arXiv subjects

Charlotte G. L. Bøttcher

Publications and source records attributed to Charlotte G. L. Bøttcher.

7 recordsLinked to original sources

arXiv2026

Dynamical Control of Superconductivity in Superconductor-Ferromagnet Bilayers

We study a simplified model of a ferromagnetic metal proximitized by a fully-gapped $s-$wave superconductor and integrated with a microwave resonator. The low-energy excitations in the combined system consist of ferromagnetic magnons, Bogoliubov excitations of the superconductor, and cavity photons. We show here that when the magnons and photons have comparable frequencies and are subject to an external drive, the hybridized driven magnon-polaritons induce a non-equilibrium crossover from the expected proximitized nodal $p-$wave superconductor to a fully gapped $(p_x+ip_y)-$superconductor. Moreover, the characteristic crossover temperature is inversely related to the magnon-photon detuning. We compute the temperature-dependent renormalization of the cavity photon frequencies across this nodal to nodeless evolution, which modifies the kinetic inductance of the resonator, and find a number of non-trivial features tied to the non-equilibrium (i.e., driven) nature of the problem. We compare and contrast these results with a recent circuit quantum electrodynamics (cQED) based experiment studying a permalloy-niobium bilayer, where a non-trivial dependence of the low-temperature cavity response on the magnon-photon detuning was observed. Our results pave the way for a principled exploration of engineering novel states of matter by coupling cavity photons to electronic collective modes in correlated two-dimensional materials and interfaces.

cond-mat.supr-con↗

arXiv2026

Recovery dynamics of a gap-engineered transmon after a quasiparticle burst

Ionizing radiation impacts create bursts of quasiparticle density in superconducting qubits. These bursts temporarily degrade qubit coherence which can be detrimental for quantum error correction. Here, we experimentally resolve quasiparticle bursts in 3D gap-engineered transmon qubits by continuously monitoring qubit transitions. Gap engineering allows us to reduce the burst detection rate by a factor of five. This reduction falls four orders of magnitude short of that expected if the quasiparticles were to quickly thermalize to the cryostat temperature. We associate the limited effect of gap engineering with the slow thermalization of the phonons in our chips after the burst.

quant-ph↗

arXiv2025

Full characterization of measurement-induced transitions of a superconducting qubit

Repeated quantum non-demolition measurement is a cornerstone of quantum error correction protocols. In superconducting qubits, the speed of dispersive state readout can be enhanced by increasing the power of the readout tone. However, such an increase has been found to result in additional qubit state transitions that violate the desired quantum non-demolition character of the measurement. Recently, the readout of a transmon superconducting qubit was improved by using a tone with frequency much larger than the qubit frequency. Here, we experimentally identify the mechanisms of readout-induced transitions in this regime. In the dominant mechanism, the energy of an incoming readout photon is partially absorbed by the transmon and partially returned to the transmission line as a photon with lower frequency. Other mechanisms involve the excitation of unwanted package modes, decay via material defects, and, at higher qubit frequencies, the activation of undesired resonances in the transmon spectrum. Our work provides a comprehensive characterization of superconducting qubit state transitions caused by a strong drive.

quant-ph↗

arXiv2025

High-frequency readout free from transmon multi-excitation resonances

Quantum computation will rely on quantum error correction to counteract decoherence. Successfully implementing an error correction protocol requires the fidelity of qubit operations to be well-above error correction thresholds. In superconducting quantum computers, measurement of the qubit state remains the lowest-fidelity operation. For the transmon, a prototypical superconducting qubit, measurement is carried out by scattering a microwave tone off the qubit. Conventionally, the frequency of this tone is of the same order as the transmon frequency. The measurement fidelity in this approach is limited by multi-excitation resonances in the transmon spectrum which are activated at high readout power. These resonances excite the qubit outside of the computational basis, violating the desired quantum non-demolition character of the measurement. Here, we find that strongly detuning the readout frequency from that of the transmon exponentially suppresses the strength of spurious multi-excitation resonances. By increasing the readout frequency up to twelve times the transmon frequency, we achieve a quantum non-demolition measurement fidelity of 99.93% with a residual probability of leakage to non-computational states of only 0.02%.

quant-ph↗

arXiv2024

Coexistence of nonequilibrium density and equilibrium energy distribution of quasiparticles in a superconducting qubit

The density of quasiparticles typically observed in superconducting qubits exceeds the value expected in equilibrium by many orders of magnitude. Can this out-of-equilibrium quasiparticle density still possess an energy distribution in equilibrium with the phonon bath? Here, we answer this question affirmatively by measuring the thermal activation of charge-parity switching in a transmon qubit with a difference in superconducting gap on the two sides of the Josephson junction. We then demonstrate how the gap asymmetry of the device can be exploited to manipulate its parity.

quant-ph↗

arXiv2022

Surface Cooper pair spin waves in triplet superconductors

We study the electrodynamics of spin triplet superconductors including dipolar interactions, which give rise to an interplay between the collective spin dynamics of the condensate and orbital Meissner screening currents. Within this theory, we identify a class of spin waves that originate from the coupled dynamics of the spin-symmetry breaking triplet order parameter and the electromagnetic field. In particular, we study magnetostatic spin wave modes that are localized to the sample surface. We show that these surface modes can be excited and detected using experimental techniques such as microwave spin wave resonance spectroscopy or nitrogen-vacancy magnetometry, and propose that the detection of these modes offers a means for the identification of spin triplet superconductivity.

cond-mat.supr-con↗

arXiv2018

Readout Of Singlet-Triplet Qubits At Large Magnetic Field Gradients

Visibility of singlet-triplet qubit readout is reduced to almost zero in large magnetic field gradients due to relaxation processes. Here we present a new readout technique that is robust against relaxation and allows for measurement when previously studied methods fail. This technique maps the qubit onto spin states that are immune to relaxation using a spin dependent electron tunneling process between the qubit and the lead. We probe this readout's performance as a function of magnetic field gradient and applied magnetic field, and optimize the pulse applied to the qubit through experiment and simulation.

cond-mat.mes-hall↗