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In a quantum spin liquid, the magnetic moments of the constituent electron spins evade classical longrange order to form an exotic state that is quantum entangled and coherent over macroscopic length scales 1-2 . Such phases offer promising perspectives for device applications in quantum information technologies, and their study can reveal fundamentally novel physics in quantum matter. Quantum spin ice is an appealing proposal of one such state, in which the fundamental ground state properties and excitations are described by an emergent U(1) lattice gauge theory 3-7 . This quantum-coherent regime has quasiparticles that are predicted to behave like magnetic and electric monopoles, along with a gauge boson playing the role of an artificial photon. However, this emergent lattice quantum electrodynamics has proved elusive in experiments. Here we report neutron scattering measurements of the rare-earth pyrochlore magnet Pr 2 Hf 2 O 7 that provide evidence for a quantum spin ice ground state. We find a quasielastic structure factor with pinch points -a signature of a classical spin ice -that are partially suppressed, as expected in the quantum-coherent regime of the lattice field theory at finite temperature.Our result allows an estimate for the speed of light associated with magnetic photon excitations. We also reveal a continuum of inelastic spin excitations, which resemble predictions for the fractionalized, topological excitations of a quantum spin ice. Taken together, these two signatures suggest that the low-energy physics of Pr 2 Hf 2 O 7 can be described by emergent quantum electrodynamics. If confirmed, the observation of a quantum spin ice ground state would constitute a concrete example of a threedimensional quantum spin liquid -a topical state of matter which has so far mostly been explored in lower dimensionalities.

Abstract. HYSPEC is a high-intensity, direct-geometry time-of-flight spectrometer at the Spallation Neutron Source, optimized for measurement of excitations in small single-crystal specimens with optional polarization analysis capabilities. The incident neutron beam is monochromated using a Fermi chopper with short, straight blades, and is then vertically focused by Bragg scattering onto the sample position by either a highly oriented pyrolitic graphite (unpolarized) or a Heusler (polarized) crystal array. Neutrons are detected by a bank of 3 He tubes that can be positioned over a wide range of scattering angles about the sample axis. HYSPEC entered the user program in February 2013 for unpolarized experiments, and is already experiencing a vibrant research program. Polarization analysis will be accomplished by using the Heusler crystal array to polarize the incident beam, and either a 3 He spin filter or a supermirror wide-angle polarization analyser to analyse the scattered beam. The 3 He spin filter employs the spin-exchange optical pumping technique. A 60• wide angle 3 He cell that matches the detector coverage will be used for polarization analysis. The polarized gas in the post-sample wide angle cell is designed to be periodically and automatically refreshed with an adjustable pressure of polarized gas, optically pumped in a separate cell and then transferred to the wide angle cell. The supermirror analyser has 960 supermirror polarizers distributed over 60• , and has been characterized at the Swiss Spallation Neutron Source. The current status of the instrument and the development of its polarization analysis capabilities are presented.

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In many areas of soft and hard matter research science, the amount of material to investigate is rather limited. Partly because the fabrication of larger samples is too expensive or not feasible, yet, partly because the interesting features depend on the size. The aim of this work is to develop a neutron reflectometer optimized for small samples and specular measurements. We present a new concept which is based on state-of-the-art neutron optical elements and which allows studies of samples 100 times smaller than with previous instrumentation. The concept is to use an elliptically focusing guide in the sample plane, and a converging beam geometry in the scattering plane. The latter allows for the simultaneous measurement of a wide angle-of-incidence range simultaneously. Here we report on the prototype setup and first measurements, where we reached a reduction of counting time by one order of magnitude. If a complete instrument is built based on the presented principle, another order of magnitude can be expected.