102 research outputs found
Room temperature electrical detection of spin coherence in C60
Journal ArticleAn experimental demonstration of electrical detection of coherent spin motion of weakly coupled, localized electron spins in thin fullerene C60 films at room temperature is presented. Pulsed electrically detected magnetic resonance experiments on vertical photocurrents through Al/C60/ZnO samples showed that an electron spin Rabi oscillation is reflected by transient current changes. The nature of possible microscopic mechanisms responsible for this spin to charge conversion as well as its implications for the readout of endohedral fullerene (N@C60) spin qubits are discussed
Room Temperature Electrical Detection of Spin Coherence in C60
An experimental demonstration of electrical detection of coherent spin motion
of weakly coupled, localized electron spins in thin Fullerene C60 films at room
temperature is presented. Pulsed electrically detected magnetic resonance
experiments on vertical photocurrents through Al/C60/ZnO samples showed that an
electron spin Rabi oscillation is reflected by transient current changes. The
nature of possible microscopic mechanisms responsible for this spin to charge
conversion as well as its implications for the readout of endohedral Fullerene
(N@C60) spin qubits are discussed.Comment: 4 pages, 3 figure
Theory for transport through a single magnetic molecule: Endohedral N@C60
We consider transport through a single N@C60 molecule, weakly coupled to
metallic leads. Employing a density-matrix formalism we derive rate equations
for the occupation probabilities of many-particle states of the molecule. We
calculate the current-voltage characteristics and the differential conductance
for N@C60 in a break junction. Our results reveal Coulomb-blockade behavior as
well as a fine structure of the Coulomb-blockade peaks due to the exchange
coupling of the C60 spin to the spin of the encapsulated nitrogen atom.Comment: 5 pages, 4 figures, v2: version as publishe
Tunneling Spectra of Individual Magnetic Endofullerene Molecules
The manipulation of single magnetic molecules may enable new strategies for
high-density information storage and quantum-state control. However, progress
in these areas depends on developing techniques for addressing individual
molecules and controlling their spin. Here we report success in making
electrical contact to individual magnetic N@C60 molecules and measuring spin
excitations in their electron tunneling spectra. We verify that the molecules
remain magnetic by observing a transition as a function of magnetic field which
changes the spin quantum number and also the existence of nonequilibrium
tunneling originating from low-energy excited states. From the tunneling
spectra, we identify the charge and spin states of the molecule. The measured
spectra can be reproduced theoretically by accounting for the exchange
interaction between the nitrogen spin and electron(s) on the C60 cage.Comment: 7 pages, 4 figures. Typeset in LaTeX, updated text of previous
versio
Bang-bang control of fullerene qubits using ultra-fast phase gates
Quantum mechanics permits an entity, such as an atom, to exist in a
superposition of multiple states simultaneously. Quantum information processing
(QIP) harnesses this profound phenomenon to manipulate information in radically
new ways. A fundamental challenge in all QIP technologies is the corruption of
superposition in a quantum bit (qubit) through interaction with its
environment. Quantum bang-bang control provides a solution by repeatedly
applying `kicks' to a qubit, thus disrupting an environmental interaction.
However, the speed and precision required for the kick operations has presented
an obstacle to experimental realization. Here we demonstrate a phase gate of
unprecedented speed on a nuclear spin qubit in a fullerene molecule (N@C60),
and use it to bang-bang decouple the qubit from a strong environmental
interaction. We can thus trap the qubit in closed cycles on the Bloch sphere,
or lock it in a given state for an arbitrary period. Our procedure uses
operations on a second qubit, an electron spin, in order to generate an
arbitrary phase on the nuclear qubit. We anticipate the approach will be vital
for QIP technologies, especially at the molecular scale where other strategies,
such as electrode switching, are unfeasible
Quantum gates using electronic and nuclear spins of Yb in a magnetic field gradient
An efficient scheme is proposed to carry out gate operations on an array of
trapped Yb ions, based on a previous proposal using both electronic and
nuclear degrees of freedom in a magnetic field gradient. For this purpose we
consider the Paschen-Back regime (strong magnetic field) and employ a
high-field approximation in this treatment. We show the possibility to suppress
the unwanted coupling between the electron spins by appropriately swapping
states between electronic and nuclear spins. The feasibility of generating the
required high magnetic field is discussed
A quantum spin transducer based on nano electro-mechancial resonator arrays
Implementation of quantum information processing faces the contradicting
requirements of combining excellent isolation to avoid decoherence with the
ability to control coherent interactions in a many-body quantum system. For
example, spin degrees of freedom of electrons and nuclei provide a good quantum
memory due to their weak magnetic interactions with the environment. However,
for the same reason it is difficult to achieve controlled entanglement of spins
over distances larger than tens of nanometers. Here we propose a universal
realization of a quantum data bus for electronic spin qubits where spins are
coupled to the motion of magnetized mechanical resonators via magnetic field
gradients. Provided that the mechanical system is charged, the magnetic moments
associated with spin qubits can be effectively amplified to enable a coherent
spin-spin coupling over long distances via Coulomb forces. Our approach is
applicable to a wide class of electronic spin qubits which can be localized
near the magnetized tips and can be used for the implementation of hybrid
quantum computing architectures
Quantum cellular automata quantum computing with endohedral fullerenes
We present a scheme to perform universal quantum computation using global
addressing techniques as applied to a physical system of endohedrally doped
fullerenes. The system consists of an ABAB linear array of Group V endohedrally
doped fullerenes. Each molecule spin site consists of a nuclear spin coupled
via a Hyperfine interaction to an electron spin. The electron spin of each
molecule is in a quartet ground state . Neighboring molecular electron
spins are coupled via a magnetic dipole interaction. We find that an
all-electron construction of a quantum cellular automata is frustrated due to
the degeneracy of the electronic transitions. However, we can construct a
quantum celluar automata quantum computing architecture using these molecules
by encoding the quantum information on the nuclear spins while using the
electron spins as a local bus. We deduce the NMR and ESR pulses required to
execute the basic cellular automata operation and obtain a rough figure of
merit for the the number of gate operations per decoherence time. We find that
this figure of merit compares well with other physical quantum computer
proposals. We argue that the proposed architecture meets well the first four
DiVincenzo criteria and we outline various routes towards meeting the fifth
criteria: qubit readout.Comment: 16 pages, Latex, 5 figures, See http://planck.thphys.may.ie/QIPDDF/
submitted to Phys. Rev.
Electron Spin-Relaxation Times of Phosphorus Donors in Silicon
Pulsed electron paramagnetic resonance measurements of donor electron spins
in natural phosphorus-doped silicon (Si:P) and isotopically-purified 28Si:P
show a strongly temperature-dependent longitudinal relaxation time, T1, due to
an Orbach process with DeltaE = 126 K. The 2-pulse echo decay is exponential in
28Si:P, with the transverse relaxation (decoherence) time, T2, controlled by
the Orbach process above ~12 K and by instantaneous diffusion at lower
temperatures. Spin echo experiments with varying pulse turning angles show that
the intrinsic T2 of an isolated spin in 28Si:P is ~60 ms at 7 K.Comment: Submitted to PRL on 02.28.200
Engineering coherent interactions in molecular nanomagnet dimers
Proposals for systems embodying condensed matter spin qubits cover a very wide range of length scales, from atomic defects in semiconductors all the way to micron-sized lithographically defined structures. Intermediate scale molecular components exhibit advantages of both limits: like atomic defects, large numbers of identical components can be fabricated; as for lithographically defined structures, each component can be tailored to optimise properties such as quantum coherence. Here we demonstrate what is perhaps the most potent advantage of molecular spin qubits, the scalability of quantum information processing structures using bottom-up chemical self-assembly. Using Cr7Ni spin qubit building blocks, we have constructed several families of two-qubit molecular structures with a range of linking strategies. For each family, long coherence times are preserved, and we demonstrate control over the inter-qubit quantum interactions that can be used to mediate two-qubit quantum gates
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