In(1-x)Mn(x)Sb - a new narrow gap ferromagnetic semiconductor

A narrow-gap ferromagnetic In(1-x)Mn(x)Sb semiconductor alloy was successfully grown by low-temperature molecular beam epitaxy on CdTe/GaAs hybrid substrates. Ferromagnetic order in In(1-x)Mn(x)Sb was unambiguously established by the observation of clear hysteresis loops both in direct magnetization measurements and in the anomalous Hall effect, with Curie temperatures T_C ranging up to 8.5 K. The observed values of T_C agree well with the existing models of carrier-induced ferromagnetism.Comment: 12 pages, 3 figure

Long-time Low-latency Quantum Memory by Dynamical Decoupling

Quantum memory is a central component for quantum information processing devices, and will be required to provide high-fidelity storage of arbitrary states, long storage times and small access latencies. Despite growing interest in applying physical-layer error-suppression strategies to boost fidelities, it has not previously been possible to meet such competing demands with a single approach. Here we use an experimentally validated theoretical framework to identify periodic repetition of a high-order dynamical decoupling sequence as a systematic strategy to meet these challenges. We provide analytic bounds-validated by numerical calculations-on the characteristics of the relevant control sequences and show that a "stroboscopic saturation" of coherence, or coherence plateau, can be engineered, even in the presence of experimental imperfection. This permits high-fidelity storage for times that can be exceptionally long, meaning that our device-independent results should prove instrumental in producing practically useful quantum technologies.Comment: abstract and authors list fixe

On the determinant representations of Gaudin models' scalar products and form factors

We propose alternative determinant representations of certain form factors and scalar products of states in rational Gaudin models realized in terms of compact spins. We use alternative pseudo-vacuums to write overlaps in terms of partition functions with domain wall boundary conditions. Contrarily to Slavnovs determinant formulas, this construction does not require that any of the involved states be solutions to the Bethe equations; a fact that could prove useful in certain non-equilibrium problems. Moreover, by using an atypical determinant representation of the partition functions, we propose expressions for the local spin raising and lowering operators form factors which only depend on the eigenvalues of the conserved charges. These eigenvalues define eigenstates via solutions of a system of quadratic equations instead of the usual Bethe equations. Consequently, the current work allows important simplifications to numerical procedures addressing decoherence in Gaudin models.Comment: 15 pages, 0 figures, Published versio

Universal quantum control of two-electron spin quantum bits using dynamic nuclear polarization

One fundamental requirement for quantum computation is to perform universal manipulations of quantum bits at rates much faster than the qubit's rate of decoherence. Recently, fast gate operations have been demonstrated in logical spin qubits composed of two electron spins where the rapid exchange of the two electrons permits electrically controllable rotations around one axis of the qubit. However, universal control of the qubit requires arbitrary rotations around at least two axes. Here we show that by subjecting each electron spin to a magnetic field of different magnitude we achieve full quantum control of the two-electron logical spin qubit with nanosecond operation times. Using a single device, a magnetic field gradient of several hundred milliTesla is generated and sustained using dynamic nuclear polarization of the underlying Ga and As nuclei. Universal control of the two-electron qubit is then demonstrated using quantum state tomography. The presented technique provides the basis for single and potentially multiple qubit operations with gate times that approach the threshold required for quantum error correction.Comment: 11 pages, 4 figures. Supplementary Material included as ancillary fil

Non-Markovian dynamics in a spin star system: The failure of thermalization

In most cases, a small system weakly interacting with a thermal bath will finally reach the thermal state with the temperature of the bath. We show that this intuitive picture is not always true by a spin star model where non-Markov effect predominates in the whole dynamical process. The spin star system consists a central spin homogeneously interacting with an ensemble of identical noninteracting spins. We find that the correlation time of the bath is infinite, which implies that the bath has a perfect memory, and that the dynamical evolution of the central spin must be non- Markovian. A direct consequence is that the final state of the central spin is not the thermal state equilibrium with the bath, but a steady state which depends on its initial state.Comment: 8 page

Correlations of spin splitting and orbital fluctuations due to 1/f charge noise in the Si/SiGe quantum dot

Theoretical Physic

Scaling of Dynamical Decoupling for Spin Qubits

We investigate scaling of coherence time, T2, with the number of {\pi}-pulses, n_{\pi}, in a singlet- triplet spin qubit using Carr-Purcell-Meiboom-Gill (CPMG) and concatenated dynamical decoupling (CDD) pulse sequences. For an even numbers of CPMG pulses, we find a power law, T2 = (n_{\pi})^({\gamma}_e), with {\gamma}_e = 0.72\pm0.01, essentially independent of the envelope function used to extract T2. From this surprisingly robust value, a power-law model of the noise spectrum of the environment, S({\omega}) ~ {\omega}^(-{\beta}), yields {\beta} = {\gamma}_e/(1 - {\gamma}_e) = 2.6 \pm 0.1. Model values for T2(n_{\pi}) using {\beta} = 2.6 for CPMG with both even and odd n_{\pi} up to 32 and CDD orders 3 through 6 compare very well with experiment.Comment: related articles at http://marcuslab.harvard.ed