Spin-echo and quantum versus classical critical fluctuations in TmVO$_4$

TL;DR

This study uses spin-echo NMR to distinguish quantum from classical critical fluctuations in TmVO$_4$, revealing that quantum fluctuations persist at high temperatures and affect nuclear spins differently than classical noise.

Contribution

It demonstrates that spin-echo techniques can selectively filter classical noise but not quantum fluctuations, providing a new way to visualize quantum critical behavior in materials.

Findings

Spin-echo filters out classical noise but not quantum fluctuations.

Quantum fluctuations persist at high temperatures near the critical point.

Quantum critical environment causes rapid decoherence in qubits.

Abstract

Using spin-echo Nuclear Magnetic Resonance in the model Transverse-Field Ising system TmVO$_4$, we show that low frequency quantum fluctuations at the quantum critical point have a very different effect on $^{51}$V nuclear-spins than classical low-frequency noise or fluctuations that arise at a finite temperature critical point. Spin-echos filter out the low frequency classical noise but not the quantum fluctuations. This allows us to directly visualize the quantum critical fan and demonstrate the persistence of quantum fluctuations at the critical coupling strength in TmVO$_4$ to high temperatures in an experiment that remains transparent to finite temperature classical phase transitions. These results show that while dynamical decoupling schemes can be quite effective in eliminating classical noise in a qubit, a quantum critical environment may lead to rapid entanglement and decoherence.