Quantum superposition of three macroscopic states and superconducting qutrit detector

Abstract

Superconducting quantum coherent circuits have opened up a novel area of fundamental low-temperature science since they could potentially be the element base for future quantum computers. Here we report a quasi-three-level coherent system, the so-called superconducting qutrit, which has some advantages over a two-level information cell (qubit) and is based on the qutrit readout circuit intended to measure individually the states of each qubit in a quantum computer. The designed and implemented radio-frequency superconducting qutrit detector (rf SQUTRID) with atomic-size ScS-type contact utilizes the coherent-state superposition in the three-well potential with energy splitting $\ensuremath{\Delta}{E}_{01}/{k}_{B}\ensuremath{\approx}1.5\phantom{\rule{0.16em}{0ex}}$K at the 30th quantized energy level with good isolation from the electromagnetic environment. The reason why large values of $\ensuremath{\Delta}{E}_{01}$ (and thus using atomic-size Nb-Nb contact) are required is to ensure an adiabatic limit for the quantum dynamics of magnetic flux in the rf SQUTRID.