Twisted superconducting quantum diodes: Towards anharmonicity and high fidelity

Abstract

As quantum technologies advance, a fundamental challenge is mitigating noise and backscattering in superconducting circuits to achieve scalable, high-fidelity operations. Conventional superconducting components lack directionality, causing energy loss and decoherence. Superconducting diodes, that allow dissipationless current in one direction and resistive flow in the other, offer a potential remedy, yet their efficiency and quantum integration remain limited. Here, we realize a quantum diode in twisted NbSe2 bilayers under in-plane and out-of-plane magnetic fields. A mere 1 degree twist yields an efficiency enhancement over pristine devices, reaching 27.6 percent. Quantum simulations reveal that this intermediate efficiency, well below 100 percent ideal, is both experimentally practical and optimal for preserving qubit anharmonicity and stabilizing two-level systems. These findings show that maximal rectification is not always beneficial for quantum information, establishing a new principle for designing the fundamental properties of twisted superconductors towards low-power, high-fidelity quantum circuits.

Figures (4)

• Figure 1: Diode design of twisted NbSe$_2$ layers. (a) Schematic illustration of twisted layers with an angle $\theta$. (b) Current and voltage measurements using four terminal setup. (c) Dependence of the resistance on temperature indicating a critical temperature of 6.87 K. (d) Typical current-voltage characteristics under an applied in-plane magnetic field close to 0 T and determination of the corresponding critical currents $I_\textrm{c}^+$ and $I_\textrm{c}^-$. (e) Out-of-plane and (f) in-plane magnetic field dependence of critical temperature. All data are from a sample with an overall thickness of 16 nm.
• Figure 2: Current-voltage characteristics of the twisted NbSe$_2$ bilayers with overall thickness of (a-f) 16 nm, (g) 30 nm, and (h) 60 nm. (a) I-V curves under the in-plane magnetic field at -20 Oe and 20 Oe applied along the current. (b) the same as in (a) but at -180 Oe and 180 Oe. (c) In-plane maximum diode efficiency is achieved when the in-plane magnetic field is -100 Oe. (d) I-V curves under the out-of-plane magnetic field at -2 Oe and 2 Oe. (e) the same as in (d) but at -20 Oe and 20 Oe. (f) The maximum diode efficiency is achieved when the out-of-plane magnetic field is -13 Oe. (g) I-V curves under the out-of-plane magnetic field at -20 Oe and 20 Oe for overall 30 nm thick sample. (h) the same as in (g) but for 60 nm thick sample. (i) Diode efficiencies and fittings as functions of the out-of-plane magnetic field $B$ for NbSe$_2$ junctions with different overall thicknesses: 16 nm (magenta), 30 nm (orange), and 60 nm (blue), see the corresponding color coding.
• Figure 3: Diode effect for various in-plane magnetic field configurations measured in a sample with overall thickness of 16$\,$nm. (a)-(c) show the magnetic field parallel to the electric current, with the critical current extracted under varying magnetic field strengths in (b), and the diode efficiency as a function of magnetic field in (c). (d)-(f) show the diode characteristics for the in-plane magnetic field perpendicular to the electric current, with the corresponding critical currents extracted in (e) and diode efficiency in (f). (g)-(i) show the same as before for a random in-plane angle between the magnetic field and current, with the critical current in (h) and diode efficiency in (i). Panels (c), (f), and (i) also show the least-squares fitting (fit) of the experimental data (exp) of the diode efficiency $\eta$, that is in the range of $B$-field from -200 to 200 Oe following $\eta = a \cdot \sin(b \cdot B + c)$, the corresponding values of $a, b$ (in units of inverse Oe) and $c$ are given.
• Figure 4: Quantum circuit design and simulation. (a) Schematics of the NbSe$_2$ device exhibiting a nonreciprocal behavior. (b) Transmon qubit chain circuit with quantum diodes. (c) Benchmark plot comparing diode efficiency and average critical current in reported 2D layered systems, with this work highlighted in red. The efficiencies were calculated from experiments wu2022fieldlee2021twistedchen2024edelsteinchen2024asymmetriczhao2023timelin2022zerolyu2021superconducting. (d) Simulation of the potential-energy landscape with the corresponding energies of the qubit levels inside the quantum wells for the three representative efficiencies $\eta$, a perfect two-level system can be achieved for $\eta$ close to 27.6%. (e) Forward and reverse quantum diode fidelity simulations---as functions of the efficiency and time---without (above) and with (below) noise (based on circuit in (b)). (f) Overall fidelity difference between forward and reverse qubit transfer without (above) and with (below) noise.