arXiv subjects

Benjamin J. Chapman

Publications and source records attributed to Benjamin J. Chapman.

23 records · Page 2Linked to original sources

arXiv2016

Closing a quantum feedback loop inside a cryostat: Autonomous state preparation and long-time memory of a superconducting qubit

We propose to use a cryogenic nonlinear resonator for the projective readout, classical memory, and feedback for a superconducting qubit. This approach sidesteps many of the inefficiencies inherent in two-way communication between temperature stages in typical systems with room temperature controllers, and avoids increasing the cryogenic heat load. This controller may find a broad range of uses in multi-qubit systems, but here we analyze two specific demonstrative cases in single qubit-control. In the first case, the nonlinear controller is used to initialize the qubit in a definite eigenstate. And in the second case, the qubit's state is read into the controller's classical memory, where it is stored for an indefinite period of time, and then used to reinstate the measured state after the qubit has decayed. We analyze the properties of this system and we show simulations of the time evolution for the full system dynamics.

quant-ph↗

arXiv2015

Out-of-Plane Spin-Orientation Dependent Magnetotransport Properties in the Anisotropic Helimagnet Cr$_{1/3}$NbS$_2$

Understanding the role of spin-orbit coupling (SOC) has been crucial to controlling magnetic anisotropy in magnetic multilayer films. It has been shown that electronic structure can be altered via interface SOC by varying the superlattice structure, resulting in spontaneous magnetization perpendicular or parallel to the plane. In lieu of magnetic thin films, we study the similarly anisotropic helimagnet Cr$_{1/3}$NbS$_2$, where the spin polarization direction, controlled by the applied magnetic field, can modify the electronic structure. As a result, the direction of spin polarization can modulate the density of states, and in turn affect the in-plane electrical conductivity. In Cr$_{1/3}$NbS$_2$, we found an enhancement of in-plane conductivity when the spin polarization is out-of-plane, as compared to in-plane spin polarization. This is consistent with the increase of density of states near the Fermi energy at the same spin configuration, found from first principles calculations. We also observe unusual field dependence of the Hall signal in the same temperature range. This is unlikely to originate from the non-collinear spin texture, but rather further indicates strong dependence of electronic structure on spin orientation relative to the plane.

cond-mat.str-el↗

arXiv2015

On-chip superconducting microwave circulator from synthetic rotation

We analyze the design of a potential replacement technology for the commercial ferrite circulators that are ubiquitous in contemporary quantum superconducting microwave experiments. The lossless, lumped element design is capable of being integrated on chip with other superconducting microwave devices, thus circumventing the many performance-limiting aspects of ferrite circulators. The design is based on the dynamic modulation of DC superconducting microwave quantum interference devices (SQUIDs) that function as nearly linear, tunable inductors. The connection to familiar ferrite-based circulators is a simple frame boost in the internal dynamics' equation of motion. In addition to the general, schematic analysis, we also give an overview of many considerations necessary to achieve a practical design with a tunable center frequency in the 4-8 GHz frequency band, a bandwidth of 240 MHz, reflections at the -20 dB level, and a maximum signal power of approximately order 100 microwave photons per inverse bandwidth.

quant-ph↗

arXiv2014

Spin Structure of the Anisotropic Helimagnet Cr$_{1/3}$NbS$_2$ in a Magnetic Field

In this letter we describe the ground-state magnetic structure of the highly anisotropic helimagnet Cr$_{1/3}$NbS$_2$ in a magnetic field. A Heisenberg spin model with Dyzaloshinkii-Moriya interactions and magne- tocrystalline anisotropy allows the ground state spin structure to be calculated for magnetic fields of arbitrary strength and direction. Comparison with magnetization measurements shows excellent agreement with the predicted spin structure.

cond-mat.str-el↗

arXiv2014

Large enhancement of emergent magnetic fields in MnSi with impurities and pressure

We report a study of the topological Hall effect (THE) in Fe-doped MnSi and compare with results from pure MnSi under pressure. We find that Fe doping increases the THE, indicating an enhancement of the magnitude of the emergent gauge field. This is consistent with the concurrent reduction in the length scale of the skyrmion lattice. For both pressurized and doped samples, we calculate the emergent magnetic field based on the size of the measured THE, and compare it with a theoretical upper-bound. We find that the ratio of these two remains more or less constant with pressure or Fe doping, but differs greatly from that of pure MnSi at ambient pressure. We discuss the implications of this ratio with respect to trends in the saturated magnetic moment and helical pitch length as T_C \rightarrow 0 via doping and pressure, respectively.

cond-mat.str-el↗