Researchers at our institute studied a new suite of protocols for measuring an elusive type of phase transition in systems of many interacting qubits.
Such systems can transition between macroscopically distinct phases, like the normal and superconducting phase of metals. A particularly interesting class of phase transitions affects the quantum entanglement of qubits which interact randomly with each other and the environment. These "entanglement transitions" are interesting because they can be experimentally tested on noisy near-term quantum computing platforms. Unfortunately, quantum entanglement is notoriously hard to detect, and the randomness makes it even harder; this "postselection problem" thwarts straightforward observations of entanglement transitions.
In our publication [1], we focus on a particular model which has been shown to feature an entanglement transition. We combine known methods for bounding quantum entanglement with sophisticated quantum error correction algorithms to devise new protocols for measuring upper and lower bounds on the (inaccessible) entanglement transition. Numerical simulations for up to 40 qubits demonstrate the effectiveness of these protocols. Crucially, the protocols scale to large systems and simulations demonstrate their robustness against certain experimental imperfections. Since the deviation of upper and lower bound only depends on the imperfections, less noisy experiments yield sharper bounds. These results pave the way for experimental observations of entanglement transitions on noisy, near-term quantum computers and simulators.
[1] Phys. Rev. Lett. 136, 140403 (2026)