Daniel came to Vienna on a Fulbright Fellowship that I had
organized for him. I remember the first morning we were sitting there together and we were thinking, “What shall we work
on?” It turned out that we both had been asking ourselves how
we could extend Bell’s theorem to new domains where it hadn’t
been applied yet. And we both thought it might be interesting to investigate whether or not more than two particles could
No one else had considered that type of experiment?
Nobody had worked on entanglement with more than two particles, though the approach is kind of obvious in hindsight. People
have worked with two photons, so why not three, you know? We
discovered that multiparticle entangled states could exist in theory
[some of them are now called Greenberger-Horne-Zeilinger, or
;;;, states], and I decided that my goal was to realize them in
the laboratory. And it turns out that most of the tools you need to
create them simply did not exist. ;e light sources and detectors
were not good enough to observe multiparticle entanglement. A
lot had to be done.
How were you eventually able to create these states?
We found a way. To do this you need four photons—two photons
entangled in one state and the other two photons entangled in
another state. Then we send one photon from each pair into a
detector, and you measure only one in such a way that you don’t
know where this one photon you measure came from, from the ;rst
pair or from the second pair. If you do that right, the remaining
three photons end up being entangled.
On the way to creating multiparticle entangled states, you
also found a way to transfer the properties of one particle to
another—the closest thing to teleportation we have. How did
you become interested in that?
Well, that’s actually a funny story. In ;;;; a proposal by theoretical
physicists came out about quantum teleportation, basically the
transfer of one particle’s properties to another that could be arbitrarily far away. When I read this I said, “What are they are talking
about? ;is is a typical theoreticians’ proposal; they don’t have
any idea how impossible this experiment is.”
What were the theorists proposing?
;ey had a simple thought experiment. You have two individuals,
by convention we call them Bob and Alice, who want to communicate via entangled pairs of particles. Alice has a quantum state,
which can represent a particular piece of information, and she
wants to transfer this quantum state to Bob. And let’s suppose that
the communication channels between the two are very bad, such
that Alice cannot send the quantum state itself because it will be
disturbed by the environment or whatever. ;ese theoreticians proposed that the way to transfer that quantum state was to entangle
a third particle with an already entangled pair. So if Alice has one
particle from an entangled pair and Bob has the other, then what
Alice does is entangle her particle with a third particle that carries the quantum state she wants to send to Bob. Just by doing this
entangling procedure, the information carried by the third particle