The Josephson Diode That Conducts Supercurrent in Only One Direction Without a Magnetic Field

Conventional semiconductor diodes operate on a simple asymmetry: send an electrical current forward, and it flows easily; flip the voltage, and the device blocks it. But that traffic control carries a metabolic tax. High resistance in the reverse state generates heat, while even forward conduction incurs a small voltage drop, bleeding energy continuously as thermal dissipation.

Superconductors eliminate electrical resistance altogether by forcing electrons into Cooper pairs that glide through a crystal lattice without scattering. For decades, however, making that lossless current move in only one direction was thought to require an external magnetic field to break time-reversal symmetry.

A team led by Mazhar N. Ali at TU Delft bypassed that requirement by constructing a non-reciprocal Josephson junction that rectifies supercurrents without any applied magnetic field.

The device relies on a van der Waals heterostructure: a thin layer of the non-centrosymmetric insulator niobium bromide ($\text_3\text_8$) sandwiched between two superconducting flakes of niobium diselenide ($\text_2$).

In a standard Josephson junction, two superconductors are separated by a thin non-superconducting barrier. Quantum mechanics allows Cooper pairs to tunnel across the barrier, supporting a dissipationless supercurrent up to a specific limit called the critical current ($I_c$). In conventional junctions, this threshold is perfectly symmetric: pushing current from left to right yields the exact same critical threshold ($|I_c^+|$) as pushing it from right to left ($|I_c^-|$).

To break that balance, the apparatus must violate two fundamental physical symmetries simultaneously: spatial inversion symmetry and time-reversal symmetry.

Spatial inversion symmetry requires that flipping the physical coordinates of the device ($x \to -x$) leaves its behavior unchanged. In the heterostructure created by Heng Wu, Yaojia Wang, and colleagues, the central $\text_3\text_8$ barrier lacks an inversion center within its crystal lattice. Because the chemical structure itself possesses an inherent structural polarity, moving from left to right across the barrier is not electronically equivalent to moving from right to left.

Breaking time-reversal symmetry without an external magnetic field proved to be the more subtle hurdle. Normally, time-reversal symmetry demands that reversing the direction of time—which flips the momentum and spin of every charge carrier—leaves the magnitude of the critical current invariant.

By utilizing the strong Rashba-type spin-orbit coupling inside the non-centrosymmetric $\text_3\text_8$ layer coupled to the superconducting order parameter of the $\text_2$ layers, the researchers induced a spontaneous phase shift in the tunneling Cooper pairs. This interaction locks the momentum of the Cooper pairs to their spin orientations, creating an intrinsic directional bias across the barrier.

The physical outcome shows up directly on the oscilloscope trace.

When a positive bias current is applied below the threshold $I_c^+$, the junction exhibits zero electrical resistance. Current flows as a pure quantum supercurrent, and the voltage drop across the junction remains precisely zero.

When the current bias is reversed, the asymmetry takes effect. Because the critical current in the reverse direction ($|I_c^-|$) is substantially lower than $I_c^+$, an equivalent reverse current exceeds the junction's backward limit. The Cooper pairs unpair, the phase coherence collapses, and the device abruptly switches into a normal ohmic state.

In this reverse state, the junction behaves like an ordinary resistor, developing a measurable voltage drop across its terminals.

By applying an alternating square-wave current bias between $|I_c^-|$ and $I_c^+$, the device acts as a half-wave rectifier for supercurrents. Charge flows without energy loss during one half of the cycle, while reverse flow is entirely choked off into a resistive state.

Subsequent work by Kun-Rok Jeon, Stuart Parkin, and collaborators demonstrated that similar zero-field non-reciprocity can be engineered using proximity-magnetized barriers, achieving zero-field diode efficiencies up to $\pm35%$ at 2 K.

This directional supercurrent transport establishes a key building block for low-power quantum electronics. Because standard superconducting logic circuits like single-flux-quantum (SFQ) architecture rely on asymmetric current paths, they traditionally require bulky magnetic coils or external bias lines to manage signal routing. Replacing those components with field-free Josephson diodes allows logic gates to route signals directional-by-design without generating stray magnetic fields that decohere neighboring qubits.

Measured.

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