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Silicon-Based NMR Quantum Computer Using Single Electron

А. А. Ларионов, Leonid Evgenevich Fedichkin, K. A. Valiev

Published Jun 1, 2001• 0 citations
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Abstract

A new NMR quantum computer scheme based on semiconductor nanostructure is proposed. Energy spectrum of the spin-qubit system was obtained. Methods of single and two qubit gates realization are discussed. This variant of quantum computer is more simple to control and more coherent.

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arXiv:quant-ph/0205135•Physical Review A•Feb 28, 2003•24 citations
A new NMR quantum computer scheme based on semiconductor nanostructures is discussed theoretically. We also propose using an infrared laser as a new independent instrument for manipulating qubits. Methods of single- and many-qubit gate realization are discussed. Resonant transfer of the electron to several quantum dots helps in implementing complex many-qubit gates. The energy spectrum of the spin-qubit system is obtained. This variant of a quantum computer is simpler to control and more coherent. The distance between qubits and hence the structural requirements are more achievable using modern nanotechnology.
Problems in realization of silicon-based solid-state NMR quantum computer with ensemble addressing to qubits are considered. It is presented the extension of Kane's scheme to ensemble approach version with strip gates. For the initialization of nuclear quantum states it is proposed to use the solid-state effect in ENDOR technique whereby the nuclear spins can be practically fully polarized or, that is the same, indirect cooled to the spin temperature less than ~ 1 mK. It is suggested the possible planar silicon topology of such ensemble quantum computer and shown that the measurement with standard NMR methods signal of L ~ 10^3 qubit system may be achieved for a number of ensemble components N >= 10^5. As another variant of ensemble silicon quantum computer the gateless architecture of cellular-automaton is also considered. The decoherence of quantum states in the ensemble quantum computers and ways of its suppression is also discussed.
As an ensemble scheme of solid-state NMR quantum computers the extension of Kane's many-qubits silicon scheme based on the array of 31 P donor atoms are spaced lengthwise of the strip gates is considered. The possible planar topology of such ensemble quantum computer is suggested. The estimation of the output NMR signal was performed and it was shown that for the number N>=10^5 of ensemble elements involving L~10^3 qubits each, the standard NMR methods are usable. As main mechanisms of decoherence for low temperature (<0.1K), the adiabatic processes of random modulation of qubit resonance frequency determined by secular part of nuclear spin hyperfine interaction with electron magnetic moment of basic atom and dipole-dipole interaction with nuclear moments of neighboring impurity atoms was considered, It was made estimations of allowed concentrations of magnetic impurities and of spin temperature whereby the required decoherence suppression is obtained. Semiclassical decoherence model of two qubit entangled states is also presented. As another variant of the solid-state ensemble quantum computer, the gateless architecture of cellular-automaton with antiferromagnetically ordered electron spins is also discussed here.
Spin is a quantum mechanical property that describes the magnetic orientation of elementary and composite particles such as electrons and nuclei. For spin-half particles placed in a magnetic field, two states with differing energies are defined. Proposals exist for developing a revolutionary kind of computer that encodes bits of information on the states of a simple quantum two-level system - like those of a spin in a magnetic field. These quantum computers exploit the fundamental principles of quantum mechanics in order to perform certain calculations exponentially faster than their classical counterparts. Key to the development of such a device is the identification of potential systems that are suitable for employment as a quantum bit (qubit). This thesis explores quantum bits constructed from the spin states of single atoms hosted in silicon semiconductor devices. Silicon is an exceptional platform for spin-based quantum computing - in addition to being the dominant material in the microelectronic industry, it is also predominantly composed of nuclear spin-zero isotopes, providing very little background magnetic disturbance. The donor-atom phosphorus (31P) is investigated here. It possesses a nucleus that binds an additional electron to its positive charge, providing two distinct spin qubits. Here, several world-first experimental milestones in spin-based quantum information processing are reported. First, the readout of the electron spin is demonstrated in single-shot, using a silicon device which combines implanted phosphorus donors with a sensitive nanoscale electrometer. Then, the electron spin is coherently operated as a qubit by delivering on-chip microwave excitations. Combining the electron spin detection and control capabilities enables a quantum non-demolition measurement of the single 31P nuclear spin. Experiments yield exceptional nuclear readout fidelities - the highest for any solid-state qubit. Through an application of nuclear magnetic resonance, high-fidelity quantum control of the nuclear spin qubit is demonstrated. Finally, a single spin-bearing silicon nucleus (29Si) coupled to the electron spin is detected in single-shot and operated as a qubit. These results, along with the clear pathways identified for improving device performance, illustrate that single-atom electron and nuclear spin qubits in silicon constitute an excellent basis on which to construct a quantum computer.
Our progress towards realization of the all‐silicon solid‐state NMR quantum computer [PRL, 89, 017901 (2002) and revised version] is reported. We show experimentally that the phase memory time TM (or phase decoherence time T2) of 29Si nuclear spins in single‐crystal Si at room temperature can be extended up to 25 s (and possibly more) using RF decoupling techniques [quant‐ph/0309164 (2003)]. A simple estimate shows that a T2 of 25 s will allow for 106 single‐qubit and 104 two‐qubit operations, by far the largest number of operations possible among existing solid‐state quantum computer schemes. Recent progress in materials science needed for the realization of the all‐silicon quantum computer is also discussed.
We propose a decoupling-free nuclear-spin quantum computer installed on a quantum electron spin chain with a singlet ground state and a finite spin gap. Qubits are I=1/2I=1/2I=1/2 nuclear spins implanted periodically along the quantum spin chain. A magnetic field gradient is applied parallel to the chain, which allows individual access to each qubit. A single-qubit operation (rotation gate) is realized with the rf field tuned to the nuclear Larmor frequency at the qubit of interest, while a two-qubit operation (controlled-NOT gate) is achieved using the Suhl-Nakamura interaction through a packet of triplet magnons, which are excited by a microwave tuned to the spin gap energy (SN(SN(SN gate). The interaction can be switched off by turning off the microwave, and a decoupling-free quantum computer is realized. The initialization is achieved with an optical pumping qubit initializer, which has a multilayered structure of the quantum spin chain and a semiconductor. Spin polarizations created by the optical pumping in the semiconducting layers are transferred to the spin chain layers through a cross polarization and a spin diffusion. The scheme allows us to separate the initialization process from the computation, enabling us to optimize the latter independently of the former.