PASADENA, Calif. — Researchers at the California Institute
of Technology (Caltech) have demonstrated quantum entanglement for
a quantum state stored in four spatially distinct atomic
memories.
Their work, described in the November 18 issue of the journal
Nature, also demonstrated a quantum interface between the
atomic memories—which represent something akin to a computer
"hard drive" for entanglement—and four beams of light,
thereby enabling the four-fold entanglement to be distributed by
photons across quantum networks. The research represents an
important achievement in quantum information science by extending
the coherent control of entanglement from two to multiple (four)
spatially separated physical systems of matter and light.
The proof-of-principle experiment, led by William L. Valentine
Professor and professor of physics H. Jeff Kimble, helps to pave
the way toward quantum networks. Similar to the Internet in our
daily life, a quantum network is a quantum "web" composed of many
interconnected quantum nodes, each of which is capable of
rudimentary quantum logic operations (similar to the "AND" and "OR"
gates in computers) utilizing "quantum transistors" and of storing
the resulting quantum states in quantum memories. The quantum nodes
are "wired" together by quantum channels that carry, for example,
beams of photons to deliver quantum information from node to node.
Such an interconnected quantum system could function as a quantum
computer, or, as proposed by the late Caltech physicist Richard
Feynman in the 1980s, as a "quantum simulator" for studying complex
problems in physics.
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Quantum entanglement is a quintessential feature of the quantum
realm and involves correlations among components of the overall
physical system that cannot be described by classical physics.
Strangely, for an entangled quantum system, there exists no
objective physical reality for the system's properties. Instead, an
entangled system contains simultaneously multiple possibilities for
its properties. Such an entangled system has been created and
stored by the Caltech researchers.
Previously, Kimble's group entangled a pair of atomic quantum
memories and coherently transferred the entangled photons into and
out of the quantum memories
(http://media.caltech.edu/press_releases/13115). For such
two-component—or bipartite—entanglement, the subsystems
are either entangled or not. But for multi-component entanglement
with more than two subsystems—or multipartite
entanglement—there are many possible ways to entangle the
subsystems. For example, with four subsystems, all of the possible
pair combinations could be bipartite entangled but not be entangled
over all four components; alternatively, they could share a
"global" quadripartite (four-part) entanglement.
Hence, multipartite entanglement is accompanied by increased
complexity in the system. While this makes the creation and
characterization of these quantum states substantially more
difficult, it also makes the entangled states more valuable for
tasks in quantum information science.
To achieve multipartite entanglement, the Caltech team used
lasers to cool four collections (or ensembles) of about one million
Cesium atoms, separated by 1 millimeter and trapped in a magnetic
field, to within a few hundred millionths of a degree above
absolute zero. Each ensemble can have atoms with internal spins
that are "up" or "down" (analogous to spinning tops) and that are
collectively described by a "spin wave" for the respective
ensemble. It is these spin waves that the Caltech researchers
succeeded in entangling among the four atomic ensembles.
The technique employed by the Caltech team for creating
quadripartite entanglement is an extension of the theoretical work
of Luming Duan, Mikhail Lukin, Ignacio Cirac, and Peter Zoller in
2001 for the generation of bipartite entanglement by the act of
quantum measurement. This kind of "measurement-induced"
entanglement for two atomic ensembles was first achieved by the
Caltech group in 2005
(http://media.caltech.edu/press_releases/12776).
In the current experiment, entanglement was "stored" in the four
atomic ensembles for a variable time, and then "read
out"—essentially, transferred—to four beams of light.
To do this, the researchers shot four "read" lasers into the four,
now-entangled, ensembles. The coherent arrangement of excitation
amplitudes for the atoms in the ensembles, described by spin waves,
enhances the matter - light interaction through a phenomenon known
as superradiant emission.
"The emitted light from each atom in an ensemble constructively
interferes with the light from other atoms in the forward
direction, allowing us to transfer the spin wave excitations of the
ensembles to single photons," says Akihisa Goban, a Caltech
graduate student and coauthor of the paper. The researchers were
therefore able to coherently move the quantum information from the
individual sets of multipartite entangled atoms to four entangled
beams of light, forming the bridge between matter and light that is
necessary for quantum networks.
The Caltech team investigated the dynamics by which the
multipartite entanglement decayed while stored in the atomic
memories. "In the zoology of entangled states, our experiment
illustrates how multipartite entangled spin waves can evolve into
various subsets of the entangled systems over time, and sheds light
on the intricacy and fragility of quantum entanglement in open
quantum systems," says Caltech graduate student Kyung Soo Choi, the
lead author of the Nature paper. The researchers suggest that the
theoretical tools developed for their studies of the dynamics of
entanglement decay could be applied for studying the entangled spin
waves in quantum magnets.
Further possibilities of their experiment include the expansion
of multipartite entanglement across quantum networks and quantum
metrology. "Our work introduces new sets of experimental
capabilities to generate, store, and transfer multipartite
entanglement from matter to light in quantum networks," Choi
explains. "It signifies the ever-increasing degree of exquisite
quantum control to study and manipulate entangled states of matter
and light."
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