To exploit the quantum coherence of electron spins in solids in future technologies such as quantum computing, it is first vital to overcome the problem of spin decoherence due to their coupling to the noisy environment. Dynamical decoupling, which uses stroboscopic spin flips to give an average coupling to the environment that is effectively zero, is a particularly promising strategy for combating decoherence because it can be naturally integrated with other desired functionalities, such as quantum gates. Errors are inevitably introduced in each spin flip, so it is desirable to minimize the number of control pulses used to realize dynamical decoupling having a given level of precision. Such optimal dynamical decoupling sequences have recently been explored. The experimental realization of optimal dynamical decoupling in solid-state systems, however, remains elusive. Here we use pulsed electron paramagnetic resonance to demonstrate experimentally optimal dynamical decoupling for preserving electron spin coherence in irradiated malonic acid crystals at temperatures from 50 K to room temperature. Using a seven-pulse optimal dynamical decoupling sequence, we prolonged the spin coherence time to about 30 mus; it would otherwise be about 0.04 mus without control or 6.2 mus under one-pulse control. By comparing experiments with microscopic theories, we have identified the relevant electron spin decoherence mechanisms in the solid. Optimal dynamical decoupling may be applied to other solid-state systems, such as diamonds with nitrogen-vacancy centres, and so lay the foundation for quantum coherence control of spins in solids at room temperature.
We have measured the magnetic correlations, susceptibility, specific heat, and thermal relaxation in the dipolar-coupled Ising system LiHo"Y& "F4. The
The a-axis optical properties of a detwinned single crystal of YBa2Cu3O6.50 in the ortho II phase (Ortho II Y123, Tc= 59 K) were determined from reflectance data over a wide frequency range (70 -42 000 cm −1 ) for nine temperature values between 28 and 295 K. Above 200 K the spectra are dominated by a broad background of scattering that extends to 1 eV. Below 200 K a shoulder in the reflectance appears and signals the onset of scattering at 400 cm −1 . In this temperature range we also observe a peak in the optical conductivity at 177 cm −1 . Below 59 K, the superconducting transition temperature, the spectra change dramatically with the appearance of the superconducting condensate. Its spectral weight is consistent, to within experimental error, with the Ferrell-GloverTinkham (FGT) sum rule. We also compare our data with magnetic neutron scattering on samples from the same source that show a strong resonance at 31 meV. We find that the scattering rates can be modeled as the combined effect of the neutron resonance and a bosonic background in the presence of a density of states with a pseudogap. The model shows that the decreasing amplitude of the neutron resonance with temperature is compensated for by an increasing of the bosonic background yielding a net temperature independent scattering rate at high frequencies. This is in agreement with the experiments. The complete phase diagram of the high temperature superconducting (HTSC) cuprates is still under intense debate. The normal state, particularly in the underdoped region, is dominated by a variety of not-well-understood cross-over phenomena that may either be precursors to superconductivity or competing states. These include the pseudogap [1], the magnetic resonance [2,3,4], the anomalous Nernst effect [5,6], stripe order [7,8,9,10] and possible superconducting fluctuations [11,12]. The situation is further complicated by the presence of disorder and the practical considerations that lead to a situation where a given cuprate is not investigated with all the available experimental techniques. Ideally, one would like to have a system where disorder is minimized and several experimental techniques can be used with the same crystals. As a step in that direction we present detailed a-axis optical data on the highly ordered ortho-II phase of YBa 2 Cu 3 O 6.50 (Ortho II Y123) and compare these data with recent results from magnetic neutron scattering and microwave spectroscopy on crystals from the same source.An important motivation for a comparison between transport properties and the magnetic neutron resonance comes from the observation that the carrier life time, as measured by infrared spectroscopy, is dominated by a bosonic mode [13,14,15,16,17] whose frequency and intensity, as a function of temperature and doping level, tracks the inelastic magnetic resonance at 41 meV with in-plane momentum transfer of (π, π) [15,16]. The magnetic resonance has also been invoked to explain other self-energy effects such as the kink in the dispersion of angle-resolved photo...
We show that nuclear spin subsystems can be completely controlled via microwave irradiation of resolved anisotropic hyperfine interactions with a nearby electron spin. Such indirect addressing of the nuclear spins via coupling to an electron allows us to create nuclear spin gates whose operational time is significantly faster than conventional direct addressing methods. We experimentally demonstrate the feasibility of this method on a solid-state ensemble system consisting of one electron and one nuclear spin.Coherent control of quantum systems promises optimal computation [1], secure communication [2], and new insight into the fundamental physics of many-body problems [3]. Solid-state proposals [4,5,6,7,8] for such quantum information processors employ isolated spin degrees of freedom which provide Hilbert spaces with long coherence times. Here we show how to exploit a local, isolated electron spin to coherently control nuclear spins. Moreover, we suggest that this approach provides a fast and reliable means of controlling nuclear spins and enables the electron spins of such solid-state systems to be used for state preparation and readout [9] of nuclear spin states, and additionally as a spin actuator for mediating nuclear-nuclear spin gates.Model System. The spin Hamiltonian of a single local electron spin with angular momentum, S = 1 2 and N nuclear spins, each with angular momentum I k = 1 2 , in the presence of a magnetic field B is [10]:Here β e is the Bohr magneton, γ k n is the gyromagnetic ratio;Ŝ andÎ k are the spin-1 2 operators. The secondrank tensors g, A k , δ, and D kl represent the electron gfactor, the hyperfine interaction, the chemical shift, and the nuclear dipole-dipole interaction respectively.In the regime where the static magnetic field B = B 0ẑ provides a good quantization axis for the electron spin, the Hamiltonian can be simplified by dropping the nonsecular terms which corresponds to keeping only electron interactions involving S z . The quantization axis of any nuclear spin depends on the magnitudes of the hyperfine interaction and the main magnetic field, as well as their relative orientations. When these two fields are comparable in magnitude [37] H 0 can be approximated by: (2) with N=1. The electron spin state is in an eigenstate of purely the Zeeman interaction, while the nuclear spin state is not an eigenfunction of the Zeeman interaction alone due to the anisotropic hyperfine interaction. Because α0|β1 = 0 and α0|β0 = 0 the electron spin operator (Ŝx) has finite probabilities between all levels (dashed arrows). This allows for universal control of the entire spin system. The filled and unfilled circles represent the relative spin state populations of the ensemble at thermal equilibrium. In our experimental setup the energy differences are ω12/2π = 7.8 MHz, ω34/2π = 40 MHz, ω14/2π = 12.005 GHz, ω23/2π = 11.954GHzThe nuclear dipole-dipole interaction is neglected as it is typically 10 2 times weaker than the hyperfine terms.As described in Figure 1 (N=1), the nuclear spin is qu...
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