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.)
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