Scientists have created a device that can release a single, precisely shaped photon of light, opening the way to a secure “quantum internet” that is incredibly powerful and impenetrable to hackers.
The vision is of a future network of quantum computers that can work together to solve problems beyond the capabilities of current machines.
The key to developing a quantum internet is the ability to encode information on single photons of light that can be produced on demand. Now Prof Andrea Fiore of Eindhoven University of Technology and colleagues have developed a nanoscale device that can do this by “sculpting” the shape of individual photons.
“This is a very important step to be able to produce single photons exactly when you want them,” said Prof Chris Phillips from Imperial College, London, who wasn’t involved in the research.
Quantum networks are highly sought after by governments around the world because they can make communication between two computers impenetrable to hackers. If someone does read the data while it’s being transmitted, it is automatically changed, making the hack easily detectable.
The shape of a photon is determined by how smoothly the energy it contains is released. Most of the energy is released at the start of the process, with the remainder trailing off towards the end. Even though the emission takes just billionths of a second, it gives the photon a lopsided shape.
Atoms naturally emit single photons but these are not precise enough. Fiore and colleagues found that they could produce photons with controlled shapes using a quantum dot, a piece of semiconductor material that can emit a photon when triggered.
To sculpt the photon, they placed the quantum dot inside a photonic crystal, which acted as a filter. In this situation, the emission depends on the energy of the quantum dot photon, which can be controlled by an applied voltage. This forces the light to escape at a steady pace. They have reported the discovery in the journal Nature Communications.
Quantum computers – which are theoretically possible but have not yet been built successfully – work by exploiting the weirdness of quantum mechanics. Traditional computers hold their data in bits as either 1 or 0. These are manipulated by programs allowing the machine to arrive at the correct answer.
Quantum computers hold this information in quantum bits (qubits) that can be 1 and 0 simultaneously, a phenomenon known as superposition. The calculation is encoded in the set-up of the qubits, which can then perform all the calculations simultaneously leading to a dramatic speed-up in performance.
“This is radically different technology than we have today,” said Prof Ian Walmsley of the University of Oxford. It will make certain computational problems, such as forecasting the weather, much simpler to perform. At present, such calculations are carried out on supercomputers but they still take too much time to do precisely, so approximations have to be made.
Simulating the behaviour of molecules would also be possible on a quantum computer and this could help pharmaceutical companies design more effective drugs. The interrogation of big databases would also be dramatically speeded up.
A quantum computer makes simultaneous calculations by synchronising its qubits using a phenomenon called entanglement. This links the quantum properties of particles in a way that defies the classical laws of physics.
For certain entangled states, measuring an entangled particle to learn its properties automatically forces its partner to assume the opposite characteristics. This happens instantaneously no matter how far apart the two particles have been separated – something that Einstein called “spooky action at a distance”.
Sharing entangled quantum data is not possible with current technology. For transmission across today’s internet, the precious qubits would have to be transformed into ordinary data, destroying their power.
“To make the most of quantum computers you will need a quantum internet,” says Fiore. His new device is a step towards sharing entangled particles, thus linking quantum computers across the world – once they have been developed.
“These nanostructures can be integrated into devices and chips. It opens the way to the efficient exchange of photons between quantum chips, which could be used in a future quantum network,” said Fiore.
“This is very good underpinning technology,” said Walmsley, “A quantum internet is a real possibility but it is not going to happen next week.”
To push the development of this technology, the UK government is investing £120m in four quantum technology hubs. In November, the universities at Birmingham, Glasgow, Oxford and York were announced as hub leaders by the science minister, Greg Clark.
