A quantum computer is like a pot of boiling water: particles are jostling around chaotically, and you need to calm them down to get a useful answer. In a new study, physicists have devised a way to cool a chain of 19 quantum spins (tiny magnets) right on a real device.
They used a trick reminiscent of a pendulum slowly coming to rest: they attach a "friction" to the system that dampens excess motion. It turns out that this friction works even with very large time steps — previously, it was thought that you had to move in tiny increments. The key is that the system has a single coldest state, and then it naturally rolls into it.
Tests on the Reimei quantum computer showed that even when errors sneak into the circuit, the chain still reaches the cold state. And if you carefully measure and subtract the errors, the result matches the ideal calculation. This means the method can be used to solve problems about materials and molecules.
as written by the authors arXiv:2601.08137
We demonstrate a dissipative protocol for ground-state preparation of a quantum spin chain on a trapped-ion quantum computer. As a first step, we derive a Kraus representation of a dissipation channel for the protocol recently proposed by Ding et al. [Phys. Rev. Res. 6, 033147 (2024)] that still holds for arbitrary temporal discretization steps, extending the analysis beyond the Lindblad dynamics regime. The protocol guarantees that the fidelity with the ground state monotonically increases (or remains unchanged) under repeated applications of the channel to an arbitrary initial state, provided that the ground state is the unique steady state of the dissipation channel. Using this framework, we implement dissipative ground-state preparation of a transverse-field Ising chain for up to 19 spins on the trapped-ion quantum computer Reimei provided by Quantinuum. Despite the presence of hardware noise, the dynamics consistently converges to a low-energy state far away from the maximally mixed state even when the corresponding quantum circuits contain as many as 4110 entangling gates, demonstrating the intrinsic robustness of the protocol. By applying zero-noise extrapolation, the resulting energy expectation values are systematically improved to agree with noiseless simulations within statistical uncertainties.
The experiment involved 4110 quantum gates — operations that entangle particles. That's like 4110 steps in a dance where every step must be precise, or the whole routine falls apart.
The idea of "cooling" a quantum system echoes the plot of the movie Inception, where characters gradually descend into deeper layers of a dream to reach the desired state.