arXiv subjects

K. Barr

Publications and source records attributed to K. Barr.

5 recordsLinked to original sources

arXiv2026

Complete coherent control of spin qubits in self-assembled InAs quantum dots under oblique magnetic fields

We demonstrate complete coherent control of a single spin qubit confined in a self-assembled InAs negatively charged quantum dot subjected to an Oblique magnetic field, and directly compare this regime with the conventional Voigt geometry. In the Oblique-field configuration, the groundstate spin eigenstates are found to be unequal superpositions of the bare electron spin, with their composition tunable via the orientation of the applied field. This tunable spin mixing provides an additional degree of freedom to engineer the spin basis and associated optical couplings in the charged quantum dot system. Although this geometry has a distinct structure with important implications, it provides a regime in which we can fully and coherently control the tailored spin qubit. We observe Rabi oscillations and Ramsey fringes, and demonstrate arbitrary single-qubit rotations, enabling a direct comparison with the Voigt case. Our results establish that spin-qubit control does not necessarily require a pure Voigt geometry and can instead be achieved under Oblique magnetic fields. This relaxes constraints on device and field alignment and offers a versatile route to design and optimize quantum information processing architectures in semiconductor quantum dots.

quant-ph↗

arXiv2025

Trion quantum coherence in site-controlled pyramidal InGaAs quantum dots

Deterministically positioned pyramidal InGaAs quantum dots (QDs) exhibit exceptional quantum properties, making them highly promising candidates for scalable on-chip quantum information processing. In this work, we investigate the coherent dynamics of positively charged excitons under the influence of strong magnetic fields in the Faraday configuration. Pyramidal quantum dots exhibit a fourfold splitting of the charged excitons even in the Faraday configuration, giving rise to an optically addressable double-Lambda{\Lambda} system akin to self-assembled quantum dots in oblique magnetic fields. Here, we investigate ultrafast complete coherent control of the trion to ground state transition utilizing advanced optical resonant excitation techniques and we observe quantum coherence over timescales that are similar to other prominent quantum dot platforms. These results pave the way towards establishing site-controlled pyramidal InGaAs QDs as scalable platforms for quantum information processing, expanding the reach of coherent control to new quantum systems.

quant-ph↗

arXiv2025

Optical pumping and initialization of a hole spin in site-controlled InGaAs pyramidal quantum dots

We investigate site-controlled In$_{0.25}$Ga$_{0.75}$As quantum dots in (111)B GaAs pyramidal recesses as spin qubits. Combining scanning confocal cryomicroscopy, magneto-photoluminescence studies and resonant excitation, we identify and isolate a positively charged exciton with a hole-spin in its ground state. Application of a strong 5 T magnetic field parallel to the growth axis, induces a fourfold splitting of the energy levels of the positively charged exciton creating an optically addressable double-lambda system. We combine weak above-band and resonant excitation to demonstrate spin pumping and high-fidelity spin initialization through all four optical transitions and study the system behavior as a function of the resonant driving strength showing the existence of a robust spin that can be optically pumped and initialized. These results demonstrate the potential of these quantum dots for precise spin manipulation and their relevance for future quantum hardware.

cond-mat.mes-hall↗

arXiv2022

Operation of a Continuous Flow Liquid Helium Magnetic Microscopy Cryostat as a Closed Cycle System

We demonstrate successful operation of a continuous flow liquid helium magnetic cryostat (Oxford Instruments, Microstat MO) in closed cycle operation using a modular cryocooling system (ColdEdge Technologies, Stinger). For the system operation, we have developed a custom gas handling manifold and we show that despite the lower cooling power of the cryocooler with respect to the nominal cryostat cooling power requirements, the magnetic cryostat can be operated in a stable manner. We provide the design of the gas handling manifold, and a detailed analysis of the system performance in terms of cooling times, magnetic field ramping rates and vibrations at the sample. Base temperature can be reached within 10 hours while the superconducting magnet can be energized at a ramping rate of 0.5 T/min. Vibrations are measured interferometrically and show amplitudes with a root mean square on the order of 5 nm permitting the use of the system for sensitive magnetic microscopy experiments.

physics.ins-det↗

arXiv2013

Periodicity and perfect state transfer in quantum walks on variants of cycles

We systematically investigated perfect state transfer between antipodal nodes of discrete time quantum walks on variants of the cycles C_4, C_6 and C_8 for three choices of coin operator. Perfect state transfer was found, in general, to be very rare, only being preserved for a very small number of ways of modifying the cycles. We observed that some of our useful modifications of C_4 could be generalised to an arbitrary number of nodes, and present three families of graphs which admit quantum walks with interesting dynamics either in the continuous time walk, or in the discrete time walk for appropriate selections of coin and initial conditions. These dynamics are either periodicity, perfect state transfer, or very high fidelity state transfer. These families are modifications of families known not to exhibit periodicity or perfect state transfer in general. The robustness of the dynamics is tested by varying the initial state, interpolating between structures and by adding decoherence.

quant-ph↗