Quantum effects are not weird


Quantum effects are not weird

Dirk Bouwmeester: being in two places at once is perfectly logical

Why can a quantum particle be in two places at once, unlike people, who cannot? Dirk Bouwmeester, Professor of Experimental Quantum Physics at Leiden University, is trying to find an answer.

Dirk BouwmeesterDirk Bouwmeester. Credits: NWO/Ivar Pel, photo may be used free of rights as long as the name of the photographer is stated

In 2014 he received the NWO Spinoza Prize worth 2.5 million euros.

You are receiving 2.5 million euros! Have you already got over the initial surprise?

‘It was indeed an enormous surprise. For a researcher it is a godsend: the possibility to go further with your research knowing that you already have the money for it. If you do experimental research, you are constantly busy trying to obtain funding for it. This prize will give me lots more space to focus even more on research.’

And quantum research is not cheap for sure?

‘That's correct. I think that in the last fifteen years I have certainly needed a budget of one million euros per year for my experiments. It seems like a lot of money but in this type of research it is soon spent. One of my experiments makes use of very low temperatures and it is incredibly expensive to keep it running.

I am therefore very grateful to NWO. And it is not for the first time that an NWO grant has supported my research. Thanks to a grant from NWO I could go to Oxford in 1996 and work with Roger Penrose, a physicist who has been an enormous source of inspiration to me.

As a PhD researcher in the quantum optics group at Leiden University I learnt to do meticulous and thorough experiments. The scintillating ideas of Penrose alone are not enough to get you there. You need to work very hard and meticulously and be incredibly persistent. Only then can you do very special experiments.’

You are now doing those experiments in both the Netherlands and the United States. How did that come about?

‘The research needs those two places. The experiments need to be carried out at extremely low temperatures. For more than one-hundred years, the Kamerlingh Onnes Laboratory in Leiden has specialised in this area. The nanostructures that are brought to those extremely low temperatures must be produced using advanced techniques. And they are very good at that at the University of California in Santa Barbara. My students in Santa Barbara work on producing and testing the materials. If these are good then they are sent to Leiden or I bring them over myself. And sometimes the students bring these materials with them if they take part in the research in Leiden.’ 

Did you know? A quantum particle, a very small particle, can be in two different places at once.

With your experiments you want to extend the boundaries of quantum mechanics. What does that involve?

‘Quantum mechanics is about the behaviour of matter and energy at the atomic level. At this scale you see weird things. For example a quantum particle (a very small particle, ed.) can be in two different places at once. But we cannot do that. People cannot be on this chair and at the same time on the chair in the room next door. An important question in quantum mechanics is why we do not observe these quantum effects in everyday life. Why can a quantum particle be in two places at once, whereas we cannot? By bringing large objects into quantum states we want to find an answer to that question. We want to make the scale at which quantum effects occur bigger.’

What does that experiment look like?

‘We observe a quantum particle that can move in two directions. It is a photon, the particle light is made up of. That photon is in quantum superposition, a quantum state in which it can move in both directions at once. We transfer this superposition to a larger object, which is a still very small mirror. How we do that? At the end of both directions of movement there is a cavity. In one of those cavities a mirror is placed on a spring. If a photon collides with the mirror then the mirror vibrates. The displacement caused by this vibration is very small, far less than the size of the mirror itself but nevertheless large enough to notice. As the photon is in superposition then the same is true for the mirror, which is in a state of vibration and non-vibration at the same time. So in effect, the mirror is in two places at once and the largest distance between these two positions is the maximum amplitude of the vibration.’

I have seen time and time again that quantum mechanics works so fantastically while at the same time being so simple

Sorry but that is simply weird.

‘If you have studied quantum mechanics for long enough then it is actually logical. Objects which we know from our everyday lives have a speed and a position. They follow a path and so they are not in two places at once. In quantum mechanics, however, this does not apply to very small particles. Instead for quantum particles a wave description is needed. A particle does not move in a line but as a wave through space-time. And the weirdest thing about this is that eventually it is detected as a particle at one location, which from a physics point of view is far weirder than the underlying quantum world.
Current physics textbooks still often assume that we have to understand how the quantum world works from the perspective of the classical world, in other words what you see around you. You should, however, do it the other way round. You should take the quantum descriptions as a starting point and then ask yourself how our classical world arose from this. Why does a particle proceed along a single pathway in space?’

And does quantum mechanics have an answer to that?

‘That is still being discussed. As soon as we measure a particle we make a jump from the quantum world, where waves play a role, to the classical world, where a particle can find itself at a certain position in space. That moment is called ‘the collapse of the wave function’. An important question currently under discussion is: what is the mechanism behind this?

In principle, quantum theory has a good answer to that. You have linked the particle you want to detect to a large measuring device that we can describe in classical terms. And that means that the particle becomes entangled with a whole range of particles in the measurement device. This entanglement is so complex that from that moment onwards we can no longer experimentally demonstrate that there is a superposition. From the quantum mechanical viewpoint that superposition is still there. However, from that moment in time the system behaves largely as a classical system without wave-like properties.

But that is not the only possible answer. It could be the case that another effect also plays a role which we do not yet know about and which insures that the quantum laws do not apply to large objects. That is what Penrose suspects. He thinks that every superposition ultimately collapses even if you do not cause that by measuring it. According to him, gravitation plays a large role in that. He suspects that the heavier a particle is, the sooner a superposition will collapse. In the case of very small particles, such as electrons and atoms, the collapse takes too long to observe in the current generation of experiments. However you might well be able to detect a collapse in large objects.

What we want to test with our experiments is whether quantum mechanics also works so well for large objects and whether the collapse of the wave function can be fully understood in quantum mechanical terms or not.’

I honestly do not know what I expect. And that is what makes the experiment so worthwhile for me
- Dirk Bouwmeester

What do you expect the outcome to be?

‘I honestly do not know what I expect. And that is what makes the experiment so worthwhile for me. I know very few experiments where it is not clear beforehand what the outcome should be. It is more usually the case that an experiment is successful if the outcome is what you expected. And if that is not the case then you try again. But that does not apply to this experiment. If it actually proves possible to make the superpositions of those tiny mirrors then I cannot predict what will happen next.
On the one hand, I lean towards the ideas of Penrose. Gravitation plays an incredibly important role in physics but I do not think it is particularly well understood yet. There is still plenty of room for surprises there. On the other hand I have seen time and time again that quantum mechanics works so fantastically while at the same time being so simple. It gives such beautiful and clear descriptions of how the microscopic world fits together that I have no difficulty in believing that large objects can also be in superposition.

I expect that the first experiments with relatively small particles will demonstrate that the quantum mechanics still works really well. We will have to wait and see whether abnormalities will be measured in the case of larger objects. And if that proves to be the case then we will need to take a serious look at the proposals of Penrose and other theoreticians with similar ideas. However if quantum mechanics also works with larger objects then we would need to make considerable adjustments to our view of the world. Then it is really possible that everything including people exist in incredibly complex superpositions.’

What would the consequences of that be?

‘In that case you can imagine the craziest of experiments. For example, I could make a device in which I bring a photon into superposition. That photon could then move both left and right at the same time. On both sides I could place a detector. Before the experiment begins I could take a decision. For example: if one detector were to go off I would say farewell to my job in Santa Barbara and work just in Leiden. If the other detector were to go off I would say farewell to my job in Leiden and work ujust in Santa Barbara. At the moment the photon is detected I will be in a superposition from a quantum mechanical viewpoint. This contains the reality that I will only work in Leiden but at the same time the reality that I will only work in Santa Barbara. What makes this so interesting is that if something like that were to happen now, we would not realise that because we cannot do a measurement where the two possibilities coexist. However if the laws of quantum mechanics also apply to large objects, as the research in Leiden might demonstrate, then that would be the conclusion. My group was the first to start this type of crazy research but now there are at least 50 groups in the world who are working towards a superposition of even larger objects still.’

A race against the clock to get there first?

‘I do not want to see science as a race. Rather I am happy that this type of research has attracted considerable interest and that a lot of people find it worthwhile to work on. This means that the question will most certainly be answered in the end. Of course I hope that our research will play a leading role in that. And with this new funding through the NWO Spinoza Prize there is a high chance of that. But that is not what motivates me. It is phenomenally inspiring to be involved in laying the foundations of a
new line of research.

It never ceases to amaze me how much has been discovered and developed in the past one-hundred years. Thanks to this knowledge the scientific world is currently exploding. Our current knowledge about pharmaceuticals and biology has been made possible thanks to quantum theory. Fantastic research is being done into new medical treatments that are only possible thanks to our precise knowledge of the DNA structure and other structures within living cells. And all of that is thanks to the quantum mechanical description of these structures. How molecules form can be calculated using quantum mechanics. Why do some molecules bind to each other, whereas others do not? Those are pure quantum mechanical calculations. And using that knowledge a considerable number of advnaces will be made in the future.’