Mainz 2026 – scientific programme
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Q: Fachverband Quantenoptik und Photonik
Q 78: Quantum Optics and Control III
Q 78.2: Talk
Friday, March 6, 2026, 14:45–15:00, P 3
Optimal Quantum States for Frequency Estimation Limited by Spontaneous Emission — •Marius Burgath1,2 and Klemens Hammerer2,1 — 1Institut für Theoretische Physik, Leibniz Universität Hannover, Germany — 2Institut für Theoretische Physik, Universität Innsbruck, Austria
We investigate ultimate bounds for frequency estimation with an ensemble of qubits subject to local spontaneous decay. For that purpose, a numerical maximization of the quantum Fisher information (QFI) over the initial state of the ensemble is carried out. For different numbers N of qubits, the QFI is maximized by very different state classes. In the regime of small N, unbalanced GHZ states with a correlated measurement and a nonlinear estimator perform close to optimal. Above a critical number of qubits, the QFI can be maximized by spin Gottesman-Kitaev-Preskill (Spin-GKP) states, which are the compact phase space analogues of the GKP states known from quantum error correction with a harmonic oscillator. The Spin-GKP states show a comb structure in phase space, and the performance of different phase space lattices is investigated. Spin-GKP-like states can also be created with a simple gate sequence. Two one-axis twisting gates and a rotation can be used to create Spin-GKP-like structures around the equator of the Bloch sphere to reach QFI values close to the ultimate bounds.
Keywords: frequency estimation; spontaneous emission; quantum Fisher information; Gottesman-Kitaev-Preskill; one-axis twisting