[1] After graduation, he spent two years in the US Army at a physics research lab connected to the NSA, working as a cryptanalyst, which eventually sparked his interest in quantum cryptography.
As the reactor had been down for maintenance, Roberto Collela, Albert Overhauser, and Sam Werner devised a better way to do the experiment using a neutron interferometer.
During a conference at Grenoble in France in 1978, Greenberger met with Michael Horne and Anton Zeilinger, which—by 1986—would eventually prove to be an important collaboration in the development of the Greenberger-Horne-Zeilinger state, a much improved version of Bell's theorem in quantum mechanics.
[2] In 1988, Greenberger won a Humboldt senior scientist award and went to Munich in 1988 to work at the Max Planck Institute of Quantum Optics in Garching.
Together with Anton Zeilinger and Michael Horne, Greenberger wrote the first paper on quantum entanglement beyond two particles.