Fermion antibunching was observed on a beam of free noninteracting neutrons. A monochromatic beam of thermal neutrons was first split by a graphite single crystal, then fed to two detectors, displaying a reduced coincidence rate. The result is a fermionic complement to the Hanbury Brown and Twiss effect for photons.PACS numbers: 03.75.Dg Over the past three decades, the research on the foundations of quantum mechanics has been enriched by many experiments on thermal neutrons, in particular by several enlightening results about the coherence properties and the physical nature of the wave function describing the behavior of a massive particle [1]. A general property of fermions is that of being characterized by an antisymmetric wave function: the second-order correlation function of a fermion gas exhibits an anticorrelation in the intensity fluctuations, in particular interference in the coincidence distributions of identical particles. The present Letter describes a new contribution in this field: an experiment on thermal neutrons that brings to light the fermion antibunching effect in a beam of free noninteracting particles. A monochromatic beam of thermal neutrons was first split by a graphite single crystal, then fed to two detectors, displaying a reduced coincidence rate. The result is a fermionic complement to the seminal Hanbury Brown and Twiss effect for bosons (photons) [2].The consequences of antisymmetry are well known in condensed matter physics, where the electronic states display a strong quantum entanglement and are confined within the Fermi surface. Interesting experiments with electron beams have confirmed these effects [3][4][5]. In the case of almost free particles, an anticorrelation was observed in the coincidence spectrum of neutrons from compound-nuclear reaction at small relative momentum [6,7]. However, such a physical system is not a good representative sample of a statistical ensemble of noninteracting identical fermions. A monochromatic beam of thermal neutrons from a nuclear reactor represents much better a statistical ensemble of free particles. Nevertheless, the observation of thermal-neutron antibunching by means of coincidence measurements on such beams with the available instrumentation did not appear to be feasible up to now, mainly because the mean number of fermions obtainable per unit cell of phase-space, to which the signal-to-noise ratio is proportional, was so low that a measurement time of several years was estimated [8].We shall show below that, with present-day available advanced instrumentation, a very accurately designed setup and a precise knowledge of the statistical properties of the neutron source, the experiment is feasible. In this Letter we shall describe some measurements carried out at the Institute Laue Langevin, Grenoble, France.How can one directly bring to light an anticorrelation effect in a neutron beam? In a gas of fermions there is a certain tendency for particles of the same spin to avoid each other, a tendency arising from the exchange antisymmet...

We present a Bell-type polarization experiment using two independent sources of polarized optical photons, and detecting the temporal coincidence of pairs of uncorrelated photons which have never been entangled in the apparatus. Very simply, our measurements have tested the quantummechanical equivalent of the classical Malus' law on an incoherent beam of polarized photons obtained from two separate and independent laser sources greatly reduced in intensities.The outcome of the experiment gives evidence of violation of the Bell-like inequalities. Drawing the conclusions of the present work, we invoke the distinction between the concepts of state-preparation and measurement to understand this result.

Oxygen implantation into n+-AlGaAs, followed by annealing above 600 °C, creates a deep acceptor level that compensates the shallow donors present in the material. Temperature-dependent Hall measurements show that the resistivity of this compensated AlGaAs has a thermal activation energy of 0.49 eV, in contrast to a value of 0.79 eV for compensation caused by ion-induced damage. The latter is stable only to 600 °C, whereas the chemically induced compensation in O-implanted AlGaAs is stable above 950 °C.

We describe an experiment confirming the evidence of the antibunching effect on a beam of noninteracting thermal neutrons. The comparison between the results recorded with a high-energy-resolution source of neutrons and those recorded with a broad-energy-resolution source enables us to clarify the role played by the beam coherence in the occurrence of the antibunching effect

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