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**You came up with the first model for a Quantum Computer.**

It’s a long story. People knew by then that, if it were possible to build a Quantum Computer, we could perform calculations which were then deemed impossible. The problem was that nobody knew how to build one. Peter Zoller -from Innsbruck University- and I suggested the first model and we saw that, in fact, it should be possible to build it. We also suggested how it could be built, in principle.

**Has anyone managed to actually build it?**

Shortly after we put forward our proposal, some experiments were done which proved that the principles we suggested worked and, with time, some small prototypes were built. Nowadays we have a small Quantum Computer prototype -though it’s still a long way from being operational- but which proves that, in principle, everything works as it should.

**Could you explain what a Quantum Computer is and why it is more powerful than a Classical one?**

A Quantum Computer works with different laws than everyday computers. The latter are based on bit logic: we put in the information we want to process as zeros and ones, which are then transformed by the computer through certain physical processes and, at the end, we get some results, which we read. Quantum Computers do not work with zeros and ones, but with other properties of microscopic particles, which are ruled by the laws of Quantum Physics. And those properties, which are very hard to explain to someone who’s not familiar with them, allow us to make millions and millions of simultaneous calculations with a single computer. That’s what makes them so powerful: the laws of Quantum Physics allow them to do things which are impossible using the logics of everyday computers.

**How did you come up with the idea for your prototype?**

I was working as a post-doc in the US with Peter Zoller, my collaborator. And we were working on Quantum Physics, though not specifically on Quantum Computers. By then, very few people knew about Quantum Computers. But both of us attended a conference in Boulder, in 1994, where somebody explained to us the power a Quantum Computer would have and, at the same time, he said nobody knew how to build one. So he challenged the audience to think about ways to do it. We sat down and, using the experience we had and some ideas we acquired when studying the problem, we gave it some thought and we came up with a proposal.

**Some philosophers use the measurement postulate to suggest consciousness and Quantum Mechanics are somehow linked. What do you think about this kind of use philosophy sometimes makes of science?**

There are two types of uses. There are some which are completely incorrect, which not only are not based on, but are actually against Quantum Mechanics, even though the ones who put them forward say they agree with it; then, there are others which are open questions within Quantum Physics and which, for the moment, we don’t know how to test, but one may have several interpretations and Philosophy may have its own view, which is perfectly valid.

In that sense, when one speaks about the measurement postulate and consciousness, one can see these two trends I mentioned: there are people who say one can decide what’s going to happen, just by using their consciousness, which is against what Quantum Physics says. Quantum Physics does not allow one to manipulate measurement outcomes. One cannot decide what the result is going to be, what’s going to happen. One cannot modify the future. But, on the other hand, there is a problem in Quantum Physics with the measurement postulate, which is when the so-called wave-function collapse happens, when the properties of objects become determined. We know that happens when an observation is made, but at what time? One may think it’s when we see it, when we feel it, when… there are several possibilities and some people think it has to be related to the fact that we are aware of it. All of those are different interpretations which, from the physicists’ point of view, are just as valid, because none of them have been falsified by an experiment. One can perfectly speak about that and I have absolutely no problem with it.

**Do you think the wave-function collapse postulate is justified? Or is it explained by decoherence theory?**

Decoherence theory does not explain the fact that, after doing a measurement, we obtain a result, and I guess every scientist who works on Quantum Physics will say the same. Decoherence explains other things, which we understand very well, but not the fact that we obtain a result after measuring. That is not explained. There are, however, physicists who have a different interpretation of Quantum Physics in which the measurement postulate is unnecessary, because they understand Quantum Physics from the point of view of information. According to them, what we do is simply to receive information from outside. An we can’t ask ourselves whether the outside world exists or whether its properties are defined or not. It’s not necessary, since all we do is receive information. If one looks at Quantum Physics from that vantage point, there is no need for any postulate. Even classically, when we acquire information, our perspective, our description of the world, changes. One has a certain probability of events happening; when one learns something, that probability changes. It’s what is called the Bayes theorem in probability theory. The measurement postulate is basically the same. That is, if one forgets about the rest of the universe and only thinks that we are receiving and processing information, there’s no need for any conflict, as far as the measurement postulate in Quantum Physics goes.

Now, if we want to speak about things beyond ourselves, if we assume that the facts we observe -of which we have some information- are happening there, that there is a reality beyond which we want to talk about, about whether that reality is defined or not, etc. then we need to assign that reality an entity and that’s what makes the measurement postulate necessary. There’s a small contradiction inside Quantum Physics because, on one hand, it tells us physical processes happen according to Schroedinger’s equation -some laws- except for the measurement process, which is described by other laws. And one thinks: why are there two different laws if we ourselves should be described by the laws of Quantum Physics and, therefore, should be subject to the former? This is the controversy around the measurement postulate and one of the proposed solutions is saying: “let’s forget about objects and speak only about information, so there’s no contradiction”, but I think that’s burying the problem instead of solving it.

**Is there a way of solving it?**

Not that we know of. What would be interesting would be to design some experiment that would allow us to falsify some of the theories: there’s the many-world interpretation, the interpretations based on the wave-function collapse, there are orthodox interpretations, there are many kinds of interpretations within Quantum Physics, but the problem is that all of them make the same physical predictions, so we cannot find out what the correct one is. It’s an arguable question, something which we physicists don’t like: we like answers. So what we need is somebody who figures out a way to be able to tell between the different interpretations but, for the moment, we don’t know how to.

**How is the notion of reality put into question in Quantum Physics?**

Quantum Physics does not question the existence of an independent reality. What it does say is that the properties of objects beyond us don’t have to be defined. And that’s what collides with classical principles. If one works with classical Physics, every object has many properties: it has movement, position, velocity, color, mass… and even when we’re not observing it, those properties are well defined. An object, even if it’s not being observed, is at some place. However, Quantum Physics says that’s not true: that objects exist but, when we’re not observing them, their properties start to become indefinite and they only become definite after an observation. And that’s something Quantum Physics claims and which, along with another property called non-locality, has been experimentally proven. It’s not only an interpretation of Quantum Physics: it’s a precise fact, the fact that objects’ properties don’t have to be defined when they’re not being observed. And, if one also believes that actions cannot be propagated at infinite velocity, those things give rise to experiments which prove that Nature does not define properties. What I mean is that my position is based on experiments, that is, on objective facts. Later, one may attempt to explain what’s going on in reality. One may even go further an ask: “if they are not defined, what is happening? Do we have many universes? Or do we have something else?” And those are the things that have a certain interpretation within Quantum Physics. There’s no interpretation I like more than any other, precisely because of what I said before about not having any evidence pointing to one or another, because they predict the same experimental outcomes.

**Mathematics seems to play an essential role in the description of the universe. Why do you think that happens? Could it be some other way?**

I don’t think so. I think a universe must have certain laws. The question is: how many? Maybe there are as many laws as possible phenomena, so we would have a description in terms of laws but it would be completely useless, since it wouldn’t be predictive. For every thing that happened we would need to have a different description. It’s pretty logical to think that the laws have to be consequent with each other and, therefore, that many phenomena should be deduced from some fundamental laws. And that’s what Mathematics does: to transcribe those laws, in terms of formulas -a different language- and using logics in order to draw consequences from the postulates, which gives rise to the description of Nature we have. And I have no problem with that. Of course, there are very interesting details, like the fact that Mathematics is not complete, in the sense that not everything that’s true in a certain theory can be proven. All of that gives rise to other very interesting questions, but it seems natural to me that the laws the universe follows be mathematical, which after all they are nothing but logical deductions.

**So Galileo was right when he said Nature is a book which is written in mathematical characters.**

In principle, yes. But there are two small details Galileo didn’t know about: one of them is Quantum Physics, and Quantum Physics breaks away from determinism. One would think that, if everything is written, then we are totally deterministic. If one knows the positions and velocities of every particle at a given moment, that completely determines what will happen later. But Quantum Physics, after introducing measurement, stops being deterministic. That gives rise to the existence of intrinsic probabilities, which implies that history is not written. That, on one hand. And on the other hand we have what I was saying before about Gödel: that, even if there were some fundamental principles, it would very hard or impossible for us to know them all, in the first place and, in the second place, even if we did know them all, it may be impossible to guess what’s going to happen next, to predict the future, because of what I said before that Mathematics is not complete, something we know through Gödel.

**Before, you talked about non-locality. Does that force us to re-think our notion of space?**

Not really. Not through non-locality, but through relativity and other phenomena, one has to start re-thinking the notion of space, but traditional Quantum Physics doesn’t raise any problems with it. Space is simply something that’s given and in which everything happens, and even though Quantum Physics has some properties that seem non-local, it can be shown that those do not violate causality, that is, that they cannot be used to produce actions at a distance. Therefore, the non-locality that exists in Quantum Mechanics is somewhat fictitious, since it doesn’t allow us to communicate a infinite velocity, which would be a tragedy because it would have dire consequences regarding causality.

**I was thinking about what happens when trying to unify Quantum Mechanics and Relativity, more specifically about the Holographic Principle, in which all the information of a Black Hole seems to be stored on its surface. Which led some physicists to think that all the information in the universe is stored on its surface.**

Yes, but I don’t think that raises any problems. Even classical Physics, Maxwell’s Electromagnetism, says that if one has a wave and measures it in some space, one will know its value in the whole of space, that is, all the information stored in a wave is simply in a plane, even though it’s moving though space. That’s not so strange. What happens is that nowadays, in terms of relativity, there is a problem when trying to unify Quantum Physics and Einstein’s theory of gravitation. We don’t know how to do that and there are people who, in order to make them agree, takes into account that space itself could be a Quantum-Physical object, an object in which there could be fluctuations of space itself. And that gave rise to those new conceptions one needs to have. But I think all of that is just lucubration, because there is no such theory yet. Nobody has managed to unify them.

**Some people associate randomness in Quantum Mechanics with free will. Is that going too far?**

It’s possible. The truth is I don’t know much about that subject. All I know is that those experiments I mentioned before, which prove Nature is strange and that objects’ properties are not defined, take free will as one of its givens. One has to believe it’s possible to obtain random results. At the end, one has to conduct experiments choosing some parameters in a random way. And, if it was impossible to produce random results -because there was no free will, for example- one could say those experiments do not prove the fact I mentioned before, that is, that Nature does not have definite properties. Then one could say free will doesn’t exist and that Nature does have well-defined properties. Some people talk about these things, the truth is I don’t understand it very well but maybe it’s a possibility.

**It seems that Quantum Mechanics operates with a logic which is slightly different to classical one. What are the differences?**

The fundamental difference is that, in classical logics, there’s only one possibility at each moment. That is, if one expresses it in binary terms, in terms of zeros and ones, one has either a zero or a one. Then, one can have a grammar that says how to combine those zeros and ones, etc. but the fact that one has a zero and a one and not both at once is fundamental. One cannot have an object represented by a zero and a one simultaneously. But Quantum Physics does allow that: it’s what we call superposition and it allows us to have one same object in two states -which we call Quantum States- at once and that’s the fundamental difference.

**And finally, on a lighter note: what is the meaning of life?**

I don’t know. I think one cannot talk about a meaning in general. Everybody has a meaning for their life. I guess each one of us can find many reasons to live, but I don’t think there’s a general meaning which is common to everybody.

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