Quantum Switch: How Discrete Graphene Modes Control Superconductivity ✛
Context
Hybrid devices combining superconductors and ballistic semiconductors open the way to controlling quantum states at the boundary between normal and superconducting matter. Bilayer graphene with an electrically tunable band gap serves as a versatile platform for such architectures: quantum dots, interferometers, and valley splitters have already been realized in it. The proximity effect in graphene has been studied, but superconducting transport through one-dimensional ballistic channels defined by gates remained unexplored. This work fills that gap, sitting alongside studies of superconductivity in graphene and the quantum Hall effect with superconducting electrodes.
Methods
The device is made of bilayer graphene encapsulated in hexagonal boron nitride, with a graphite bottom gate and two layers of metallic top gates separated by an Al2O3 dielectric. Two split gates with a gap of 75 nm form a one-dimensional channel, and a channel gate controls the number of filled modes. A superconducting aluminum contact (Ti/Al, 5/40 nm) is located about 350 nm from the constriction. Measurements were performed at 1.7 K (above aluminum's critical temperature) to characterize the normal state and at 10 mK to study superconducting transport. Differential conductance was recorded as a function of bias voltage and magnetic field, along with differential resistance under current bias.
Results
At 1.7 K, the QPC shows conductance plateaus at (4e²/h)N, where the factor 4 reflects spin and valley degeneracy. At 10 mK, the plateaus are enhanced to about 1.25×(4e²/h)N due to Andreev reflections, corresponding to an interface transparency of about 0.9. Bias spectroscopy reveals the superconducting gap of aluminum, Δ ≈ 120 μeV, and an anomalous conductance dip at voltages above the gap, associated with suppression of excess current when the contact transitions to the normal state. The excess current, extracted from current-voltage characteristics, increases in steps of about 9 nA per mode, which is lower than the theoretical estimate for the clean limit (28 nA) but consistent with reduced transparency and local heating. In current-biased measurements, the switching current also changes discretely, following the number of open modes, and is well described by a dissipative transition model with a constant critical power Pc ≈ 0.5 nW.
Implications
This work shows for the first time that quantized one-dimensional QPC modes in bilayer graphene govern both equilibrium Andreev transport and nonequilibrium destruction of superconductivity. Unlike neighboring studies that examined Josephson junctions or the proximity effect in graphene without a ballistic channel, here direct spectroscopic resolution of modes in a hybrid system is achieved. This opens the way to studying mode-resolved superconductivity and dissipative mechanisms in more complex ballistic nanostructures.
Future development
The topic could develop toward studying Andreev processes in multimode channels with tunable transparency, as well as toward quantum circuits based on hybrid QPCs. Also of interest is investigating the influence of the valley degree of freedom on superconducting transport, especially in the presence of a magnetic field.
Impact
The results will influence the development of hybrid superconductor-semiconductor devices for quantum computing and the understanding of nonequilibrium superconductivity in nanostructures.
Next steps
Next steps include measuring shot noise to directly confirm Andreev reflection and studying the system's behavior at finite temperatures and in microwave fields.
Key open problems
This work connects to the unresolved problem of precisely controlling the transparency of the superconductor-graphene interface and to understanding dissipative transitions in hybrid nanostructures, where heating competes with quantum coherence.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study presents a quantum point contact (QPC) fabricated in bilayer graphene with a superconducting aluminum electrode. At 10 mK, the conductance steps are enhanced due to Andreev reflections, and at finite bias, an anomalous dip appears, linked to the disappearance of the excess current. The excess current and the switching current increase in steps as the one-dimensional modes of the QPC are populated, indicating control over both the equilibrium proximity effect and the non-equilibrium breakdown of superconductivity via Joule heating.
Where its strength is
This work is the first to directly resolve quantized one-dimensional modes spectroscopically in a hybrid superconductor–graphene system, connecting them to Andreev transport and the dissipative transition. It lays the groundwork for studying mode-resolved superconductivity and the mechanisms of superconductivity breakdown in ballistic channels.
Where it could go next
- It might be interesting to see how defects in graphene affect the superconducting order and transport in such hybrid devices: in our archive, there is a nearby work on the role of defects in forming a superconducting web.based on: Superconducting Web: How Defects in Graphene Weave Quantum Order arXiv:2607.02267
- To further understand the mechanisms of superconductivity in graphene, one could compare the observed step-like behavior with results on how the geometry of electronic orbits affects superconductivity.based on: How the Geometry of Electron Orbits Gives Rise to Superconductivity arXiv:2606.21166
- It is worth considering how similar approaches to controlling superconductivity via one-dimensional channels could be applied to other materials, for example, gold-based systems where induced superconductivity was recently observed.based on: How to Make Gold and Copper Conduct Without Resistance arXiv:2606.03663
Worth trying
- Investigate how introducing controlled defects near the QPC alters the excess current and switching current, using techniques from the work on the superconducting web.
- Test superconducting transport measurements under different gate configurations to create asymmetric channels, which may reveal new mode-related effects.
- Study the influence of a magnetic field on the conductance steps in the superconducting state, comparing with observations in other graphene superconducting systems.
Why this may matter
The results could underpin the development of mode-resolved superconducting devices, where individual quantum channels are used to manipulate quantum states, which is important for quantum technologies and for probing fundamental properties of hybrid systems.
Nearby works in our archive
- Superconducting Web: How Defects in Graphene Weave Quantum Ordersimilarity 0.65
- How the Geometry of Electron Orbits Gives Rise to Superconductivitysimilarity 0.65
- How to Make Gold and Copper Conduct Without Resistancesimilarity 0.64
- How to Make Gold a Superconductorsimilarity 0.64
- Strange Current in Graphene: When Weaker Means Strongersimilarity 0.64
- Graphene Sandwich Accelerates Electronssimilarity 0.63
The closest work in the archive sits at 0.65 by meaning — the usual level for our corpus.
Original abstract
as written by the authors arXiv:2608.16711v1
We report the realization of a gate-defined quantum point contact (QPC) in bilayer graphene proximitized by a single aluminum superconducting electrode. Superconducting correlations induced in the ballistic channel enhance the conductance plateaus beyond their normal-state values. In addition, we observe a pronounced above-gap conductance anomaly which serves as a spectroscopic signature of the loss of superconductivity and the associated collapse of the Andreev excess current. By reconstructing the nonlinear current-voltage characteristics, we find that the magnitude of the excess current increases as successive QPC modes are populated. Additionally, we find that the switching current associated with the loss of superconductivity follows the underlying mode structure of the QPC, exhibiting discrete levels consistent with a heat dissipation-driven transition. These results demonstrate that the one-dimensional transport modes of the QPC govern both the equilibrium proximity effect and the non-equilibrium dynamics of the hybrid system.
The superconducting gap of aluminum in this device is only 120 microvolts — about 100 times smaller than the voltage from a typical AA battery. Yet the switching current changes in steps like a staircase as new quantum modes open.
Key numbers
- superconducting gap: 120 μeV
- interface transparency: 0.9
- critical switching power: 0.5 nW
- excess current step per mode: 9 nA
- split gate gap: 75 nm