Einstein was wrong and in Olomouc today thus photon twins are researched

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For the tenth year, Ondřej Haderka heads the Joint Laboratory of Optics (SLO) in Olomouc  – a workplace that is operated by the Division of Optics of the Institute of Physics together with the Faculty of Sciences of the Palacky University. A place where huge mirrors for space observatories are created and astrophysics is focused on, but also where research is conducted in the field of quantum optics and lasers. Addressed are, therefore, also the topics for which this year three scientists were awarded the Nobel Prize for Physics. His goal is to keep the SLO a place where it's a joy to explore. In addition, he likes taking photos, especially of the universe. And recently he was appointed a professor.

Let's start with a current topic – the Nobel Prize in Physics was awarded to Alain Aspect, John Clauser and Anton Zeilinger for their research and experiments with entangled photons. It is a topic that you are also dealing with. Are you happy that your field has received such attention?

Yes, I am. The work of this trio has accompanied me and my colleagues throughout our careers. Due to geographical proximity, we have the closest relationship with Anton Zeilinger, whom I have had the opportunity to meet several times. The work of his group has taken quantum optics a huge step forward.

What is the difference between their research and yours?

In Olomouc, we deal with closely related topics and use very similar tools. We focus in particular on ways to investigate quantum correlations, quantum entanglement, using a financially achievable photon detection method with the identification of their number.  

What have you researched in quantum entanglement?

We mainly deal with the investigation of photon twins, photon pairs. These are interesting physical objects that consist of two particles, but from the point of view of physics, they are represented by a single notation. It is a single object composed of two particles. Its quantumness resides in the fact that when we do something to one member of a pair, it affects the object as a whole. So also the second photon, which can already be half a universe away.

This is fascinating, but a bit beyond the normal human understanding…

The key to understanding is seeing it as one object all the time. Both particles are bound by a common history of formation. Maybe that's one of the reasons we call them twins. There is a certain parallel to what is said about identical twins that when something happens to one of them, the other somehow mystically knows about it. Yet here, it is no mysticism, it's physics because, since it's one object, any intervention will affect it as a whole.

How is it possible?

In quantum physics, we move in a space of superposition of states. From a classical logic point of view, we have two bit states. Zero or one. In quantum physics, we work with superpositions, where it's not just zero and one, but anything in between. It can be any superposition of zero and one, and even though each of those photons is in one of the superpositions, you don't know exactly which one, on condition you create it right. And if you interfere with the state of one of those photons by measuring, you force a certain value on it, and you also affect the object as a whole, because the states of both photons are correlated. Even Einstein coined a term for this – "spooky action at a distance". And he was convinced that it was not possible.

But wasn’t it calculated back then?

Einstein was aware of the results that came from the founding fathers of quantum mechanics. He was not directly involved in them, but they seemed impossible to him. He considered quantum mechanics an incomplete theory since it allowed such things. But it gradually turned out that the founding fathers were right. At least that's what we think today, and there's a lot of experimental evidence that suggests that this is indeed the case, that this is the essence of the quantum microworld, which is a bit fuzzy, indeterminate, and allows, among other things, for non-local coupling at a distance.

So how old is the field?

The cornerstones were laid at the turn of the 19th and 20th centuries, when quantum mechanics was first formulated in its basic form. The quantum optics we deal with gradually developed from this, and experimental quantum optics emerged after the discovery of the laser, so in the 60s and 70s of the last century, when experimental quantum optics received tools powerful enough to be implemented in a laboratory. This enabled us to generate the aforementioned photon twins.

And how do you generate them?

Making a photon twin is a trivial process. If we let a photon enter a non-linear crystal, that is, a special environment that increases the probability that this phenomenon will occur, it may happen that, still with a small probability of about ten to the power of minus 11, it will decay into two photons of lower energy. One particle decays into two, while the law of conservation of energy and the law of conservation of momentum are satisfied. One becomes two, and these are the twins, the photon pair.

How do you observe them then?

A photon is visible, even the human eye is almost sensitive to individual photons. If a person stays in the dark for a long time and lets their eyes adapt to it, they can see them – the eye is a relatively sensitive detector. But we need something objective that allows us to evaluate the signal electronically, so we use very sensitive detectors that are capable of detecting individual photons. However, it is one thing to detect photons and another to count them. And that's another big problem. One photon carries very little energy, so the detector has to perform a very strong amplification process, which turns the impact of one photon into a strong enough electrical pulse to be detectable. And in that amplification process, information about how many photons were there is usually lost. That's another problem that we've been partly dealing with for the fifteen years we've been looking for ways and tools to count photons.

And do you have them now?

Yes, we do. There are many approaches to this issue. Some are exotic and expensive requiring very demanding experimental equipment that is not available to us, so we looked for simpler and cheaper ones. What we use today is based on a very simple idea. Instead of a detector that can count photons, we use one that tells us when photons hit it, but doesn't know how many there were. And then, of course, there is an option to have a large number of these simple detectors, so that each always captures exactly one photon. The idea is nice, but even this detector is not so cheap that we could have a huge number of them.

How can this problem be solved?

There are two options available at present. Either we use a fibre optic delay loop where we send the cloud of photons and in each round of the cloud we chip off a tiny bit of it, diluting it with a large number of paths offered. Then one detector is sufficient. Instead of there being a lot of paths in space, there are a lot of them in time. This is the way it works for small quantities. When we have a lot of them, we use a device that was originally developed for a completely different purpose – an intensified CCD camera. When we put a so-called image intensifier, which can amplify the incident light, in front of a CCD chip, which is itself not sensitive to photons, we get a large matrix of detectors sensitive to individual photons, and these allow us to count them. And as a bonus, we get location information, which means that we not only count photons, but we can also track their spatial correlations. And if we add a spectrometer to the device, we can observe the correlations in the spectrum. We get a good tool for affordable money that allows us to do photon counting experiments.

Quantum technologies, the generation of photon twins, we have already heard also about their teleportation – where can it all go, where will we get? Will people teleport from one end of the world to the other?

I think this is complete science fiction, but we can already do a lot of things. Quantum technologies are being talked about more and more. The laser and the CCD chip are first-generation quantum technologies. Today we are talking about quantum technologies 2.0 – they include things like quantum computers, quantum encryption, quantum imaging, quantum metrology. There are a number of things that we already know today in varying degrees of sophistication. For example, quantum encryption, by the way, it is what we started with in our lab. This was the first experiment we did here after the creation of our quantum optical laboratories at the end of the last century. Today it is application technology sold by several companies on the market, and if you have enough money you can buy it.

How does it work?

Quantum encryption can create a communication system the security of which is protected by the laws of quantum theory. This means that it is at a higher level than classic encryption systems. Classic encryption methods, at least some of them, can be broken with the use of a quantum computer, there is an algorithm for this. Here we see how clever quantum physicists are. Not only will they attack the cipher, but they will also offer a system that is resistant to an attack by a quantum computer and where security is guaranteed by the laws of physics.

What else can we expect from quantum physics?

We can already do a lot of things now. Quantum computers are no longer a complete fantasy, several are on the market. Then, for example, the field of quantum metrology. Quantum physics can push the accuracy of measurements a little further than what is possible within the framework of classical physics. For example, interferometry based on so-called compressed states is used in gravitational wave detectors. It is a new window of astronomy open into the universe. Today we can detect gravitational waves predicted by Albert Einstein's general theory of relativity. We can measure the ripples in spacetime that occur in dramatic cosmic events, such as the collision of two black holes. For this, we need extremely sensitive and accurate length measurement. We therefore use gravitational wave detectors, such as the LIGO detector, and they help themselves to high precision with quantum technologies based on compressed states.

This brings us to another topic of your interest – your photographs of the universe, you have photos of nebulae, space bodies and phenomena on your website. How are they created?

This is really just an amateur thing, there are a number of other astrophotographers who can do it on a completely different level than I do. Photos of the universe are created by pointing a telescope with a detection device, preferably a CCD camera, somewhere in the sky. One has to make the exposure long enough, because today the sky is over-lit by public lighting and other sources of radiation, in addition, tens or hundreds of satellites of the Musk system fly over there constantly. Then one gets beautiful pictures. And by the way, it's nice to see where the instrument technology has developed, because what for many decades, or perhaps even centuries, was the privilege of highly professional astronomical observatories, which with huge telescopes were able to take those beautiful colour photos published in books, now almost any amateur astrophotographer can, because CCD detectors are available enough, cheap enough, and good enough to take beautiful photos of nebulae at a city centre.

Does one have to know where to aim, or wherever you aim, you take a photo of something?

Almost anywhere you point, you can take a photo, but of course you usually have a specific goal in mind. Today, we have robotic telescopes that do a lot of the work for you. This means that they point where you tell them to, find out if they are pointed correctly, correct their position, and then follow the object for the entire duration of the exposure, because you need hours, or rather dozens of hours of exposure to get one nice photo like that.

How deep a knowledge of physics does one need to take such a photo?

Deep knowledge is not necessary yet it helps because it provides a better understanding of what is happening and what one is doing, but I also know amateur photographers who do not have a deep knowledge of physics and still take beautiful pictures of the universe. It's more about technology than knowledge of physics, but for me it was the other way around; my interest in physics inspired me, and I'm not alone at the Institute of Physics, to start taking photos of space.

My last question is about the Joint Laboratory of Optics – what are your visions and plans for the future? What do you consider crucial for the future?

My plan is to keep the SLO a place where doing research is joy. That's the most important thing. I have always considered us physicists having an undeserved privilege that we can make a living doing what we enjoy. But whether I succeed, and this is where scepticism enters my speech, does not depend only on whether or not I do my job well. I have a feeling that science is starting to experience difficult times. That science needs to fight for its credibility that is being lost, I am afraid. Science in general, not just in the Czech Republic. I don't think it's only science that’s at fault, but the whole world is becoming more superficial, as if the essence of things no longer matters. We settle for the fact that it somehow works, but we do not strive to go in depth. And I'm afraid this is a death trap we're willingly walking into. I would like to believe that science will be an exception to this trend, that it will be honest, that it will go after the essence of things, but I fear that this is not quite the case and that science follows the social trend and begins to become superficial. This is where my big concern comes from.