7 research outputs found
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
Decoherence in Crystals of Quantum Molecular Magnets
Decoherence in Nature has become one of the most pressing problems in
physics. Many applications, including quantum information processing, depend on
understanding it; and fundamental theories going beyond quantum mechanics have
been suggested [1-3], where the breakdown of quantum theory appears as an
'intrinsic decoherence', mimicking environmental decoherence [4]. Such theories
cannot be tested until we have a handle on ordinary environmental decoherence
processes. Here we show that the theory for insulating electronic spin systems
can make accurate predictions for environmental decoherence in molecular-based
quantum magnets [5]. Experimental understanding of decoherence in molecular
magnets has been limited by short decoherence times, which make coherent spin
manipulation extremely difficult [6-9]. Here we reduce the decoherence by
applying a strong magnetic field. The theory predicts the contributions to the
decoherence from phonons, nuclear spins, and intermolecular dipolar
interactions, for a single crystal of the Fe8 molecular magnet. In experiments
we find that the decoherence time varies strongly as a function of temperature
and magnetic field. The theoretical predictions are fully verified
experimentally - there are no other visible decoherence sources. Our
investigation suggests that the decoherence time is ultimately limited by
nuclear spins, and can be extended up to about 500 microseconds, by optimizing
the temperature, magnetic field, and nuclear isotopic concentrations.Comment: Submitted version including 11 pages, 3 figures and online supporting
materials. Appeared on Nature Advance Online Publication (AOP) on July 20th,
2011.
(http://www.nature.com/nature/journal/vaop/ncurrent/full/nature10314.html
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Solutions of the two-dimensional hubbard model: Benchmarks and results from a wide range of numerical algorithms
Numerical results for ground-state and excited-state properties (energies, double occupancies, and Matsubara-axis self-energies) of the single-orbital Hubbard model on a two-dimensional square lattice are presented, in order to provide an assessment of our ability to compute accurate results in the thermodynamic limit. Many methods are employed, including auxiliary-field quantum Monte Carlo, bare and bold-line diagrammatic Monte Carlo, method of dual fermions, density matrix embedding theory, density matrix renormalization group, dynamical cluster approximation, diffusion Monte Carlo within a fixed-node approximation, unrestricted coupled cluster theory, and multireference projected Hartree-Fock methods. Comparison of results obtained by different methods allows for the identification of uncertainties and systematic errors. The importance of extrapolation to converged thermodynamic-limit values is emphasized. Cases where agreement between different methods is obtained establish benchmark results that may be useful in the validation of new approaches and the improvement of existing methods
Quantum oscillations in a molecular magnet
Bertaina S, Gambarelli S, Mitra T, Tsukerblat B, Müller A, Barbara B. Quantum oscillations in a molecular magnet. NATURE. 2008;453(7192):203-206.The term 'molecular magnet' generally refers to a molecular entity containing several magnetic ions whose coupled spins generate a collective spin, S ( ref. 1). Such complex multi- spin systems provide attractive targets for the study of quantum effects at the mesoscopic scale. In these molecules, the large energy barriers between collective spin states can be crossed by thermal activation or quantum tunnelling, depending on the temperature or an applied magnetic field(2-4). There is the hope that these mesoscopic spin states can be harnessed for the realization of quantum bits 'qubits', the basic building blocks of a quantum computer - based on molecular magnets(5-8). But strong decoherence(9) must be overcome if the envisaged applications are to become practical. Here we report the observation and analysis of Rabi oscillations ( quantum oscillations resulting from the coherent absorption and emission of photons driven by an electromagnetic wave(10)) of a molecular magnet in a hybrid system, in which discrete and well-separated magnetic V-15(IV) clusters are embedded in a self- organized non- magnetic environment. Each cluster contains 15 antiferromagnetically coupled S = 1/ 2 spins, leading to an S = 1/ 2 collective ground state(11-13). When this system is placed into a resonant cavity, the microwave field induces oscillatory transitions between the ground and excited collective spin states, indicative of long-lived quantum coherence. The present observation of quantum oscillations suggests that low- dimension self- organized qubit networks having coherence times of the order of 100 mu s ( at liquid helium temperatures) are a realistic prospect