Science > Physics > Physics News Update -- Number 655, September 26, 2003
| Topic: |
Science > Physics |
| User: |
"Sam Wormley" |
| Date: |
26 Sep 2003 02:33:13 PM |
| Object: |
Physics News Update -- Number 655, September 26, 2003 |
Physics News Update -- Number 655, September 26, 2003
Ref: http://www.aip.org/physnews/update/
Number 655, September 26, 2003 by Phillip F.
Schewe, Ben Stein, and James Riordon
An Ultrabright Tunable Photon-Pair Source
An ultrabright tunable photon-pair source created at MIT is the best
generator so far of entangled photon pairs, a development which
should help quantum communications systems to do their job more
smoothly. Entangled photons possess a special correlation unlike
anything in classical physics: if, say, we measure the spin
(polarization) of one photon, then we automatically know the
polarization of the other photon, even though it might be on the
other side of the galaxy and even if, until the moment of
measurement, the spins of both photons had been indeterminate. This
weird property of quantum reality, it is hoped, will be a boon to
encryption (perhaps in a "quantum teleportation" scheme - see
Physics News Update 350) and future quantum computers. Indeed, for
some time now quantum effects have been an important factor in
communications engineering applications, especially insofar as
quantum fluctuations (uncertainty in our knowledge of where an
electron is or the value of its energy) can produce levels of
electrical noise that can limit the effectiveness of practical
devices. The use of entangled photons might be able to mitigate this
problem. Quantum limitations are already a problem in such devices
as optical amplifiers (whose amplified spontaneous emission noise
limits communication performance) or soliton pulses (supposedly
non-dispersing light pulses that are subject to quantum-induced
timing jitter accumulation) used in fiber-optic communications.
MIT's Research Laboratory of Electronics is a place where quantum
aspects of electrical engineering are taken very seriously. The head
of the lab, Jeffrey H. Shapiro (617-253-4179), will report on
progress in a program aimed at developing a system for
long-distance, high-fidelity teleportation of photon states at the
upcoming Frontiers in Optics meeting of the Optical Society of
America. As part of this work the MIT team has developed a source of
entangled photons some ten times brighter than previous sources. The
correlated photons are engendered by shooting a laser beam into a
nonlinear optical crystal, where incoming photons are, in effect,
split into two related photons of half the wavelength. This
"down-conversion" process is even tunable over a certain wavelength
range. Up to 12,000 photon pairs per second per milliwatt of input
power have been produced. (Paper MI3, OSA meeting 5-9 October in
Tucson, AZ; meeting website)
The Relativity of Time
The relativity of time, as set forth in Einstein's theory, has been
affirmed once again, with new higher precision. Time dilation is the
name for the notion that elapsed time as recorded by two observers
with identical clocks will differ if one of the observers is
traveling at a velocity v with respect to the other. The amount of
dilation will become more noticeable as v becomes a larger fraction
of the speed of light. In an experiment performed by Gerald Gwinner,
Dirk Schwalm and their colleagues at the Max Planck Institute for
Nuclear Physics in Heidelberg the clocks are lithium ions. The ions
are struck by laser light from in front and from the back, putting
them temporarily into an excited state and inducing fluorescence. By
comparing the resonant laser wavelengths with the transition
wavelength of the stationary ion, and by taking into account the
Doppler effect (the apparent wavelength of a wave emitted from a
traveling source will always be different from a stationary source
owing to bunching or thinning of the wave crests - but this has
nothing to do with relativity) the researchers can arrive at a value
for time dilation. In the Heidelberg experiment, the lithium ions
moved with a speed of 19,000 km/sec, or about 6.4 % of the speed of
light (and corresponding to an energy of 13.3 MeV, the largest
energy obtainable at the local heavy-ion storage ring). The
precision of the new time dilation measurement, an uncertainty of
2.2 x 10-7, is about a factor of four better than the best previous
value. (Saathoff et al., Physical Review Letters, upcoming article;
contact Guido Saathoff, 49-6221-516-547; website)
Malleability of Spacetime
Malleability of Spacetime, as set forth in Einstein's general
relativity theory, has been affirmed, once again, by watching radio
waves from the Cassini spacecraft, on its way toward Saturn, be
deflected by the sun. Einstein said that a massive object would
distort the fabric of spacetime in its vicinity, and that this
distortion would slightly redirect the trajectory of light waves
passing the object. Scientists from three Italian universities
(those of Pavia, Rome, and Bologna) have carefully scrutinized
Cassini's radio report and found that the observed light deflection
is in accordance with the conventional form of relativity.
Furthermore, the sensitivity of their measurements is at a level
where some alternative gravity models can be probed for veracity.
(Bertotti et al., Nature, 25 November 2003.)
Physics News Update is a digest of physics news items arising from
physics meetings, physics journals, newspapers and magazines, and
other news sources. Subscriptions are free as a way of broadly
disseminating information about physics and physicists. Feel free to
post it where others can read it; please credit the American
Institute of Physics. Physics News Update appears approximately once
a week. Questions? Contact the editors at physnews@aip.org.
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| User: "Uncle Al" |
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| Title: Re: Physics News Update -- Number 655, September 26, 2003 |
26 Sep 2003 04:49:47 PM |
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Sam Wormley wrote:
Physics News Update -- Number 655, September 26, 2003
Ref: http://www.aip.org/physnews/update/
Number 655, September 26, 2003 by Phillip F.
Schewe, Ben Stein, and James Riordon
An Ultrabright Tunable Photon-Pair Source
An ultrabright tunable photon-pair source created at MIT is the best
generator so far of entangled photon pairs, a development which
should help quantum communications systems to do their job more
smoothly.
[snip]
Would you like to buy a bridge?
As part of this work the MIT team has developed a source of
entangled photons some ten times brighter than previous sources. The
correlated photons are engendered by shooting a laser beam into a
nonlinear optical crystal, where incoming photons are, in effect,
split into two related photons of half the wavelength. This
"down-conversion" process is even tunable over a certain wavelength
range. Up to 12,000 photon pairs per second per milliwatt of input
power have been produced. (Paper MI3, OSA meeting 5-9 October in
Tucson, AZ; meeting website)
1) The wavelength doubles not halves. Energy is conserved.
2) 12,000 photons/milliwatt-sec is putrid as a source of anything.
It's laughable.
[snip]
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!
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| User: "tj Frazir" |
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| Title: Re: Physics News Update -- Number 655, September 26, 2003 |
26 Sep 2003 06:22:13 PM |
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What is MIT on thses days ?
Some one should test the water.
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