# Now You See It: How To Spot Quantum Behavior

Distinguishing quantum behavior from its classical counterpart is harder than it sounds. Now a group of theorists have worked out how a few simple measurements can do the trick.

Imagine you want to measure the quantum behavior of electrons in some kind of nanostructure, a quantum wire, say.

You send a few electrons through the wire and measure the current. You see a few oscillations and assume they’re the Rabi oscillations you were looking for. These oscillations occur when a quantum system rapidly switches between one state and another so they’re a pretty good indicator that you’ve got some decent quantum behavior to study in your wire. Right?

Not quite. There are plenty of ways that ordinary classical currents can oscillate too. So who’s to say you haven’t got a classical current in your wire?

And therein lies the problem: how to tell a quantum current from a classical one. It’s of more than passing interesting for physicists building nanostructures designed to work as quantum playgrounds for electrons. There must be some way to easily distinguish quantum from classical behavior, but how?

Today, Neill Lambert at the Institute of Physical and Chemical Research (RIKEN) in Japan and few buddies say they’ve solved the problem. What they’ve done is “formulate a set of inequalities that would allow an experimentalist to exclude the possibility of a classical description of transport through a nanostructure.”

All an experimentalist has to do is measure the local charge in the device as well as the current flow through it. If the results violate the Lambert team’s inequalities, then there’s definitely quantum behavior in the air.

That’s a useful trick to have up your sleeve but it may have wider application. Lambert and company say that similar inequalities can be derived to test the quantum behavior of other systems such as atom-field interactions in quantum optics and the quantum behavior of networks of quantum dots, Cooper pair boxes and even molecules.

It’s this last one that really catches the eye. Testing the quantum behavior of networks of molecules is exactly the kind of thing that quantum biologists want to do. Some have even managed it in certain systems, such as light harvesting in photosynthesis.

But the experiments are hard and the results sometimes difficult to interpret. A set of inequalities for quantum behavior could make life much easier.

Ref: arxiv.org/abs/1002.3020: Distinguishing Quantum And Classical Transport Through Nanostructures