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update.250 (fwd)
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From [email protected] Fri Dec 1 18:18:29 1995
Date: Fri, 1 Dec 95 14:44:34 EST
From: [email protected] (AIP listserver)
Message-Id: <[email protected]>
To: [email protected]
Subject: update.250
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
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, [email protected]) 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.,
two upcoming articles in Physical Review Letters, tentatively Dec.
11; journalists should contact AIP Public Information at
[email protected])
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.)