arXiv subjects

Parvathy Gireesan

Publications and source records attributed to Parvathy Gireesan.

3 recordsLinked to original sources

arXiv2026

Excited-state-induced nonlinearity and polarity reversals in the thermocurrent through a quantum dot

Quantum dots are electrically tuneable thermal machines capable of both energy harvesting and cooling at the nanoscale in the cryogenic regime. At large thermal biases, their thermoelectric transport shows a complex behaviour, including additional extrema and polarity reversals in the thermocurrent. Though these asymmetries and nonlinearities directly affect rectification and power conversion, their origin has not been studied systematically. We address this through measurements and modelling of a single GaAs/AlGaAs quantum dot whose source is heated by white-noise excitation. Using Pauli master equation for sequential tunnelling through a two-level dot, retaining the full Fermi distributions of both leads, distinct tunnel rates for the two levels, and relaxation between them, we show that the excited state acts as a second transport channel which competes with the ground-state contribution. Strong excited-state coupling combined with slow relaxation produces polarity reversals in the thermocurrent; without both, the excited state yields only additional extrema and an amplitude asymmetry across the charge degeneracy point. Experimentally, as the source temperature increases, an additional extremum emerges in the thermocurrent which develops into a polarity reversal. Using the excited-state level spacing and ground-state tunnel rate obtained independently from pulsed-gate and bias spectroscopies, the model reproduces the positions of the observed extrema and their amplitudes to within a factor of about three, placing the device in the regime of slow relaxation and strong excited-state coupling. Our results establish thermocurrent line-shape as a probe of excited-state tunnelling and relaxation in quantum dots. Excited states, often treated as a minor correction, can dominate quantum-dot thermoelectric transport far from equilibrium.

cond-mat.mes-hall↗

arXiv2026

Tunnel-rate controlled local heat distribution in mesoscopic circuits

Solid-state quantum technologies, including qubits and quantum metrology circuits, demand milli-Kelvin operation to preserve fragile quantum states from classical noise. While the negligible electron-phonon coupling is the major impediment, reaching 50 mK electron temperature is further suffered by the high electrical resistance and sub-micron-scale dimensions of typical devices, limiting conventional heat dissipation. Though the phonons are effectively frozen, thermoelectric techniques could offer a viable path for heat management.This work explores thermally driven electrical transport in a gated quantum dot (QD) on a GaAs-AlGaAs two-dimensional electron gas (2DEG), to control heat flow between the source and drain reservoirs.By exploiting the QD's discrete energy spectrum and tuneable tunnel rates, a precise control over the polarity and magnitude of the resulting thermoelectric current is demonstrated. A temperature difference of 650 mK is maintained across the QD, a separation of 400 nm, by tuning the tunnel-rates. An experimental gate pulsing method is also introduced to directly measure the electron temperature differences across the QD, bypassing the need for any theoretical fits. The results presented here show that tuneable tunnel barriers can be used for local heat control, and could lead to advanced quantum refrigerators that work efficiently in mesoscopic circuits.

cond-mat.mes-hall↗

arXiv2025

Active Noise Reduction in Si/SiGe Gated Quantum Dots

Solid-state quantum technologies such as quantum dot qubits and quantum electrical metrology circuits rely on quantum phenomena at ultra-low energies, making them highly sensitive to various forms of environmental noise. Conventional passive filtering schemes can reduce high-frequency noise but are often ineffective against low-frequency interference, like powerline or instrument-induced. Extending such filters to lower frequencies causes practical issues such as longer stabilization times, slower system response, and increased Johnson noise, which impede low-frequency transport measurements. To address these limitations, we propose and experimentally demonstrate a generalized active noise cancellation scheme for quantum devices operating at sub-Kelvin temperatures. Our approach compensates periodic environmental interference by dynamically injecting a phase-coherent anti-noise signal directly into the device. We employ an automated feedback protocol featuring beat-frequency reduction and adaptive phase-amplitude tuning, enabling real-time compensation without any manual intervention. Unlike post-processing or passive filtering, this method suppresses noise at the device level without introducing additional time constants. We implement the scheme on a gate-defined Si/SiGe quantum dot subject to strong 50 Hz powerline interference and validate its effectiveness through acquiring Coulomb Blockade Oscillations and Coulomb diamond plots. The technique achieves substantial suppression of both the targeted interference and the overall noise floor, thereby stabilizing transport characteristics and enhancing device fidelity. While demonstrated on a quantum dot, the proposed framework is broadly applicable to a wide class of solid-state quantum devices where deterministic noise presents a critical bottleneck.

cond-mat.mes-hall↗