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Intro: One of the best ways to imagine this is a black box, imagine the quantum memory as a black box, and then from outside as a user, you don't care what is inside, right? But then this black box, whatever you put in, it just shouldn't do anything on it, right? And at a later time, you said, I want it back, and then you get it back. AskDifferent, the podcast by the Einstein Foundation.

Marie Röder: Today with Marie Röder. Welcome to the podcast. We use the Internet every day. We send messages, store data, share information, often without thinking about how it actually works. But what if there was a completely new kind of Internet, one that is fundamentally more secure, one where information is carried by light and can't be secretly intercepted? To make that possible, scientists need something that sounds almost impossible: a way to store light. And that's exactly what our guest is working on. Mustafa Gündoğan is an Einstein Independent Researcher at Humboldt University in Berlin, and he's trying to build a key technology for the future: quantum memory. Welcome to the podcast. 

Mustafa Gündoğan: Thank you, Marie, and looking forward to chat with you today. Yeah, thanks.

Röder: Okay, when people hear quantum, they might think of quantum computers, and we actually have an episode, episode 36, that talks about quantum computers. It's with Dr. Jens Eisert. I highly recommend going back to that episode for whoever wants to learn more about quantum computers or really quantum physics in a more general, more basic sense. But you work on quantum communication and quantum memory. Can you explain this in very simple terms? What is quantum memory? 

Gündoğan: When I say quantum communication, so it's in a more kind of strict sense of the word. Basically, it is to distribute secret keys across long distances. So just imagine we two want to exchange secret messages across this room, like across this like two meters. So basically, what we need to have is a pre-shared key, right, with which we will then decode our secret messages, right? So quantum communications, and then there's this protocol or specific protocol called quantum key distribution or one key application. It basically lets users exchange such keys without being interrupted. Or if someone really interrupts them, then it is noticed, right? And then you immediately cut the line, you cut the protocol, you stop the protocol. And then coming back to quantum memories, in order to extend this range, because optical fibers have losses, right? But in classical physics or in classical communication, we use amplifiers. But in quantum physics, you can't do that simply, because all the quantum states that you prepare, that you want to send to the other party, so they are in this quantum state. If you want to amplify them, right, first you want to measure to learn what they are, right? But then you simply destroy them. So you can't just use a simple amplifier or repeater in the classical sense, which brings us to the topic of memories, basically. So here the idea is similar, but it is not amplifier per se. It is not really an amplifier, but that's why we call them quantum repeaters. So you repeat the signal in a way, but I don't go into technical details there, how you do it. But basically, you need to store these single photons, single particles of light with which we encode information, and then take them back from the memories at a later time when you really need them. That is the key. 

Röder: So I just want to go back a little bit again and ask, if you had to explain to a child what quantum memory is, how would you do that? 

Gündoğan: So it is actually not, I mean, it might sound a bit difficult, or the idea sounds a bit difficult to me at first, to explain this to a child. But in the end, I think one of the best ways to imagine this is a black box. Imagine the quantum memory is a black box. And then from outside, as a user, you don't care what is inside. But then this black box, whatever you put in, it just shouldn't do anything on it. And at a later time, you say, I want it back, and then you get it back. And then in terms of what makes it quantum, is that we want to store the quantum states of certain particles. I mean, in our case, it is light, basically, single photons, single particles of light. And then one can encode information in these photons. So basically, this is how you create these zeros and ones. And in quantum physics, you know, we always say there's a superposition. It can take any value between these two classical values, zero and one. So basically, you prepare such a quantum state, and then you put this in a black box. And then at any time, at some time later, you want to press a button and then get it back. And then now you need to see the user who puts it inside the box at the beginning. Then he, she checks it. Okay, so this is the exact state that I put in, right? That means that actually the box is not doing anything. Basically, it is not disturbing it. And so to sum it up, basically, it should be a device that shouldn't be disturbing the input quantum, the fragile input quantum state.

Röder: Okay, what I understood so far is that quantum communication is kind of eavesdropping-proof, and it has huge capacities. But why? We've prepared a little bit of an explanation. 

A quantum information unit is called a qubit. These qubits behave differently from normal bits, which only recognize zero and one. Qubits can assume a vast number of different states, and they have the unique characteristic that only the sender and receiver of information want to access them. Anyone else who tries to intercept or read the information will automatically and irrevocably alter it. Therefore, quantum communication is inherently eavesdropping-proof. 

Röder: From what I understood, this technology is, it sounds to me that something big tech or government will use. So I'm trying to understand why is this technology important for our everyday lives? Can you give us an example? 

Gündoğan: So this is a bit difficult. I mean, to me, at least personally, I find a bit difficult. To find use cases for people like us. But on the other hand, it will certainly help in accelerating scientific discoveries. So, many results from fundamental science, in the end, transformed our lives in a technological sense. I mean, even if you imagine, for example, biological imaging, like high resolution imaging. So these people, I think back in the day, many people were interested in how to detect single molecules, just out of curiosity, maybe. Because even as a fundamental science question, this is something really interesting, like how you see a single molecule. But in the end, once you develop it to a certain level, then it has huge applications in medicine, for example. And then likewise, I think some of the technologies or some of the experiments we are pushing, I mean, when I say we, I mean the whole community, basically, not us here in Berlin, would have some implications in, or would find its use in science again, but this time as a tool. So this is how I see it. 

Röder: Storing this kind of information is still very challenging, and you've identified a basic trade-off in quantum memory. The better the storage efficiency, the shorter the storage time. So let us take a listen to this dilemma. 

Scientists are building a deep freeze cloud of atoms. The atoms now move almost in unison because of the extremely low temperature. This cloud serves as the storage medium, a quantum memory device, almost like an exotic hard drive for quantum information. The incoming information, qubits, is encoded in individual light particles called photons. Light enters the cloud. The atoms of the cloud capture the light and collectively store the quantum state of these photons. The entire cloud now contains the information that was contained in the light. But there is a problem. The closer the atoms of the storage cloud are to each other, the more information they can absorb. At the same time, the information is erased more quickly because the atoms collide with each other more frequently. 

Röder: So that really sounds like a dilemma. How do you approach solving this problem? 

Gündoğan: So this problem is, it's actually quite specific to one type of quantum memories, what we call Bose-Einstein condensate, again Einstein's name. So basically this is a quantum phase of matter where, and then you see it only when you cooled a group of atoms, a cloud of atoms, below a certain threshold. And as we know, temperature means movement, right? I mean, as the molecules just move around with really high speeds, it means that actually they are, I mean, this is all, of course, the simplest way to explain it. It means they are hot, right? And then if you cool them down, they tend to slow down. And then if you really cool them down below a certain level, then all the atoms within this cloud, within this small cloud of atoms, now they start sharing the same quantum wave function, basically. That means that like the whole group of atoms, let's say a hundred thousand of them, a million of them, would act as a single quantum object, right? But then this doesn't mean that they don't collide with each other. I mean, even though they kind of share the same quantum state, they still see each other, they still collide with each other. And then as they collide with each other, then the tendency of losing this information gets higher, right? But then we need a high number of atoms to increase the probability of absorbing this incoming light, right? Because, I mean, again, for the listeners, now this cloud of atoms is our black box, right? To store the incoming single light particle, right? But then if we have a lot of atoms, and if we really make them packed, if we pack them at a like densely in a small volume, that means that actually it is nice because a single photon now, single light particle would see a lot of atoms, right? So the likelihood of it getting absorbed is higher. But then once it is absorbed, now there are a lot of atoms and then these atoms would collide with each other. So it is lost faster. And then here, the solution that we came up with is in the context of microgravity and space research, basically. So after storing the light, what happens if you just make them expand? And then again, maybe I take a step back. And then how do you trap these atoms? How do you make them really tight? Is what we call, we are using these optical traps, the so-called optical traps. So it is like a balloon. I like this example of a balloon. And then because in balloons, you are basically trapping a group of atoms with this plastic, right? With this rubber, with this plastic thingy. And then what we came up with is if you just pop the balloon, if you just remove the trap, the atoms will just expand. But on Earth, you can't do this because, I mean, you can do this, but they will just fall, right? Fall off from your sight, right? Because I mean, you then need to send lasers to see what's happening to these atoms. But then if they just fall, like due to gravity, then you can't use them. Or the amount of time is extremely limited. But in space or in microgravity environment, you can just pop the balloon, just remove the trap. And then since they were initially really like tightly packed, right? They just start expanding due to this initial internal pressure. And then this is already what happens to a balloon that is popped, right? So all the gas molecules inside, they will just go everywhere. I mean, I don't know if this is 100% exact, correct analogy, but I think this is the one that I can get, I mean, as close as possible that I can find. And then once they are now expanded, so once this cloud is now expanded, that means that actually now atoms are colliding with each other at a much, much slower rate, right? So this information that you store initially, it can live much longer. It can survive for much, much, much longer durations. And then we show that actually you can increase the time by, I mean, from one second or so to minutes. But then, I mean, this would bring me to the other point. And actually, this is quite a good candidate for operation in space then. 

Röder: And if we look into the future, let's say in the next 20, 30 years, what would be the best imaginable outcome of this technology? 

Gündoğan: If I take the question as a whole, then I think at some point we would like to see interconnected quantum computers across really large distances. And also, these key exchanges that I mentioned at the beginning, the secret key exchanges for secure communication. I think the other main thing that, or breakthrough that scientists would like to see is the realization of such key exchanges across global distances. Because at the moment, this is limited to a couple of thousands of kilometers, which is already quite high, which is already quite high. But then to push it to truly global distances would be a technological and also scientific feat. Because even from a basic scientific point of view, it is quite interesting to imagine like these entangled particles, you know, like these two light particles or two memory devices are entangled, but across like two different parts of the globe, like two opposite sides of the globe. So even if there's a technological use case, even as a science case, it's quite interesting on its own.

Röder: And you've argued that quantum memories that are satellite based could actually enable like a global quantum internet, if I understood correctly. 

Gündoğan: Yeah, more or less. Yeah, correct.

Röder: What would that internet actually do? 

Gündoğan: So the internet here is a bit of a, I don't know if that's that kind of conveys the message 100% correctly. I think at some point it was, I think it was coined back in 2008 or so, one of the fathers of the field, Jeff Kimble, who passed a couple of years ago, a great figure. Actually, one of my academic grandfathers, he was the postdoc advisor of my PhD advisor. I think he coined the term quantum internet in a more like a popular but still technical article. And then I think he was envisioning a world where different quantum processes can be connected via such devices and these single particles of light, these single photons. In that sense, I don't think that it will replace the internet that we know today. I mean, I don't think that I would be sending like WhatsApp messages over the quantum internet, but then in a way it will be still similar to today's Internet in the sense that there will be separate quantum processes working together to reach a common goal or to do a common calculation. Because in the end, building a single really powerful quantum processor at a single node is not that easy. I mean, if you already know from today, right, all these supercomputers, data centers, they are basically connecting like thousands of classical computers together. And then in the end, it kind of becomes a supercomputer. I mean, again, simplifying it a lot, but we can also think of quantum internet in a similar way. Or one user from one location may want to run a quantum simulation on a quantum computer. But then the data she wants to send over to this quantum computer that is located, I don't know, maybe some hundreds of kilometers away from her, that data has to be encrypted in a quantum way too. Or that connection has to be quantum, too. So in that sense, this is all a quantum network. And then I think this is kind of the quantum internet that I imagine when I hear the term. Not necessarily like on our cell phones, but also I don't think that we really necessarily need it at a super personal level. But I think for some certain applications, it will be great.

Röder: And we already mentioned now in this conversation, Albert Einstein a few times, he theoretically described 100 years ago, some of the problems that you're tackling with your work now. So as an Einstein Independent Researcher, this is a grant you earned as an excellent postdoc. What is your attitude towards Einstein's work? 

Gündoğan: May also sound a bit cliché, but it's a great honor to carry this title. But for I mean, his work really was revolutionary across all domains. Because I mean, he contributed to physics across, I think, like from quantum physics to general relativity to statistical physics, photoelectric effect for which he got the Nobel Prize. He started a lot of stuff that people can't even imagine. And but even then those people like we all know, like Einstein was great. But even then, like you get surprised as you progress in physics, like how much he really contributed and how deep insights he had, his skepticism towards quantum physics. It's actually what basically set this whole thing off, right, the whole quantum entanglement, quantum information, both I mean, theory, experiments, technology, because again, for the listeners who are not really familiar with the topic, so he was skeptical of this whole strange character of quantum physics, like action at a distance, like how come wave function can collapse at a place where you don't even have physical influence on it basically, right. But then like this, this skepticism triggered a series of extremely smart people to tackle really fundamental questions. But it's, I truly am honored to have his name as a title, basically. And I feel quite a bit of pressure for that.

Röder: Thank you so much for giving us insights on your research today, Mustafa Gündoğan. 

Gündoğan: Thank you for having me. It was really great. Yeah, thanks again.

Röder: If you like this podcast, please feel free to comment and to share. Thanks for tuning in. Looking forward to the next time.

AskDifferent, the podcast by the Einstein Foundation.