Avoiding entanglement loss when two-qubit quantum gates are controlled by electronic excitation

Rodriquez, Roberta; Fisher, A. J.; Greenland, P. T.; Stoneham, A. M.

doi:10.1088/0953-8984/16/16/001

Cited by 17 publications

(40 citation statements)

References 25 publications

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“…Both of the proposals use optical methods to control spins, but do so in wholly different ways. The first is a scheme for optically controlled spintronics that I, Andrew Fisher, and Thornton Greenland proposed [11,12]. The second route exploits entanglement of states of distant atoms by interference [13] in the context of measurement-based quantum computing [14].…”

Section: Hopes For Higher Temperature Operationmentioning

confidence: 99%

Is a room-temperature, solid-state quantum computer mere fantasy?

Stoneham

2009

Physics

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Creating a practical solid-state quantum computer is seriously hard. Getting such a computer to operate at room temperature is even more challenging. Is such a quantum computer possible at all? If so, which schemes might have a chance of success?Subject Areas: Quantum Information, SpintronicsIn his 2008 Newton Medal talk, Anton Zeilinger of the University of Vienna said: "We have to find ways to build quantum computers in the solid state at room temperature-that's the challenge." [1] This challenge spawns further challenges: Why do we need a quantum computer anyway? What would constitute a quantum computer? Why does the solid state seem essential? And would a cooled system, perhaps with thermoelectric cooling, be good enough?Some will say the answer is obvious. But these answers vary from "It's been done already" to "It can't be done at all." Some of the "not at all" group believe high temperatures just don't agree with quantum mechanics. Others recognize that their favored systems cannot work at room temperature. Some scientists doubt that serious quantum computing is possible anyway. Are there methods that might just be able to meet Zeilinger's challenge? The questions that challengeWhat is a computer? Standard classical computers use bits for encoding numbers, and the bits are manipulated by the classical gates that can execute AND and OR operations, for example. A classical bit has a value of 0 or 1, according to whether some small subunit is electrically charged or uncharged. Other forms are possible: the bits for a classical spintronic computer might be spins along or opposite to a magnetic field. Even the most modest computers on sale today incorporate complex networks of a few types of gates to control huge numbers of bits. If there are so few bits that you can count them on your fingers, it can't seriously be considered a computer.What do we mean by quantum? Being sure a phenomenon is "quantum" isn't simple. Quantum ideas aren't intuitive yet. Could you convince your banker that quantum physics could improve her bank's security? Perhaps three questions identify the issues. First, how do you describe the state of a system? The usual descriptors, wave functions and density matrices, underlie wavelike interference and entanglement. Entanglement describes the correlations between local measurements on two particles, which I call their "quantum dance." Entanglement is the resource that could make quantum computing worthwhile. The enemy of entanglement is decoherence, just as friction is the enemy of mechanical computers. Second, how does this quantum state change if it is not observed? It evolves deterministically, described by the Schrödinger equation. The probabilistic results of measurements emerge when one asks the third question: how to describe observations and their effects. Measurement modifies entanglement, often destroying it, as it singles out a specific state. This is one way that you can tell if an eavesdropper intercepted your message in a quantum communications system.Proposed quantum compute...

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Section: Hopes For Higher Temperature Operationmentioning

confidence: 99%

Is a room-temperature, solid-state quantum computer mere fantasy?

Stoneham

2009

Physics

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“…As with atoms in traps the ground states are tightly confined and well isolated from the environment, giving rise to extraordinarily sharp transitions (3)(4)(5) and very long spin coherence times (6,7), measured with magnetic resonance experiments. There are several proposals for quantum information processing based on the spin of silicon donors (8)(9)(10)(11)(12)(13) and such impurities can now be placed singly with atomic precision (14).…”

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confidence: 99%

Silicon as a model ion trap: Time domain measurements of donor Rydberg states

Vinh

Greenland

Litvinenko

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

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One of the great successes of quantum physics is the description of the long-lived Rydberg states of atoms and ions. The Bohr model is equally applicable to donor impurity atoms in semiconductor physics, where the conduction band corresponds to the vacuum, and the loosely bound electron orbiting a singly charged core has a hydrogen-like spectrum according to the usual Bohr-Sommerfeld formula, shifted to the far-infrared because of the small effective mass and high dielectric constant. Manipulation of Rydberg states in free atoms and ions by single and multiphoton processes has been tremendously productive since the development of pulsed visible laser spectroscopy. The analogous manipulations have not been conducted for donor impurities in silicon. Here, we use the FELIX pulsed free electron laser to perform time-domain measurements of the Rydberg state dynamics in phosphorus-and arsenicdoped silicon and we have obtained lifetimes consistent with frequency domain linewidths for isotopically purified silicon. This implies that the dominant decoherence mechanism for excited Rydberg states is lifetime broadening, just as for atoms in ion traps. The experiments are important because they represent a step toward coherent control and manipulation of atomic-like quantum levels in the most common semiconductor and complement magnetic resonance experiments in the literature, which show extraordinarily long spin lattice relaxation times-key to many well known schemes for quantum computing qubits-for the same impurities. Our results, taken together with the magnetic resonance data and progress in precise placement of single impurities, suggest that doped silicon, the basis for modern microelectronics, is also a model ion trap.coherence ͉ free electron laser ͉ quantum information ͉ picosecond population dynamics ͉ hydrogenic donor impurity H omogenous lifetime-broadened two-level atoms in ion traps (1) have become favorite objects of study for quantum optics with a view toward both fundamental physics and the eventual development of a quantum computer. Among the many schemes proposed (2), the states of ions in trap systems are attractive for the realization of quantum information ''qubits'' (quantum bits) because they are well isolated from the decohering effects of the environment and can be coherently controlled by lasers. The Bohr model is equally applicable to donor impurity atoms in semiconductor physics, where the conduction band corresponds to the vacuum, and the loosely bound electron orbiting a singly charged core has a hydrogen-like spectrum according to the usual Bohr-Sommerfeld formula, shifted to the far-infrared because of the small effective mass and high dielectric constant. As with atoms in traps the ground states are tightly confined and well isolated from the environment, giving rise to extraordinarily sharp transitions (3-5) and very long spin coherence times (6, 7), measured with magnetic resonance experiments. There are several proposals for quantum information processing based on the spin of silicon do...

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“…We note that an effect of these corrected states is a significant reduction in the donor-donor exchange couplings, with an order of magnitude difference between the HEMT and Bi donor couplings. As a result the variation of exchange coupling with separation is very different from that assumed in [8]. …”

Section: The Model Potentialmentioning

confidence: 68%

“…We assume a perfectly symmetric system, with the qubits equally spaced either side of the control and in identical environments, so that the strengths of their couplings to the control are identical and given by 2 J C . It is evident that for this system the energies of states |2 and |4 are identical, a fact which allows U e to be written in closed form (see [8] for details). We require that the control bit C is disentangled from the qubits at the end of the gate operation, and this is ensured by imposing one further condition, namely that the time between pulses is given by…”

Section: An Analytical Modelmentioning

confidence: 99%

“…Note that although we obtain these isolated states via scaled HF equations, there is currently no SCF cycle within our time dependent calculations. In keeping with [8] we only consider the evolution of the spin states, and so in this sense our simulations should be viewed as a time dependent configuration interaction calculation, with the contributing configurations defined in terms of their spin components. Each configuration has an identical spatial component, with the only degree of freedom being the choice as to whether the control electron is in its ground or excited state.…”

Section: 'Realistic' Gate Simulationsmentioning

confidence: 99%

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Time dependent quantum simulations of two-qubit gates based on donor states in silicon

Kerridge¹,

Savory²,

Harker³

et al. 2006

J. Phys.: Condens. Matter

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Many quantum gate proposals make physical assumptions to ease analysis. Here we explicitly consider the effect of these assumptions for a particular two-qubit gate proposal, a cube-root-of-unity gate, in which the two qubits are donors in a semiconductor coupled via an intermediate 'control' spin. Our approach considers directly the electronic structures of the qubit and control impurity systems. We find that such gates are highly sensitive to environmental factors overlooked in analytically soluble models, but that there are regimes in which simplifying assumptions are valid and lead to high fidelity gates.

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Avoiding entanglement loss when two-qubit quantum gates are controlled by electronic excitation

Cited by 17 publications

References 25 publications

Is a room-temperature, solid-state quantum computer mere fantasy?

Is a room-temperature, solid-state quantum computer mere fantasy?

Silicon as a model ion trap: Time domain measurements of donor Rydberg states

Time dependent quantum simulations of two-qubit gates based on donor states in silicon

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