The ground state of a superconductor is a macroscopic quantum state that can extend coherently over substantial distances. As a result, electrons tunnelling from two different points (separated by macroscopic length) on the surface of a superconductor remain coherent in phase and so are able to interfere: this property forms the basis of superconducting quantum interference devices (SQUIDs). Another characteristic of electrons tunnelling from a superconductor is that they are monochromatic, their energy being determined by the ground-state energy of the superconducting state. Monochromatic electrons have been observed tunnelling from a superconductor to a normal metal, and the resulting currents have been used to probe the dynamics of atoms and molecules at interfaces. Here we report the results of field-emission experiments that confirm the prediction that monochromatic electrons can similarly be emitted from a superconductor into vacuum. Monochromatic emissions of this type might find application as the sources in a range of electron-based spectroscopies.
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