Nnews

I M A G E B Y H I D E O K O S A K A ; C O M M U N I C A T I O N S P H Y S I C S , 2 , 7 4 ( 2 0 1 9 ) . D O I : 1 0 . 1 0 3 8 / S 4 2 0 0 5 - 0 1 9 - 0 1 5 8 - 0

EFFICIENT ERROR-CORRECTING CODES for quantum comput- ing recently emerged from athematical models used to study black holes. This surprising finding joins to a long list of profound connections between information and physics.

The most intriguing examples began as paradoxes or “thought experiments” that are hard to test experimentally. Physicists take them seriously because they challenge core concepts and may require revolutionary theoretical changes that could have practical consequences.

 

The Physics of Computation

The first hints that information has physical significance emerged in the 1800s, as researchers connected the somewhat mysterious thermodynamic quantity known as entropy to the information needed to describe a particular physical configuration. In this view, the progressive loss of information about an orderly initial state leads to the inexorable increases in entropy of an isolated system demanded by the Second Law of Thermodynamics, which constrains the efficiency of engines.

Beyond such statistical accounting, individual bits of information can have direct physical consequences, as illustrated by thermodynamics pioneer Information Is Physics

Individual bits of information can have direct physical consequences.

Science | DOI: 10.1145/3360909 Don Monroe

James Clerk Maxwell. He suggested
that a “demon” that could see ap-
proaching molecules could merely
open or close a trapdoor between two
compartments of gas to let slow and
fast molecules accumulate on oppo-
sites sides. The resulting temperature
difference, seemingly without any en-
tropy increase elsewhere, would violate
the Second Law. (Maxwell’s tricky crea-
ture also inspired the name of pro-
grams that operate behind the scenes
in some operating systems.)
Physicists resolved the paradox by
noting that Maxwell’s demon eventu-
ally would need to erase the informa-
tion it had gleaned about the mol-
ecules, and that this erasure would
create enough entropy to preserve the
Second Law. Overcoming the erasure
The lattice structure of carbon atoms in a diamond crystal contains a nitrogen-vacancy center
with surrounding carbon nuclear spins. Researchers have demonstrated reliable quantum
state transfer of photon polarization into a carbon isotope nuclear spin coupled to the nitrogen-
vacancy center, based on photon-electron Bell state measurement by photon absorption.