Quantum Leaps Revealed: Three Clear Steps Define How Observation Collapses Reality

Scientists have long sought to understand the precise mechanisms governing quantum jumps, the seemingly instantaneous transitions between quantum states induced by measurement. Barkay Guttel, Danielle Gov, and Noam Netzer, from the Department of Condensed Matter Physics at the Weizmann Institute of Science, alongside Uri Goldblatt et al., now demonstrate that these jumps do not arise from a smooth shift in behaviour, but rather through a series of three clearly defined dynamical transitions. Their research, focused on a continuously monitored qubit, reveals a cascade beginning with the suppression of coherent oscillations, followed by state freezing, and ultimately the counterintuitive quantum Zeno effect, in which increased measurement inhibits decay. Significantly, the team discovered that decoherence doesn’t simply obscure these transitions, but actively reshapes the system’s behaviour, altering the expected order of these phases and providing a new understanding of measurement-induced transitions in quantum systems. Abrupt dynamical transitions define the onset of continuous quantum jumps in many-body systems Scientists have uncovered a cascade of three distinct dynamical transitions marking the emergence of continuous quantum jumps in a continuously monitored superconducting qubit. Quantum jumps, representing the collapse of a quantum system upon measurement, are a fundamental consequence of quantum mechanics, and recent experiments have demonstrated their continuous nature. However, the precise crossover from coherent dynamics to measurement-dominated behaviour has remained an open question. This work details the tuning of measurement strength in a superconducting qubit, revealing that quantum jumps do not arise from a gradual shift, but instead through a series of abrupt changes in the system’s behaviour. The initial transition manifests as an exceptional point where coherent oscillations cease, giving way to jumps towards a stable quantum state. Further increasing the measurement strength leads to dynamical state freezing, where the qubit’s dwell time near this stable state diverges, indicating a prolonged period of stability before a measurement event. A third, critical threshold signals entry into the quantum Zeno regime, a paradoxical scenario where increased measurement actually inhibits the natural tendency of the system to relax. Strikingly, researchers found that decoherence, a process that typically degrades quantum information, does not simply obscure these transitions but fundamentally restructures the dynamical phase diagram, even inverting their expected order.

This research maps measurement-induced transitions in a monitored qubit, demonstrating that the interplay between coherent driving, measurement, and decoherence generates a hierarchy of distinct dynamical phases. The study employed a superconducting circuit comprising a system qubit coupled to an ancillary detector qubit, allowing precise control over the measurement strength by varying the amplitude of a driving signal. By combining binary detector click records with conditional quantum state tomography, the team reconstructed the qubit’s dynamics, revealing features obscured in ensemble averages. These findings elucidate the emergence of continuous quantum jumps and provide a detailed map of dynamical phases within a monitored qubit system. Reconstructing qubit dynamics via conditional tomography and variable measurement strength offers improved state characterization A 72-qubit superconducting processor underpins this work, enabling precise control and continuous monitoring of a qubit’s quantum state. Researchers employed binary detector click records coupled with conditional quantum state tomography to reconstruct the qubit dynamics contingent on specific measurement outcomes. This technique facilitated the observation of dynamical features obscured in ensemble averages, revealing subtle transitions in the system’s behaviour. The experimental setup involved tuning the measurement strength of the continuously monitored qubit, allowing for the observation of quantum jumps and the identification of distinct dynamical transitions. Initially, the qubit was driven coherently while simultaneously subjected to continuous measurement. By varying the measurement strength, the researchers mapped the crossover from coherent oscillations to measurement-dominated behaviour. Tomographic data, specifically the excited state population, was gathered as a function of no-click sequence duration and measurement strength, revealing a critical point at λobs 1 = 0.99 ±0.01 where oscillatory behaviour abruptly ceased. One-dimensional cuts of the data below and above this transition confirmed the shift to jump-like dynamics, aligning with simulations incorporating decoherence. Further analysis involved histograms of no-click durations, initialising the qubit in the |1⟩ state to minimise errors in long intervals. These histograms were fitted using a sum of three complex exponentials, providing additional confirmation of the observed transition. The integration time for single measurements was consistently maintained at Tint = 320ns, with a coherent drive strength of ΩS/2π = 100kHz throughout the experiments. Prior to tomographic measurement, a verification step ensured the system remained within the qubit manifold, enhancing the accuracy of the results. This meticulous approach elucidated the emergence of continuous quantum jumps and mapped the dynamical phases within the monitored qubit. Exceptional points, state freezing and prolonged residence in monitored qubit dynamics reveal underlying quantum phenomena Researchers observed a cascade of three distinct dynamical transitions while tuning the measurement strength of a continuously monitored qubit. The first transition manifested as an exceptional point where coherent oscillations abruptly ceased, giving way to jumps towards a stable eigenstate. This cessation of oscillations marked a fundamental shift in the qubit’s behaviour as measurement influence increased. Beyond this point, the system began to evolve deterministically towards a specific eigenstate, characteristic of continuous quantum jumps. The second transition identified was the onset of dynamical state freezing, where the qubit’s dwell time near the eigenstate diverged. This divergence indicates that trajectories frequently reached and remained near the eigenstate before a detector click occurred, effectively halting further evolution for extended periods. Consequently, the system exhibited a prolonged residence within a specific quantum state, a phenomenon termed dynamical freezing. A third threshold then signalled entry into the quantum Zeno regime, where stronger measurement paradoxically suppressed relaxation. In this regime, increased monitoring slowed the natural tendency of the qubit to decay from its excited state. Analysis combining detector click records with conditional quantum state tomography revealed these dynamical features. Researchers reconstructed the qubit’s dynamics conditioned on specific measurement outcomes, allowing for detailed observation of the transitions. Notably, decoherence did not blur these transitions but fundamentally restructured the dynamical phase diagram, inverting their order as predicted by theoretical models. These results map measurement-induced transitions in a monitored qubit, demonstrating that the interplay between coherent driving, measurement, and decoherence gives rise to a hierarchy of distinct dynamical phases. Decoherence reverses dynamical transitions governing quantum jump emergence and leads to classicality Researchers have identified a cascade of three distinct dynamical transitions governing the emergence of quantum jumps in a continuously monitored system. These transitions delineate the shift from coherent quantum behaviour to a regime dominated by measurement, revealing a more nuanced process than previously understood. Initial investigations revealed an exceptional point where oscillations cease, followed by dynamical state freezing characterised by extended dwell times, and finally entry into the quantum Zeno regime where increased measurement suppresses relaxation. Notably, the inclusion of decoherence fundamentally alters the expected order of these transitions, inverting the initial two critical points. This restructuring of the dynamical phase diagram demonstrates that decoherence does not simply obscure the transitions but actively reshapes the system’s behaviour. The observed critical measurement strengths differ from ideal theoretical predictions due to the finite waiting time of the detector, introducing non-Hermitian dynamics and lowering the thresholds for these transitions. Acknowledging potential residual systematic errors in calibration and data analysis, future research could employ additional probes such as click counting fields or trajectory-resolved entropy production to uncover further structural details within these dynamical phases. The cascaded emergence of measurement-induced dynamics is expected to extend beyond single-qubit systems, potentially impacting areas like measurement-enhanced entanglement generation and measurement-induced phase transitions in more complex quantum systems. 👉 More information 🗞 Unravelling the emergence of quantum jumps in a monitored qubit 🧠 ArXiv: https://arxiv.org/abs/2602.02672 Tags: