Dicke states $|J=N/2,m\rangle$ of a collection of $N$ atoms were central to Dicke's theory of superradiance. Except for the fully polarized end states, they are entangled many-body states; in particular, the single-excitation Dicke state is the W state well known in quantum information science. However, deterministic preparation of Dicke states with a prescribed spin projection remains challenging, especially without relying on postselection or heralding. Here we present a detuning-programmed Hamiltonian protocol using atoms or qubits in a dispersive cavity subject to a coherent transverse drive. In the absence of the drive, the off-resonant cavity produces an effective collective-spin interaction. By combining this cavity-mediated interaction with the coherent drive, tuning the atom--drive detuning enables the protocol, in principle, to target any allowed Dicke state along the symmetric Dicke ladder. Starting from the transverse-drive ground state, adiabatic ground-state interpolation prepares the selected Dicke state by ramping down the drive strength while ramping up the cavity-mediated interaction. After preparation, tuning the cavity into resonance with the atoms or qubits provides a direct way to probe the collective-emission response of the prepared Dicke state. We discuss implementation with superconducting circuit QED and show that the prepared states, especially the central Dicke state with $m=0$, provide resources for quantum sensing with Heisenberg-limited scaling.
Dicke states are multipartite entangled states in which a fixed number of quantum excitations is coherently shared among many qubits. Originally introduced in the context of cooperative emission and superradiance, they are now important resources for quantum sensing, networking, and collective quantum phenomena. Preparing prescribed Dicke states with high fidelity, however, remains challenging, particularly as the system size and excitation number increase. Here we present an open dataset and accompanying code for preparing Dicke states using a collision-based quantum protocol. The dataset covers systems from five to fourteen qubits over a broad range of excitation numbers and records how the best-found noiseless preparation fidelity changes with circuit depth. It also provides circuit-resource estimates and experimental measurements for selected states on the 54-qubit IQM Emerald superconducting processor. The accompanying code reproduces the processed data and validation checks, providing a reusable benchmark for studying the trade-off between state-preparation fidelity, circuit cost, and hardware noise.
Duc-Kha Vu, M. Nguyen, Ş. K. Özdemir et al.· 0 citations
Efficient, deterministic, and high-fidelity preparation of large Fock states is essential for scaling bosonic quantum technologies and exploring quantum phenomena at large excitation energies. We introduce a deterministic one-parameter (D1p) protocol that maps Fock-state preparation in an infinite-dimensional Hilbert space onto two-dimensional amplitude amplification. Starting from a coherent state with $|\alpha|\simeq\sqrt{n}$, the initial target-state population scales as $n^{-1/2}$, yielding an iteration count and circuit depth of $\mathcal{O}(n^{1/4})$. Phase matching guarantees unit fidelity in the ideal model; remarkably, preparing $|{10^6}\rangle$ requires only 39 iterations. The protocol uses only displacements and number-selective phase operations, requires no numerical optimization, and further extends to state transfer, general superpositions, finite-dimensional systems, and multipartite entangled states. In the large-amplitude regime, its multi-target form prepares $L$-legged cat states with an iteration count determined only by $L$; cats with up to ten legs require only two iterations, independent of the coherent-state amplitude. This framework provides a broadly applicable route to highly excited bosonic states on platforms supporting these elementary controls.
The quantum superpositions of coherent states offer an alternative to the conventional qubit-based encodings by harnessing the large Hilbert space available in bosonic modes, including those realised in microwave and optical cavities, magnons, and mechanical resonators. Beyond their advantages for local information processing, establishing long-distance quantum networks for such bosonic states is crucial for scalable quantum communication and distributed quantum computation. In this work, we propose an entanglement-swapping-based protocol to generate a bipartite magnonic cat state shared between spatially separated subsystems. Each subsystem comprises a hybrid architecture consisting of a superconducting transmon qubit coupled to a yttrium iron garnet (YIG) sphere that supports magnon modes. By performing a projective Bell-state measurement on the qubits, the initially established magnon-qubit entanglement is coherently transferred to the remote magnon modes, resulting in a nonlocal magnonic cat state. For experimental characterisation of the gener- ated states, we perform quantum state tomography through reconstruction of the Wigner function using joint displaced parity measurements of the magnon modes. Our scheme provides a feasible route towards realising long-distance magnonic entanglement and contributes to the advancement of hybrid quantum network architectures.
Urmimala Dewan, Roson Nongthombam, S. Kalita et al.· 0 citations
Quantum circuits with measurements and unitary feedback (MF) can prepare long-range entangled states in constant depth, but a systematic construction of the MF preparation circuit for a given target state remains underexplored. We develop such a scheme for one-dimensional states, based on the notion of pushable defects: virtual-bond operators of a matrix product state that can be pushed through the tensor at the price of a physical feedback unitary. We show that the set of pushable defects, together with their pushing relations classifies finite-depth MF-preparable states and dictates their preparation circuits. To each class of the target state $|A\rangle$, we associate a state $|B\rangle$ from which $|A\rangle$ can be prepared using a 1-round MF circuit; in particular, $|A\rangle$ is preparable from a product state using a circuit with 1 round of MF whenever $|B\rangle$ is preparable by a finite-depth local unitary (FDLU) circuit. For a general target state, the scheme is obtained by iterating this procedure until the associated state is FDLU-preparable. For open-boundary matrix product states, the scheme is complete: it constructs a preparation circuit whenever finite-depth MF preparation with left-conditioned feedback corrections is possible. Pushable defects and pushing relations thus emerge as a unifying principle for quantum state preparation via measurements and feedback. This characterization further reveals an intrinsic connection between MF circuits and non-invertible symmetries: states with certain classes of pushing relations are related to a product state by Tambara-Yamagami duality operators, or by continuous cosine symmetry operators with fusion rules $L_{\alpha} L_{\alpha'} = L_{\alpha+\alpha'} + L_{\alpha-\alpha'}$, together with their generalizations up to (not necessarily transversal) gates.
The tripartite quantum Rabi model couples two qubits to a bosonic mode through a collective spin-oscillator interaction, providing a simple setting for studying two-qubit entanglement. In the zero-detuning limit, the triplet part of the spectrum splits into branches with zero and maximal entanglement, while the antisymmetric singlet ladder remains exactly decoupled. Within the triplet sector, finite detuning turns the crossings between these branches into avoided crossings and redistributes this entanglement. We identify an eigenbranch whose entanglement grows from nearly zero to a nearly maximal value through such avoided-crossing mixing. The weak-coupling level ordering yields a simple analytic criterion for whether this eigenbranch has a separable weak-coupling endpoint. A three-state effective model explains how the Bell-state component becomes dominant as the coupling increases. We further use a finite-time linear ramp of the collective coupling to benchmark the final coupling and ramp time required for high Bell-state fidelity. These results show how collective spin-oscillator coupling reorganizes spectral entanglement and connects static branch structure to finite-time Bell-state preparation.
Li-Li Gao, Zi-Xuan Chen, Longlong Li et al.· 0 citations
Dissipation, though often detrimental to quantum entanglement, can be manipulated for the preparation of entangled states, wherein ingeniously designed quantum jump processes drive the system toward the desired steady state. Here we venture beyond this paradigm, and demonstrate a new type of entanglement generation in dissipative quantum dynamics. Combining driven-dissipative steady-state engineering and adiabatic passage, we propose a general protocol where the final entangled state depends on the chirality of the evolution path in the parameter space, a scheme that is further extendable to multipartite entanglement. By simulating the Liouvillian dynamics through the quantum Langevin equation for a pair of photons, we experimentally confirm the noise-resistant chiral preparation of various entangled states with high fidelity and concurrence. Our work establishes parametric chiral dynamics as a scalable and robust tool for controllable entanglement generation, paving the way for its applications in quantum information.
Huixia Gao, Kong-Hao Sun, Yiwen Han et al.· 0 citations
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