Free Electron Sources and Diffraction in Time

The quantum revolution of the last century advanced synergistically with technology, for example, with control of the temporal and spatial coherence, and the polarization state of light. Indeed, experimental confirmation of the quirks of quantum theory, as originally highlighted by Einstein, Podolsky, and Rosen, through Bohm, and then Bell, have been performed with photons, i.e., electromagnetic wave packets prepared in the same quantum states. Experimental tests of quantum mechanics with matter wave packets have been limited due to challenges in preparing all of the packets with similar quantum states. While great strides have been made for trapped atoms and Bose-Einstein condensates, the technology for electron matter waves has not kept pace. In other words, electron sources typically have a low quantum degeneracy. As new techniques to control the coherence of electron wave packets are developed, new avenues to test quantum theory become available. To better understand the temporal coherence of a pulsed electron source, we have studied electron emission from metallic and semiconductor structures. Ultrafast electron emission was obtained by focusing femtosecond laser pulses on metallic nanotips and nanoribbons, as well as semiconductor shards. The various emission mechanisms from these sources were investigated in efforts to control the coherence of the emitted electron wave packets. The electronic band structure of semiconductor materials provides an additional feature in that the spin state of emitted electrons can be optically controlled. Controlling the spin polarization in a pulsed electron source could lead to a source with the highest quantum degeneracy yet achieved. The development of a quantum degenerate source of matter waves is a step toward realizing fundamental tests of quantum mechanics with electron wave packets. As quantum degenerate electron sources do not yet exist, their potential and utility remains to be explored. An ultrafast spin-polarized source of electrons is itself intriguing for studying nanoscale magnetic systems with unprecedented temporal resolution. These advancements suggest that an entirely new field of free electron quantum optics may be within our reach, with all of its possibilities open for exploration. The quantum revolution may yet have some surprises in store