Cambridge 12 Jan 2006
Breakthrough enables long-distance secure communication, high resolution imaging and quantum computing.
Researchers at Toshiba Research Europe Ltd (TREL) and the University of Cambridge have discovered that light possessing quantum entanglement can be generated by a simple semiconductor device. It could lead to long-distance, highly-secure optical networks safe from hacking, more sensitive medical diagnosis, more powerful computer chips and scalable quantum computing.
Unlike normal light in which the photons (the ‘particles’, or quanta, of light) can be regarded as distinct, the new source emits a stream of photons in pairs at regulated times with ‘entangled’, or interrelated, properties. The breakthrough will be reported today in the scientific journal Nature.
The new entangled photon source is similar to an ordinary semiconductor light source, but contains a tiny, nanometer-sized quantum dot that emits the coupled photons. TREL researcher and lead author, Dr Mark Stevenson, said: “we discovered that only dots with a certain shape can emit photon pairs which are entangled and that the required shape can be engineered by controlling its growth process.”
Entangled photon pairs have the essential attribute that their properties are inter-related. Although measuring either photon in the pair produces a random result, these two seemingly random results are always the same for the two photons of an entangled pair. This means that measuring one photon appears to alter the property of its entangled twin, even if it is 100's of kilometres away. This peculiar feature of quantum theory was referred to by Albert Einstein as ‘spooky action at a distance’.
Dr Andrew Shields, head of the Quantum Information Group at TREL remarks: “Generators of entangled photons are essential components in future IT systems that exploit quantum effects. For example, they could allow us to overcome the current distance limitation in quantum cryptography by teleporting the quantum keys from any location to another. A simple device for generating entangled photons will greatly accelerate these technologies, as well as stimulate new ones. Indeed, analogy with developments after the invention of the semiconductor laser suggests there may be many more applications that we have not yet even imagined.”
Another attractive application of entangled photons is in optical imaging. The detail that can be resolved in ordinary images is limited by the wavelength of the light used. Using entangled photons, however, it is possible to produce finer images. This could be useful for very high-resolution microscopy of cell structure in medical diagnosis or to produce finer patterns in the manufacture of ultra-dense computer chips.
Until now, the most reliable method of producing entangled photons was by pumping a non-linear crystal with a strong laser beam. However, this process is inefficient and cannot produce photon pairs at controlled times. In contrast, the new device produces photons in response to an external trigger pulse, which is essential for many applications. The semiconductor device is also more compact, robust and potentially much cheaper, since it can be mass manufactured with a similar process to ordinary semiconductor light emitting devices. The breakthrough could therefore lead to photon entanglement being exploited in a wide range of light-based applications.
The Managing Director of TREL's Cambridge Research Laboratory, Professor Sir Michael Pepper, commented “In Toshiba we have been convinced for a long time that fundamental physics could be used for a new generation of communication systems that give entirely new functions such as inherent security. The ability to generate entangled photons on demand, by pressing a switch, opens the door to many new applications. These include quantum computers based on novel principles and may even extend to consumer electronics as entangled photons can be used to make devices with much smaller features than normal photons.”
The work was partially funded by the UK Engineering and Physical Sciences Research Council through the IRC in Quantum Information Processing and EC Future and Emerging Technologies programme.
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