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Photons Under Control

Photons Under Control - Physics Forum

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Old 10-28-2004, 01:01 PM
Ken Kubos
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Default Photons Under Control

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Photons Under Control
October 28, 2004

Building block created for quantum-computing, secure communication and
quantum Internet

Researchers at the Max Planck Institute of Quantum Optics (MPQ) in
Garching, Germany have achieved unprecedented control over the creation of
single photons (Nature, October 28, 2004). By using a tightly trapped single
calcium ion, localized between two ultra-high reflectivity mirrors, and
subjecting it to an external laser pulse, the scientists could emit photons
one by one. The emission time and the pulse shape of each photon were
completely user-controlled. Remarkably, the device was operated without
interruption over a period limited only by the trapping time of the ion,
typically many hours. The achievement has important applications in quantum
information processing. A controlled quantum interface between atoms and
photons has become feasible. In this way, local ion-based operations on
quantum states can be combined with long distance quantum information
exchange, a key requirement for the implementation of a secure quantum

Fig. 1: Ion trap used in the experiment. The ion is loaded at the rear end
and pushed along the trap axis to the centre of the cavity. Photon emission
is achieved by means of a pump pulse injected from the side. Single photons
are emitted through the output mirror. Image: Max Planck Institute for
Quantum Optics

Next year the 100th anniversary of Einstein's discovery of the photoelectric
effect will be celebrated. This discovery was at the time an important
additional proof of Max Planck's quantum hypothesis, which he formulated in
the year 1900. According to this hypothesis the energy of an electromagnetic
wave does not consist of a continuous flow but of discrete energy packages,
the photons. Photons are emitted in an uncontrolled way by atoms. In the
past, this has not been a problem, because in the macroscopic world, we only
experience the effect of light as the sum of trillions of photons each
second, so that fluctuations are averaged out. New types of light sources
have recently been developed in the laboratory however, that emit photons
one by one. These experiments are motivated by schemes proposing to use the
quantum states of photons to process information with unparalleled
efficiency, or to realize secure communication. To work reliably, quantum
processing schemes require emission and absorption of the photons in a fully
controlled way.

One method to create a single photon is to place a single atom between two
mirrors, which form a cavity, resonantly supporting the photon to be
generated. From a suitable excited state, the atom emits a single photon
into the cavity mode. The main problem with using an atom is the lack of
control over its position in the cavity due to limitations of trapping
technology. This leads to randomly fluctuating conditions for photon
generation and hence random properties of the emitted photons.

Matthias Keller, Birgit Lange, Kazuhiro Hayasaka, Wolfgang Lange and Herbert
Walther of the Max Planck Institute of Quantum Optics have overcome the
limitations of trapped atoms in cavities. They used a single calcium ion,
confined in a radio frequency trap (Fig. 1). By means of laser cooling, the
ion's motion was restrained to a region 40 nm in diameter. This is only a
fraction of the wavelength of the photons to be generated (866 nm) and
provides optimum conditions for controlling the interaction of ion and

The ion was placed between two high-reflectivity mirrors (see Fig. 1). The
distance between the mirrors is adjusted, so that a standing light wave can
form between them, coinciding with a suitable atomic transition. Initially,
the cavity contains no light. Energy must be supplied externally by exciting
the ion with a laser beam injected from the side of the cavity. When the
system parameters are set correctly, the ion absorbs a photon from the
external laser. Subsequently, the strong interaction with the cavity mode
induces the ion to emit a single photon into the cavity mode. After the
emission, the ion is in a state in which it does not absorb the exciting
laser light anymore. In this way, creation of a second photon is prohibited.
In order to deliver the photon to the outside world, one of the mirrors is
made partially transparent, causing the photon to leak out of the cavity,
thus completing the process of single-photon generation.

Fig. 2: Single photon pulse shapes, obtained by statistically evaluating the
detection times of identically prepared photons. (a) single-peaked
pump-pulse. (b) twin-peaked pump-pulse. Image: Max Planck Institute for
Quantum Optics

Since the photon emission is triggered by the external laser pulse, the
researchers could create the photon at the push of a button. But not only
the emission time, the shape of the single-photon pulse is also linked to
the shape of the excitation pulse. But how can a single-photon pulse shape
be measured? In the experiment, a single photon reveals itself by producing
a click in a detector at a certain time. At this moment, all other
information about the photon is irretrievably lost. However, at the Max
Planck Institute, the researchers took advantage of the fact that their
control over the initial preparation of the ion is so good, that every
photon emitted from the apparatus has identical properties. This allows them
to probe the pulse shape by performing repeated measurements on subsequent
photons. By statistically evaluating the arrival times of the photons, which
are spread out over 2 microseconds, an image of the shape of the photon
pulse is obtained. Two examples of measured pulse shapes are shown in Fig.
2. The blue trace represents the measured photon arrival times, to be
compared with the superimposed red trace, obtained from a quantum mechanical
calculation. The precise coincidence between the two curves illustrates the
degree of control that was achieved in the experiment. Note that the pulse
shape in Fig. 2b belongs to just a single photon, which was cast in a shape
with two maxima by a corresponding pump pulse.

An additional major advantage is the long storage time of ions, usually
several hours. This is in contrast to atoms with trapping times below one
second. The Max Planck group has extracted a continuous stream of single
photons for an unprecedented 90 minutes, which is 10,000 times longer than
for atoms. This is important for a reliable operation of the device in
quantum information processing. The coupling of ions and photons in a
controlled way is required in schemes linking optical long-distance quantum
communication with ion-trap quantum processors, both of which have been
successfully demonstrated in the past. The result could be a quantum version
of the Internet, in which local processing sites are connected with each
other by photonic channels.

Original work:

Matthias Keller, Birgit Lange, Kazuhiro Hayasaka, Wolfgang Lange & Herbert
Continuous generation of single photons with controlled waveform in an
ion-trap cavity system
Nature, 28 October 2004

Source: Max Planck Society


"Buddhism elucidates why we are sentient."

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Old 10-28-2004, 07:20 PM
Posts: n/a
Default Photons Under Control

"Ken Kubos" <[Only registered users see links. ]> wrote in message news:<[Only registered users see links. ]>...

Thanks, Ken, for the information you posted. Do you have an opinion
as to the wavelength of a single photon? I know it depends on the
energy of the photon, but I am referring to the photons in this
particular experiment.
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Old 10-28-2004, 11:54 PM
N:dlzc D:aol T:com \(dlzc\)
Posts: n/a
Default Photons Under Control

Dear TomGee:

"TomGee" <[Only registered users see links. ]> wrote in message
news:cc2dde17.0410281120.1bf1162f@posting.google.c om...

It was an excited calcium atom. It has a few hundred associated
characteristic energy levels, each of which could mathematically be
back-calculated to a wavelength. It wouldn't *mean* anything, however.
Only the energy does.

David A. Smith

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