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Applied Quantum Physics with Single Photons: How Light Particles Can Make Communication Safe from Interception

Por: | 05 de mayo de 2014


By Thomas Aichele, Humboldt-Universität zu Berlin

In 1905, Albert Einstein published his ingenious explanation of the photoelectric effect, for which he was awarded the Nobel Prize in Physics in 1921. In this work he explained the emission of electrons from a metal plate by the absorption of light particles (or photons); this was in opposition to the classical wave picture. Even today, the photon is the workhorse used to test the foundations of quantum physics against a purely classical interpretation of nature.

At the beginning of the 1980s, proposals were first made in which conventional, classical applications in the field of information processing were extended into the quantum world. Nobel Prize laureate R. P. Feynman suggested using quantum mechanical states to replace classical bits in a computer. Soon, algorithms were discovered that proved the superiority of such quantum computers to classical computers in certain situations. Although such systems today are still in a very initial development state, their fascinating promises have greatly increased our knowledge of quantum mechanics. In modern quantum computation concepts, photons are especially important as transmitters for quantum bits.

Quantum cryptography, a second important application of single photons, was discovered in 1984 by C. H. Bennett and G. Brassard. It enables the secure transmission of private data without the possibility of wiretapping. In this scheme, a cryptographic key is encoded on the polarization of a sequence of single, isolated photons, which are transferred between remote communication partners. The security of the scheme relies on the so-called no-cloning theorem, a fundamental law in quantum physics. This prevents a third party from intercepting the quantum key and perfectly copying the detected (and thus annihilated) photons. Instead, additional noise will be introduced into the transmission channel that can be discovered by both sender and receiver in time. In such a case they can abort their communication at a point where only an unused key, but no private data was transmitted.

For transmission over larger distances, the photon is currently the only reasonable quantum information carrier. Transmissions over 200 km have been recently demonstrated by research groups in the USA. Today several commercial quantum communication systems are available and have been utilized—for example, systems for securely transmitted bank transfers or for the transmission of election results in Geneva, the Swiss canton.

In spite of their fundamental character, single photons are not easy to generate. Most natural light sources tend to emit multiple photons instead of individual particles per time unit, even for the case of high attenuation.

A widely used method to sort photons is parametric down-conversion, where a non-linear crystal is ‘pumped’ by a strong laser field. With a certain probability, a laser photon is down-converted to a photon-pair of half frequency. The detection of the first photon heralds the presence of the second with a high accuracy. Yet, as this is a random process, no on-demand single-photon generation can be achieved.

An alternative is single quantum emitters — physical systems with discrete energy levels, which emit one single photon at a time upon spontaneous decay. A discrete electronic transition in an atom is the prototype of such a quantum emitter. In such systems, a laser pulse moves an atom into an excited state in which an electron possesses increased energy. From this state, the electron can decay back to its ground state, with the energy being carried away by the emission of a single photon. The emission of an isolated single atom is characterized by a high mode pureness. However, the technical complexity of traps for single atoms has restricted this system to fundamental research, so far. Nowadays, a multitude of further systems have been identified: dye molecules and color defects in diamond crystals, for example, can be utilized as single-photon sources using standard microscopy techniques at ambient conditions.

Applications such as quantum cryptography transmissions, however, require single-photon devices of high efficiency combined with a low cost of materials. Here, so-called quantum dots have a great potential of fulfilling these demands. Quantum dots are semiconductor structures with a few nanometers in size, that are prepared on semiconductor wafers. Because of their small size, a quantum dot possesses optical properties similar to a single atom. By appropriate choice of shape and material, quantum dots can offer a broad accessible wavelength range from ultraviolet to infrared. But their biggest advantage is their compatibility with current chip technology. Hence, integration in small microelectronic devices is possible, which makes quantum dots appealing for the development commercial devices.

However, this integration has been hindered so far by the necessity to cryogenically cool quantum dot sources in order to get efficient single-photon emission. Typically, systems have to be cooled to below -170 °C using liquid helium or liquid nitrogen. Recently, at Institut Néel in Grenoble (France), we developed quantum dots integrated in 10 nm-wide semiconductor wires with which we achieved the operation of a single photon source at -50 °C. Although still below the aspired goal at room temperature, this is in reach for cost-efficient and compact electrical cooling using Peltier elements.

Quantum technologies have the potential to influence our society in a similar way as the development of computers have done over the last centuries. While the potential impact of quantum computers on future technologies is still difficult to foretell, secure communication based on quantum cryptographic transmission has already passed initial demonstrations. Single-photon sources have proven to be an especially important factor in this fascinating and still emerging field.

Thomas Aichele
Humboldt-Universität zu Berlin

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