85 research outputs found
Quantum Computing in Molecular Magnets
Shor and Grover demonstrated that a quantum computer can outperform any
classical computer in factoring numbers and in searching a database by
exploiting the parallelism of quantum mechanics. Whereas Shor's algorithm
requires both superposition and entanglement of a many-particle system, the
superposition of single-particle quantum states is sufficient for Grover's
algorithm. Recently, the latter has been successfully implemented using Rydberg
atoms. Here we propose an implementation of Grover's algorithm that uses
molecular magnets, which are solid-state systems with a large spin; their spin
eigenstates make them natural candidates for single-particle systems. We show
theoretically that molecular magnets can be used to build dense and efficient
memory devices based on the Grover algorithm. In particular, one single crystal
can serve as a storage unit of a dynamic random access memory device. Fast
electron spin resonance pulses can be used to decode and read out stored
numbers of up to 10^5, with access times as short as 10^{-10} seconds. We show
that our proposal should be feasible using the molecular magnets Fe8 and Mn12.Comment: 13 pages, 2 figures, PDF, version published in Nature, typos
correcte
Spintronic magnetic anisotropy
An attractive feature of magnetic adatoms and molecules for nanoscale
applications is their superparamagnetism, the preferred alignment of their spin
along an easy axis preventing undesired spin reversal. The underlying magnetic
anisotropy barrier --a quadrupolar energy splitting-- is internally generated
by spin-orbit interaction and can nowadays be probed by electronic transport.
Here we predict that in a much broader class of quantum-dot systems with spin
larger than one-half, superparamagnetism may arise without spin-orbit
interaction: by attaching ferromagnets a spintronic exchange field of
quadrupolar nature is generated locally. It can be observed in conductance
measurements and surprisingly leads to enhanced spin filtering even in a state
with zero average spin. Analogously to the spintronic dipolar exchange field,
responsible for a local spin torque, the effect is susceptible to electric
control and increases with tunnel coupling as well as with spin polarization.Comment: 6 pages with 4 figures + 26 pages of Supplementary Informatio
Spin qubits with electrically gated polyoxometalate molecules
Spin qubits offer one of the most promising routes to the implementation of
quantum computers. Very recent results in semiconductor quantum dots show that
electrically-controlled gating schemes are particularly well-suited for the
realization of a universal set of quantum logical gates. Scalability to a
larger number of qubits, however, remains an issue for such semiconductor
quantum dots. In contrast, a chemical bottom-up approach allows one to produce
identical units in which localized spins represent the qubits. Molecular
magnetism has produced a wide range of systems with tailored properties, but
molecules permitting electrical gating have been lacking. Here we propose to
use the polyoxometalate [PMo12O40(VO)2]q-, where two localized spins-1/2 can be
coupled through the electrons of the central core. Via electrical manipulation
of the molecular redox potential, the charge of the core can be changed. With
this setup, two-qubit gates and qubit readout can be implemented.Comment: 9 pages, 6 figures, to appear in Nature Nanotechnolog
Spin dynamics of molecular nanomagnets fully unraveled by four-dimensional inelastic neutron scattering
Molecular nanomagnets are among the first examples of spin systems of finite
size and have been test-beds for addressing a range of elusive but important
phenomena in quantum dynamics. In fact, for short-enough timescales the spin
wavefunctions evolve coherently according to the an appropriate cluster
spin-Hamiltonian, whose structure can be tailored at the synthetic level to
meet specific requirements. Unfortunately, to this point it has been impossible
to determine the spin dynamics directly. If the molecule is sufficiently
simple, the spin motion can be indirectly assessed by an approximate model
Hamiltonian fitted to experimental measurements of various types. Here we show
that recently-developed instrumentation yields the four-dimensional
inelastic-neutron scattering function S(Q,E) in vast portions of reciprocal
space and enables the spin dynamics to be determined with no need of any model
Hamiltonian. We exploit the Cr8 antiferromagnetic ring as a benchmark to
demonstrate the potential of this new approach. For the first time we extract a
model-free picture of the quantum dynamics of a molecular nanomagnet. This
allows us, for example, to examine how a quantum fluctuation propagates along
the ring and to directly test the degree of validity of the
N\'{e}el-vector-tunneling description of the spin dynamics
Controlling spins in adsorbed molecules by a chemical switch
The development of chemical systems with switchable molecular spins could lead to the architecture of materials with controllable magnetic or spintronic properties. Here, we present conclusive evidence that the spin of an organometallic molecule coupled to a ferromagnetic substrate can be switched between magnetic off and on states by a chemical stimulus. This is achieved by nitric oxide (NO) functioning as an axial ligand of cobalt(II)tetraphenylporphyrin (CoTPP) ferromagnetically coupled to nickel thin-film (Ni(001)). On NO addition, the coordination sphere of Co2+ is modified and a NO–CoTPP nitrosyl complex is formed, which corresponds to an off state of the Co spin. Thermal dissociation of NO from the nitrosyl complex restores the on state of the Co spin. The NO-induced reversible off–on switching of surface-adsorbed molecular spins observed here is attributed to a spin trans effect
Quantum Computing
Quantum mechanics---the theory describing the fundamental workings of
nature---is famously counterintuitive: it predicts that a particle can be in
two places at the same time, and that two remote particles can be inextricably
and instantaneously linked. These predictions have been the topic of intense
metaphysical debate ever since the theory's inception early last century.
However, supreme predictive power combined with direct experimental observation
of some of these unusual phenomena leave little doubt as to its fundamental
correctness. In fact, without quantum mechanics we could not explain the
workings of a laser, nor indeed how a fridge magnet operates. Over the last
several decades quantum information science has emerged to seek answers to the
question: can we gain some advantage by storing, transmitting and processing
information encoded in systems that exhibit these unique quantum properties?
Today it is understood that the answer is yes. Many research groups around the
world are working towards one of the most ambitious goals humankind has ever
embarked upon: a quantum computer that promises to exponentially improve
computational power for particular tasks. A number of physical systems,
spanning much of modern physics, are being developed for this task---ranging
from single particles of light to superconducting circuits---and it is not yet
clear which, if any, will ultimately prove successful. Here we describe the
latest developments for each of the leading approaches and explain what the
major challenges are for the future.Comment: 26 pages, 7 figures, 291 references. Early draft of Nature 464, 45-53
(4 March 2010). Published version is more up-to-date and has several
corrections, but is half the length with far fewer reference
Superconductivity in a single C60 transistor
Single molecule transistors (SMTs) are currently attracting enormous
attention as possible quantum information processing devices. An intrinsic
limitation to the prospects of these however is associated to the presence of a
small number of quantized conductance channels, each channel having a high
access resistance of at best =12.9 k. When the
contacting leads become superconducting, these correlations can extend
throughout the whole system by the proximity effect. This not only lifts the
resistive limitation of normal state contacts, but further paves a new way to
probe electron transport through a single molecule. In this work, we
demonstrate the realization of superconducting SMTs involving a single C60
fullerene molecule. The last few years have seen gate-controlled Josephson
supercurrents induced in the family of low dimensional carbon structures such
as flakes of two-dimensional graphene and portions of one-dimensional carbon
nanotubes. The present study involving a full zero-dimensionnal fullerene
completes the picture.Comment: 12 pages, 3 figure
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
Rare-earth solid-state qubits
Quantum bits (qubits) are the basic building blocks of any quantum computer.
Superconducting qubits have been created with a 'top-down' approach that
integrates superconducting devices into macroscopic electrical circuits [1-3],
whereas electron-spin qubits have been demonstrated in quantum dots [4-6]. The
phase coherence time (Tau2) and the single qubit figure of merit (QM) of
superconducting and electron-spin qubits are similar -- Tau2 ~ microseconds and
QM ~10-1000 below 100mK -- and it should be possible to scale-up these systems,
which is essential for the development of any useful quantum computer.
Bottom-up approaches based on dilute ensembles of spins have achieved much
larger values of tau2 (up to tens of ms) [7, 8], but these systems cannot be
scaled up, although some proposals for qubits based on 2D nanostructures should
be scalable [9-11]. Here we report that a new family of spin qubits based on
rare-earth ions demonstrates values of Tau2 (~ 50microseconds) and QM (~1400)
at 2.5 K, which suggests that rare-earth qubits may, in principle, be suitable
for scalable quantum information processing at 4He temperatures
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
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