Certifying Multilevel Coherence in the Motional State of a Trapped Ion

TL;DR

This paper experimentally certifies the superposition of three motional states in a trapped ion, using a robust interference-based method that does not require perfect control, advancing quantum coherence verification techniques.

Contribution

It introduces a robust, interference-based scheme to certify multilevel coherence in a trapped ion's motional state, suitable for noisy quantum devices.

Findings

Successfully certified three-level superposition in a trapped ion

Demonstrated robustness against imperfect operations

Provided a method to infer minimum superposition levels from interference patterns

Abstract

Quantum coherence is one of the clearest departures from classical physics, exhibited when a system is in a superposition of different basis states. Here the coherent superposition of three motional Fock states of a single trapped ion is experimentally certified, with a procedure provably robust against imperfect operation. As the motional state cannot be directly interrogated, our scheme uses an interference pattern generated by projective measurement of the coupled qubit state. The minimum number of coherently superposed states is inferred from a series of threshold values based on analysis of the interference pattern. This demonstrates that high-level coherence can be verified and investigated with simple, nonideal control methods well-suited to noisy intermediate-scale quantum devices.