*“With a quantum computer we will be able to solve problems which are imposible with present day supercomputers”*

*“For our day to day transactions by internet (purchases, private messages etc) we code the information using mult-digit keys which ensures that no current computer can decipher them”*

*“The experiments with quantum entanglement carried out at the IAC’s Observatories, between telescopes on Tenerife and La Palma will help us to develop technologies based on entanglement and quantum teletransportation”.*

By Hector Socas (IAC)

**The discovery of quantum physics gave rise to one of the most important scientific and technological revolutions in human history. As examples, it led to the discovery of lasers, of semiconductors, and of nuclear energy. In recent years there has been a “second quantum revolution” which not only confirms the most exotic aspects of quantum physics, but will also produce bigger technological consequences. In particular new ways to implement cryptography and computation are emerging which would not be possible with other technology. At the present time there is a major international effort to build quantum computers, cryptographic systems and other devices. Juan Ignacio Cirac Sasturáin, Director of the Theoretical Division of the Max Planck Institute for Quantum Optics in Garching (Germany), invited by the Instituto de Astrofísica de Canarias, explained the basis of these devices and their potential applications, as well as the present state of the international effort and its perspectives with respect to new technology.**

**Question: You have said that we are at the beginning of a second quantum revolution. ****What do you mean?**

**Answer:** A major part of our electronic technology, such as semiconductors, lasers, and other advances, are based on quantum physics. It would be difficult to imagine present day society without computers, smartphones, GPS, satellite telecommunications etc. This was the first quantum revolution. Quantum computing is the most obvious example of new technology based on these effects, but they will certainly be developed in others, some of which we cannnot even imagine right now.

**Q: How does a quantum computer differ from a classical one?**

**A:** In a classical computer the memory is made up of a large number of bits. Each bit may take the values o or 1. In a quantum computer we have qubits, or quantum bits, which can take the values 0 or 1 or a superposition of both states simultaneously. Just as with Schroedinger’s famous cat, which es alive and dead at the same time, until someone observes it, the qubits are in a superposition of states until we make a measurement. The interesting point is that we can manipulate these bits to make very complicated calculations, giving rise to an infinite number of combinations of superposed states among the different qubits. All these possible states coexist and they are used in the operation. The algorithm is programmed so that the qubits interfere in such a way that we obtain the desired result with high probability. The complexity of the calculations that we can do increases exponentially with the number of qubits, and for that reason it is possible to resolve problems that are impossible for present day supercomputers. But technically it is very difficult to produce these qubits, and above all to keep them sufficiently aisolated so that they don’t interfere with one another and spoil the computation.

**Q: When do you think that we will be able to have the first quantum computers?**

**A:** At the present time there are some prototypes, but make no mistake, these are only experimental systems to develop the techniques for creating and maintaining the qubits. They are unable to do anything useful. I hope that quite son, in a year or two, the first quantum computer capable of doing something better than a classical computer will be constructed, This is often termed “quantum supremancy” but this refers to only one specific problema. This will be, for sure, a purely academic problem, without any practical interest, which the computer will be able to solve. To have quantum computers that can really do better than present day computers we will have to wait longer. I estimate between ten and twenty years.

**Q: People say that quantum computing will put an end to privacy on internet, and will preclude the possibility of carrying out transactions such as buying and selling? ****Is that so?**

**A:** It will end privacy and transactions as we know them today, but at the same time quantum technology will give us much more powerful methods. At present we use techniques for encrypting messages and transactions based on a public key. These methods are very easy to implement and work because, to decipher the messages we would need to solve a mathematical problem called factorization, which at the moment would take too much time. With current supercomputers it takes years to factorize numbers with 200 digits. For our daily transacions via internet (purchases, private messages etc.) we encrypt the information using keys with many digits, which guarantees that no present day computer can decipher them. However the arrival of quantum computers is a threat to this type of cryptography because they will be able to solve factorization problems, and so decipher these keys in a matter of minutes.

Luckily there are other methods of cyphering which we do not know how to resolve even with quantum computers. So there is no need to worry about the collapse of communications. What will happen is that there will have to be a change in the encryption technique. There will be practical problems to solve because these new methods are more costly and complex than present method. What will occur is that when quantum computers are introduced it will be possible to decipher older messages, which we have been sending up to now, for the past 20 years. I am sure that there are individuals and organizations storing all the encrypted traffic on the internet in the hope of being able to decipher them in the near future. I don’t think that this should cause the normal user to lose sleep, but I imagine that the major world leaders and intelligence agencies must be very worried about the question.

**Q: So will there be a race between methods of encryption and computers which decipher them?**

**A:** Maybe. As I said, there are encryption methods which today we do not know how to decipher even with quantum computers. But nobody can guarantee that in the future an algorithm will not be discovered with which they can be deciphered. A cryptographic race could occur. Another possibility is that we will begin to use what is known as quantum crypotgraphy, which we can indeed guarantee to be always inviololable. Not that we do not know how to decipher it, but the laws of quantum physics guarantee that we cannot do so. This is a technique which is feasible in principle even today, but technically complex. It is based on sending coded information using quantum systems, such as photons, along a fibre optic cable. With present technology there are practical limitations on distance, typically of some 20 km. It would be possible to use relays, which which we could build a type of global network of encrypted communications. An alternative is to used satellites to send signals between distant points.

**Q: Can we really understand quantum physics?**

**A:** This is a question which has given rise to a great deal of philosophical debate. The answer really depends on the definition of “understand” In general we say that we understand something when we can find an analogy with everyday concepts. For example if I say that an atom is a tiny ball in the nucleus with another tiny ball orbiting around it I may think that I understand it, although this model is not valid. From this point of view it is hard to find satisfactory analogies for quantum physics because its ideas clash directly with our day to day intuition. However, we do understand it in the sense that its laws are perfectly clear and that one can predict with complete accuracy what will be the result of a specific experiment. A number of interpretations hae been proposed for quantum physics. All of them produce the same observable effects and therefore they are indistinguishable from the scientific point of view. The arguments for preferring one or another are entirely subjective, or philosophical. I don’t have a strong opinion about this. I think that without objective criteria it makes no sense to prefer one interpretatin or another. Perhaps for purely personal taste I have a slight preference for the interpretation of non-local hidden variables.

**Q: What do you think when you hear people talk about quantum cures, or quantum entanglement between souls?**

**A:** I don’t know what they are talking about, and in any case this has nothing to do with what I do, or with quantum physics.

**Q: Are you up to date about the experiments on quantum entanglement which have been performed at the Canary Island Observatories, between telescopes on Tenerife and La Palma?**

**A:** Yes, I know them well, and my institute has been involved in this work. I think that this is a most interesting study which whill help in the development of technologies based on entanglement and quantum teleportation. One obvious example is the quantum encryption which I mentioned before. It is a question of sending entangled photons over large distances. I hope that we will be able to continue collaborating with the IAC to perform these quantum experiemnts in the future.

**Q: What impression have you received of the Institute and the Canary Observatories after your visit?**

**A:** I knew the Institute and the Obsevatories, but not personally. I have been very impressed by the high level installations and pioneering reseach which is being carried out here in many areas of physics. I have been able to talk to IAC experts about matters such as the origin of the universo, the physics of magnetized plasma in the Sun, the latest techniques in astronomical observation, and of course have learned about distant galaxies and the new exoplanets which are being discovered. It has been an exceptional experience. I am really proud that there is a centre such as the IAC in Spain.

You can listen to an extended interview (in Spanish) on the podcast of the weekly discussion group “Coffee Break Señal y Ruido” of the IAC from Friday 28th April 2017 on: **http://señalyruido.com**