Ultrashort pulses pushed superconductors to the edge where electron pairs break

Niobium nitride carried a current 2.2 times its ordinary critical limit for a few trillionths of a second, then stopped superconducting all at once. That abrupt failure is the depairing current, the point at which the current is large enough to tear apart the electron pairs that make a superconductor work. A team at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg reached it by switching the current on and off faster than the usual failure mechanism can act.

Eryin Wang, the first author, and Andrea Cavalleri, who leads the group, report the measurements in Nature Physics, in a paper called “Probing picosecond depairing currents in type-II superconductors.” The paper was published on September 24, 2026.

Vortex motion and the critical current

A superconductor carries current without resistance because its electrons form Cooper pairs that move together. In theory the pairs survive until the current becomes large enough to break them, the depairing limit. In practice a real wire fails long before that. In type-II superconductors, the family that includes both niobium nitride and the copper oxides, magnetic vortices slip in, start to move, and generate resistance and heat. The current at which that happens, the conventional critical current, sits well below the theoretical ceiling.

The gap between the two is the subject of the Nature Physics paper. The authors write that picosecond electrical pulses “act on timescales too short for vortices to move” and so “drive supercurrents up to the intrinsic depairing limit.” Wang described the strategy in plain terms in the Max Planck account of the work: the aim was to outrun the vortex dynamics, since magnetic vortices move only tens of nanometers during a picosecond. A current that lasts only that long is over before the vortices have gone anywhere. Guido Meier, a co-author on the paper, is listed with the team alongside collaborators from several institutions.

Photoconductive switches and 300-femtosecond laser flashes

The pulses were generated with photoconductive switches triggered by 300-femtosecond laser flashes at a wavelength of 515 nanometers, according to that account. Each switch closes for an instant when the green laser hits it and lets a short burst of current through the sample, with a duration of a few picoseconds. Because the current is applied for so short a time, neither vortex motion nor self-heating has a chance to build up.

The scheme is described in the arXiv version of the work, whose abstract states that the technique reveals microscopic superconducting properties that standard transport measurements cannot reach. The authors frame it as a way to close the long-standing gap between theoretical and measured critical current densities.

NbN snaps, YBCO fades

Two materials were tested, and they behaved differently. Niobium nitride, an s-wave superconductor whose electron-pairing gap is the same in every direction, held steady until the current crossed a sharp threshold and then failed abruptly. The abstract puts that onset at a current density of 2.2 times the conventional critical current density. Quantum Zeitgeist’s summary of the paper gives the conventional value for NbN as about 100 gigaamperes per square meter at 7 kelvin, and says the depairing current falls by less than an order of magnitude as the temperature rises to 80% of the critical temperature.

YBCO, the copper-oxide material YBa2Cu3O7, is a d-wave superconductor whose gap varies with direction. It did not snap. Superconductivity weakened gradually as the current rose, a pattern the authors tie to that directional gap. The same summary lists its conventional critical current density as slightly below 50 gigaamperes per square meter at 50 kelvin.

So the “edge” has two shapes. For niobium nitride it is a cliff at a measurable multiple of the ordinary limit, while for YBCO it is a slope with no single breaking point.

Depairing as a design ceiling

A Lifeboat News write-up of the Max Planck work carries the line that on these ultrashort timescales the team “were able to access a much higher, intrinsic limit known as the depairing current.” The practical interest is a ceiling, and the paper’s own framing is that superconducting electronics could in principle operate at higher current limits once that ceiling is known. The measurement tells designers what the material itself allows: how much current a given superconductor could carry if the vortex problem were engineered away, and how the pair-breaking behavior differs between an s-wave and a d-wave material.

The paper stops at measurement, and none of its reported numbers describe a working wire or chip. It does not claim a device that runs continuously at these currents, and the 2.2 multiple is specific to niobium nitride, and the quoted conventional values come from single temperatures, 7 kelvin for niobium nitride and 50 kelvin for YBCO. Whether the sharp-versus-gradual contrast holds for other superconductors is a question the two-material comparison cannot answer.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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