Photo-Assisted Quasiparticle Tunneling

• Photo-assisted quasiparticle tunneling is a quantum phenomenon where photon absorption or emission modulates charge transfer, directly probing quasiparticle statistics and energies.
• The Floquet–Keldysh framework and Tien–Gordon model are used to analyze multi-photon sidebands that encode key parameters like effective tunneling charge and scaling dimensions.
• Applications include quantum transport in superconducting qubits, topological systems, and quantum dots, enabling precise diagnostics from phase shifts to noise spectroscopy.

Photo-assisted quasiparticle tunneling refers to the process in which the tunneling of fractionally charged or integer quasiparticles across a quantum barrier is enabled or modulated by the absorption or emission of photons, typically through the application of an AC (microwave or optical) external drive. This phenomenon provides a highly sensitive probe of quasiparticle properties, many-body correlations, statistics, and device-specific microscopic structure, with applications spanning quantum transport, superconducting nanoelectronics, topological systems, and quantum information devices.

1. Fundamental Principles and Theoretical Framework

In photo-assisted quasiparticle tunneling (PAQT), quantum conductors—such as quantum Hall edge states, Josephson junctions, or hybrid quantum dots—are subject to time-dependent voltages or gate modulations that induce additional channels for charge transfer, each corresponding to the absorption or emission of a quantized external field photon of frequency $\Omega$ (energy $\hbar\Omega$). The central theoretical tool is the Floquet–Keldysh framework, in which the time-dependent system is expanded in harmonics of the drive frequency, and the tunneling Hamiltonian is expressed as a sum over processes with different photon numbers (Bertin-Johannet et al., 2023, Bertin-Johannet et al., 2022). For a cosinusoidal drive, the tunneling amplitude is expanded as

$\lambda(t) = \lambda_0 \sum_l p_l\,e^{i l\Omega t}$

where $p_l$ are the Floquet weights (typically Bessel functions or directly determined by the drive waveform).

The generic expression for the time-averaged current (for weak tunneling) takes the Tien–Gordon form:

$$\langle I(t)\rangle = \sum_n P_n(q)\,I^0_\mathrm{DC}(V_\mathrm{DC} + n\Omega/e)$$

where $P_n(q) = |p_n|^2$ is the weight for $n$ absorbed/emitted photons, and $I^0_\mathrm{DC}(V)$ is the static (dark) current (Bertin-Johannet et al., 2022). In strongly correlated or fractionalized systems, the formula must be generalized to account for the effective tunneling charge and the scaling dimension of the quasiparticle operator (Bertin-Johannet et al., 2023).

PAQT also produces sidebands in the spectral response of the system, with features shifted in energy by multiples of $\hbar\Omega$, thus encoding information about the energies and charges of the participating quasiparticles.

2. Photo-Assisted Tunneling in Fractional Quantum Hall Systems

In Laughlin fractional quantum Hall (FQH) edge states, quasiparticle tunneling through a quantum point contact (QPC) can be photo-assisted by an AC drive applied either through a voltage or a gate modulation. The effective low-energy Hamiltonian for a single-mode $\nu=1/m$ Laughlin edge is

$\hbar\Omega$0

with quasiparticle operators $\hbar\Omega$1, scaling dimension $\hbar\Omega$2, and fractional charge $\hbar\Omega$3.

When the tunneling amplitude is modulated as $\hbar\Omega$4, the backscattered current contains harmonics at integer multiples of $\hbar\Omega$5. The phase shift of the $\hbar\Omega$6 harmonic ($\hbar\Omega$7) is found to be uniquely and universally related to the scaling dimension,

$\hbar\Omega$8

allowing direct experimental determination of the anyonic statistics angle $\hbar\Omega$9 from a single-phase measurement in a weakly pinched QPC. This circumvents the need for multi-QPC interferometers or shot-noise analysis and provides a robust probe of the fundamental properties of FQH quasiparticles (Bertin-Johannet et al., 2023).

3. Multi-Quasiparticle and Andreev Reflection Processes

In hybrid superconductor–semiconductor structures, PAQT enables quantification of multi-particle processes such as multiple Andreev reflections (MAR). When an $\lambda(t) = \lambda_0 \sum_l p_l\,e^{i l\Omega t}$0-electron MAR process undergoes photo-assisted modulation, the periodic drive modifies the Tien–Gordon formula by replacing $\lambda(t) = \lambda_0 \sum_l p_l\,e^{i l\Omega t}$1 with $\lambda(t) = \lambda_0 \sum_l p_l\,e^{i l\Omega t}$2:

$\lambda(t) = \lambda_0 \sum_l p_l\,e^{i l\Omega t}$3

The energy spacing of the resulting PAT sidebands scales as $\lambda(t) = \lambda_0 \sum_l p_l\,e^{i l\Omega t}$4, directly revealing the correlated charge quanta involved in each MAR order. Systematic measurement of these spacings enables unambiguous identification of subgap transport mechanisms—distinguishing single quasiparticle, Cooper-pair, or higher-charge processes (Carrad et al., 2022).

4. Applications in Quantum Devices and Probing Exotic Quasiparticles

PAQT is a powerful spectroscopic tool for probing hybrid nanostructures, superconducting qubits, topological systems, and quantum dots:

• Superconducting qubit reset: By engineering SINIS (superconductor–insulator–normal–insulator–superconductor) junctions to act as quantum-circuit refrigerators, PAQT enables rapid, unconditional qubit reset with microsecond-scale dead times and reset fidelities limited by thermal excitation and device engineering (Sevriuk et al., 2022).
• Single-Microwave-Photon Detection: Charge-parity switches induced by photon-assisted quasiparticle injection can serve as the basis for single-microwave-photon detectors with sub-50 ns resolution and 10% quantum efficiency by monitoring quantum jumps on small superconducting islands (Basset et al., 21 Nov 2025).
• Topological qubit validation: In proximitized nanowires, PAT spectroscopy tracks the photon-induced splitting of ground state degeneracies associated with Majorana zero modes, providing direct measurement of hybridization energies and topological protection in Majorana devices (Zanten et al., 2019).

Additionally, the presence and structure of PAT sidebands serve as diagnostic signatures for identifying subgap states, distinguishing between trivial Andreev bound states, fractionalized Majorana modes, and quasi-1D Josephson physics.

5. Noise, Coherence, and Many-Body Correlations

PAQT modifies not only the time-averaged current but also introduces characteristic signatures in current noise and finite-frequency correlations. In resonant-tunneling and quantum dot systems, the photo-assisted noise spectra develop step and peak structures, whose location, amplitude, and asymmetry encode both the charge of the tunneling species and quantum-coherent electron–hole interference, sensitive to the scattering phase and device asymmetry (Hammer et al., 2011). In regimes with strong correlations or non-classical light, the PAT-induced transitions can probe the negativity of environmental quasi-probabilities, revealing the non-trivial many-body quantum state of the electromagnetic field (Souquet et al., 2014).

Crossover regimes where photon energy becomes comparable to the superconducting gap introduce additional complexity: multiple Floquet channels interact, yielding enhanced noise, negative excess noise, and loss of simple Tien–Gordon factorization. In Josephson junctions with magnetic impurities, photon-assisted tunneling through subgap Yu–Shiba–Rusinov states rapidly increases dissipation and destroys phase coherence, manifesting as suppression of Shapiro steps and onset of incoherent transport (Trahms et al., 30 Sep 2025).

6. Coulomb Interaction, Quantum Dots, and Nonlinear Regimes

In systems with strong Coulomb interaction, such as quantum dots, PAQT is modulated by many-body effects. The position and intensity of PAT peaks are split and shifted by onsite interaction $\lambda(t) = \lambda_0 \sum_l p_l\,e^{i l\Omega t}$5, resulting in new resonances such as photon-induced excited-state resonances (PIER), SATs (satellite peaks), and multi-photon pump effects. The resonance pattern provides a direct measure of $\lambda(t) = \lambda_0 \sum_l p_l\,e^{i l\Omega t}$6, level spacings, and spin-orbit, and opens strategies for all-electrical pumping of quantized charge and spin currents (Yuan et al., 2014, Tang et al., 2014).

The density matrix transfer Hamiltonian formalism provides a general framework to derive the nonlinear I–E characteristics of photo-assisted tunneling, treating multi-photon processes systematically. In asymmetric barriers and nanostructures with spatially confined fields, the photon energy acts as an effective, temperature-dependent chemical potential, enhancing tunneling even without any static bias (Davids et al., 2017).

7. Experimental Realizations and Diagnostics

Modern quantum devices exploit photo-assisted quasiparticle tunneling for:

• Measuring the fractional statistics angle in FQH edge states by phase-resolved lock-in detection at $\lambda(t) = \lambda_0 \sum_l p_l\,e^{i l\Omega t}$7 (Bertin-Johannet et al., 2023).
• Extracting the charge of multi-particle tunneling events in Josephson MAR via bias period analysis under irradiation (Carrad et al., 2022).
• Real-time continuous detection of individual itinerant microwave photons through charge-parity measurement in superconducting islands, reaching single-photon sensitivity (Basset et al., 21 Nov 2025).
• Fast and unconditional qubit reset through SINIS-based quantum refrigerators (Sevriuk et al., 2022).
• PAT spectroscopy in topological nanowires as a diagnostic for Majorana hybridization (Zanten et al., 2019).

A summary of representative physical systems, charge transfer mechanisms, and PAQT spectroscopic observables is provided below:

System/Device	Transfer Mechanism	PAQT Diagnostic
Laughlin QHE edge (QPC)	Fractional $\lambda(t) = \lambda_0 \sum_l p_l\,e^{i l\Omega t}$8	$\lambda(t) = \lambda_0 \sum_l p_l\,e^{i l\Omega t}$9 phase
S–N, Josephson junction	MAR, $p_l$0 correlated	Sideband period $p_l$1
Quantum dot, Coulomb block.	$p_l$2 splitting, SOC	Multiplet PAT satellites
SINIS, superconducting qubit	Quasiparticle tunneling	Qubit $p_l$3 control/reset
Majorana double island	Single-electron parity	PAT-resonance vs detuning
Superconducting island	QP injection, parity	Single-photon detection

This comprehensive landscape demonstrates that photo-assisted quasiparticle tunneling is a universal phenomenon bridging quantum transport, quantum optics, and electronic many-body physics, underpinning both metrological and quantum information applications across a wide range of condensed matter systems.

References: (Bertin-Johannet et al., 2023, Carrad et al., 2022, Bertin-Johannet et al., 2022, Sevriuk et al., 2022, Basset et al., 21 Nov 2025, Trahms et al., 30 Sep 2025, Zanten et al., 2019, Tang et al., 2014, Yuan et al., 2014, Hammer et al., 2011, Davids et al., 2017, Souquet et al., 2014, Vu et al., 2010)