Photons under control

October 28, 2004

This release is also available in German

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 Internet.

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 field.

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.

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 Walther
Continuous generation of single photons with controlled waveform in an ion-trap cavity system Nature, 28 October 2004


Related Photons Articles from Brightsurf:

An electrical trigger fires single, identical photons
Researchers at Berkeley Lab have found a way to generate single, identical photons on demand.

Single photons from a silicon chip
Quantum technology holds great promise: Quantum computers are expected to revolutionize database searches, AI systems, and computational simulations.

Physicists "trick" photons into behaving like electrons using a "synthetic" magnetic field
Scientists have discovered an elegant way of manipulating light using a ''synthetic'' Lorentz force -- which in nature is responsible for many fascinating phenomena including the Aurora Borealis.

Scientists use photons as threads to weave novel forms of matter
New research from the University of Southampton has successful discovered a way to bind two negatively charged electron-like particles which could create opportunities to form novel materials for use in new technological developments.

The nature of nuclear forces imprinted in photons
IFJ PAN scientists together with colleagues from the University of Milano (Italy) and other countries confirmed the need to include the three-nucleon interactions in the description of electromagnetic transitions in the 20O atomic nucleus.

Pushing photons
UC Santa Barbara researchers continue to push the boundaries of LED design a little further with a new method that could pave the way toward more efficient and versatile LED display and lighting technology.

Photons and electrons one on one
The dynamics of electrons changes ever so slightly on each interaction with a photon.

An advance in molecular moviemaking shows how molecules respond to two photons of light
Some of the molecules' responses were surprising and others had been seen before with other techniques, but never in such detail or so directly, without relying on advance knowledge of what they should look like.

The imitation game: Scientists describe and emulate new quantum state of entangled photons
A research team from ITMO University, MIPT and Politecnico di Torino, has predicted a novel type of topological quantum state of two photons.

What if we could teach photons to behave like electrons?
The researchers tricked photons - which are intrinsically non-magnetic - into behaving like charged electrons.

Read More: Photons News and Photons Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to