1202.4958 (Shao-Wen Chen et al.)

Quantum criticality at infinite temperature revealed by spin echo    [PDF]

Shao-Wen Chen, Zhan-Feng Jiang, Ren-Bao Liu

Criticality occurs widely at transitions between qualitatively different
states of matters. Quantum criticality due to diverging quantum fluctuations,
which can be observed by varying system parameters at zero temperature, is
particularly important as it indicates the emergence of new orders of quantum
matters. However, at temperature higher than the system's interaction strength
thermal fluctuations will dominate over the quantum fluctuations and hence
conceal the quantum criticality. Extremely low temperatures are required for
quantum criticality to occur in many interesting systems. For example, for
nuclear spins in solids and cold atoms in optical lattices, temperatures of
$10^{-9}$ or even $10^{-12}$ Kelvin are required. Here we show that quantum
criticality can be observed at high or even infinite temperature by measuring
the echo signal of a probe spin coupled to a quantum many-body system, because
the spin echo can remove the thermal fluctuation effect and therefore reveal
the quantum fluctuation effect. We also establish the correspondence between
the probe coherence time and the inverse temperature required for the onset of
quantum criticality. In particular, quantum criticality that would occur below
$10^{-9}$ or even $10^{-12}$ Kelvin can be detected by spin echo with coherence
time longer than milliseconds or seconds, respectively. This discovery provides
a new route to a vast land of quantum matters, via trading long coherence time
of a quantum probe for extremely low temperature.

View original: http://arxiv.org/abs/1202.4958