arXiv subjects

Charlie Oncebay

Publications and source records attributed to Charlie Oncebay.

3 recordsLinked to original sources

arXiv2026

Operational modes of a Raman-coupled two-qubit quantum thermal machine

We investigate a quantum thermal machine composed of two qubits coupled through a Raman-induced exchange interaction and driven by inhomogeneous transition frequencies. The system is analyzed within Carnot, Otto, and Stirling thermodynamic cycles, including the Stirling cycle with and without regeneration. We identify the conditions under which the device operates as a heat engine, refrigerator, thermal accelerator, or heater. Efficiency maps and operational-mode diagrams reveal well-defined boundaries in parameter space, governed by the frequency ratio $r=\bar{\omega}/\omega$, the coupling strength $g$, and the thermal gradient between reservoirs. The Carnot cycle exhibits sharp transitions between engine and refrigerator regimes, while the Otto cycle displays a richer structure with the coexistence of all operational modes. The Stirling cycle shows enhanced versatility and performance, particularly when assisted by a regenerator, where near-ideal efficiencies are achieved. Overall, the Raman-type interaction introduces a controllable left-right asymmetry that enables nontrivial manipulation of thermodynamic behavior through frequency tuning.

quant-ph↗

arXiv2022

Localized Nitrogen-Vacancy centers generated by low-repetition rate fs-laser pulses

Among hundreds of impurities and defects in diamond, the nitrogen-vacancy (NV) center is one of the most interesting to be used as a platform for quantum technologies and nanosensing. Traditionally, synthetic diamond is irradiated with high-energy electrons or nitrogen ions to generate these color-centers. For precise positioning of the NV centers, fs-laser irradiation has been proposed as an alternative approach to produce spatially localized NV centers in diamond. However, most of the studies reported so far used high-repetition rate fs-laser systems. Here, we studied the influence of the irradiation conditions on the generation of NV$^-$. Specifically, we varied pulse fluence, laser focusing, and the number of pulses upon irradiation with 150 fs pulses at 775 nm from a Ti:sapphire laser amplifier operating at 1 kHz repetition rate. Optically Detected Magnetic Resonance (ODMR) was used to investigate the produced NV centers, revealing a sizeable zero-field splitting in the spectra and indicating the conditions in which the lattice strain produced in the ablation process may be deleterious for quantum information applications.

physics.optics↗

arXiv2017

Nonlinear optical spectrum of diamond at femtosecond regime

Although diamond photonics has driven considerable interest and useful applications, as shown in frequency generation devices and single photon emitters, fundamental studies on the third-order optical nonlinearities of diamond are still scarce, stalling the development of an integrated platform for nonlinear and quantum optics. The purpose of this paper is to contribute to those studies by measuring the spectra of two-photon absorption coefficient ($β$) and the nonlinear index of refraction (n$_2$) of diamond using femtosecond laser pulses, in a wide spectral range. These measurements show the magnitude of $β$ increasing from 0.07 to 0.23 cm/GW, as it approaches the bandgap energy, in the region from 3.18 to 4.77 eV (390 - 260 nm), whereas the n$_2$ varies from zero to 1.7E-19 m$^2$/W in the full measured range, from 0.83 - 4.77 eV (1500 - 260 nm). The experimental results are compared with theoretical models for nonlinear absorption and refraction in indirect gap semiconductors, indicating the two-photon absorption as the dominant effect in the dispersion of the third-order nonlinear susceptibility. These data, together with optical Kerr gate measurements, also provided here, are of foremost relevance to the understanding of ultrafast optical processes in diamond and its nonlinear properties.

physics.optics↗