Quantum Technology

From Objection to Resource

In 1935 Einstein and two colleagues pointed out that quantum mechanics allows two particles to share a single state, so that measuring one seems to fix the properties of the other at once, however far apart they are. They intended it as proof that the theory was missing something. See The EPR Paradox and Entanglement.

Experiments from the 1970s onward showed that the connection is real. Once physicists accepted that, a new question followed. If entanglement is a genuine feature of nature, what can be done with it?

Unbreakable Keys

Two parties who share entangled particles can use them to generate a random encryption key known only to them. Any attempt by an eavesdropper to intercept the particles disturbs the correlations in a way that can be detected, using the same test John Bell devised to settle the Einstein–Bohr argument. Entanglement-based systems have been run over optical fiber and, by satellite, between ground stations more than 1,100 kilometers apart. A related satellite method has linked continents.

Quantum Computers

A quantum computer stores information in quantum bits that can be entangled with one another. For certain problems, such as simulating molecules or factoring large numbers, a machine exploiting entanglement could in principle outperform any conventional computer. Working devices exist with hundreds of quantum bits, and some with more. Making them reliable enough to be useful is one of the major engineering efforts of the present day.

Teleportation

In 1997 Anton Zeilinger's group demonstrated quantum teleportation: using an entangled pair to transfer the exact quantum state of one particle onto another some distance away. No matter travels, and no information moves faster than light, but the state is transferred intact. It is a basic operation for the quantum networks now being built.

The Nobel Prize

The 2022 Nobel Prize in Physics went to Alain Aspect, John Clauser, and Anton Zeilinger for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science.

How Direct Is the Link?

Indirect, and there is irony in it. Einstein was the first to see clearly what quantum mechanics implied, and he described it precisely. He was wrong to conclude that nature could not be like that. A whole technology now rests on the thing he refused to accept, which is an odd and rather fitting tribute to the quality of his objection.