Quantum Decoherence at the Femtosecond Level in Liquids and Solids Observed by Neutron Compton Scattering

Abstract

About 10 years ago, it was found that neutron scattering on hydrogen showed anomalously low cross sections in many materials when it was observed under Compton scattering conditions (i.e. with neutron energies larger than 10 eV, where the duration of the scattering process falls in the τ _sc = 10⁻¹⁶ to 10⁻¹⁵ s range). The anomalies decreased with the neutron energy, which means that the cross sections approached normal values for long scattering times.

This phenomenon is interpreted here as due to an entanglement between the protons (because of their indistinguishability) during the scattering process, by which certain terms in the cross section are cancelled through the large zero-point motion of the protons. The anomalies disappear gradually as the proton states decohere in contact with the local environment. Fitted decoherence times range from 4•10⁻¹⁵ s for proton pairs in liquid hydrogen to 5•10⁻¹⁶ s in metal hydrides. For the proton pairs in water, the data are compared with a theoretical estimate for decoherence based on the influence of fluctuations in hydrogen bonding to nearby molecules.

The fast decoherence of locally prepared entangled states in condensed media studied here is compared with decoherence (in the 10⁻⁶ to 10⁻³ s range) in objects studied in quantum optics in high vacuum, with the disappearance of the superposition state in NH₃ or ND₃ molecules in dilute gases, and with the lifetime of superconducting qubits in solids (10⁻⁷ s) at low temperature.

In recent experiments, there are also indications for an energy shift in connection with the breaking of the n-p entanglement in neutron Compton scattering. Comments on this possibility will be given at the end of this chapter.