# Pulsed Quantum-State Reconstruction of Dark Systems

## TL;DR

This paper introduces a new method for reconstructing the quantum states of systems that cannot be directly measured, using a controlled probe and pulse sequences to extract detailed quantum information.

## Contribution

It presents a novel pulse-based scheme for quantum-state reconstruction of dark systems, applicable to spins and oscillators, with robustness against noise.

## Key findings

- Successfully reconstructs dark spin states and harmonic oscillator Wigner functions
- Scheme is robust against slow noise in the probe
- Feasible in solid-state spins and trapped ions

## Abstract

We propose a novel strategy to reconstruct the quantum state of dark systems, i.e., degrees of freedom that are not directly accessible for measurement or control. Our scheme relies on the quantum control of a two-level probe that exerts a state-dependent potential on the dark system. Using a sequence of control pulses applied to the probe makes it possible to tailor the information one can obtain and, for example, allows us to reconstruct the density operator of a dark spin as well as the Wigner characteristic function of a harmonic oscillator. Because of the symmetry of the applied pulse sequence, this scheme is robust against slow noise on the probe. The proof-of-principle experiments are readily feasible in solid-state spins and trapped ions.