Controlling superconducting transistor by coherent light

Abstract

The Josephson junction is typically tuned by a magnetic field or electrostatic gates to realize a superconducting transistor, which manipulates the supercurrent in integrated superconducting circuits. However, this tunable method does not achieve simultaneous control for the supercurrent phase (phase difference between two superconductors) and magnitude. Here, we propose a novel scheme for the light-controlled superconducting transistor, which is composed of two superconductor leads linked by a coherent light-driven quantum dot. We discover a Josephson-like relation for supercurrent $I_{\mathrm{s}}=I_{c}(Φ)\,\sinΦ$, where both supercurrent phase $Φ$ and magnitude $I_{c}$ could be entirely controlled by the phase, intensity, and detuning of the driving light. Additionally, the supercurrent magnitude displays a Fano profile with the increase of the driving light intensity, which is clearly understood by comparing the level splitting of the quantum dot under light driving and the superconducting gap. Moreover, when two such superconducting transistors form a loop, they make up a light-controlled superconducting quantum interference device (SQUID). Such a light-controlled SQUID could demonstrate the Josephson diode effect, and the optimized non-reciprocal efficiency achieves up to $54\%$, surpassing the maximum record reported in recent literature. Thus, our feasible scheme delivers a promising platform to perform diverse and flexible manipulations in superconducting circuits.

Figures (4)

• Figure 1: (a) Two superconductors are linked by a light-driven quantum dot with two levels, whose levels are illustrated in (b). (c) The change of critical current $I_{c}$ with the the driving strength $\mathcal{E}_{\mathrm{d}}$ and the light detuning $\delta\omega$. (d) The critical current $I_{c}$ as the driving strength $\mathcal{E}_{\mathrm{d}}$ increase shows the Fano profile with the fixed light detuning $\mathrm{\delta\omega=0,0.4,0.8,1.2\Delta_{\mathrm{s}}}$. Hereafter, the same coupling strength always are set as $\Gamma_{\mathrm{L}}=\Gamma_{\mathrm{R}}=\Gamma=0.1\Delta_{\mathrm{s}}$.
• Figure 2: (a) The level distribution of the superconducting transistor setup in the rotating frame. (b) The split of energy levels $E_{+}$ (dashed line) and $E_{-}$ (dashdotted line) of QD with the driving strength $\mathcal{E}_{\mathrm{d}}$ increase. The intersections of the purple and gray dotted lines corresponds to $E_{\pm}=\Delta_{\mathrm{s}}$. The Fano profile of critical current is because the flow direction of Cooper pairs reverses the back [from lead-R to lead-L in (c)] and forth [from lead-L to lead-R in (d)] between two superconducting leads with increasing driving strength.
• Figure 3: The supercurrent through superconducting quantum interference device controlled by the coherent light (a) and the magnetic field (b). (c) The total phase-asymmetric current (red solid line) is sum of the phase-symmetric current (blue dotted line) through QD-$1$ and the phase-symmetric current (light blue dashed line) through QD-$2$ with the driving strength $\mathcal{E}_{\mathrm{d},1}=1.0\Delta_{\mathrm{s}}$ and $\mathcal{E}_{\mathrm{d},2}=1.1\Delta_{\mathrm{s}}$. (d) Diode efficiency $\eta$ versus driving strength $\mathcal{E}_{\mathrm{d},1}$ and $\mathcal{E}_{\mathrm{d},2}$ for QD-$1$ and QD-$2$. The optical phase difference and light detuning have set: $\varDelta\phi_{\mathrm{d}}=\pi/10$ and $\delta\omega=0$.
• Figure 4: The energy level (a) of the two superconducting leads connected by a light-controlled quantum dot in Eq. (\ref{['eq:total-H-light']}) by the unitary transformation $\mathbf{U}(t)$ becomes the effective energy level (b) with the reduced lower level $\bar{\Omega}_{\mathrm{g}}=\Omega_{\mathrm{g}}-\mu_{\mathrm{L}}$and upper level $\bar{\Omega}_{\mathrm{e}}=\Omega_{\mathrm{e}}-\mu_{\mathrm{R}}$, and the phase $\bar{\varphi}_{\mathrm{R}}=\varphi_{\mathrm{R}}+2\phi_{\mathrm{d}}$ of the right superconducting of (\ref{['eq:U-H']}) in the rotation representation. Plotted (c) shows that the main contribution of the integrand $\rho(\nu)$ in Eq. (\ref{['eq:intergrand']}) to the integral is concentrated between $-1.2\Delta_{\mathrm{s}}$ and $-\Delta_{\mathrm{s}}$ when the driving strength $\mathcal{E}_{\mathrm{d}}$ changes from $0.3\Delta_{\mathrm{s}}$ to $1.8\Delta_{\mathrm{s}}$, which is shown more clearly in the inset.