BOULDER,
Colo. – Scientists at the Commerce Department’s
National Institute of Standards and Technology (NIST) have
coaxed six atoms into spinning together in two opposite directions
at the same time, a so-called Schrödinger “cat”
state that obeys the unusual laws of quantum physics. The
ambitious choreography could be useful in applications such
as quantum computing and cryptography, as well as ultra-sensitive
measurement techniques, all of which rely on exquisite control
of nature’s smallest particles.
The experiment,
which was unusually challenging even for scientists accustomed
to crossing the boundary between the macroscopic and quantum
worlds, is described in the Dec. 1 issue of Nature.* NIST
scientists entangled six beryllium ions (charged atoms) so
that their nuclei were collectively spinning clockwise and
counterclockwise at the same time. Entanglement, which Albert
Einstein called “spooky action at a distance,”
occurs when the quantum properties of two or more particles
are correlated. The NIST work, along with a paper by Austrian
scientists published in the same issue of Nature, breaks new
ground for entanglement of multiple particles in the laboratory.
The previous record was five entangled photons, the smallest
particles of light.
“It
is very difficult to control six ions precisely for a long
enough time to do an experiment like this,” says physicist
Dietrich Leibfried, lead author of the NIST paper.
The
ability to exist in two states at once is another peculiar
property of quantum physics known as “superposition.”
The NIST ions were placed in the most extreme superposition
of spin states possible with six ions. All six nuclei are
spinning in one direction and the opposite direction simultaneously
or what physicists call Schrödinger cat states. The name
was coined in a famous 1935 essay in which Austrin physicist
Erwin Schrödinger described an extreme theoretical case
of being in two states simultaneously, namely a cat that is
both dead and alive at the same time.
Schrödinger’s
point was that cats are never observed in such states in the
macroscopic “real world,” so there seems to be
a boundary where the strange properties of quantum mechanics—the
rule book for Nature’s smallest particles—give
way to everyday experience. The NIST work, while a long way
from full entanglement of a real cat’s roughly 1026
atoms, extends the domain where Schrödinger cat states
can exist to at least six atoms. The Austrian team used a
different approach to entangle more ions (eight) but in a
less sensitive state.
In the
NIST experiment, the ions are held a few micrometers apart
in an electromagnetic trap. Ultraviolet lasers are used to
cool the ions to near absolute zero and manipulate them in
three steps. To create and maintain the cat states, the researchers
fine-tuned trap conditions to reduce unwanted heating of the
ions, improved cooling methods, and automated some of the
calibrations and other formerly manual processes. One run
of the experiment takes about 1 millisecond; the cat states
last about 50 microseconds (about 1/20 as long). The team
ran the experiment successfully tens of thousands of times,
including numerous runs that entangled four, five, or six
ions.
Entanglement
and superpositions are being exploited in laboratories around
the world in the development of new technologies such as quantum
computers. If they can be built, quantum computers could solve
certain problems in an exponentially shorter time than conventional
computers of a similar size. For example, current supercomputers
would require years to break today’s best encryption
codes, (which are used to keep bank transactions and other
important information secret) while quantum computers could
quickly decipher the codes. Quantum computers also may be
useful for optimizing complex systems such as airline schedules
and database searching, developing "fraud-proof"
digital signatures, or simulating complex biological systems
for use in drug design.
Cat states,
because they are superpositions of opposite overall properties
that are relatively easy to verify, could be useful in a NIST-proposed
design for fault-tolerant quantum computers. In addition,
cat states are more sensitive to disturbance than other types
of superpositions, a potentially useful feature in certain
forms of quantum encryption, a new method for protecting information
by making virtually all eavesdropping detectable.
The entangled
cat states created by the NIST researchers also might be used
to improve precision instruments, such as atomic clocks or
interferometers that measure microscopic distances. Six ions
entangled in a cat state are about 2½ times more sensitive
to external magnetic fields than six unentangled ions, offering
the possibility of better magnetic field sensors, or (for
fixed external magnetic fields) better frequency sensors,
which are components of atomic clocks. In addition, correlations
between entangled ions could improve measurement precision,
because a measurement of the spin of one of the entangled
ions makes it possible to predict the spin of all remaining
ions with certainty.
The research
was funded by the Advanced Research and Development Activity/National Security Agency, the Department of Defense Multidisciplinary
University Research Initiative Program administered by the
Office of Naval Research, and NIST.
More
information about NIST research on quantum computing and cryptography,
and spin-off applications in measurement science, is available
at http://qubit.nist.gov.
As a
non-regulatory agency of the Commerce Department’s Technology
Administration, NIST promotes U.S. innovation and industrial
competitiveness by advancing measurement science, standards
and technology in ways that enhance economic security and
improve our quality of life.
* D.
Leibfried, E. Knill, S. Seidelin, J. Britton, R.B. Blakestad,
J. Chiaverini, D. Hume, W.M. Itano, J.D. Jost, C. Langer,
R. Ozeri, R. Reichle, and D.J. Wineland. 2005. Creation of
a six atom Schrödinger cat state. Nature. Dec.
1.
Background:
Creating Entangled Cat States with Six Ions
STEP
1
Ions
have a property called spin. Spin can be visualized as a rotating
top, which can be pointing up, down, or any direction in between
to represent a combination of up and down at the same time.
(The spin can point in any direction, so there are many possibilities.)
The NIST
experiment begins with all six ions spin down. Then they are
hit with two parallel laser pulses, which places each of the
ions in an equal superposition of spin up and spin down. This
means that each ion would have a 50/50 chance of being measured
as spin up or spin down. (A measurement always causes a superposition
to collapse to one direction or the other.) A measurement
of all six ions would have 64 (26) possible outcomes, or combinations
of up and down spins. But the ions are not measured at this
point in the experiment. Instead, they remain in the superposition
of all 64 possibilities.
STEP
2
All six
ions are entangled using a NIST technique originally developed
several years ago to entangle two or three ions. Two laser
beams are positioned at right angles to apply an oscillating
force to all six ions. The lasers are tuned so the difference
between their frequencies is very close to the frequency of
one of the natural vibrational motions of the six-ion string.
Based on differences in the spin up and spin down components
of the evolving 64 states, the ions “feel” a differing
laser force that cause the ions to oscillate in a particular
way. This coupling of the superposition of spin states to
the motion of the ion string has the global effect of entangling
the ions in a controlled way.
“During
this process the ions all 'talk’ to each other at the
same time, like in a conference call,” says NIST physicist
Dietrich Leibfried. “The common motion can be thought
of as the ‘phone line.’ (The Austrian experiment
is more like a series of individual phone calls to ‘the
boss,’ or the motion.)"
STEP
3
A final
laser pulse places all six entangled ions in the cat state,
where they stop evolving and remain briefly in the superposition
of all spins up (rotating to the right) and all spins down
(rotating to the left); that is, the original 64 possibilities
have been reduced to two.
VERIFICATION
NIST
scientists used two techniques to prove indirectly that the
ions were in cat states. (Direct measurements would cause
the cat states to collapse.) Both techniques rely on the fact
that any spinning object oscillates in an external magnetic
field at a rate proportional to its internal magnetic properties.
In a
superposition of all six spins up and down simultaneously
(a cat state), the “all up” component will oscillate
at six times the rate of a single spin up, and the “all
down” component will oscillate at six times the rate
of a single spin down—but in the direction opposite
to “all up.” Therefore, a cat-like superposition
of six ions will spin apart six times faster than a superposition
of a single atom. Scientists can evaluate how cleanly the
prepared cat states execute the oscillation at the sixfold
speed, and thus determine how purely the original “all
up and all down” state was prepared.
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