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Quantum Electronics

Quantum computer based on activated dielectric nanoparticles selectively interacting with short optical pulses

O. N. Gadomskiĭ, Yu Ya Kharitonov

Published Mar 31, 2004• 16 citations
Publisher

Abstract

The operation principle of a quantum computer is proposed based on a system of dielectric nanoparticles activated with two-level atoms — cubits, in which electric dipole transitions are excited by short intense optical pulses. It is proved that the logical operation (logical operator) CNOT (controlled NOT) is performed by means of time-dependent transfer of quantum information over `long' (of the order of 10 4 nm) distances between spherical nanoparticles owing to the delayed interaction between them in the optical radiation field. It is shown that one-cubit and two-cubit logical operators required for quantum calculations can be realised by selectively exciting dielectric particles with short optical pulses.

DOI
10.1070/qe2004v034n03abeh002622

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The operation principle of a quantum computer is proposed based on a system of dielectric nanoparticles activated with two-level atoms — cubits, in which electric dipole transitions are excited by short intense optical pulses. It is proved that the logical operation (logical operator) CNOT (controlled NOT) is performed by means of time-dependent transfer of quantum information over `long' (of the order of 10 4 nm) distances between spherical nanoparticles owing to the delayed interaction between them in the optical radiation field. It is shown that one-cubit and two-cubit logical operators required for quantum calculations can be realised by selectively exciting dielectric particles with short optical pulses.
Equations of quantum computing in a system of two ensemble qubits are solved. The qubits are spherical nanoparticles activated by two-level atoms. It is proved that a photon echo may occur during quantum computations in an optical quantum computer when the ensemble qubits are selectively excited by short optical pulses. The photon echo phenomenon is caused by time reversal of oscillations of induced dipole moments of the qubits. It is shown that this effect can be observed in the case of interference of oscillating dipoles in the wave zone.
It is shown that it is preferable to perform quantum computations on a system of two-level atoms with metastable states using optical dipole transitions that occur under the effect of ultrashort light pulses. It is suggested to measure the quantum information that is passed to qubits using Bloch, rather than pure, quantum states of two-level atoms. Moreover, the inversion of atoms can be used as the measure of quantum information. In order to describe the logical operators NOT and CNOT in the system of interacting two-level atoms (qubits), modified optical equations for the Bloch vectors of individual qubits are derived. These equations are solved in combination with field equations, without using the slowly varying amplitude approximation, for a small two-qubit system in the field of ultrashort intense optical pulses of arbitrary shape. A numerical analysis of the solution shows that it is possible to control the recording of information on individual qubits in a small quantum system of a dimension much smaller than the length of the optical wave by smoothly varying the irradiation conditions of qubits.
Controlling the dynamics of open quantum systems; i.e. quantum systems that decohere because of interactions with the environment, is an active area of research with many applications in quantum optics and quantum computation. My thesis expands the scope of this inquiry by seeking to control open systems in proximity to an additional system. The latter could be a classical system such as metal nanoparticles, or a quantum system such as a cluster of similar atoms. By modelling the interactions between the systems, we are able to expand the accessible state space of the quantum system in question.For a single, three-level quantum system, I examine isolated systems that have only normal spontaneous emission. I then show that intensity-intensity correlation spectra, which depend directly on the density matrix of the system, can be used detect whether transitions share a common energy level. This detection is possible due to the presence of quantum interference effects between two transitions if they are connected. This effect allows one to asses energy level structure diagrams in complex atoms/molecules. By placing an open quantum system near a nanoparticle dimer, I show that the spontaneous emission rate of the system can be changed ``on demand" by changing the polarization of an incident, driving field. In a three-level, Λ\LambdaΛ system, this allows a qubit to both retain high qubit fidelity when it is operating, and to be rapidly initialized to a pure state once it is rendered unusable by decoherence. This type of behaviour is not possible in a single open quantum system; therefore adding a classical system nearby extends the overall control space of the quantum system. An open quantum system near identical neighbours in a dense ensemble is another example of how the accessible state space can be expanded. I show that a dense ensemble of atoms rapidly becomes disordered with states that are not directly excited by an incident field becoming significantly populated. This effect motivates the need for using multi-directional basis sets in theoretical analysis of dense quantum systems. My results demonstrate the shortcomings of short-pulse techniques used in many recent studies. Based on my numerical studies, I hypothesize that the dense ensemble can be modelled by an effective single quantum system that has a decoherence rate that changes over time. My effective single particle model provides a way in which computational time can be reduced, and also a model in which the underlying physical processes involved in the system's evolution are much easier to understand. I then use this model to provide an elegant theoretical explanation for an unusual experimental result called ``transverse optical magnetism''. My effective single particle model's predictions match very well with experimental data.
A brief overview of the current state of the experimental research on the development of the element base of quantum computers with qubits based on single neutral atoms trapped in optical traps. The requirements for qubits, peculiarities of single neutral atoms as qubits, methods for the quantum register development and for the implementation of single-qubit quantum logic operations in the laser and microwave fields, and two-qubit operations through the dipole–dipole interaction after a short laser excitation of atoms to the Rydberg states are discussed. The results of the experiments on the observation of the interaction of two Rydberg atoms at a Förster resonance controlled by the dc and radio-frequency electric field are presented.
In nanophotonic device operations, characteristic features on a nanometer scale, such as locally excited states, dependence on the excitation number, and spatial symmetry of a system, play an important role. Using these features, selective excitation energy transfer via an optical near field is shown for a quantum-dot system with discrete energy levels. This selectivity strongly depends on a dipole-inactive state of an exciton, which cannot be excited by the far-field light. Operation principles of logic gates, photon storage, and quantum information processing device, which are based on the selectivity, are proposed, and the temporal dynamics are investigated analytically and numerically by using quantum theory. Nanophotonic devices, which are constructed from quantum mechanical and classical dissipative systems, are expected to become one of a key technologies in future device architecture.
We propose a scheme of a quantum computer based on nanophotonic elements: two buses in the form of nanowaveguide resonators, two nanosized units of multiatom multiqubit quantum memory and a set of nanoprocessors in the form of photonic quantum transistors, each containing a pair of nanowaveguide ring resonators coupled via a quantum dot. The operation modes of nanoprocessor photonic quantum transistors are theoretically studied and the execution of main logical operations by means of them is demonstrated. We also discuss the prospects of the proposed nanophotonic quantum computer for operating in high-speed optical fibre networks.