Short-Depth Circuits for Dicke State Preparation

We present short-depth circuits to deterministically prepare any Dicke state |Dn,k>, which is the equal-amplitude superposition of all n-qubit computational basis states with Hamming Weight k. Dicke states are an important class of entangled quantum states with a large variety of applications, and a long history of experimental creation in physical systems. On the other hand, not much is known regarding efficient scalable quantum circuits for Dicke state preparation on realistic quantum computing hardware connectivities. Here we present preparation circuits for Dicke states |Dn,k> with (i) a depth of O(k log(n/k)) for All-to-All connectivity (such as on current ion trap devices); (ii) a depth of O(k sqrt(n/k)) = O(sqrt(nk) for Grid connectivity on grids of size Omega(sqrt(n/s)) x O(sqrt(ns)) with s<=k (such as on current superconducting qubit devices). Both approaches have a total gate count of O(kn), need no ancilla qubits, and generalize to both the preparation and compression of symmetric pure states in which all non-zero amplitudes correspond to states with Hamming weight at most k. Thus our work significantly improves and expands previous state-of-the art circuits which had depth O(n) on a Linear Nearest Neighbor connectivity for arbitrary k (Fundamentals of Computation Theory 2019) and depth O(log n) on All-to-All connectivity for k=1 (Advanced Quantum Technologies 2019).