Physicists Create Quantum Link Between Photons That Don’t Exist at the Same Time

Physicists Create Quantum Link Between Photons That Don’t Exist at the Same Time

Photons. Image: NASA/Sonoma State University/Aurore Simonnet

Now they’re just messing with us. Physicists have long known that
quantum mechanics allows for a subtle connection between quantum
particles called entanglement, in which measuring one particle can
instantly set the otherwise uncertain condition, or “state,” of another
particle—even if it’s light years away. Now, experimenters in Israel
have shown that they can entangle two photons that don’t even exist at
the same time.

“It’s
really cool,” says Jeremy O’Brien, an experimenter at the University of
Bristol in the United Kingdom, who was not involved in the work. Such
time-separated entanglement is predicted by standard quantum theory,
O’Brien says, “but it’s certainly not widely appreciated, and I don’t
know if it’s been clearly articulated before.”

Entanglement is a kind of order that lurks within the uncertainty of
quantum theory. Suppose you have a quantum particle of light, or photon.
It can be polarized so that it wriggles either vertically or
horizontally. The quantum realm is also hazed over with unavoidable
uncertainty, and thanks to such quantum uncertainty, a photon can also
be polarized vertically and horizontally at the same time. If you then
measure the photon, however, you will find it either horizontally
polarized or vertically polarized, as the two-ways-at-once state
randomly “collapses” one way or the other.



Entanglement can come in if you have two photons. Each can be put
into the uncertain vertical-and-horizontal state. However, the photons
can be entangled so that their polarizations are correlated even while
they remain undetermined. For example, if you measure the first photon
and find it horizontally polarized, you’ll know that the other photon
has instantaneously collapsed into the vertical state and vice versa—no
matter how far away it is. Because the collapse happens instantly,
Albert Einstein dubbed the effect “spooky action at a distance.” It
doesn’t violate relativity, though: It’s impossible to control the
outcome of the measurement of the first photon, so the quantum link
can’t be used to send a message faster than light.

In
standard entanglement swapping (top), entanglement (blue shading) is
transferred to photons 1 and 4 by making a measurement on photons 2 and
3. The new experiment (bottom) shows that the scheme still works even if
photon 1 is destroyed before photon 4 is created. Image: AAAS/Science

Now Eli Megidish, Hagai Eisenberg, and colleagues at the Hebrew
University of Jerusalem have entangled two photons that don’t exist at
the same time. They start with a scheme known as entanglement swapping.
To begin, researchers zap a special crystal with laser light a couple of
times to create two entangled pairs of photons, pair 1 and 2 and pair 3
and 4. At the start, photons 1 and 4 are not tangled. But they can be
if physicists play the right trick with 2 and 3.

The key is that a measurement “projects” a particle into a definite
state — just as the measurement of a photon collapses it into either
vertical or horizontal polarization. So even though photons 2 and 3
start out unentangled, physicists can set up a “projective measurement”
that asks, are the two in one of two distinct entangled states or the
other? That measurement entangles the photons, even as it absorbs and
destroys them. If the researchers select only the events in which
photons 2 and 3 end up in, say, the first entangled state, then the
measurement also entangles photons 1 and 4. (See diagram, top.) The
effect is a bit like joining two pairs of gears to form a four-gear
chain: Enmeshing two inner gears establishes a link between the outer
two.

In recent years, physicists have played with the timing in the
scheme. For example, last year a team showed that entanglement swapping
still works even if they make the projective measurement after they’ve
already measured the polarizations of photons 1 and 4. Now, Eisenberg
and colleagues have shown that photons 1 and 4 don’t even have to exist
at the same time, as they report in a paper in press at Physical Review
Letters.

To do that, they first create entangled pair 1 and 2 and measure the
polarization of 1 right away. Only after that do they create entangled
pair 3 and 4 and perform the key projective measurement. Finally, they
measure the polarization of photon 4. And even though photons 1 and 4
never coexist, the measurements show that their polarizations still end
up entangled. Eisenberg emphasizes that even though in relativity, time
measured differently by observers traveling at different speeds, no
observer would ever see the two photons as coexisting.

The experiment shows that it’s not strictly logical to think of
entanglement as a tangible physical property, Eisenberg says. “There is
no moment in time in which the two photons coexist,” he says, “so you
cannot say that the system is entangled at this or that moment.” Yet,
the phenomenon definitely exists. Anton Zeilinger, a physicist at the
University of Vienna, agrees that the experiment demonstrates just how
slippery the concepts of quantum mechanics are. “It’s really neat
because it shows more or less that quantum events are outside our
everyday notions of space and time.”

So what’s the advance good for? Physicists hope to create quantum
networks in which protocols like entanglement swapping are used to
create quantum links among distant users and transmit uncrackable (but
slower than light) secret communications. The new result suggests that
when sharing entangled pairs of photons on such a network, a user
wouldn’t have to wait to see what happens to the photons sent down the
line before manipulating the ones kept behind, Eisenberg says. Zeilinger
says the result might have other unexpected uses: “This sort of thing
opens up people’s minds and suddenly somebody has an idea to use it in
quantum computing or something.”


source:-
http://www.wired.com/wiredscience/2013/05/quantum-linked-photons/