Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


A Quantum Low Pass for Photons


Physicists in Garching observe novel quantum effect that limits the number of emitted photons.

The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called Poisson-distribution. There are, however, light sources with non-classical photon number distributions that can only be described by the laws of quantum mechanics.

Illustration of the two-photon blockade.

Top: Irradiated by a laser pulse a single atom in free space can absorb and emit only one photon at a time, without constraints on the direction of the photons.

Middle: A system consisting of a cavity can absorb and emit an unlimited number of photons.

Below: In case of the strongly coupled atom-cavity system the frequency of the laser light can be chosen such that the system can store and emit two photons at

MPQ, Quantum Dynamics Division

A well-known example is the single-photon source that may find application in quantum cryptography for secret key distribution or in quantum networks for connecting quantum memories and processors. However, for many applications in nonlinear quantum optics light pulses with a certain fixed number of photons, e.g. two, three or four, are highly desirable.

A team of scientists from the Quantum Dynamics Division of Professor Gerhard Rempe at the Max Planck Institute of Quantum Optics (Garching near Munich) has now succeeded to make the first steps in this direction. Using a strongly coupled atom-cavity system, they were the first to observe the so-called two-photon blockade: the system emits at most two photons at the same time since its storage capacity is limited to that number (PRL, 31 March 2017).

A naive approach for generating a stream of single photons would be to sufficiently attenuate the intensity of a laser beam. But in this case the number of photons still varies from pulse to pulse, and only when averaging over many pulses a mean photon number of one is observed. Applications instead require a fixed number of exactly one photon per pulse.

The fluctuations of the photon number per pulse can be strongly reduced by using a single atom as a single-photon source. When the atom is illuminated by a laser beam, it can absorb only one photon at a time, thereby making a transition from the ground state to an excited state. A second photon can only be absorbed after the atom has fallen back to the ground state by emitting a photon. Therefore, no more than one photon is detected in the emitted light field at the same time, an effect that is known as “single-photon blockade”.

In order to extend this principle to a “two-photon blockade” one has to go beyond a single atom and look for a system that can store more than one photon, but not more than two. To this end, the MPQ physicists combine the single atom with a cavity that provides additional storage capacities. A cavity can absorb an unlimited number of photons and exhibits a correspondingly large number of energy states that lie – similar to a “ladder” – in exactly the same distance from each other.

Inserting a single atom into the cavity introduces a nonlinear element. This causes the energy levels to split by a different amount for each of the ‘ladder steps’. Hence, laser light can excite the system only up to the level to which it is tuned to. The number of photons that can be stored is thus limited to a certain number, and therefore, not more photons than that can be emitted.

In the experiment, the physicists hold a single rubidium atom in an optical trap inside a cavity made of two high-finesse mirrors. The frequency of the incoming laser beam is tuned to an energy level requiring the absorption of two photons for its excitation. During the five seconds of atom storage time around 5000 measurement cycles are carried out, during which the system is irradiated by a probe laser and emission from the cavity is recorded via single-photon detectors.

“Interestingly, the fluctuations in the number of emitted photons does strongly depend on whether we excite the cavity or the atom,” points out the project leader Dr. Tatjana Wilk. “The effect that the absorption of two photons suppresses further absorption leading to emission of two or less photons is only achieved in case of atomic excitation. This quantum effect does not appear when we excite the cavity. In this case, we observe an enhanced signal of three and more photons per light pulse.”

Christoph Hamsen, doctoral candidate at the experiment, explains the underlying processes: “When the atom is excited we are dealing with the interplay between two conflicting mechanisms. On the one hand, the atom can absorb only one photon at a time. On the other hand, the strongly coupled atom-cavity system is resonant with a two-photon transition. This interplay leads to a sequence of light pluses with a non-classical photon distribution.”

And Nicolas Tolazzi, another doctoral candidate, adds: “We were able to observe this behaviour in correlations between detected photons where the coincidence of three photons was significantly suppressed compared to the expectation for the classical case.”

Prof. Gerhard Rempe gives an outlook on possible extensions of the experiment: “At present, our system emits light pulses with two photons at maximum, but also pulses with fewer, one or even zero, photons. It acts like a kind of ‘low pass’. There are, however, a number of applications for quantum communicating and quantum information processing where exactly two, three or four photons are required. Our ultimate goal is the generation of pure states where each light pulse contains exactly the same desired number of photons. The two-photon blockade demonstrated in our experiment is the first step in this direction." Olivia Meyer-Streng

Picture caption:
Illustration of the two-photon blockade.
Top: Irradiated by a laser pulse a single atom in free space can absorb and emit only one photon at a time, without constraints on the direction of the photons.
Middle: A system consisting of a cavity can absorb and emit an unlimited number of photons.
Below: In case of the strongly coupled atom-cavity system the frequency of the laser light can be chosen such that the system can store and emit two photons at maximum.

Original Publication:
Christoph Hamsen, Karl Nicolas Tolazzi, Tatjana Wilk, and Gerhard Rempe
Two-Photon Blockade in an Atom-Driven Cavity QED System
Physical Review Letters 118, 133604 (31.03.2017)


Dr. Tatjana Wilk
Max Planck Institute of Quantum Optics
Phone: +49 (0)89 / 32 905 - 670

Prof. Dr. Gerhard Rempe
Director at the Max Planck Institute of Quantum Optics
Hans-Kopfermann-Str. 1
85748 Garching, Germany
Phone: +49 (0)89 / 32 905 - 701

Dr. Olivia Meyer-Streng
Press & Public Relations
Max Planck Institute of Quantum Optics
Phone: +49 (0)89 / 32 905 - 213

Dr. Olivia Meyer-Streng | Max-Planck-Institut für Quantenoptik
Further information:

More articles from Physics and Astronomy:

nachricht Witnessing turbulent motion in the atmosphere of a distant star
23.08.2017 | Max-Planck-Institut für Radioastronomie

nachricht Heating quantum matter: A novel view on topology
22.08.2017 | Université libre de Bruxelles

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Fizzy soda water could be key to clean manufacture of flat wonder material: Graphene

Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.

As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...

Im Focus: Exotic quantum states made from light: Physicists create optical “wells” for a super-photon

Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.

Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...

Im Focus: Circular RNA linked to brain function

For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.

While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...

Im Focus: RAVAN CubeSat measures Earth's outgoing energy

An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.

The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...

Im Focus: Scientists shine new light on the “other high temperature superconductor”

A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.

Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...

All Focus news of the innovation-report >>>



Event News

Call for Papers – ICNFT 2018, 5th International Conference on New Forming Technology

16.08.2017 | Event News

Sustainability is the business model of tomorrow

04.08.2017 | Event News

Clash of Realities 2017: Registration now open. International Conference at TH Köln

26.07.2017 | Event News

Latest News

Researchers devise microreactor to study formation of methane hydrate

23.08.2017 | Materials Sciences

ShAPEing the future of magnesium car parts

23.08.2017 | Automotive Engineering

New insights into the world of trypanosomes

23.08.2017 | Life Sciences

More VideoLinks >>>