  Quantum criticality is a central concept in condensed matter physics, but the
direct observation of quantum critical fluctuations has remained elusive. Here
we present an x-ray diffraction study of the charge density wave (CDW) in
2H-NbSe2 at high pressure and low temperature, where we observe a broad regime
of order parameter fluctuations that are controlled by proximity to a quantum
critical point. X-rays can track the CDW despite the fact that the quantum
critical regime is shrouded inside a superconducting phase, and, in contrast to
transport probes, allow direct measurement of the critical fluctuations of the
charge order. Concurrent measurements of the crystal lattice point to a
critical transition that is continuous in nature. Our results confirm the
longstanding expectations of enhanced quantum fluctuations in low dimensional
systems, and may help to constrain theories of the quantum critical Fermi
surface.
