is teleported over to Bob’s. ;is is a very elegant idea, but there was
no way to do the experiment in the old days.
But on the road to realizing multiparticle entangled states, we
actually ended up developing the tools that enabled us to do the
teleportation experiment. ;is was amazing. For teleportation
you have to start with two entangled photons, and then you have
to be able to entangle one of those with a third photon. ;at’s the
idea. And the big surprise is that this has implications for the new
;eld of quantum information science.
What is the connection between quantum teleportation and
A quantum computer needs what is called a universal quantum
gate, a device where the quantum state of one system changes
depending on the state of another system. It’s similar to logic gates,
the circuits that regulate output in classical computers. And teleportation can actually serve as that universal quantum gate.
Before this, what would you tell people if they asked you
about the practical implications of your research into the
fundamentals of quantum mechanics?
I would openly tell them that it had no use whatsoever.
You would say that you were just satisfying your natural
;is is part of being human. It must go back to prehistoric times
when people stood there and looked up at the sky and wondered
what’s going on up there. We would not have our civilization if
people weren’t curious about things. To me this is the most
important driving force in science. Even for developing new technology, if you really want to do something new, then by its very
nature future technology cannot be imagined.
I mean, who had thought of the mobile phone? When the computer chip was developed, nobody had any idea that this would be
in a mobile phone, that you would be able to see photographs of
other people and download whole books. Our fantasies just aren’t
strong enough. We can imagine a lot about the future but they’re
usually the wrong things. If you look at things written in the ;;;;s
about the future, most of it was simply wrong. And the same is
true when people talk about the possible future today.
Some of your most recent work involved sending entangled
photons across long distances in the Canary Islands. What
did you get out of those experiments?
Well, two things. One is making progress in developing methodology for worldwide quantum communication with satellites,
because the distance [from the ground to low-earth orbit] is
about the same. ;e second is closing another loophole in Bell’s
theorem—the freedom of choice loophole. ;is loophole assumes
that the source that creates the two particles in the experiment
also somehow creates information that in;uences the choice of
what measurement to make on the particles. ;is is a completely
logically consistent position, and the way to rule it out is to make
the decision of what to measure before any information can have
reached you from the source.
We would not
have our civilization
if people weren’t
curious about things.
To me this is
the most important
driving force in science.”
La Palma and Tenerife, which are separated by ;; miles. On La
Palma we created a pair of photons and sent one of the photons
over to Tenerife using a free space telescope link, and on Tenerife
we decided a long time before the photon arrived which polarization would be measured.
And when you say a long time, you mean. . .
A long time means that—for modern electronics, it’s a long time—
it takes half a millisecond for the photon to go from A to B, so if
you decide something like a few tenths of a microsecond before,
then that’s a long time. ;en the other photon is kept locally in
a glass-fiber spool and measured at a later time in La Palma.
And which kind of measurement to perform on this photon is
decided by a random-number generator at the same moment
when the photon pairs are created at a distant location, about a
mile away, which means that no signal could travel fast enough
to in;uence this.
So were you able to close that loophole?
;at loophole was closed, and now the big carrot is to do a de;ni-tive experiment that closes all loopholes at once. ;e most important point in closing all the loopholes is that it would lead to
completely secure quantum cryptography. ;e loopholes can in
principle be used by an eavesdropper to hide, to cover up his presence and get some of the information. But if you have a loophole-free test of Bell’s theorem, then this is not possible anymore. So
there is a practical consequence to these experiments.
What are the philosophical implications of your work?
;e quantum state represents measurement results; it represents
information about a concrete situation, and it allows me to make
predictions about future measurement results. So it is information both about a situation that I know and information about
the future. I often say that quantum theory is information theory,
and that the separation between reality and information is an