43 research outputs found
Modular nanomagnet design for spin qubits confined in a linear chain
On-chip micromagnets enable electrically controlled quantum gates on electron
spin qubits. Extending the concept to a large number of qubits is challenging
in terms of providing large enough driving gradients and individual
addressability. Here we present a design aimed at driving spin qubits arranged
in a linear chain and strongly confined in directions lateral to the chain.
Nanomagnets are placed laterally to one side of the qubit chain, one nanomagnet
per two qubits. The individual magnets are "U"-shaped, such that the magnetic
shape anisotropy orients the magnetization alternately towards and against the
qubit chain even if an external magnetic field is applied along the qubit
chain. The longitudinal and transversal stray field components serve as
addressability and driving fields. Using micromagnetic simulations we calculate
driving and dephasing rates and the corresponding qubit quality factor. The
concept is validated with spin-polarized scanning electron microscopy of Fe
nanomagnets fabricated on silicon substrates, finding excellent agreement with
micromagnetic simulations. Several features required for a scalable spin qubit
design are met in our approach: strong driving and weak dephasing gradients,
reduced crosstalk and operation at low external magnetic field.Comment: 6 pages, 4 figure
Adiabatic quantum simulations with driven superconducting qubits
We propose a quantum simulator based on driven superconducting qubits where
the interactions are generated parametrically by a polychromatic magnetic flux
modulation of a tunable bus element. Using a time-dependent Schrieffer-Wolff
transformation, we analytically derive a multi-qubit Hamiltonian which features
independently tunable and -type interactions as well as local bias
fields over a large parameter range. We demonstrate the adiabatic simulation of
the ground state of a hydrogen molecule using two superconducting qubits and
one tunable bus element. The time required to reach chemical accuracy lies in
the few microsecond range and therefore could be implemented on currently
available superconducting circuits. Further applications of this technique may
also be found in the simulation of interacting spin systems.Comment: 11 pages, 6 figure
Universal nuclear focusing of confined electron spins
For spin-based quantum computation in semiconductors, dephasing of electron
spins by a fluctuating background of nuclear spins is a main obstacle. Here we
show that this nuclear background can be precisely controlled in generic
quantum dots by periodically exciting electron spins. We demonstrate this
universal phenomenon in many-electron GaAs/AlGaAs quantum dot ensembles using
optical pump-probe spectroscopy. A feedback mechanism between the saturable
electron spin polarization and the nuclear system focuses the electron spin
precession frequency into discrete spin modes. Employing such control of
nuclear spin polarization, the electron spin lifetime within individual dots
can surpass the limit of nuclear background fluctuations, thus substantially
enhancing the spin coherence time. This opens the door to achieve long electron
spin coherence times also in lithographically-defined many-electron systems
that can be controlled in shape, size and position
Time-resolved tomography of a driven adiabatic quantum simulation
A typical goal of a quantum simulation is to find the energy levels and
eigenstates of a given Hamiltonian. This can be realized by adiabatically
varying the system control parameters to steer an initial eigenstate into the
eigenstate of the target Hamiltonian. Such an adiabatic quantum simulation is
demonstrated by directly implementing a controllable and smoothly varying
Hamiltonian in the rotating frame of two superconducting qubits, including
longitudinal and transverse fields and iSWAP-type two-qubit interactions. The
evolution of each eigenstate is tracked using time-resolved state tomography.
The energy gaps between instantaneous eigenstates are chosen such that
depending on the energy transition rate either diabatic or adiabatic passages
are observed in the measured energies and correlators. Errors in the obtained
energy values induced by finite and times of the qubits are
mitigated by extrapolation to short protocol times.Comment: 5 pages, 4 figure
Sex-Gender Differences Are Completely Neglected in Treatments for Neuropathic Pain
As sex-gender differences have been described in the responses of patients to certain medications, we hypothesized that the responses to medications recommended for neuropathic pain may differ between men and women. We conducted a literature review to identify articles reporting potential sex-gender differences in the efficacy and safety of these medications. Only a limited number of studies investigated potential sex-gender differences. Our results show that women seem to achieve higher blood concentrations than men during treatment with amitriptyline, nortriptyline, duloxetine, venlafaxine, and pregabalin. Compared to men, higher rates of women develop side effects during treatment with gabapentin, lidocaine, and tramadol. Globally, the sex-gender differences would suggest initially administering smaller doses of these medications to women with neuropathic pain compared to those administered to men. However, most of these differences have been revealed by studies focused on the treatment of other diseases (e.g., depression). Studies focused on neuropathic pain have overlooked potential sex-gender differences in patient responses to medications. Despite the fact that up to 60% of patients with neuropathic pain fail to achieve an adequate response to medications, the potential role of sex-gender differences in the efficacy and safety of pharmacotherapy has not adequately been investigated. Targeted studies should be implemented to facilitate personalized treatments for neuropathic pain
Breakdown of the Korringa Law of Nuclear Spin Relaxation in Metallic GaAs
We present nuclear spin relaxation measurements in GaAs epilayers using a new
pump-probe technique in all-electrical, lateral spin-valve devices. The
measured T1 times agree very well with NMR data available for T > 1 K. However,
the nuclear spin relaxation rate clearly deviates from the well-established
Korringa law expected in metallic samples and follows a sub-linear temperature
dependence 1/T1 ~ T^0.6 for 0.1 K < T < 10 K. Further, we investigate nuclear
spin inhomogeneities.Comment: 5 pages, 4 (color) figures. arXiv admin note: text overlap with
arXiv:1109.633