Contact: Collier Smith, smithcn@boulder.nist.gov
EMBARGOED FOR RELEASE UNTIL: NIST 96-18
5 p.m. EDT, Thursday, May 23, 1996
Contact: Collier Smith (Boulder) SCHROEDINGER'S CAT
(303) 497-3198 IN AN ATOMIC CAGE
collier.smith@nist.gov
They say, "You can't have your cake and eat it, too."
They say, "You can't be in two places at one time."
"They" may be wrong, however, since scientists at the Commerce
Department's National Institute of Standards and Technology have just
disproved the latter by preparing a beryllium atom that is
simultaneously located in two widely separated places.
At the NIST laboratories in Boulder, Colo., Christopher Monroe,
Dawn Meekhof, Brian King and David Wineland isolated a single beryllium
ion (an atom with one of its two outer electrons stripped away) in an
electromagnetic trap and cooled it nearly to absolute zero with
precisely tuned laser beams. This confined it to a tiny region of space
less than a millionth of a centimeter across, where it rested almost
without motion.
The ion's single remaining outer electron can be in two internal
quantum states: "up" and "down." These states correspond to different
orientations of the spin of the electron. The laws of quantum mechanics
also allow the electron to be placed in a "superposition" of the two
states, in which the two states both exist and are, in a sense, sort of
stacked upon one another. Until the particle is disturbed by an outside
agent, there is an equal probability that it is in either state, and
thus it is considered to be in both states.
Next, additional pulses of laser radiation were delicately applied,
producing a tiny force in a manner that pushed one way on the "up"
electron state, and the opposite way on the "down" state. This force, in
effect, gently shoved the two states apart without collapsing them to a
single entity, so that the states that were superimposed on each other
in the original ion became two physically separated states. The
separation was more than 80 nanometers, or 11 times the size of the
original ion.
This bizarre state, of being in two well-separated places at once,
can be visualized by imagining a large, shallow, round-bottomed bowl
with a marble simultaneously at opposite sides of the bowl, rolling from
side to side and through itself at the center. The experiment provides a
glimpse of quantum superposition states at a scale never seen before.
As detailed in the May 24 issue of Science, this experiment has
connections to the works of Albert Einstein and Erwin Schroedinger, both
of whom in 1935 described hypothetical scenarios allowed by quantum
mechanics which seemed to defy reality. Schroedinger, for example,
considered the possibility that a cat could be made to be both dead and
alive at the same time. "Schroedinger's cat" soon became a shorthand way
to refer to a whole class of superposed states, and quantum particles in
microscopic superposed states have been observed for many years. Until
now, however, no one has ever prepared a particle where the
superposition was transformed into a physical separation on so large a
scale and under such controlled conditions.
Schroedinger cat states are extremely fragile. Any interaction with
the surroundings will destroy the superposition and the ion collapses
into a single entity (becomes "decoherent"). As the separation is made
larger, the cat state becomes more fragile; this is interpreted as the
reason such states are never seen in the larger world of common
experience.
Given that fragility, how did the scientists detect that the
Schroedinger cat state really existed? If observing the ion collapses
it, how did they know?
Their technique was to repeat the experiment many times while
slowly varying the direction of the force that shoved the states apart,
and to look at the end result after each trial. They found that when the
direction was such that the two states began to physically overlap, they
produced an interference pattern, and a narrower pattern demonstrated a
greater amount of original separation of the superposed states.
The NIST experiment is an outgrowth of advanced research on trapped
atoms aimed at developing new kinds of atomic clocks. NIST operates the
U.S. primary standard of time and frequency, and is charged with
providing the country with a standard that meets the ever-advancing
demands of industry and technology. Trapped atoms are one of the
mechanisms being studied for their potential to provide time and
frequency thousands of times more precisely than today's standards.
The experiment also may provide a route to the first controlled
studies of quantum decoherence, a topic that has received much interest
lately for its importance in the fields of quantum computers and quantum
cryptography.
However, this work probably won't offer any hope to greedy birthday
celebrants or overscheduled modern folks. As Monroe says, "I don't think
this is likely to lead to our being able to attend a son's piano recital
and a daughter's softball game at the same time, or eat a cake and store
its superposed double in the freezer. The quantum states of birthday
cakes and people are just too complicated to separate like we can a
single atom."
An agency of the Commerce Department's Technology Administration,
NIST promotes economic growth by working with industry to develop and
apply technology, measurements and standards.
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