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How to See Quantum Entanglement

By Lisa
Grossman

June 2, 2010  |

4:03 pm  |

Categories: Physics

Human eyes can detect the spooky phenomenon of quantum entanglement — but
only sometimes, a new study on the physics preprint website arXiv.org claims.
While eyes can help determine if two individual photons were recently entangled,
they can’t tell if the brighter bunch of photons that actually hit the retina
are in this bizarre quantum state.

“In general you think these quantum phenomena that involve only a few
particles, they’re really far removed from us. That is actually not so true
anymore,” said physicist Nicolas Brunner of the University of Bristol. “You
could really go to an experiment by just having people look at these photons,
and from there really actually see entanglement.”

In an earlier paper, Brunner and colleagues at the University of Geneva in
Switzerland sketched out an experiment in which a human observer could replace a standard
quantum detector. This isn’t as far-fetched as it sounds, they say, because
the eye’s most important job is to be a sensitive photon detector.

The researchers would first prepare two entangled photons — photons whose
quantum properties are so intimately linked that one always knows what the other
is doing. When an aspect of one photon’s quantum state is measured, the other
photon changes in response, even when the two photons are separated by large
distances.

The researchers would send one photon to a standard detector and the other to
a human observer in a dark room. The human would see a dim point of light in
either the right or left field of view, depending on the photon’s quantum state.
If those flashes of light correlate strongly enough with the output of the
ordinary photon detector, then the scientists can conclude that the photons are
entangled.

“This is a standard way of measuring
and detecting entanglement,” says physicist Nicolas Gisin of the University
of Geneva, a coauthor of the new paper.

There’s just one problem: Humans can’t see individual photons. The retina
needs at least seven photons to hit it at once before it sends signals to the
brain. Also, 90 percent of photons are lost or scattered on the way through the
gelatinous part of the eye to the retina. These restrictions mean that you need
a lot of photons — at least hundreds, preferably thousands — to make a practical
human quantum detector.

In 2008 a group in Rome found a way to clone an entangled photon that
preserves the entanglement. If you treat the big bunch of clones as a single
quantum state, the entire bunch is entangled with the other original photon, the
researchers claimed.

“It’s like having a Schrodinger’s cat,” says Brunner, referring to Erwin Schrodinger’s
famous 1935 thought experiment in which a cat in a box has a 50-50 chance of
living or dying depending on whether a radioactive atom decays. In this case,
the microscopic state of the atom is entangled with the macroscopic state of the
cat: Either the atom decays and the cat is dead, or the atom doesn’t decay and
the cat is alive. Until someone opens the box, the only way to describe the
system is by including both the atom and the cat.

Gisin and colleagues thought this photon-cloning method would be perfect for
their human quantum-detector experiments. All they would have to do is make a
few thousand copies of one member of the original entangled photon pair, and
send all those copies to the human observer.

But because entanglement is easy to break, the team was unsure if the photons
that reach the observer’s eyes would still be entangled with the other
photon.

To test this idea, Gisin and colleagues imagined what would happen if instead
of cloning the original photon, they made the equivalent of a photocopy. Like a
black-and-white Xerox of a color picture, some information about the original
photon would be lost. Because the copied photons were never entangled with the
original, they would still not be entangled when they reached the observer’s
eyes.

The researchers compared the theoretical results using photocopied photons
and a real quantum cloner, and found that they looked exactly the same. The
human observer would see the same thing, even when the bunch of photons were
just Xeroxes that couldn’t possibly be entangled with the other photon.

The group concluded that human eyes can’t see quantum entanglement between a
macro-state and a micro-state. Schrodinger’s cat may well be entangled with the
atom, but a human detector can’t tell.

But the human eye can reliably tell whether the original two photons
were entangled. That’s still “seeing” entanglement, the authors say.

“Macro-micro is almost out of the question. But the micro-micro is nice as
well,” said study coauthor Christoph Simon of the University of Calgary in
Canada. “You’re bringing the observer a little bit closer to the quantum
physics.”

The researchers are now working on ways to perform the experiment in the lab
and expect it to be ready within two years.

“The theoretical paper is certainly sound and of good quality,” comments
physicist Dirk Bouwmeester of the University of California, Santa Barbara.

But Gisin acknowledges that replacing quantum detectors with eyeballs would
not lead to any new applications.

“Why do we do it nevertheless?” he says. “We find entanglement
fascinating.”

Image: ~Dezz~/flickr

See Also:

Quantum
Physics Used to Control Mechanical System

Quantum
Entanglement Visible to the Naked Eye

Quantum
Computer Simulates Hydrogen Molecule Just Right

Reverse-Engineering
the Quantum Compass of Birds

Photonic
Six Pack Provides Better Quantum Communication

Tags: Christoph
Simon, Nicolas Brunner, Nicolas
Gisin, quantum entanglement, schrodinger's cat, University of Geneva

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Posted by: jmacster | 06/2/10 | 4:21 pm |

huh?

Posted by: gekido | 06/2/10 | 5:50 pm |

^my thoughts exactly^

Posted by: zeroexcelcior | 06/2/10 | 6:33 pm |

So what exactly does an entangled photon look like?

Posted by: seibzehn | 06/2/10 | 6:57 pm |

uhh, what?

Posted by: cecilgreen | 06/2/10 | 7:35 pm |

There’s a cat. There was an experiment, or the idea of an experiment, with
a box and a cat.

Humans can’t see individual photos because they are small. Need hundreds
for the light to register in our brainz.

Hope you enjoyed my summary of this article.

Posted by: lordslackarse | 06/2/10 | 8:09 pm |

L2Physics you noobs.

Posted by: ReviewThis | 06/2/10 | 11:05 pm |

I thought quantum entanglement was a theory? What the hell would it even
look like? WTF man, I’m trippin out

Posted by: zigloo99 | 06/3/10 | 12:46 am |

I need a quantum entanglement device before the next blackout!

Posted by: sddengineer | 06/3/10 | 8:00 am |

There are other kinds of entanglement. Entanglement doesn’t need to be
generated; it can be attracted. One example: the phenomenon of what Leary
called the “contact high” – It draws in towards an attractor.

Posted by: Bahlkris | 06/3/10 | 9:13 am |

Does anyone else think that quantum physics is just another way for man to
put himself at the center of the universe just like religon?

Posted by: puddin | 06/3/10 | 9:51 am |

^ No. ^

Posted by: jmacster | 06/3/10 | 10:38 am |

perhaps we have been “punked” with this piece and the author is just
laughing at us scratching our heads

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