Quantum Criticality Unlocked at Room Temperature

H Hannan

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Quantum Criticality Unlocked at Room Temperature
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The paper proposes a new method to detect quantum criticality (abrupt change in the ground state of a quantum system) at high temperatures. Typically, observing quantum criticality requires cooling the system to extremely low temperatures. The new method involves using a quantum probe spin with a long coherence time, coupled to the quantum system of interest. By applying spin echo control to the probe spin, the effects of thermal fluctuations can be eliminated, allowing the probe to detect quantum criticality in the coupled system even at high temperatures.

Quantum criticality is the abrupt change in the ground state of a quantum system as a parameter is tuned. It signals the emergence of new quantum phases and is important to understand the novel quantum matter. However, observing quantum criticality requires impractically low temperatures. This work shows quantum criticality can be detected using long-lived quantum probes, even at room temperature.

The method is first demonstrated on an exactly solvable transverse field Ising model. It is found that by measuring the spin echo signal of the probe over time, the quantum criticality of the Ising model can be observed even at infinite temperatures, seen as an enhanced decoherence of the probe spin at the critical point. This is because spin echo removes the effect of thermal noise, which is static while retaining the effect of dynamic quantum fluctuations which exhibit critical behaviour.

Further, a correspondence between the low temperature required in conventional techniques and the long coherence time required in the new technique is established – the coherence time acts like an inverse temperature. So quantum criticality requiring temperatures of nano- or pico-Kelvin in standard methods can now be probed at ordinary temperatures using coherence times of milliseconds or seconds.

The method is also verified on a more complex dipolar coupled spin ring model. Despite strong thermal noise, the spin echo of the probe spin still shows a sharp transition in decoherence at the critical point, indicating potential applicability to realistic systems.

Possible experimental realizations are proposed using systems like cold atoms in optical lattices, spins in solids like diamond or silicon, or trapped ions, where long-lived probe spins as well as tunable spin model systems exist or can be engineered.

In conclusion, harnessing long coherence times to suppress thermal noise provides a route to uncover hidden quantum critical phenomena in complex and macroscopic spin systems at attainable temperatures. Dynamical decoupling techniques can help further push the frontiers and study novel quantum matter.

Source research paper DOI10.1088/1367-2630/15/4/043032

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