This site uses cookies. By continuing to use this site you agree to our use of cookies. To find out more, see our Privacy and Cookies policy.
Skip to the content

Share this

Free weekly newswire

Sign up to receive all our latest news direct to your inbox.

Physics on film

100 Second Science Your scientific questions answered simply by specialists in less than 100 seconds.

Watch now

Bright Recruits

At all stages of your career – whether you're an undergraduate, graduate, researcher or industry professional – can help find the job for you.

Find your perfect job

Physics connect

Are you looking for a supplier? Physics Connect lists thousands of scientific companies, businesses, non-profit organizations, institutions and experts worldwide.

Start your search today


Quantum Cheshire cat spotted in Grenoble

An illustration of a Cheshire cat

A most curious thing is the quantum Cheshire cat. (Courtesy: iStockphoto/koffeezilla)

By Hamish Johnston

Three months ago we ran a news article about a “quantum Cheshire cat” experiment that was proposed by Yakir Aharonov of Tel Aviv University and colleagues. Now, an international team of physicists has created a quantum Cheshire cat using polarized neutrons at the Institut Laue-Langevin (ILL) in Grenoble, France.

The work was done by Yuji Hasegawa and colleagues at the Vienna University of Technology, ILL, the University of Cergy-Pontoise and Chapman University.

In Lewis Carroll’s story Alice’s Adventures in Wonderland, Alice encounters a smiling Cheshire cat that fades away leaving behind just its grin. A quantum Cheshire cat is a physical system that can somehow be separated from its quantum state, only to be reunited at a later time.

In Hasegawa’s experiment the cat is a neutron and the quantum state is the polarization of the neutron. The experiment involves sending a beam of polarized neutrons through an interferometer. “Inside the interferometer the cat goes though the upper beam path, while its grin travels along the lower beam path,” he says.

The team says that it was able to confirm that the neutron and its polarization took different paths by making “weak measurements” on the neutrons as they pass through the experiment – something that was suggested by Aharonov and colleagues. Unlike a conventional measurement in quantum mechanics, which would have a profound effect on the outcome of an interferometry experiment, a weak measurement provides a small amount of information about an intermediate state without having a significant effect on the final outcome.

Hasegawa and colleagues describe their experiment in a preprint on arXiv entitled “Observation of a quantum Cheshire cat in a matter wave interferometer experiment“.

You can find out much more about weak measurement in the Physics World feature article “In praise of weakness” by Aephraim Steinberg, Amir Feizpour, Lee Rozema, Dylan Mahler and Alex Hayat.

This entry was posted in General and tagged , , . Bookmark the permalink.
View all posts by this author  | View this author's profile


  1. M. Asghar

    Nice work to observe the separation of the body of neutron and its spin, while passing through an interferometer. However, due to the many-body effect, one has been able to separate an electron from its spin and charge reducing it to a naked and smileless Cheshire cat!

  2. If indeed “weak measurement provides a small amount of information about an intermediate state,” how can one make a firm conclusion regarding the actual separation of a neutron and its polarization based on little info?

    You need to explain clearly what is meant by saying polarization (a quantum observable) itself taking a different path from the neutron. It’s like saying that the neutron’s momentum took a different path from the neutron, which makes no sense.


  • Comments should be relevant to the article and not be used to promote your own work, products or services.
  • Please keep your comments brief (we recommend a maximum of 250 words).
  • We reserve the right to remove excessively long, inappropriate or offensive entries.

Show/hide formatting guidelines

Tag Description Example Output
<a> Hyperlink <a href="">google</a> google
<abbr> Abbreviation <abbr title="World Health Organisation" >WHO</abbr> WHO
<acronym> Acronym <acronym title="as soon as possible">ASAP</acronym> ASAP
<b> Bold <b>Some text</b> Some text
<blockquote> Quoted from another source <blockquote cite="">IOP</blockquote>
<cite> Cite <cite>Diagram 1</cite> Diagram 1
<del> Deleted text From this line<del datetime="2012-12-17"> this text was deleted</del> From this line this text was deleted
<em> Emphasized text In this line<em> this text was emphasised</em> In this line this text was emphasised
<i> Italic <i>Some text</i> Some text
<q> Quotation WWF goal is to build a future <q cite="">
where people live in harmony with nature and animals</q>
WWF goal is to build a future
where people live in harmony with nature and animals
<strike> Strike text <strike>Some text</strike> Some text
<strong> Stronger emphasis of text <strong>Some text</strong> Some text