New record in quantum spin.

The macroscopic quantum spin state
of caesium atoms held in a vessel
has been teleported to a second
vessel 50 cm away – according
physicists in Denmark, Spain and the
UK, who have performed the feat.
Although this distance is far smaller
than the 143 km record for the
quantum teleportation of relatively
simple states, the experiment
achieves a different type of
teleportation that had previously
been achieved only across
microscopic distances. The technique
can teleport complex quantum states
and could therefore have a range of
technological applications –
including quantum computing, long-
distance quantum communication
and remote sensing.
Quantum teleportation was first
proposed in 1993 by Charles
Bennett, of the IBM Thomas
J Watson Research Center in New
York, and colleagues. It allows one
person (Alice) to send information
about an unknown quantum state to
another person (Bob) by exchanging
purely classical information. It
utilizes the quantum entanglement
between two particles; one with
Alice and one with Bob. Alice
interacts the unknown quantum
state with her half of the entangled
state, measures the combined
quantum state and sends the result
through a classical channel to Bob.
The act of measurement alters the
state of Bob’s half of the entangled
pair and this, combined with the
result of Alice’s measurement, allows
Bob to reconstruct the unknown
quantum state.
Collective spin
This is usually demonstrated with
discrete quantum states, such as
single atomic spins that can be up,
down or a superposition of these two
states. In principle, however, it is
possible to teleport quantum states
that are effectively continuous, such
as the collective spin of a large
atomic ensemble. Furthermore, doing
so would have interesting practical
consequences for the development of
technologies based on the
teleportation process.
For Alice and Bob to send
information using quantum
teleportation, they must first be in
possession of entangled particles
(usually photons). Swapping
entangled photons inevitably results
in some being lost and this will have
an effect on the reconstruction that
Bob can make of Alice’s mystery
quantum state. If the information
being exchanged concerns a discrete
state, it will be entangled with a
single photon, which will either
arrive or not arrive, and Bob will
either make a perfect reproduction
or no reproduction of the state. This
is known as probabilistic quantum
teleportation. If the information
concerns a continuous state, it will
be entangled with a pulse of light
containing many photons. Some will
arrive and others will not. Bob can
always make a reconstruction of
Alice’s quantum state but if losses
are high then it will be less than
perfect. This is deterministic
quantum teleportation.
A key question is whether or not the
fidelity with which Bob can
reproduce Alice’s unknown quantum
state exceeds the maximum possible
fidelity achievable if Alice simply
measured the state and told Bob the
result – a limit imposed by the
Heisenberg’s uncertainty principle.
This will depend not just on the
proportion of photons lost but also
on other experimental parameters,
such as the length of time the
quantum states can be preserved for
interactions between the unknown
quantum state and the entangled
particles.
Room-temperature samples
This deterministic continuous-
variable teleportation was proposed
and realized in the lab by Eugene
Polzik and colleagues at the Niels
Bohr Institute in Copenhagen,
together with researchers at the
Institute of Photonic Sciences (ICFO)
in Barcelona and the University of
Nottingham. Their experimental set-
up involves two room-temperature
samples of caesium-133 gas held in
glass containers and separated by
about 50 cm. The aim of the
experiment is to use light to teleport
the collective quantum spin state of
1012 atoms from one container to
the other. The team extended the
life of the state by coating the
insides of the containers with a
special material that does not
absorb angular momentum from the
atoms.
Precise control over the spin states
of the system was done using
constant and oscillating magnetic
fields. They also collaborated with
theorists Christine Muschik at the
ICFO and Ignacio Cirac of the Max
Planck Institute for Quantum Optics,
near Munich, to develop a new
model of the interaction between
the atoms and the light. Using these
advances, they teleported multiple
collective spin states between the
two canisters and looked at the
variance in their measurements.
When they compared this with the
theoretical minimum variance that
could be achieved by sending the
spin state information in a purely
classical manner, they found that
the variance from their process was
lower. “We have achieved the first
deterministic, atomic-to-atomic
teleportation over a macroscopic
distance,” says Polzik.
Hugues de Riedmatten, a quantum-
optics expert at the ICFO – who was
not involved with the experiment –
says that the research is “very
significant”, describing the results
as “convincing”. He cautions,
however, that it is “a proof of
principle”, saying “I think it’s a first
step. If you would like to use it for
doing useful things in quantum-
information science, for example, you
would need to transport much more
complicated quantum states. It
remains to be seen whether this will
be possible or not.”