Thinking about the quantum internet
Quantum computers deserve their own quantum internet. This is a network that dispatches information not in the form of bits - ones and zeros - but rather as qubits, just like in the quantum computer itself. In the view of Dirk Bouwmeester, a professor in Leiden’s Quantum Matter & Optics research group, a cloud of cheap mini-satellites is going to roll out this network across the whole world. Among other things, this network will make it possible to send messages in a way that is absolutely secure.
No more doubts
‘The quantum computer is coming,’ Bouwmeester assures us. ‘You can be 100% sure about that.’ A couple of years ago he wouldn’t have said that with the same confidence, but since then there have been so many technical breakthroughs that all his doubts have disappeared. The first prototype could even be built within five years or so. In 2014, Bouwmeester won a Spinoza prize, the highest scientific distinction in the Netherlands, for his research programme on the utmost consequences of quantum theory. He is also closely involved in research on ‘artificial atoms’ that communicate via infrared photons.
You can’t amplify a qubit
He wants to use the same principle to link up two quantum computers over long distances. If these computers can communicate with each other using qubits, a quantum internet will be created that will raise the quantum computer’s possibilities to an entirely new level, just as the Internet has done with the conventional computer. The quantum internet will not make the current Internet obsolete, but will rather complement it.
Computers currently communicate with light signals passing through glass fibres. A light signal is a sort of Morse code, with alternations of on’s and off’s corresponding to a bit’s value of 0 or 1. Over great distances, such as from Europe to the US, signal amplifiers need to be deployed in the glass fibre cable at regular intervals to prevent the signal from fading. This sort of amplifier measures the sequence of zeros and ones and sends them on further in amplified form.
But this isn’t possible with qubits. As soon as you measure the value of a qubit, a 0 or 1 is emitted and the qubit is no longer a mixture of 0 and 1 at the same time; it’s simply a normal bit. A quantum repeater is needed to get quantum computers to communicate in their own language.
Chain of artificial atoms
A quantum repeater is considerably more complex than a glass fibre cable with amplifiers. Essentially, two quantum computers are connected using a chain of intermediary stations. Each intermediary station emits pairs of photons that are entangled with each other to the neighbouring station to its left and its right. These photons become entrapped by an artificial atom in the neighbouring station and are subsequently read out in such a way that a quantum entanglement is created between stations, which are set increasingly further away from each other, until the entire distance has been covered. The ultimate result is that you can teleport the quantum information that is in the qubit at one end of the chain to the other end, without any disturbance and without the signal fading.
It is obvious that this should no longer be done with glass fibre, but with satellites. One of Bouwmeester’s former PhD students, William Marshall, has set up a company (Planet Labs) that has a large number of mini-satellites in space for making photo images of our whole planet. This kind of cloud of optical mini-satellites, each weighing no more than a kilogram, would be able to cover the planet with a network of quantum repeaters.
The Internet as we know it would continue to exist, as there is no reason to send Facebook and everyday e-mail over the quantum internet. But with the quantum internet you could merge multiple quantum computers into an even more powerful machine, and you could use it to exchange coded messages that are absolutely secure, because it becomes immediately evident when some attempts to intercept the signal. This last issue is called quantum cryptography, which is already in an advanced stage of development.
Besides developing elements for a quantum internet, Bouwmeester is also in search of fundamental limitations to the occurrence of quantum entanglement. We still do not know how quantum theory affects the evolution of the universe and life as we know it. For example, we cannot exclude the possibility that gravity plays a role in the transition from quantum laws to classical laws of physics for large-scale objects. To develop the experiments necessary to study this topic, we may need to turn to optical experiments in space. In the near future, this will lead to research on the quantum internet, quantum code and large-scale quantum effects being combined.
‘What I’m aiming at is research that is extremely difficult for a university from the technical angle. But thanks to the combined expertise of my research group in Santa Barbara in the area of manufacturing semiconductors and my group in Leiden in the area of quantum optics, we are in a unique position to make this a success.’
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