Quantum Computers Distinguish Synthetic Unravelings, Revealing Dynamics Beyond Ensemble Averages

Scientists are increasingly exploring how quantum trajectories, the individual realisations of a quantum system’s evolution, can reveal more than just average behaviour. Eloy Piñol, Piotr Sierant, and Dustin Keys, alongside colleagues including Romain Veyron, Miguel Angel García-March, and Tanner Reese, demonstrate a novel method for distinguishing between different ways of ‘unravelling’ the same quantum dynamics , essentially, different interpretations of how a system evolves at a granular level. Their research, detailed in a new paper, introduces synthetic unravelings implemented on quantum computers using one and two qubits, showing that subtle differences in these trajectories can be detected through measurements of variance and von Neumann entropy. This is significant because it provides an experimentally accessible demonstration, using IBM superconducting qubit hardware, that trajectories encode information about measurement backaction beyond what is predicted by standard quantum theory, potentially opening new avenues for quantum sensing and control.

The study unveils a digital approach to explore the consequences of different measurement schemes on quantum systems, circumventing the limitations of traditional optical experiments. They implemented protocols on superconducting-qubit hardware, meticulously combining circuit design, readout-error mitigation, and classical post-processing techniques to accurately capture the Stochastic dynamics of the system. This work establishes that while the average behaviour of a quantum system is independent of the chosen unraveling, the detailed statistics of individual trajectories are not.

The research team designed two unravelings that, despite producing the same overall evolution of the quantum state, generate different distributions of conditional states along each trajectory. By analysing these distributions, they were able to demonstrate that the choice of measurement scheme fundamentally alters the information encoded in the quantum trajectories. Furthermore, the successful implementation on IBM Quantum hardware highlights the potential of near-term quantum computers for exploring fundamental questions in quantum mechanics. Combining repeated circuit executions, the researchers were able to characterise the unravelings and provide a clear demonstration of measurement backaction effects that are invisible to linear average dynamics. This achievement opens avenues for investigating more complex quantum phenomena and developing new quantum technologies that exploit the subtle interplay between measurement and quantum evolution, potentially leading to advancements in quantum sensing and information processing.

Quantum Trajectories in Superconducting Qubit Systems reveal fascinating

Scientists engineered a novel experimental platform to investigate the subtle differences between quantum trajectories generated by distinct unravelings of the same master equation. The study pioneered a digital approach, translating the complexities of resonance fluorescence, traditionally demanding high experimental precision, into a controllable superconducting-qubit system provided by IBM. This innovative design allowed for the direct comparison of trajectory-level behaviour, circumventing the limitations of optical setups. Experiments employed precisely calibrated single- and two-qubit gates to enact the projective measurements and random unitary kicks, effectively simulating the stochastic processes inherent in different detection schemes.

Crucially, the team harnessed the capabilities of the superconducting qubit hardware to access and record individual quantum trajectories, capturing the conditional states at each step of the evolution. Data collection involved repeated execution of the circuits, 1024 repetitions for the 1Q protocols, to build up statistically significant ensembles of trajectories for each unraveling. The technique reveals that nonlinear functions of the conditional state are essential for detecting these subtle differences, confirming theoretical predictions about the role of trajectory ensembles in quantum mechanics. This accessible demonstration provides a powerful tool for exploring the foundations of quantum measurement and information.,.

Unravelings reveal measurement backaction in quantum trajectories, fundamentally

Scientists have demonstrated that distinct intervention schemes can generate different stochastic quantum trajectories, even when sharing identical unconditional dynamics. This research unravels the essence of Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) master equations, revealing that trajectory-ensemble averages of functions linear in the conditional state are entirely determined by the unconditional density matrix. However, applying nonlinear functions before averaging yields unraveling-dependent results, exposing measurement backaction beyond the average evolution. Experiments conducted on superconducting-qubit hardware provided by IBM Quantum successfully accessed trajectory-level information through a combination of circuit design, readout-error mitigation, and classical post-processing.

Crucially, the unconditional state and ensemble-averaged expectation values linear in the state remained identical across both unravelings, confirming the sensitivity of the measurements to trajectory-specific information. Measurements confirm that the trajectory variance provides a clear signal of measurement backaction, a phenomenon invisible to traditional linear average dynamics. The study meticulously implemented protocols on superconducting qubits, achieving access to detailed trajectory-level data. The team measured the variance across trajectories, revealing a quantifiable difference between the projective and random-unitary unravelings.

Specifically, the observed variance served as a key indicator of the distinct stochastic processes occurring within each unraveling. This divergence is directly attributable to the differing measurement backaction inherent in each unraveling scheme. This work provides a platform for investigating the fundamental interplay between measurement, dynamics, and information in quantum systems, with potential applications in quantum sensing and control.,.

Unravelings shape quantum trajectories differently, influencing measurement outcomes

Scientists have demonstrated that distinct measurement schemes, termed ‘unravelings’, can generate differing stochastic trajectories even when the overall average dynamics remain identical. This research focuses on the subtle differences arising from how quantum systems evolve when observed, highlighting that the method of observation, the unraveling, can influence trajectory-level statistics beyond what is captured by average system properties. The key finding is that while both unravelings yield the same average system evolution and identical averages of linear functions of the system’s state, they produce demonstrably different nonlinear trajectory statistics. The authors acknowledge that accessing and analysing individual trajectories requires substantial experimental precision and that their current protocols are limited to relatively small quantum systems. Future work will likely focus on scaling these techniques to larger systems and exploring measurement-driven entanglement phenomena, such as measurement-induced phase transitions. This work provides an accessible experimental basis for investigating conditional physics and offers insights into the fundamental role of measurement in quantum mechanics.