Realization of a Binary-Outcome Projection Measurement of a Three-Level Superconducting Quantum System

Abstract

Learning the state of a multilevel quantum system without changing that state, as in quantum computing, is tricky. Ordinarily a dispersive measurement of such a system destroys all coherence. By exploiting the two excited states of a three-level system (qutrit), the authors show how to determine whether it is in the ground state or an excited state, while preserving coherence between the excited states. This $d\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e$ $m\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t$ can be used to detect leakage errors, and to test quantum contextuality, the critical resource behind the exponential speedup of a quantum computer.