues and perform experiments relevant to the considered problems. Among them are the study of entanglement of trapped atoms (the methods for production of stable entanglement methods, analysis of experimental techniques for entanglement production, and elaboration of methods for characterization of the created entangled states), quantum decoherence and relaxation suppression (for both simple model systems of two atoms and an ensemble of atoms), and application of quantum information approach to the trapped atoms (nature of quantum information and calculation of the quantum information quantities in experiments with trapped atoms, optimisation of the experimental set up parameters, and search for new definitions of quantum information).
As a result of the studies performed in the course of the current proposal, a deep insight into using trapped neutral atoms as an experimental prototype of physical system for quantum information storage and possibly quantum computing will be achieved.
142190 Troitsk, Moscow Region
195251 St. Petersburg