$W$- and Dicke-state engineering using optimal global control in nearest-neighbor coupled ring-shaped qubit arrays

TL;DR

This paper develops optimal control methods for efficiently creating $W$ and Dicke states in a ring-shaped qubit array with nearest-neighbor interactions, demonstrating robustness and speed suitable for Rydberg-atom systems.

Contribution

It introduces two optimal control schemes leveraging symmetry and global optimization to prepare complex quantum states in a ring qubit array, improving speed and robustness.

Findings

Successful preparation of $W$ and Dicke states using both control schemes.

Control sequences are robust against typical errors.

Preparation times are shorter than coherence times in Rydberg systems.

Abstract

Motivated by a compelling need for time-efficient and robust schemes for quantum-state engineering in systems of neutral atoms in optical tweezers, we consider a ring-shaped array of qubits with nearest-neighbor Ising-type ($zz$) coupling and transverse ($x$ and $y$) global control fields. This system to a large extent mimics -- outside of the Rydberg-blockade regime -- a circular array of neutral atoms interacting through van-der-Waals type interaction. We investigate the preparation of $W$ and Dicke states in this system starting from the default initial state $|00\ldots 0\rangle$ using two different optimal-control approaches: (i) NMR-like pulse sequence, which consists of instantaneous (delta-shaped) control- and Ising-interaction pulses, and (ii) time-dependent control scheme, which entails shaped control pulses in the presence of always-on Ising interaction between adjacent qubits. By making use of the underlying dihedral symmetry of this system -- which allows one to use a symmetry-adapted computational basis with $\mathcal{O}(2^N / N)$ states in an $N$-qubit system -- and utilizing advanced global-optimization methods, we find optimal sequences of pulses for realizing $W$ and Dicke states within both approaches. In addition, we demonstrate robustness of these sequences against unavoidable control errors. Finally, using typical values of parameters in realistic Rydberg-atom systems, we show that our control schemes enable the preparation of the desired multiqubit states on time scales much shorter than the relevant coherence times of those systems.