Fig. 1: Measurement setup and expected Josephson current flow.
a Schematics showing SC-normal-SC junction measured by scanning NV center embedded in a diamond tip. The SC wave function can be described by an amplitude and phase Ψ = ∣Ψ∣eiθ. Under external magnetic field Bz, the screening current near the JJ (red lines) induces a phase difference ϕe(x). The bias current causes a phase difference between the SC electrodes ϕbias. b Measured differential resistance dV/dI versus perpendicular magnetic field Bz and bias current Idc, at T = 7 K. Dashed lines are the expected critical current (see Supplementary Note 2), where red is ϕbias = π/2, blue is ϕbias = −π/2. The Ic nodes are denoted as ±Bn. c, d Calculated Josephson current normalized by critical current density for 0- and 1-JV states, at external Bz = 1.10 mT in (c), and Bz = 1.91 mT in (d). The current flow is sine-like at zero bias current (bold lines), shifts along x direction at a finite bias current, and becomes cosine-like at the critical current. e Simulations showing the Josephson current flow (top) and local SC phase (bottom) of the 0- and 1-JV states. The screening current near the junction Jx (red arrows) is reduced by the Josephson current Jy (cyan arrows) in the 0-JV state, and enhanced in the 1-JV state. This causes the Josephson current-induced phase. Ibias = 0, Bz ≈ 1.2 mT in this simulation. f, Simulated ϕe(x) for 0- and 1-JV states, at the same Bz as (e). ϕe(x) is the difference of θ taken along the two dashed lines in each sub-panel of (e). g NV control and current bias sequence based on the ac magnetometry protocol. The X(Y) microwave (MW) pulses rotate the qubit around the X(Y) axis by \(\frac{\pi }{2}\) or π. NV qubit is put on the equator of the Bloch sphere and rotated by the magnetic field generated by current flow. Pulses of different bias current are synced with the MW pulses such that the final signal is the difference between the two current flow patterns.
