A research group of the Max Planck Institute of Quantum Optics and the Munich University, have for the first time realized Planck’s oscillators in an experiment (nature, 17 February 2000).
After 100 years, an idea of Max Planck’s (1858-1947) has been successfully demonstrated experimentally for the first time. The founder of the quantum theory postulated that oscillators in matter are responsible for the emission of radiation in the form of photons. Scientists at the Max Planck Institute for Quantum Optics and the Munich University now succeeded to store individual photons in a resonator. This resonator can be considered being equivalent to the oscillators assumed by Max Planck. The research group - Herbert Walther, Ben Varcoe and Simon Brattke - using the one-atom maser (Microwave Amplification by Simulated Emission of Radiation) developed at the Max Planck Institute of Quantum Optics, experimentally produced radiation fields with a specific number of photons. Since these fields can only be described by the laws of quantum physics, this radiation is also called non-classical radiation. It differs from the light of a laser, which obeys the laws of classical physics. New applications in quantum communication and quantum cryptography, which are not possible with normal light or laser light, are now conceivable with this non-classical radiation. Noise reduced signals can be transmitted via individual photons, and messages can be sent without the risk of line tapping, as with quantum fields each line tap of a signal channel would immediately be recognised.
Light or - more generally - electromagnetic radiation, consists of individual energy quanta. Therefore the radiation is not emitted in a continuous flow, but rather is sent in discrete packages whereby the energy of a packet depends on the frequency of the radiation. In accordance with Planck’s opinion of 100 years ago, photons were emitted by the above mentioned oscillators.
The experiment to isolate individual photons is very complex. The radiation field, which consists of a fixed number of photons, must, with as minimal a loss as possible, be stored in the resonator of the one-atom maser (micromaser). This is achieved using a super-conducting niobium resonator. The principle, that is the basis for the micromaser, is the interaction of an individual atom with an individual oscillatory mode of the resonator.
The photons are generated in the resonator. A precise number of atoms are sent into the cavity and release their energy there in the form of photons. The emission of the photons is caused by the so-called vacuum fluctuations representing the ground state of the quantum field. The control of the interaction time of the atom with the resonator guarantees that a photon is emitted with high probability. The precise number of photons generated and stored in the resonator depends thus on the number of atoms sent through the cavity. The average lifetime of a light quanta in a resonator amounts to 0.2 seconds, which is substantially longer than the interaction time of an atom with the resonator. This storage time is orders of magnitude larger than those achieved in previous experiments.
The number of photons present are measured by the scientists with the help of a rubidium atom, which is sent with defined velocity through the cavity. The atom is excited into an highly excited state called a Rydberg state, which allows the transitions in the microwave region. It enters the resonator in an excited state, emits and then reabsorbs a photon, thereby, leaving the resonator in an excited state. What is particular about the method, is that the photons are not destroyed by the measurement. The quantum states of the radiation field, achieved was of high purity. Fields of up to three photons could be generated.
One hundred years ago, Max Planck predicted the existence of photons or light quanta. Since their energy depends on the wavelength of the radiation, they have higher energy in the blue area of the visible spectrum than in the red section. Planck made these assumptions in order to explain the spectrum of a hot radiating body. At a temperature of about 800°C, a body begins to send out visible light. The light is initially red but changes its colour with increasing temperature, up to the point of incandescence. With increasing temperature further colours of the spectrum are added; these changes occur until eventually the entire spectrum is emitted. At the time, there was a lot of interest in an understanding of the spectrum of hot bodies, as one wanted to make the future use of the new electric light bulb as effective as possible.
Although the structure of the atom and therefore the mechanisms of light radiation were not particularly well known 100 years ago, Max Planck found an explanation for the spectrum of a hot body. In Planck’s vision, the radiation is produced through oscillators which emit the radiation. The oscillators - in accordance with the idea of oscillating electrons at the time - had an energy which corresponds to a precise number of photons. The energy distribution of the oscillators was already given in the theory of thermodynamics.
With this assumption, Planck could explain the spectrum of a hot body. Through his revolutionary assumption, he had intuitively anticipated what would become fact in future years, with the development of a full understanding of quantum mechanics. Today we know that Planck oscillators correspond to the quantum states of the electromagnetic field, which is exactly what the scientists of the Max Planck Institute for Quantum Optics and the Munich University have experimentally realized.
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