Number 250, December 1, 1995 by Phillip F. Schewe and Ben Stein
TWO-BIT QUANTUM LOGIC GATES have been experimentally demonstrated for
the first time. Analogous to conventional electronic logic gates in personal
computers but different in that they follow the strange rules of quantum
mechanics, a quantum logic gate, in its simplest form, consists of two
"qubits." Each qubit is a quantum system (for example an atom
or a photon) having two states corresponding to the 0 and 1 of a conventional
gate. Unlike an ordinary digital bit, a qubit can be in a combination or
"superposition" of 0 and 1, offering the potential for unique
kinds of calculations. A NIST team (Chris Monroe, 303-497-7415) uses a
single trapped beryllium ion to demonstrate a two-bit quantum logic gate.
One bit, the control bit, is specified by the (quantized) external vibrations
of the ion in the atom trap; the two lowest vibrational levels correspond
to values 0 and 1. The other bit (the target bit) is specified by an internal
state of one of the ion's electrons; it has a "spin-down" state
(0) and a "spin-up" state (1). Shooting laser pulses at the single
ion causes it to act as a two-bit "controlled NOT" gate. If the
control bit is 0 then the target bit is left alone. If the control bit
is 1 then the target bit flips its spin. Meanwhile, a Caltech group (Quentin
Turchette, 818-395-8343) has demonstrated the feasibility of using a pair
of electromagnetic fields (each representing a single photon or less) as
a two-bit quantum gate. When the two fields interact with an atomic beam
in between a narrow cavity, the first field, having one of two orientations,
or "polarizations," can control the phase of the second field;
switching the polarization prevents the first field from controlling the
phase. Finally, in a paper submitted to Physical Review Letters, a team
at the Ecole Normale Superieure (Serge Haroche, haroche@physique.ens.fr)
reports a quantum logic gate in which a two-level electromagnetic field
in a cavity changes the energy level of a Rydberg atom (an atom in a highly
excited state) in the cavity. All groups are currently attempting to string
together multiple gates, but this remains a major challenge. Performing
the powerful calculations envisioned with quantum computers would probably
require thousands of gates, but Haroche warns that systems of quantum gates
are likely to become "decoherent," or lose their quantum properties,
beyond several tens or hundreds of gates. While practical "quantum
computers" might be difficult to realize with present concepts, physicists
believe these two-bit experiments may pay off by opening possibilities
for practical schemes of quantum teleportation and quantum cryptography
and by bringing new insights into, as Haroche puts it, "the fuzzy
boundary between the classical and quantum worlds." (C. Monroe et
al. and Q. A. Turchette et al., Physical Review Letters, Dec. 18; journalists
should contact AIP Public Information at physnews@aip.org)
EVIDENCE FOR COSMIC RAYS COMING FROM A SUPERNOVA has finally been observed.
The standard opinion about cosmic rays is that the lower-energy rays (up
to an energy of 10**15 eV) probably originate in our galaxy and consist
of electrons and ions accelerated to high speeds by supernova shocks. (Higher-energy
cosmic rays may be extragalactic in origin.) New pictures of supernova
SN1006 recorded by the orbiting ASCA x-ray telescope reveal both thermal
x rays---the radiation coming from supernova remnant material at high temperature---and
non-thermal x rays from the limb of the supernova---synchrotron radiation
from high energy electrons (100 TeV), presumably energized by the outward-moving
shock front from the supernova. The ASCA scientists expect that ions too
are being accelerated by the same mechanism. (K. Koyama et al., Nature,
16 November.)
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