Synopsis: Classical vs Quantum

An experimental realization of one of Einstein’s thought experiments shows that the quantum world is even more complex than previously thought.
Synopsis figure
L. Schmidt et al., Phys. Rev. Lett. (2013)

A particle passes through a single slit and then through one opening of a double slit. Can the pathway of the particle be determined without destroying the interference structure? This was a question first debated by Einstein and Bohr as they tried to understand the newly developed ideas of quantum physics. Einstein argued that classical physics was sufficient; you could determine the particle’s path by measuring the momentum transfer imparted from the deflection of the particle by the first slit. Bohr claimed instead that the slits, as well as the particle, behave as quantum objects, whose position and momentum are uncertain—we can either know which path the particle takes through the slit maze, or how long the path is, but not both.

So who was right? Writing in Physical Review Letters, Lothar Schmidt and colleagues, from Goethe University in Germany, show that Bohr was right. In their experiments, the team replaced the slits with hydrogen-deuteron molecular ions and bombarded them with helium atoms. As the atoms collided with the ions, an electron was exchanged between the atom and the ion. By measuring this exchange they could determine the positions and orientation of the atoms and ions. The scattering of the atoms was consistent with Bohr’s view; you need a quantum description of the slits and particles to understand the results. However, the results could still be correctly predicted using classical slits, but only if the particle simultaneously passed through both holes of the double slit and transferred half of its momentum to each path. – Katherine Thomas


Announcements

More Announcements »

Subject Areas

Quantum Physics

Previous Synopsis

Next Synopsis

Particles and Fields

Heavy Quark Model Lands on Solid Footing

Read More »

Related Articles

Viewpoint: A Close Look at the Fermi-Hubbard Model
Quantum Physics

Viewpoint: A Close Look at the Fermi-Hubbard Model

The engineered simplicity of a cold-atom system described by the 2D Fermi-Hubbard model allows for a precision test of the model’s equation of state. Read More »

Focus: Water Molecule Spreads Out When Caged
Quantum Physics

Focus: Water Molecule Spreads Out When Caged

Water molecules confined in nanochannels exhibit tunneling behavior that smears out the positions of the hydrogen atoms into a pair of corrugated rings. Read More »

Synopsis: One-Way Quantumness
Quantum Physics

Synopsis: One-Way Quantumness

Experiments provide evidence for one-way quantum steering—an effect by which distant entangled systems can influence one another in a directional way. Read More »

More Articles