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arXiv:2601.00402

$Δ_T$ Noise from Electron-Hole Asymmetry in Normal and Superconducting Quantum Point Contacts

Sachiraj Mishra, Colin Benjamin

cond-mat.mes-hallmath-phphysics.app-phquant-phSubmitted Jan 1, 2026• Announced Jan 2, 2026
v1 · Jan 1, 2026v2 · Sep 9, 2026
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Abstract

This work examines $Δ_T$ noise in two-terminal hybrid nanostructures featuring a quantum point contact (QPC), realized either between two normal metals (NQN) or between a normal metal and a superconductor (NQS). The inclusion of a QPC breaks electron-hole (e-h) symmetry, leading to a finite thermovoltage. In contrast, earlier studies on hybrid junctions incorporating insulating barriers, as e-h symmetry is preserved, have vanishing thermovoltage, and consequently, $Δ_T$ noise is calculated at zero thermovoltage. In our setup, the broken e-h symmetry allows for a finite thermovoltage, at which we compute the corresponding $Δ_T$ noise. Unlike earlier studies restricted by e-h symmetry and vanishing thermovoltage, our work establishes a self-consistent framework in mesoscopic hybrid junctions, revealing how Andreev reflection fundamentally reshapes $Δ_T$ noise once e-h symmetry is broken. This broad access to charge fluctuation signatures provides a more comprehensive understanding of non-equilibrium transport in linear response. To our knowledge, this work provides the first self-consistent analysis of $Δ_T$ noise in superconducting hybrid junctions where e-h symmetry is broken, explicitly revealing how Andreev reflection modifies $Δ_T$ noise beyond the symmetry-protected zero-thermovoltage regime.

Journal reference
Phys. Rev. B 114, 165422 (2026)
DOI
10.1103/gtjm-rsqk
Comments
15 pages, 9 figures, 1 table

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ΔTΔ_TΔT​ Noise from Electron-Hole Asymmetry in Normal and Superconducting Quantum Point Contacts
This work examines ΔTΔ_TΔT​ noise in two-terminal hybrid nanostructures featuring a quantum point contact (QPC), realized either between two normal metals (NQN) or between a normal metal and a superconductor (NQS). The inclusion of a QPC breaks electron-hole (e-h) symmetry, leading to a finite thermovoltage. In contrast, earlier studies on hybrid junctions incorporating insulating barriers, as e-h symmetry is preserved, have vanishing thermovoltage, and consequently, ΔTΔ_TΔT​ noise is calculated at zero thermovoltage. In our setup, the broken e-h symmetry allows for a finite thermovoltage, at which we compute the corresponding ΔTΔ_TΔT​ noise. Unlike earlier studies restricted by e-h symmetry and vanishing thermovoltage, our work establishes a self-consistent framework in mesoscopic hybrid junctions, revealing how Andreev reflection fundamentally reshapes ΔTΔ_TΔT​ noise once e-h symmetry is broken. This broad access to charge fluctuation signatures provides a more comprehensive understanding of non-equilibrium transport in linear response. To our knowledge, this work provides the first self-consistent analysis of ΔTΔ_TΔT​ noise in superconducting hybrid junctions where e-h symmetry is broken, explicitly revealing how Andreev reflection modifies ΔTΔ_TΔT​ noise beyond the symmetry-protected zero-thermovoltage regime.
Quantum noise has long served as a powerful probe of quantum transport in mesoscopic junctions. Recently, temperature-driven noise, or ΔTΔ_TΔT​ noise, has attracted growing interest due to its presence even in the absence of average charge current. In this work, we investigate a normal metal-insulator-iron-pnictide junction and demonstrate how zero temperature quantum shot noise, finite temperature quantum noise and ΔTΔ_TΔT​ noise can discriminate between S++S_{++}S++​ and S+−S_{+-}S+−​ pairing symmetries, which are relevant to iron-based superconductors. We introduce ΔTΔ_TΔT​ noise as a novel probe for distinguishing between the two pairing symmetries. In contrast to conductance, which exhibits a single peak for both S++S_{++}S++​ and S+−S_{+-}S+−​ states with only a difference in magnitude, the ΔTΔ_TΔT​ noise reveals qualitatively distinct features: a twin-peak structure for the S++S_{++}S++​ pairing symmetry and a single-peak profile for the S+−S_{+-}S+−​ state. A similar symmetry-dependent contrast is observed in both zero temperature quantum shot noise and finite temperature quantum noise, where the S++S_{++}S++​ state consistently exhibits a twin-peak structure, while the S+−S_{+-}S+−​ state shows a single-peak response. Our results demonstrate that noise-based measurements form a mutually reinforcing set of probes that enables reliable identification of superconducting gap symmetry in Iron Pnictide superconductors.
Andreev reflection, which corresponds to the tunneling of two electrons from a metallic lead to a superconductor lead as a Cooper pair (or vice versa), can be exploited to measure high frequency noise. A detector is proposed, which consists of a normal lead--superconductor circuit, which is capacitively coupled to a mesoscopic circuit where noise is to be measured. We discuss two detector circuits: a single normal metal-superconductor tunnel junction and a normal metal separated from a superconductor by a quantum dot operating in the Coulomb blockade regime. A substantial dc current flows in the detector circuit when an appropriate photon is provided or absorbed by the mesoscopic circuit, which plays the role of an environment for the junction to which it couples. Results for the current can be cast in all cases in the form of a frequency integral of the excess noise of the environment weighted by a kernel, which is specific to the transport process (quasiparticle tunneling, Andreev reflection, etc) considered. We apply these ideas to the measurement of the excess noise of a quantum point contact, and we provide numerical estimates of the detector current.
Non-local conductance experiments using tunnel junctions can provide valuable spectroscopic information on both the transport and relaxation of quasiparticles in superconductors, as these techniques directly probe the quasiparticle charge and energy imbalance even at mK temperatures. In this work, we employ mesoscopic three terminal Cu and Al NIS devices to study non-local quasiparticle transport over length-scales on the order of the superconducting coherence length in this regime. Via a dual-bias scheme, which utilizes detector biases both above and below the superconducting gap, we are able to extract the effect of quasiparticle energy imbalance via its impact on the self consistent pair potential by symmetry considerations. We observe non-local conductance features due to pair-breaking which are anti-symmetric with respect to the polarity of the voltage bias, with a sharp onset during single electron tunneling at energies around 3Δ3Δ3Δ. We compare these findings with quasiclassical simulations including inelastic effects to obtain estimates of the energy dependent inelastic scattering time. In addition, we demonstrate kinetic effects due to a large applied supercurrent which can also be captured in this formalism and decomposed with respect to the particle-hole symmetry and supercurrent direction, and discuss further opportunities for the advancement of this method.
We study the interplay between Coulomb blockade and superconductivity in a tunable superconductor-superconductor-normal-metal single-electron transistor. The device is realized by connecting the superconducting island via an oxide barrier to the normal-metal lead and with a break junction to the superconducting lead. The latter enables Cooper pair transport and (multiple) Andreev reflection. We show that these processes are relevant also far above the superconducting gap and that signatures of Coulomb blockade may reoccur at high bias while they are absent for small bias in the strong-coupling regime. Our experimental findings agree with simulations using a rate equation approach in combination with the full counting statistics of multiple Andreev reflection.
We investigate how particle-hole symmetry, quantum interference, and tunnel barriers shape thermodynamic uncertainty relations in chaotic Andreev billiards. Using random-matrix theory and the Mahaux-Weidenmüller scattering approach, we study charge conductance and shot noise across the four Altland-Zirnbauer symmetry classes, from the single-channel extreme quantum limit to the multichannel semiclassical regime and from ideal to opaque contacts. We characterize thermodynamic precision through two complementary ensemble observables: either by averaging the sample-resolved noise-to-conductance ratio, or from the separately averaging noise and conductance. Their pronounced discrepancy in the extreme quantum regime reveals the non-self-averaging character of mesoscopic transport and persists over a broad range of barrier transparencies. For ideal contacts, the first provides a sensitive fingerprint of the Altland-Zirnbauer symmetry class. In the opaque regime, this hierarchy changes. Despite these strong symmetry- and barrier-dependent effects, the standard thermodynamic uncertainty relation remains satisfied throughout all regimes investigated. Our results establish thermodynamic uncertainty as a symmetry-sensitive probe of universal transport in chaotic normal-superconducting systems.
We investigate quantum noise in a mesoscopic quantum dot serving as a realization of the charged Sachdev-Ye-Kitaev (SYK) model weakly coupled to a fermionic lead via a tunnel contact. We find noise signatures under voltage and temperature biases that can serve as clear markers of the SYK physics in experiments with related setups. We develop a linear response theory that treats all types of noise on the same footing and generalizes a concept of transport coefficients for charge and heat currents, as well as relations between them, to equilibrium noise power. Within this theory, we find characteristic scaling of the with temperature in all regimes that can be relevant for experimental realizations of the SYK dots, find a set of universal constants, with their values being unique to the SYK physics, that connect these coefficients, and characterize noise manifestations of the Coulomb blockade. Beyond SYK systems, these results may serve as a general framework for identification of non-Fermi-liquid signatures in mesoscopic transport and provide additional observables for experiments on thermoelectric phenomena.