Researchers at the International Quantum Academy in Shenzhen, China, collaborating with Peking University and Southern University of Science and Technology, report achieving 98.7% fidelity in global quantum measurements, a crucial step toward building stable quantum computers. This high level of accuracy addresses a significant hurdle in quantum error correction, which requires essential mid-circuit measurements and feedback operations during computation. The team developed a superconducting quantum processor enabling not only these high-fidelity measurements but also fast conditional feedback with a 200 nanosecond latency. Utilizing this platform, the researchers demonstrated the coexistence of an absorbing-state transition in the quantum channel and a measurement-induced entanglement transition, demonstrating how adaptive quantum circuits provide a powerful platform for exploring non-equilibrium quantum many-body dynamics.
Quantum Processor for Mid-Circuit Measurement & Feedback
A superconducting quantum processor enables global mid-circuit measurement with an average quantum non-demolition (QND) fidelity of 98.7% and fast conditional feedback with a 200 nanosecond real-time decision latency. Utilizing this platform, the researchers demonstrate the coexistence of an absorbing-state transition in the quantum channel and a measurement-induced entanglement transition at the level of individual quantum trajectories. For the absorbing-state transition, they experimentally extract a set of critical exponents at the transition point, which is in excellent agreement with the directed percolation universality class. Crucially, the two transitions occur at distinct values of the tuning parameter. These results demonstrate that adaptive quantum circuits provide a powerful platform for exploring non-equilibrium quantum many-body dynamics. The newly developed processor utilizes 66 transmon qubits arranged in a 6×11 lattice, with a focused implementation leveraging 30 qubits to minimize signal interference and maximize operational fidelity.
The team reports realizing both an absorbing-state transition and a measurement-induced entanglement transition on the same platform, a feat previously hindered by the need for both a large qubit count and high-fidelity operations. Experimental extraction of critical exponents at the transition point is in excellent agreement with the directed percolation universality class. The team confirmed that the two observed transitions, the absorbing-state transition and the measurement-induced entanglement transition, occur at distinct parameter values, solidifying their separate physical origins. The processor’s performance is quantified by cumulative error rates of 0.07%, 0.7%, and 1.3% for synchronized SQG, iSWAP, and mid-circuit RO operations, respectively, showcasing the precision of the implemented control and measurement sequences.
Adaptive Quantum Circuits Enable Non-Equilibrium Dynamics
Beyond the established capabilities of quantum processors for tasks like simulation and optimization, a new area of research is emerging: the exploration of dynamic quantum systems far from equilibrium. Researchers from Peking University, Southern University of Science and Technology, and the International Quantum Academy are now leveraging adaptive quantum circuits, those incorporating mid-circuit measurements and feedback, to probe previously inaccessible regimes of quantum many-body physics. A key enabler of this progress is achieving high-fidelity measurements coupled with rapid feedback. The team reports demonstrating global mid-circuit measurement with an average quantum non-demolition (QND) fidelity of 98.7%. This combination is critical, as limitations in either area previously hindered the observation of subtle dynamic phenomena. The absorbing-state transition, a form of non-equilibrium criticality, exhibited critical exponents in excellent agreement with the directed percolation universality class. Importantly, the two transitions occur at distinct parameter values.
The experimental setup utilized a 30-qubit superconducting processor to minimize crosstalk and maximize fidelity. The adaptive circuit, consisting of alternating unitary layers and measurement-feedback loops, allowed for dynamic tracking of qubit activity. These results demonstrate that adaptive quantum circuits provide a powerful platform for exploring non-equilibrium quantum many-body dynamics.
Absorbing-State Transition & Directed Percolation Class
This research focuses not merely on building better qubits, but on understanding the fundamental physics governing their behavior, specifically in the context of absorbing-state transitions and their connection to the directed percolation (DP) universality class. The researchers are interested not just in the final result of a calculation, but in the process itself, and how measurements taken during the computation influence subsequent steps. The absorbing-state transition, in particular, is characterized by a shift from an active fluctuating phase to a target absorbing state, and the team has successfully extracted critical exponents at the transition point, which is in excellent agreement with the directed percolation universality class. The ability to observe these transitions, coupled with the high fidelity measurements, positions this platform as a powerful tool for probing the intricacies of non-equilibrium quantum dynamics.
Measurement-Induced Entanglement Transition
The ability to maintain quantum information long enough to perform useful calculations hinges on overcoming decoherence, and recent advances in quantum processors are providing new tools to probe the very nature of quantum transitions. This work isn’t simply about building more powerful quantum computers; it’s about understanding the fundamental behavior of quantum systems as they evolve under observation. A key focus of this research is the measurement-induced entanglement transition, where increasing the rate of measurement fundamentally alters the entanglement structure of a quantum system. Below a critical measurement rate, pc, states remain highly entangled, while above this threshold, repeated measurements collapse the system into a less entangled state. Simultaneously, the team investigated an absorbing-state transition, a phenomenon where the system is driven towards a specific, stable configuration. The researchers achieved 98.7% fidelity in global mid-circuit measurements, coupled with a remarkably swift 200 nanosecond real-time decision latency for feedback operations. The two transitions occur at distinct values of the tuning parameter. The researchers utilized a 30-qubit setup to study both the measurement-induced entanglement transition and the absorbing-state transition.
Critical Exponents at the Absorbing-State Transition Point
Beyond simply demonstrating a quantum system’s ability to undergo a phase transition, researchers are now pinpointing the precise characteristics of those transitions, revealing underlying principles of quantum dynamics. While phase transitions are commonplace in everyday materials, such as water freezing into ice, observing them within the fleeting, fragile realm of quantum systems presents a unique challenge. This combination is crucial; accurate mid-circuit measurements allow for adaptive control, while minimal latency prevents the quantum state from collapsing before feedback can be applied. The researchers focused on an absorbing-state transition, a type of non-equilibrium criticality where the system rapidly falls into a stable, inactive state above a certain threshold. Their findings indicate this transition is in excellent agreement with the directed percolation (DP) universality class, a well-established theoretical framework. The significance lies in the quantitative agreement between experimental results and theoretical predictions.
By varying initial conditions, the team found the extracted critical exponents matched those expected for DP, in excellent agreement with the universality class. The absorbing-state transition occurs at a demonstrably different parameter value than the measurement-induced entanglement transition, indicating their distinct physical origins.
Superconducting Qubit Lattice & Operation Fidelity
A 98.7% fidelity in global quantum measurements achieved by researchers signifies a substantial leap toward realizing practical quantum error correction. Mid-circuit measurements and feedback operations conditioned on the measurement outcomes are essential for implementing quantum error-correction on quantum hardware. The team’s processor features a 6×11 lattice of 66 transmon qubits, with a carefully selected subset of 30 utilized to minimize crosstalk and maximize performance during mid-circuit measurements. These measurements, taken during the computation itself, are crucial for implementing feedback loops that correct errors and guide the quantum evolution. This speed is essential for maintaining quantum coherence while applying corrective actions. The 30-qubit processor allowed for the observation of a clear separation between the critical points of the two transitions, confirming their distinct physical nature and paving the way for more sophisticated quantum simulations.
