Quantum Coherence in Superconducting Vortex States Exhibits Microsecond Coherence and Millisecond Relaxation Times

The behaviour of magnetic vortices within superconducting materials presents a long-standing challenge to physicists, as these typically act as sources of energy loss. However, Ameya Nambisan, Simon Günzler, and Dennis Rieger, all from Karlsruhe Institute of Technology, alongside Nicolas Gosling, Simon Geisert, and Victor Carpentier, now demonstrate that vortices trapped within granular superconducting films exhibit surprisingly quantum properties. Their research reveals these vortices can behave as coherent quantum systems, maintaining superposition for microseconds and relaxing over fractions of a millisecond, a behaviour previously unseen in such structures. This discovery supports the idea that granular superconductors function as networks of tiny tunnel junctions, creating vortices with distinct quantum characteristics and opening exciting possibilities for new approaches to information processing and materials analysis.

Single Vortex Pinning Confirms Qubit Origin

This research provides extensive evidence supporting the claim that the observed qubit behaviour originates from a single pinned vortex within a superconducting film. The team meticulously ruled out alternative explanations, such as contributions from multiple vortices, and demonstrated that the observed properties, including energy relaxation, coherence, and quantum jumps, align with theoretical models of a single vortex qubit. They also showed that the fabrication process, whether field cooling or zero-field cooling, affects qubit characteristics but does not fundamentally alter the underlying physics. The data confirms that the observed qubit is dominated by the behaviour of a single vortex, allowing scientists to confidently attribute the observed properties to this isolated entity.

Detailed measurements of qubit coherence and relaxation reveal sensitivity to magnetic field detuning, consistent with flux-noise-induced dephasing. Time-resolved measurements confirm clear quantum jumps between qubit states, confirming the discrete nature of the system. The consistency of results obtained from samples fabricated using both field-cooled and zero-field-cooled methods reinforces the robustness of the qubit behaviour and its independence from the specific fabrication process. These measurements provide strong evidence for a well-defined qubit with measurable coherence and relaxation properties.

Vortex Qubits Demonstrate Unexpected Quantum Coherence

Scientists have demonstrated that vortices trapped within granular superconducting films can exhibit quantum coherence lasting for hundreds of microseconds, a remarkable finding that challenges conventional understanding of these phenomena. These vortices, traditionally considered dissipative, surprisingly behave as two-level quantum systems, displaying coherence times reaching fractions of a millisecond. The research team achieved this breakthrough by utilizing circuit electrodynamics to coherently manipulate and non-destructively read out the states of these vortex qubits within granular aluminum microwave resonators. The observed quantum properties of these vortex qubits are remarkably similar to those of engineered superconducting qubits, despite arising from a fundamentally different physical system.

Experiments reveal that the energy relaxation times of these vortex qubits are on the order of hundreds of microseconds, comparable to those of engineered superconducting qubits. This is a significant departure from the expected dissipation associated with Abrikosov vortex dynamics, supporting a model where granular aluminum acts as a three-dimensional network of Josephson junctions capable of hosting gapful vortices. The observed dispersive shifts and spectral characteristics are accurately described by an asymmetric quantum Rabi model, consistent with a two-level system residing in a double-well potential, potentially arising from vortex tunneling between pinning sites modulated by the magnetic field. The team’s measurements confirm that these vortex qubits exhibit quantum coherence, opening exciting avenues in quantum science and materials characterization.

Beyond granular aluminum, disordered superconductors or engineered two-dimensional networks of Josephson junctions may also host similar vortex qubits, providing new insights into the complex physics near the superconductor-to-insulator transition. Furthermore, the ability to measure quantum coherence in vortex states offers a novel embedded tool for microscopic material characterization and potentially, nanoscale sensing, if the observed dynamics stem from single-vortex tunneling. Ultimately, engineering the pinning landscape and device geometry, combined with noise spectroscopy and susceptibility measurements, will be crucial to enhance vortex qubit coherence and potentially establish a vortex-based quantum information platform.

Vortex Qubits Exhibit Microsecond Quantum Coherence

Researchers have demonstrated that vortices trapped within granular superconducting films exhibit behaviour consistent with two-level quantum systems, displaying coherence lasting for microseconds and energy relaxation times reaching hundreds of microseconds. This finding challenges the conventional understanding of vortices as dissipative entities and supports theoretical models describing granular superconductors as networks of Josephson junctions hosting gapful vortices. The team achieved coherent manipulation and non-destructive measurement of these vortex states using circuit electrodynamics techniques, opening new possibilities for exploring fundamental physics and developing novel technologies. The observed quantum properties of these vortex qubits are remarkably similar to those of engineered superconducting qubits, despite arising from a fundamentally different physical system.

Analysis using a quantum Rabi model accurately captures the observed dispersive shifts and spectra, suggesting that the vortices exist within a double-well potential, potentially due to tunneling between pinning sites modulated by magnetic fields. While these measurements support this hypothesis, the researchers acknowledge that direct observation via techniques like scanning tunneling or SQUID microscopy is needed for confirmation. Looking forward, the team suggests that similar vortex qubits may exist in other disordered superconductors or engineered Josephson junction networks, potentially offering insights into the superconductor-insulator transition. Furthermore, they propose that these vortex states could serve as embedded tools for microscopic material characterization or nanoscale sensors. Future work will focus on engineering the pinning landscape and device geometry, alongside detailed noise spectroscopy and susceptibility measurements, to enhance coherence and explore the potential for a vortex-based quantum information platform.