Experimental Methods for Studying Quantum Complexity

Chinese Researchers Slow Quantum Chaos Using 78-Qubit Processor Scientists at the Chinese Academy of Sciences have used their 78-qubit superconducting processor, Chuang-tzu 2.0, to directly observe and control a key transitional phenomenon in quantum systems known as prethermalisation. The work offers a new pathway to manage quantum decoherence—the core obstacle to scalable quantum computing. The Core Challenge In quantum systems, stored information naturally disperses through a process called decoherence. Once decoherence dominates, qubits lose their usable state information, undermining computational reliability. Modeling this process on classical computers is computationally infeasible for systems approaching 100 qubits due to the exponential growth of state space. Using Quantum Hardware as a Physics Laboratory Instead of simulating decoherence classically, the team used their quantum processor itself as a physical simulator. For large quantum systems, the processor effectively becomes an experimental platform to observe complex dynamical laws directly—analogous to a wind tunnel for aerodynamics. Discovery of the Prethermalisation Plateau The researchers observed an intermediate stage before full thermalisation: • A temporary plateau where quantum chaos is suppressed. • Information remains partially localized rather than fully scrambled. • Decoherence progression slows before complexity rapidly increases. This “prethermalisation plateau” creates a controllable time window during which quantum information can be utilized before it dissipates irreversibly. Control and Tunability Critically, the team demonstrated that this stage is not merely observable but adjustable: • Tailored control sequences altered both the duration and structure of the plateau. • Researchers were able to extend or shorten the prethermalisation phase. • This suggests active engineering of decoherence timelines may be feasible. Strategic Implications The findings matter for three reasons: Extending Coherence Windows Controlled prethermalisation could lengthen usable qubit lifetimes. Improving Error Correction Understanding how complexity spreads may inform better quantum error-correction architectures. Hardware as Fundamental Science Tool The experiment highlights a broader shift: quantum processors are becoming instruments for probing physics beyond classical computational limits. Perspective If decoherence is the central scaling barrier in superconducting quantum computing, then controllable prethermalisation introduces a new lever. Rather than merely fighting noise, engineers may be able to shape the temporal structure of quantum chaos itself. In a competitive global landscape, advances like this underscore how quantum hardware is evolving from prototype processors into platforms for exploring—and potentially mastering—the dynamics that limit quantum advantage.

In an international collaboration, researchers from BasQ, CERN, UAM–CSIC, the Wigner Research Centre for Physics, and IBM have simulated the real-time dynamics of confining strings in a (2+1)-dimensional Z2-Higgs gauge theory with dynamical matter, leveraging a superconducting quantum processor with up to 144 qubits and 192 two-qubit layers (totaling 7,872 two-qubit gates). This work tackles a longstanding challenge in high-energy physics: understanding the real-time dynamics of confinement in gauge theories with dynamical matter—a crucial aspect of non-perturbative quantum field theory, including quantum chromodynamics (QCD). Classical methods face fundamental limitations in simulating these dynamics, often requiring indirect approaches such as asymptotic in-out probes in collider experiments. Quantum processors, by contrast, now offer the opportunity to observe the microscopic evolution of confining strings directly, opening new pathways for studying these complex phenomena in real time. To accomplish this, matter and gauge fields were encoded into superconducting qubits through an optimized mapping onto IBM’s heavy-hex architecture. By exploiting local gauge symmetries, the team applied a robust combination of error suppression, mitigation, and correction techniques—including novel methods such as gauge dynamical decoupling (GDD) and Gauss sector correction (GSC)—enabling high-fidelity observations of string dynamics, supported by 600,000 measurement shots per time step. The results reveal both longitudinal and transverse string dynamics—including yo-yo oscillations and endpoint bending—as well as more complex processes such as string fragmentation and recombination, which are essential to understanding hadronization and rotational meson spectra from first principles. To predict large-scale real-time behavior and benchmark the experimental results, the study integrates state-of-the-art tensor network simulations using the basis update and Galerkin methods. Altogether, this paper marks a significant milestone in the quantum simulation of non-perturbative gauge dynamics, showcasing how current quantum hardware can be used to explore real-time phenomena in fundamental physics. paper is here https://lnkd.in/eD89BKqi

Last week, we shared exciting new results studying operator dynamics on structured circuits designed by our collaborators at Algorithmiq. Our experiments on up to 70 qubit, high-fidelity, heavy-hex layouts, with heuristic error mitigation methods, produced accurate results at short depths that were verified with classical simulation. At larger circuit depths (up to 1872 CZ gates), the circuits were seen to be challenging for Belief propagation-based tensor network methods in the Schrödinger picture, even at fairly large bond dimensions, while the experiments produced data points that were within theoretical bounds. These experiments were enabled, in part, by a 10x reduction in median 2Q error rates from the utility experiment — now at 0.101% in simultaneous operation across the layout! Thanks to our collaborators at Algorithmiq, Simons Foundation Flatiron Institute. We shared these results in the new open community Quantum advantage tracker (https://lnkd.in/eG6Ue3sg), that includes the theoretical background for the experiment, classical simulation and experimental details, run-times, open-source code, etc. This tracks progress towards observable estimation with rigorous error bounds, ground state problems with variational solutions, and problems with efficient classical verification, and also invites proposals for new advantage candidates! Looking forward to sharing upcoming results from experiments and simulations, as they roll in, in this new open "lab notebook". I hope this accelerates the feedback loop between quantum experiments and classical simulation, without boundaries, and ultimately advances the pace of scientific discovery.

By driving a quantum processor with laser pulses arranged according to the Fibonacci sequence, physicists observed the emergence of an entirely new phase of matter—one that displays extraordinary stability in a domain where fragility is the norm. Quantum computers operate using qubits, which differ radically from classical bits. A qubit can exist in superposition, occupying multiple states at once, and can become entangled with others across space. These properties enable immense computational power, but they come with a cost: quantum states are notoriously short-lived. Environmental noise, microscopic imperfections, and edge effects rapidly degrade coherence, limiting how long quantum information can survive. Seeking a new way to protect fragile quantum states, scientists at the Flatiron Institute, instead of applying laser pulses at regular intervals, they used a rhythm governed by the Fibonacci sequence—an ordered but non-repeating pattern long known to appear in biological growth, crystal structures, and wave interference. The experiment was carried out on a chain of ten trapped-ion qubits, driven by precisely timed laser pulses. The result was the formation of what is described as a time quasicrystal. Unlike ordinary crystals, which repeat periodically in space, a time quasicrystal exhibits structure in time without repeating in a simple cycle. The Fibonacci-based driving created a temporal order that resisted disruption, allowing the quantum system to remain coherent far longer than expected. The improvement was significant. Under standard conditions, the quantum state persisted for roughly 1.5 seconds. When driven by the Fibonacci pulse sequence, coherence times stretched to approximately 5.5 seconds—more than a threefold increase. Even more intriguing was the system’s temporal behavior. Measurements indicated that the quantum dynamics unfolded as if time itself possessed two independent structural directions. This does not imply time flowing backward, but rather that the system’s evolution followed two intertwined temporal pathways—an emergent property arising purely from the Fibonacci drive. The researchers propose that the non-repeating structure of the Fibonacci sequence suppresses errors that typically accumulate at the boundaries of quantum systems. By distributing disturbances in a highly ordered yet aperiodic way, the sequence stabilizes the collective behavior of the qubits. In effect, a mathematical pattern found throughout nature acts as a self-organizing error-management protocol. The findings suggest a powerful new strategy for quantum control. Rather than fighting noise solely with complex correction algorithms, future quantum technologies may harness structured patterns—drawn from mathematics and natural order—to achieve resilience at a fundamental level. https://lnkd.in/dVxp7R8J https://lnkd.in/dDVNRsPk