arXiv subjects

Ross D. Monaghan

Publications and source records attributed to Ross D. Monaghan.

3 recordsLinked to original sources

arXiv2025

Synthetic areas spread in two-dimensional Superconducting Quantum Interference Filter Arrays

Superconducting Quantum Interference Devices (SQUIDs), formed by incorporating Josephson junctions into loops of superconducting material, are the backbone of many modern quantum sensing systems. It has been demonstrated that, by combining multiple SQUID loops into a two-dimensional (2D) array, it is possible to fabricate ultra-high-performing radio-frequency sensors. However, to function as absolute magnetometers, current-in-use arrays require the area of each SQUID loop in the array to be incommensurate. Doing so forbids the achievement of their full potential of performance, limited only by the standard quantum limit. This is because imposing incommensurability in the areas contrasts with optimised performance in each single SQUID loop. In this work, we report that by selectively inserting bare sections of a superconducting circuit with no Josephson junctions, 2D SQUID arrays can operate as an absolute magnetometer even when no physical area spread is applied. Based on a generalisation of currently available theories, a complete analytical formulation for the one-to-one correspondence between the distribution of these bare loops and what we call a synthetic areas spread is unveiled. This synthetic spread represents the equivalent physical spread of incommensurate SQUID loops that you would use to obtain the absolute Voltage-Magnetic Flux response if no bare loops were in use. Our work opens the way to a broader use of this technology for the fabrication of ultra-high-performance absolute quantum sensors. Our approach is also experimentally verified by fabricating several 2D Superconducting Quantum Interference Filter (SQIF) arrays incorporating bare superconducting loops and by demonstrating that they behave in alignment with what is suggested by our theory.

cond-mat.supr-con↗

arXiv2024

The role of Rashba spin-orbit induced spin textures in the anomalous Josephson effect

This work reports the theoretical investigation into the mechanism underpinning the anomalous Josephson effect. The prototypical system we study is a ballistic two-dimensional junction containing a two-dimensional Rashba spin-orbit interaction. In this paper we demonstrate how this two-dimensional Rashba interaction mixes the spins of adjacent transverse subbands which leads to significant spin-asymmetry within the junction. Under an external magnetic field, applied perpendicular to both the axis of transport and the normal vector of the junction, the sinusoidal Josephson current can then experience an anomalous phase shift. The role of this spin mixing in the limit of a single sub-band is initially explored by deriving an analytical expression for the resulting anomalous phase shift. The analysis is then extended to systems with multiple occupied sub-bands; in this later section, starting from a microscopic model, we derive an analytic formula for the resulting anomalous phase shift indicating it is linear in both magnetic field and spin-orbit strength. We then verify and validate all findings by comparing them with numerical results evaluated by a tight-binding model.

cond-mat.mes-hall↗

arXiv2021

Non-adiabatic quantum control of valley states in silicon

Non-adiabatic quantum effects, often experimentally observed in semiconductors nano-devices such as single-electron pumps operating at high frequencies, can result in undesirable and uncontrollable behaviour. However, when combined with the valley degree of freedom inherent to silicon, these unfavourable effects may be leveraged for quantum information processing schemes. By using an explicit time evolution of the Schrodinger equation, we study numerically non-adiabatic transitions between the two lowest valley states of an electron in a quantum dot formed in a SiGe/Si heterostructure. The presence of a single atomic layer step at the top SiGe/Si interface opens an anti-crossing in the electronic spectrum as the centre of the quantum dot is varied. We show that an electric field applied perpendicularly to the interface allows tuning of the anti-crossing energy gap. As a result, by moving the electron through this anti-crossing, and by electrically varying the energy gap, it is possible to electrically control the probabilities of the two lowest valley states.

cond-mat.mes-hall↗