October 28, 2004
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.