Quantum Computers Spoof Thermalization with Almost No Effort

We show that even simple quantum computers can mimic one of nature’s most mysterious processes, thermalization. In everyday life, hot objects cool down and reach equilibrium, losing all memory of how they started. A similar idea, called quantum thermalization, explains why isolated quantum systems appear random and “thermal” when many particles are involved. But recent experiments have revealed a deeper mystery: some quantum states stay random-looking after parts of them are measured, suggesting an unexpectedly persistent form of apparent randomness. This stronger effect, called deep thermalization, has typically been linked to extreme entanglement and enormous complexity at the heart of quantum matter. Our work shows that such extreme complexity may not be necessary. Instead, deep thermalization can arise computationally, i.e., from the limits of what observers can efficiently calculate, rather than from true physical chaos. Using only low-depth quantum circuits with minimal entanglement, we generate states that appear perfectly random to any realistic observer, including those equipped with a quantum computer. After measurement, these states still look indistinguishable from thermal ones. This discovery reveals that “thermal” behaviour can emerge from structured, low-complexity dynamics, implying that the universe can look chaotic and random, not because it truly is, but because no observer has the computational power to tell otherwise.

Publication: S. Chakraborty, S. Choi, S. Ghosh, and T. Giurgică-Tiron, Fast computational deep thermalization, Accepted in Physical Review Letters (2025). arXiv.