SEMICONDUCTOR NANOCRYSTALBASED QUANTUM COMPUTATION Nithya Subrahmanyam Eric Isaacs Physics Chemistry Computer Science C191 December 8 2008 QUANTUM DOTS Semiconductor nanocrystals with size in the range of 10 nanometers to one micrometer Constitutes a well conducting region surrounded in all directions by a poor conducting region Physics excitons electron hole bound pairs are bound in three directions PROPERTIES OF QUANTUM DOTS Just like an atom electronic energy levels are quantized density of states electron energy Quantum dots contain a countable number of electrons Addition of single electron can drastically change the system s properties Coulomb blockade Quantum Dot Advantages The wavelength at which they absorb or emit electromagnetic radiation can be adjusted They can be synthesized into a variety of different shapes and forms e g pyramidal quantum dots They can improve the efficiency of solar cells in two ways Extending band gap to absorb more light in spectrum Generating more charges from single photon SYNTHESIS OF QUANTUM DOTS Photolithography creates a grid like pattern followed by selective growth Bottom up approach colloidal chemistry used to engineer reactions to precipitate quantum dots from polymer precursor surface must be capped to ensure chemical stability Also molecular beam epitaxy MBE CLASSICAL VERSUS QUANTUM While a transistor in a conventional computer is used to switch between on and off the classical bit a quantum computer takes advantage of quantum theory the qubit can be simultaneously in both states i e a superposition of the two This enables the computer to work faster and to hold more memory Conventional computer relies on electrical current Quantum computer made from quantum dots relies on manipulation of spin Size approximately 180 nm 1 grain of sand 5000 quantum dots QUANTUM DOT QUBITS Size of quantum dots results in large Coulomb blockade allowing precise control of of electrons The dot has an excess electron in its outermost energy level This electron can either be in the up spin or down spin or as a superposition of course These two states are the basis of the qubit e e e ee e e ee QUBIT INTERACTION Electrical gating used to control interaction with other qubits High gate voltage prevents tunneling low gate voltage results in timedependent Hamiltonian where J t is a function of the tunneling matrix Hopping to ferromagnetic quantum dot enables single qubit operations Spin dependent tunneling to 3 for measurement DESIGN Gate electrodes confine the electrons changing voltage pushes electron wavefunction into mag layer for Zeeman splitting 1 qubit gates or current wires can be used for local magnetic field We use these fields to perform spin rotations which are sufficient for one qubit gates Pulsed in plane magnetic field in resonance w Zeeman splitting also can address individual spins Only electrical switching is required to control spin dynamics and quantum computation DIVINCENZO CRITERIA 1 Well characterized scalable qubits Spin of the excess electron is well characterized and arrays of quantum dots is feasible with best techniques for defining nanostructures in semiconductors 2 Ability to initiate state Sure large magnetic field can be used to relax quantum dot into thermal ground state 3 Decoherence time longer than operation time Decoherence times in ms range gate operation time in ns range 4 Universal set of gates use local magnetic field for one qubit gates use interaction from tunneling for two qubit gates DIVINCENZO CRITERIA 5 Qubit readout Idea is to transfer spin information into charge information Charge distribution on a reference quantum dot which is a function of spin dependent tunneling from the dot of interest can be used Need electrometer 6 Conversion of stationary and flying qubits 7 Transmission of flying qubits
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