Researchers Achieve 96.7%-Fidelity Bell States Using Quantum Cellular Automata

Scientists are tackling the escalating challenge of coherent control in increasingly large quantum devices, and a new study demonstrates a promising solution using quantum cellular automata (QCAs). Ryan White, Vikram Ramesh, and Alexander Impertro, from the University of Chicago and the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, alongside et al., have successfully realised QCAs on a dual-species Rydberg array of rubidium and cesium atoms. This innovative approach bypasses complex individual qubit control by utilising a static array and global operations, allowing the researchers to generate highly entangled states , including GHZ states, Bell states with 96.7(1.7)% fidelity, and even 17-qubit cluster states , with remarkably simple pulse sequences. The versatility and scalability of this QCA implementation offers a compelling pathway towards building larger, more manageable quantum systems and opens exciting new avenues for exploring complex quantum dynamics.

Dual-species Rydberg arrays enable scalable quantum control

This breakthrough addresses the escalating challenge of coherently controlling increasingly large quantum devices, offering a pathway to scalability by utilising only static qubit arrays and global control operations. The research team achieved this by leveraging independent global control of each atomic species, enabling the performance of a myriad of quantum protocols with simple pulse sequences. Information is encoded in ground-Rydberg qubits, prepared using optical pumping and excited with species-selective lasers, while a nearest-neighbor Rydberg blockade is exploited to implement controlled-rotation unitaries. Crucially, the researchers implemented dual-species rearrangement using acousto-optic deflectors, creating defect-free arrays and enabling precise control over qubit interactions.
This innovative combination of techniques allows for the exploration of many-body dynamics and the generation of complex entangled states using a minimal set of global controls. This work establishes QCAs as a powerful tool for scaling quantum information systems, offering a compelling alternative to architectures requiring individual qubit addressing. By investigating quasiparticle dynamics in a PXP automaton, the team explored deviations from integrable regimes, gaining insights into complex quantum behaviours. These experiments highlight that a minimal set of global controls suffices to realise a plethora of advanced quantum protocols, opening new doors for globally-controlled explorations of quantum information and simulation.

Researchers utilised AC Stark shifts from acousto-optic deflector-generated tweezers to strongly detune the Rydberg state at selected sites, preventing resonant excitation and providing a means for selective qubit measurement. Rabi oscillations between ground and excited states were driven by species-selective lasers, demonstrating the precise control achievable over individual qubits within the array. The team’s ability to create and manipulate these complex quantum states with simple pulse sequences represents a significant step towards building larger, more robust quantum computers, and offers new perspectives on quantum many-body dynamics, including phase transitions and quantum chaos.

Dual-species Rydberg arrays for scalable quantum computation offer

This work realizes cellular automata (QCAs), a promising framework circumventing conventional control challenges, by leveraging independent global control of each atomic species to execute a diverse range of protocols. This versatility and scalability demonstrate compelling routes for scaling information processing with global controls and offer new insights into dynamic processes. To meticulously characterise the system, researchers performed numerical simulations employing the quantum trajectories method on systems of up to 11 atoms, comprising 6 rubidium and 5 cesium, modelling each atom as a three-level system. The dynamics of each trajectory were computed exactly using sparse-matrix methods, incorporating off-resonant scattering from the intermediate state, Rydberg-state decay, laser phase noise, and fluctuations in both atomic position and laser intensity.

Decay from both excited states was assumed to occur via a single channel into ground states outside the three-level manifold, while laser phase noise was modelled as white frequency noise with amplitudes determined by frequency-averaged power spectral densities. The study pioneered tensor-network methods to model the preparation of a 17-site cluster state, a task beyond the reach of exact numerical simulations; these simulations were conducted without noise to isolate the effects of finite Rydberg blockade strength, next-nearest-neighbor interactions, and detuning of the intermediate state. Scientists defined a unitary step for the PXP automaton, assuming infinite nearest-neighbor interactions and vanishing interactions beyond this range, utilising two pulses to define a Floquet step with a system-size-independent cycle repeating every three steps. Experiments employed a precise measurement approach to track staggered magnetization, calculated as the difference between rubidium and cesium atom averages, to assess the impact of van der Waals interactions versus noise on population damping. Simulations of vacuum evolution on an 11-atom chain were compared with experimental data, revealing a decay timescale of 141(10) π-pulses due to van der Waals interactions, reduced to 68(5) pulses with the inclusion of the noise model, and observed experimentally as 17(1) pulses. This detailed analysis enabled the team to refine their understanding of the system’s behaviour and optimise performance.

Rydberg Array Realises High-Fidelity Entangled States for quantum

The team successfully implemented universal dynamics using a static qubit array and global control operations, bypassing traditional scaling challenges in coherent control. Researchers meticulously tracked the dynamics of quasiparticles within the QCA, initializing states with varying numbers of domain walls to observe their behaviour. Measurements confirm that single domain wall states on 11-atom arrays exhibit linear motion and reflection after application of up to 21 π-pulses, with the position histogram revealing a clear trajectory. When two quasiparticles collided, scientists recorded a modification in their trajectory, requiring only two π-pulses for a single step compared to the three π-pulses needed for independent particles.

This interaction provides insight into the system’s non-integrable behaviour. Tests prove that deviating from precise π-pulses induces non-integrable behaviour, causing a rapid saturation of the quasiparticle number in 15-atom chains. Data shows that increasing the deviation from π results in an increasingly fast saturation, as observed through exponential fits to the measured quasiparticle proliferation. Analysis of a 35-atom array at 6 π-pulses revealed a clear upward shift in the quasiparticle number distribution as the rotation angle deviated from π, with a minimal shift observed at the ideal π angle.

The breakthrough delivers a method for growing GHZ states by initializing a single Cs atom in the |+⟩ state and applying PXP unitary steps. Scientists observed a light cone forming in the Rydberg population, consistent with the expected behaviour of a GHZ state, with a peak population of approximately 50%. Fidelity measurements, using population and parity measurements, verified entanglement over multiple time steps, achieving results with up to 4 single-species qubits and 5 dual-species qubits, a significant step towards scalable quantum information processing.

Dual-Species Rydberg Arrays Generate Complex Entanglement networks

The dual-species platform is particularly well-suited for implementing these automata due to the ease of global control afforded by laser-driven Rydberg transitions and independent control of the two atomic species. Authors acknowledge current limitations include restriction to one-dimensional atomic chains and coherence times measured in microseconds, though they suggest extending protocols to two-dimensional arrays with modified trapping geometries and laser shaping. Future research directions include exploring many-body dynamics, quantum chaos, and quantum many-body scars, as well as variational approaches to quantum state preparation and combinatorial optimisation, potentially leading to measurement-free error correction and the study of measurement-induced phase transitions.