In a new paper published in the journal Nature Communications, researchers from the Universities of Glasgow, Stanford, Tokyo and Würzburg describe how they have implemented a novel tool for a long-distance telecommunication link which is impossible for hackers to breach. The technique could also underpin the creation of a new form of ‘quantum internet’.
Scientists have previously used the phenomenon of quantum entanglement – also known as ‘spooky action at a distance’ - to allow the exchange of information over short distances. Entanglement allows particles which are physically separated to nonetheless share properties – for example, the direction of one electron’s spin will be related to the direction of spin of its entangled partner.
This process of entanglement also allows scientists to encode information in quantum particles, similar to the way in which the ones and zeroes (known as bits) of digital communication are used to encode all kinds of data. Two computers sharing quantum information are much more secure, as any interception by a third party will change the properties of the data itself, allowing easy detection by the intended recipient.
To allow quantum computers to communicate with each other, a new type of quantum internet capable of transmitting the special quantum bits (known as qubits) over long distances will need to be built.
The team, co-ordinated by University of Glasgow postdoctoral research fellow Dr Chandra Mouli Natarajan, together with colleagues at Stanford, managed to create long distance telecommunication link for a stationary quantum bit for the first time. They created correlations between a spin of an electron stored in a tiny crystal of semiconducting material known as a ‘quantum dot’ and the arrival time of single photon across two kilometres of fiber-optic cable used in standard telecoms industry.
Their work is built on three components. The first is quantum dots – special crystals just a few nanometers, or billionths of a meter, in size. The second is a tailored waveguide, which allows us to precisely control and convert the energy of individual particles of light, also known as photons. The final component is a highly-sensitive detector capable of sensing single photons.
Quantum dots are commonly used to generate individual photons. However, these types of photons can’t travel very far in the standard fiber optic network used in telecommunications industry because they tend to leak out along the way. Quantum dots are also capable of trapping electrons, and previously our research group had shown that entanglement can be created between the trapped electron and a photon generated by the quantum dot.
For the first time, they were able to establish a long-distance correlation between the trapped electron and the photon in the telecommunications band. Firstly, the information encoded in the orientation of the emitted photon was translated to the arrival time of the photon. Secondly, the photon’s energy was lowered to a type more suitable for the telecommunications industry without altering the encoded information. In the near future, these implementations would enable execution of challenging long distance quantum teleportation experiments involving quantum dots.
The physics behind quantum communication, by their very nature, make data transfer utterly secure– any tampering with either side of the communication will be immediately apparent because it will affect the quantum correlations. This work is an important step towards creating architectures for the future hybrid quantum internet.
Dr Natarajan carried out the experiments at Stanford University, enabled by a SU2P Science Bridges Entrepreneurial Fellowship funded by National Institute of Informatics, Tokyo. Prof. Robert Hadfield’s group in the School of Engineering at the University of Glasgow provided key technical expertise in advanced single photon detector technology in the telecom band.
More information about this approach can be seen in the paper - Two-photon interference at telecom wavelengths for time-bin-encoded single photons from quantum-dot spin qubits. This has been published in Nature Communications.
