How could quantum mechanics revolutionalise our communications? Take a listen to Season 2, Episode 10 of insideQuantum to find out!

This week, Dr Sumeet Khatri, a postdoctoral researcher at Freie Universität Berlin, tells us all about the fast-paced research area of quantum communications, and explains to us how science-fiction-sounding concepts like teleportation actually arise in real-life quantum communication systems.

Dr Sumeet Khatri obtained his undergraduate degree from the University of Waterloo, followed by a PhD at Louisiana State University and a postdoctoral position at Freie Universität Berlin.

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🟢 Steven Thomson (00:06): Hi there and welcome to Inside Quantum, the podcast telling the human stories behind the latest developments in quantum technologies. I’m Dr. Steven Thomson, and I’ll be your host for this episode.

In previous episodes, we’ve talked a lot about individual quantum systems such as different types of quantum computing platforms, but just like in the classical world, quantum systems will need to communicate with each other. Unlike in the classical world, however, communication between quantum devices is not straightforward. Between the impossibility to copy quantum states – known as the no cloning theorem – and how fragile most quantum phenomena are, figuring out how to get quantum devices to talk to each other is a major challenge in the development of practical quantum technologies.

Fortunately, today’s guest is an expert on quantum communication and is here to tell us all about it. It’s a pleasure to be joined today by Dr. Sumeet Khatri, a postdoctoral researcher at Freie Universität Berlin. Hi Sumeet, and thank you so much for joining us today.

🟣 Sumeet Khatri (00:58): Hi, Steven. Thank you. Thank you for having me. It’s great to be here.

🟢 Steven Thomson (01:01): So before we get into the details of quantum communications theory and concepts such as teleportation, let’s first talk about your journey to this point, and let’s go right back to the very beginning. What first got you interested in quantum physics?

🟣 Sumeet Khatri (01:15): Yeah, so I did my undergraduate degree in physics and I took a lot of courses in quantum physics, but I wasn’t immediately interested in quantum physics. I found many of those courses to be somewhat boring, and a lot of the calculations and things that they got us to do in those courses were a little bit mechanical and I couldn’t quite see the relevance to real life or to anything practical. And it was only later on in my undergraduate career when I started taking courses in quantum information and in particular also quantum computing, that I started to really see a real life connection, and that was when the subject of quantum physics and in particular then quantum information, quantum computing and then quantum communication, that’s when that whole realm started to really come alive for me.

🟢 Steven Thomson (02:03): Do you think that had you been exposed to that earlier, you still would’ve found it interesting? Or do you think you still needed to go through the building blocks of quantum theory before you would be able to find quantum computing as interesting as you do?

🟣 Sumeet Khatri (02:16): That’s a good question. I think the building blocks of quantum information or – not quantum information, quantum physics – that I was taught early on, I think those were still very helpful and they were helpful in understanding not just from the physical standpoint why the classical world is different from the quantum world, but then also how it’s different mathematically. And so those foundations were very important, but I think having that tangible connection to something that is real and some connection to real life, I think would’ve made the subject more interesting right off the bat as opposed to something that sort of came almost accidentally later on.

🟢 Steven Thomson (02:58): Yeah, I see. So was that the point that you decided you wanted to go into quantum physics as a career towards the end of your undergrad, or was that just the point where you started to think it’s interesting, maybe it’s worth following up a little bit.

🟣 Sumeet Khatri (03:12): Yeah, actually, when I first started my undergraduate career, I was really interested in astrophysics, so I was really fascinated by the stars, the universe, and I had this sort of romantic sort of idea about the universe and wanting to study it and so on. And so I was really into astrophysics very much, probably for the first three years, for the vast majority of my undergraduate career, I was really into astrophysics and I was desperate to get into an astrophysics research group, and I tried really hard to do it, and eventually I did, and then I start to realise that maybe the day-to-day work isn’t exactly the way I thought it would be. So I had this romantic fantasy and vision of what I might be doing as an astrophysicist, and the day-to-day work was interesting, but it wasn’t satisfying the appetite that I had for the kind of physics that I wanted to be doing and also the kind of mathematics that I wanted to be doing.

(04:05): So after that research opportunity, I started to wonder, okay, so what else could I do? Then in the final year of my undergraduate, I took a course in quantum theory, and it was actually taught by a prominent researcher in quantum information at that time at the University of Waterloo who’s now in Munich, and he taught this course. It wasn’t necessarily about quantum information, but he brought out a lot of concepts from quantum information and also the way he taught math in terms of the mathematics and how it’s relevant to real life and to experiments that really brought things alive for me, and that was when I started to really find quantum physics more interesting, and therefore also quantum information and quantum communication more interesting. So that very next summer I asked him if I could do a research project with him and he agreed. Luckily I didn’t have to try so hard as I had to do for astrophysics, and then that was pretty much it. I was on the way.

🟢 Steven Thomson (05:06): It’s interesting to hear that you started off in one field and then you decided to go in such a different direction. And it’s interesting also that I think several of our guests have had similar experiences that they started off in something, maybe astronomy or particle physics, these more glamorous fields, the ones that we see these documentaries on TV or we read these books and then we think, wow, that sounds really exciting, and it’s very interesting to see that after being exposed to some physics, people’s opinions start to change, and then sometimes they go into quantum theory, into quantum information, connects matter computing, all these things that maybe don’t have such a high profile, they don’t have these big sort of documentaries and these big famous sort of celebrity scientists.

🟣 Sumeet Khatri (05:46): Right, that’s right.

🟢 Steven Thomson (05:47): I guess we don’t see so much of that in the media perhaps, and we don’t know that that’s an opportunity. Is that how it went for you? Were you aware of these fields at all before you had courses in them or was that your first exposure to them?

🟣 Sumeet Khatri (05:59): So I mean certainly my exposure to astrophysics was based a lot on this kind of the glamorous side of it, but when it comes to quantum information and computing and so on, not so much. I mean in Waterloo it was already one of those places where quantum information and quantum computing was quite prominent already. So there was the Institute for Quantum Computing, which was kind of at that time when I started still on the fringes, both sort of figuratively, but also literally. So they had their institute kind of outside campus, and it was when I started my undergraduate career that they actually started to build a building right in the central part of campus. So I was there for all those four years when that building was actually coming up, and then by the time it was actually ready, it was the end of my undergraduate career and the timing was kind of all lined up well, and the hype was already starting to build up as well. So it was that combination of being at the right place at the right time, but then also taking that course that I did in my final year of undergrad that really made me realise that, yeah, I think I want to do this. And of course, being in a new flashy building and having the opportunity to work there was certainly something that I didn’t want to let go.

🟢 Steven Thomson (07:22): That sounds quite exciting. So you mentioned Waterloo there. Can you give us a quick summary of your career to date? How did you get to where you are now?

🟣 Sumeet Khatri (07:31): So I grew up in Toronto, Canada, and I did my primary, high school and so on over there. And then I went to Waterloo, which is like an hour or two away. That’s where I did my undergraduate degree in physics, in mathematical physics, and then my master’s degree in quantum information, and in particular it was on quantum communications. And then after that I decided to go to Louisiana and the US to do my PhD, and I was there for about four and a half years. I graduated in 2021, and then a few months later I came here to Berlin to do my postdoc.

🟢 Steven Thomson (08:05): And you’ve been here ever since.

🟣 Sumeet Khatri (08:06): And I’ve been here ever since.

🟢 Steven Thomson (08:08): So if you weren’t doing your current job then if you weren’t a postdoc here in Berlin, what do you think you might be doing instead?

🟣 Sumeet Khatri (08:14): I’m not sure to be honest. And one thing which is interesting is that I never really thought so carefully about what I wanted to do as a career. I sort of guided myself by what I liked in terms of subjects at school, but they were not necessarily tied concretely to a particular career. So I knew I liked math, I knew I liked physics, and I did well in those subjects, but those things didn’t necessarily translate into a specific career option. Although when I was younger, I thought maybe I might become an airline pilot because I liked planes a lot at that time. I have very fond memories of being by the airport and with my uncles and just watching the planes land at the airport, and I had really a tremendous fascination towards aeroplanes. So maybe I thought I wanted to be a pilot, and then I like computers a lot, so I thought I maybe would want to be a computer engineer. So yeah, maybe I would’ve been one of those things if I wasn’t doing what I’m doing right now, but I’m not sure, to be honest.

🟢 Steven Thomson (09:13): Still kind of difficult, quite technological jobs, I guess, things that demand quite a high degree of skill in whatever area that you go into, I guess. That’s interesting. So let’s turn to the research that you do for a minute. Can you tell us what’s the big picture goal of the field that you work in and where does your work fit into that big picture?

🟣 Sumeet Khatri (09:35): So what’s emerging nowadays is a keen interest in how the rules of quantum physics, rules of quantum theory can be used to achieve some kind of practical advantage in real life. So this is what’s driving all of the research efforts into quantum computing because we want to know, can we actually use these qubits to do something that we can’t just do with our usual classical computers that we have already right now? And the same question can be asked in the realm of communication, which is not so different from computing, but in quantum communication, what we are really asking about is using qubits to communicate and what actually does that mean? So that means it could mean several things. So we could actually use qubits to transmit classical information. We could use qubits as carriers for bits themselves, and then we can ask ourselves, can we do, for example, secure classical communication better using quantum bits and quantum strategies as opposed to what we do now?

(10:33): Another thing we might want to do is transmit qubits themselves. So you might have two spatially separated parties, two spatially separated people. Say we call them Alice and Bob, and Alice wants to send a qubit to Bob, and we might imagine that this is important because when we actually have quantum computers, someday we might want to transmit the state of one quantum computer to the state of another quantum computer that is really far away. That’s kind of what we’re doing these days as well. We have computers that are very far away from each other and they need to talk to each other. So part of quantum communication is to figure out not only how can we exploit the rules of quantum physics to do better classical communication, but then also just within the quantum realm itself, how do we transmit qubits? And once quantum computers are around, we might envision that we might have a quantum internet where we might be able to do this. So my work has been focused on two parts. So there’s one part which is really sort of on the theoretical fundamental side. So if we have a communication medium for qubits, then we might ask ourselves, what’s the highest rate at which I can transmit either qubits or classical bits or even secret classical bits, so classical bits that are secure against an eavesdropper.

(11:53): And so this is a very basic and fundamental question, and it involves very heavily the tools of information theory. So the information, the tools of information theory as pioneered by Claude Shannon in the 1940s.

🟢 Steven Thomson (12:06): So are these ultimately classical tools from classical information theory that are now being applied to quantum systems or has there always been a quantum element to information theory?

🟣 Sumeet Khatri (12:15): Well, the tools of classical information theory, but adapted to work under the rules of quantum mechanics. So basically that’s why we call it quantum Shannon theory because it’s Shannon theory, but it’s with the rules of quantum mechanics as opposed to the rules of classical physics. So that’s one aspect of my research. The other aspect of my research is actually trying to make a quantum internet into a reality. So if we really want to be able to communicate not only classical bits but quantum bits, then how will we actually do this? So a lot of my research has been focused on how to distribute entanglement. And so why is entanglement important? Entanglement is important because of quantum teleportation, and I know you alluded to this, so maybe I can just talk about it now.

🟢 Steven Thomson (13:10): Yeah, sure. I mean, it sounds like some sort of crazy sci-fi concept, right? Teleportation…surely that can’t exist in real life, but in quantum communications it does, right?

🟣 Sumeet Khatri (13:19): It does. However, unfortunately it doesn’t exist in the way that our minds might conjure up immediately. So those who are old enough, or maybe not even so old, but know anything about Star Trek might have one particular vision in mind when they think of teleportation. But quantum teleportation is still cool, but it’s not exactly that. So it’s not exactly the Star Trekky kind of teleportation that you might have seen, but it’s quite different. And what it involves is actually having two people, Alice and Bob, who are spatially separated to communicate qubits to each other without actually physically sending any qubits at all. And this is the interesting aspect of it. So of course this comes at the cost of having entanglement shared between them. So if for example, you have two particles which are entangled with each other and Alice and Bob each hold one half of that pair of entangled particles, then whatever qubit that Alice wants to send, a separate one, she can do some interaction with that one half of that entangled qubit pair, and she can use that to send an arbitrary state of that qubit to Bob. And when I mean the state, it really just means that Alice is not sending a physical qubit to Bob. Bob already has a qubit on his side, but she’s really sending in some sense the soul of the qubit, and we call that the state of the qubit. It really just contains all the information about that qubit, which is just as good as sending the qubit directly, but without having to actually do that.

🟢 Steven Thomson (14:46): I see. Okay. So Bob already has a qubit that is in some state. So qubits, it’s a quantum bit, right? It’s one of the building blocks of quantum computers. That’s right. It’s in some state, we don’t know. We don’t care what it is. Alice wants to, Alice already has a qubit that has a state that maybe contains some information or some useful property, and she wants Bob’s qubit to take the same state. She wants to just copy that state and send that arbitrary long distances, I guess, and have Bob’s qubit exactly mimic it.

🟣 Sumeet Khatri (15:14): Yeah. For example, Alice’s qubit could be the result of some computation that she did on a quantum computer. Now Alice wants to send the result of that quantum computation to Bob, and this is one way that she can do that without actually physically sending the qubit. And the reason you might not want to physically send the qubit over some transmission medium is that this could be very lossy. And what could happen is if she’s decided to send it, for example, through some kind of fibre optics or something like that, then the state of the qubit could become easily corrupted or even lost. Whereas if the entanglement that they share, if that is perfect, she can do this without losing any information at all. And of course, there’s no free lunch. And there the caveat of course is that you need this entanglement. And so a lot of my work in the past few years has been focused on how to distribute entanglement when you are sort of in the realm that we are today where quantum devices are noisy and lossy and just imperfect. So how best can we do this and how far can we get away with doing this with the devices and with the technology that we have available to us today?

🟢 Steven Thomson (16:27): So then why is this a bigger challenge for quantum systems than it is for classical systems? Because you mentioned they’re fibre optic optics and the internet. We already have the internet for classical computers, fibre optic optics for classical computers. That seems to work pretty well. Why does this not work for quantum computers? Why do we need a whole new technology? What’s the difference here that means that the standard existing networks just do not work for quantum technologies?

🟣 Sumeet Khatri (16:52): Right, so yeah, that’s a very good question. It has to do with the fact that quantum information is very fragile. So keeping a quantum system in its nice pristine state containing all the information that you want it to contain is very tough, really difficult. And this is the reason why we don’t yet have proper full-scale quantum computers that can do something useful. So we need different tools because of the fact that quantum systems fundamentally behave differently from classical systems, and in particular, they’re extremely fragile and prone to becoming corrupted by interacting with their environments. So a lot of what happens in quantum information science, especially within the context of quantum computing and quantum communication, is really how to fight the noise and how to protect our qubits. The whole game is how to keep our qubits protected as much as possible for as long as possible so that we can extract whatever information we need from them before they become so corrupted that they lose all their information and are useless.

🟢 Steven Thomson (18:01): And then in terms of sending qubits and sending information, I think I mentioned in the intro, copying quantum states exactly is not possible. This is something that I remember from undergrad and I didn’t study quantum information. So it sounds like it’s almost a non-starter. If I can’t copy a quantum state, how can I possibly send a quantum state? So how do you resolve this paradox?

🟣 Sumeet Khatri (18:23): Right, yeah, this is a very good point. So unlike in the classical realm, and this is actually also one of the things that makes it different from classical communication, is unlike in classical communication where we can just easily copy whatever information we want – we’re doing this all the time – in the quantum case, we can’t quite do this. So we need different tools. And one of the things that we can do, or that people have come up with, is the concept of a quantum repeater. So a quantum repeater is in spirit very similar to a classical repeater, which would be kind of like a signal amplifier kind of thing. And it’s really meant to not necessarily copy the information, but in a sense relay the information. So one of the ways that we can think about a quantum repeater is that it not only can protect entangled states from becoming corrupted, because entangled states, as we might remember, are the resources that we need in order to do quantum teleportation.

(19:22): So the quantum repeaters not only do that, but they also act as a kind of relay to extend the entanglement over long distances. And this happens through a protocol that’s called entanglement swapping. So you might have, for example, two pairs of entangled quantum bits – qubits – and they have a common node in between. Now, what you can do is you can do a sequence of operations that allows you to take these two entangled pairs, which are sort of spanning certain lengths, and you can extend the range of that entanglement to incorporate the end-to-end distance between the two end points. So this kind of entanglement swapping, this kind of quantum repeater protocol, can be used to extend the range of your entanglement, and therefore you can extend the range over which you can do quantum communication.

🟢 Steven Thomson (20:11): And I guess you probably need a lot of quantum repeaters if you want to send a message a very long distance.

🟣 Sumeet Khatri (20:17): Right, and especially if you want, I mean either we need great qubits or the great ability to protect qubits or yeah, we need lots of repeaters – in a sense, we need both, because every time we do this kind of repeater operation, there’s a chance that you will lose the information, and this has to do with the physics of how this entanglement swapping works.

🟢 Steven Thomson (20:37): I was going to ask if there’s one sort of big outstanding challenge in your field, which is the question I often ask, but it sounds like the big challenge is just protecting this information because it sounds like anything you do to a qubit risks destroying information or losing information, right? I mean, that feels like a big challenge. If there’s almost anything that you do as soon as you touch them, as soon as you look at them, you lose information. Yeah, that sounds, again, sounds like an impossible challenge to overcome.

🟣 Sumeet Khatri (21:05): Yeah, that’s right. And people, we have been coming a long way as a community in terms of coming up with quantum error correction codes, which are essentially the most general way to do this, and this would apply even in the case of quantum communication. But one of the things that we can also do, and this is the kind of work that I also do, is really try to understand what we can do in the presence of these, presence of this noise. So instead of actually fighting the noise, let’s try to, because we don’t quite have the resources yet to do it, instead of fighting the noise, let’s try to see how much noise we can get away with in a sense, what we can still do with the noise that we have and how best we might be able to design our devices such that even if they’re not perfect, we can still do something useful. (21:57): And we have actually come quite a long way, I would say, even though we don’t have full scale quantum computers and our qubits aren’t perfect, there are still networks of systems doing quantum key distribution, which is a form of quantum communication all around the world. So there are quantum key distribution networks in the US and Canada, in China, in Japan, also in Europe, and in the Netherlands, people are doing this. So it is possible on small scales to still have quantum communication networks and quantum communication systems, even though we don’t have perfect qubits, so we can still get away with some of the noise in a sense without actually fully getting rid of it.

🟢 Steven Thomson (22:40): And you mentioned there quantum key distribution, I guess that touches on one of the applications of quantum communications, right? So I mentioned earlier quantum systems just talking to each other. They naturally speak in the language of quantum information if you like, but then there’s this whole other field of cryptography and key distribution. Can you tell us a bit about what is quantum key distribution and why is it a useful thing? Why do we care why we need quantum communications to achieve this goal?

🟣 Sumeet Khatri (23:06): Yeah. So I’ll start with what key distribution is more generally. So the whole goal in key distribution and in cryptography in general is to send information without it being tampered with or with some guarantee that the information that is being sent is not being eavesdropped on by some malicious individual. So the whole game is to protect the information that we want to send to each other from malicious people. And we’re doing this all the time. When we are on our computers and we’re doing financial transactions and all this type of stuff, we are distributing keys sort of in real time as this is happening. And a key, what it does or it’s meant to do, it’s meant to protect the information that you send such that a person at the other end who also shares that same key can then decrypt the information that you send such that what they get at the end is what you intended to send in such a way so that whoever was trying to intercept that message, the encrypted message – and the encryption happens with the key – whoever tries to intercept that encrypted message to them, the message will look just like gibberish, or should look like gibberish, to the extent that they won’t really be able to figure out what the intended message was.

🟢 Steven Thomson (24:27): I see. So in the classical world, if there is an eavesdrop or some malicious third party, they can copy the message, they can eavesdrop on the message, they can get a complete copy of the message, but it’s encoded, it’s encrypted, and ideally they want to be able to decrypt. That’s the idea?

🟣 Sumeet Khatri (24:43): Ideally. Yeah, that’s right. That’s right. And the way to achieve this in a perfect sort of technical sense, what we call information theoretic security, is through this concept called a one-time pad. So a one-time pad, what that is is a very simple protocol. So the two parties, Alice and Bob, want to communicate with each other should share the secret key, and as long as this key is random – has high entropy, as we say – then as long as Alice and Bob share the secret key, Alice uses the key to encrypt her message, and then that message gets sent over to Bob and then Bob, because he shares that same key, can use it to decrypt the message. Now, if the eavesdropper in the middle tried to intercept it, because in principle the key is completely random and only Alice and Bob know it, if it is truly random, then the eavesdropper should not be able to decrypt that message.

(25:34): So that’s a very simple protocol, and that’s known to be information-theoretically secure. I see. Okay. In principle, so now the whole game is how do you distribute this key? So now in the olden days, what I’ve heard is that what used to happen is people would literally have a briefcase with these secret keys, secret codes, and they would be handcuffed to these special people who would then be responsible for transmitting or to literally transmitting physically if they would actually go and take these codes, these keys to the people that it was relevant to. So what quantum key distribution aims to solve is this task of how to exchange the key. I see, okay. So in a sense, it’s a purely classical thing that we still aim to do. So quantum key distribution is still aiming to achieve a classical task.

🟢 Steven Thomson (26:20): I see. Okay.

🟣 Sumeet Khatri (26:20): A classical task of distributing a key, a key which is completely classical. So the key is just a string of bits of zeros and ones, but the point of quantum key distribution is to distribute these bits in a quantum way and the point of doing this in a quantum way so that we can achieve this ideal information theoretic security without having to have someone change to a suitcase and transporting this key. So that’s the aim of quantum key distribution: to use qubits to, so that Alice and Bob can actually share this, share a secret key, and in such a way so that it is guaranteed that regardless of whatever tampering that might have occurred in between during the key exchange process, even Alice and Bob have a key that is guaranteed to be secure, such that their messages are also then going to be secure.

🟢 Steven Thomson (27:10): I see. Okay. I actually didn’t realise quantum key distribution was exchanging keys that would be used for classical communication.

🟣 Sumeet Khatri (27:15): That’s right.

🟢 Steven Thomson (27:15): Yeah. Yeah, that’s interesting. Yeah. Then why does quantum mechanics help in this case? Why does it help the procedure to be more secure than, as you say, just handcuffing, right? Handcuffing a briefcase to someone and sending it off across the world.

🟣 Sumeet Khatri (27:26): Yeah, exactly. It helps because of what you mentioned earlier, which is the no-cloning theorem. So the way quantum key distribution works is Alice sends a bunch of qubits in particular states to Bob, and suppose that Alice [Editor’s note: This ‘Alice’ should be ‘Eve’, the eavesdropper.] then wanted to intercept these qubits. Because of the no cloning theorem, she can’t just copy them. If she tries to copy them, she’s going to interrupt the state of those qubits. And if she does do that, then what Alice and Bob can do after the fact, after they have distributed these qubits to each other, is that they can exchange classical information about what they actually sent and the measurements that they did on their two ends. And through that, and this requires some work to realise this, they can actually figure out if their qubits are mismatching too much that they can figure out that somebody has eavesdropped.

🟢 Steven Thomson (28:17): I see. So it’s almost like…

🟣 Sumeet Khatri (28:18): And they can abort the procedure.

🟢 Steven Thomson (28:19): So it’s almost like the key is transmitted and then they’re comparing the key in a smart way. And because the key was quantum mechanical, if anyone interacted with it, if anyone looked at it essentially it would’ve got changed…

🟣 Sumeet Khatri (28:30): It would’ve gotten changed from what it should have been in the ideal case. I see. So Alice will have some bits on her end based on the qubits and the measurements she did. Bob will have some bits on his end, and if they disagree too much, because remember at the end they should have the same string of bits, if they differ too much, if they differ beyond the point that they can fix it – and this is where the kind of fixing the noise and kind of dealing with the noise comes in – if it’s beyond the point that it can be fixed, then basically they can conclude that there was probably some eavesdropping and then they can abort the procedure and just try again, I guess.

🟢 Steven Thomson (29:08): I see. Okay.

🟣 Sumeet Khatri (29:10): So that’s where quantum mechanics comes in, because it provides you with a way of detecting whether somebody has eavesdropped, through the no-cloning theorem.

🟢 Steven Thomson (29:19): So this sounds like it’s an addition to standard communication networks then if you’re swapping private keys, I guess this is probably overkill for most purposes, it seems like only if you’re exchanging particularly sensitive information.

🟣 Sumeet Khatri (29:30): Yeah.

🟢 Steven Thomson (29:31): Is that the same as what you meant earlier when you talked about a quantum internet or for a quantum internet? Do you mean a communications channel that is purely quantum and has no classical component?

🟣 Sumeet Khatri (29:42): Well, we can use the quantum internet also to do quantum key distribution. So the entangled states that I was talking about could actually be used not only for teleportation, but they can actually be used for quantum key distribution as well. So that’s why entanglement is so versatile, because it can be used for multiple different things, not just for teleportation. So entanglement could actually even be used to do quantum computing itself. So there’s a form of quantum computing called measurement-based or distributed quantum computing, where you actually do it by implementing measurements on a very big sort of entangled state of several qubits. So you can even do computing in that way. And so this is one of the ways in which entanglement can be used as a resource, not just for teleportation as I described earlier, but also for other things like quantum key distribution and even quantum computing. So quantum internet would be useful then for transmitting entanglement for various applications.

🟢 Steven Thomson (30:33): So once you have these quantum communications networks, then you can apply them to all these different applications, you can use these different protocols, and yeah, once you’ve got these quantum communication channels, the world is your oyster. You can do all these things. So where are we in terms of actually achieving this? Because again, these things like teleportation, it sounds like science fiction. Is this work still very theoretical? Is it still very abstract, or are there real communications networks already using this technology?

🟣 Sumeet Khatri (31:06): Yeah, fortunately it’s not just a theoretical construct anymore. So that’s great, and that makes me very happy to see that over the past, I guess five to 10 years, there have been, there’s been lots of progress on actually implementing, for example, quantum key distribution networks. As I mentioned in various countries in North America, Asia, and Europe, these systems are now in place, but now even more broadly in terms of entanglement distribution, there have been experiments done that make use of satellites to transmit entangled states. So I think that’s really cool, and over long distances as well. So there was a nice sort of experimental breakthrough from a group in China in 2017 where they successfully distributed entanglement over 1200 kilometres.

🟢 Steven Thomson (31:54): Was that between satellites or was that between a satellite and some party on the ground?

🟣 Sumeet Khatri (31:58): It was a satellite distributing entangled states to two stations on the ground. So you have a satellite up top and it sends one half of the entangled pair to one ground station and another half to the other.

🟢 Steven Thomson (32:07): This is probably a very naive question, but coming from the sort of many body low temperature background, a lot of the interesting quantum phenomena that we see happens at low temperatures, where the thermal fluctuations play less of a role. If you put a satellite in space…space is pretty cold. Does that gain you anything? Does that help at all to have the qubits in a cold environment or are there enough other challenges that that’s kind of an irrelevant concern?

🟣 Sumeet Khatri (32:31): That’s a good question. I actually never really thought about that so much. I mean, it could help, but I’m not sure if it would still be cold enough on its own to be up in space. I mean, these satellites are not in space directly, they’re sort of high up in the atmosphere. So I don’t know if they’re so far up that the temperatures would be cold enough on its own, be helpful. The entanglement sources, they still need to be whatever they are in terms of how they’re made, and they still need to be at low enough temperatures. So the sources that are put on board these satellites, they still need to function, I think, in the same way that they would have to function on the ground.

🟢 Steven Thomson (33:07): I see. Yeah. I suppose the electronics and the satellite would also be generating heat and…

🟣 Sumeet Khatri (33:13): Yeah, that’s right. Absolutely.

🟢 Steven Thomson (33:15): Yeah. I have no idea how the temperature changes as you gradually move away from the earth. I’m just remembering, as you probably know from an astronomy undergrad, deep space is extremely cold, colder, I think, than liquid helium is. So right. There’s something appealing about just putting your quantum device in a satellite and letting the temperature of deep space do the work.

🟣 Sumeet Khatri (33:30): Right, right. Yeah, that’s true. I mean, yeah, I guess people say that space is on the order of two Kelvin or something like that.

🟢 Steven Thomson (33:39): I’m trying to remember. There’s a two, a three and a seven in it, and I can’t remember what order the digits come in.

🟣 Sumeet Khatri (33:43): Yeah, yeah, exactly. Maybe it’s like 2.713. I’ll just go out there and say that even if I’m wrong.

🟢 Steven Thomson (33:48): 2.7 sounds about right. It’s all roughly three, right? We’re theorists, it’s roughly three.

🟣 Sumeet Khatri (33:53): So yeah, it might be interesting to actually try to put a quantum computer in space. Maybe that’s even easier to do it then. I don’t know. But…

🟢 Steven Thomson (34:02): There’s got to be some engineer who just heard you say putting a quantum computer in space is easy, disagreeing with that…

🟣 Sumeet Khatri (34:11): Right. It’s hard enough to do it on the ground. Right? I mean, then you have to add the additional challenge of actually going into space and then…yeah, but I’m a theorist. I can imagine wherever I want.

🟢 Steven Thomson (34:24): Fair point. Well, on that note, as a theorist working on quantum information, your work lies at the intersection between computer science and theoretical physics or quantum physics. Historically, these feel like they’ve been two quite different communities. The information science, you mentioned, this sort of Shannon information theory developed quite extensively on classical computers and then quantum physics, which came, I suppose more from many body physics and materials. So now the fields are starting to collide in this quantum computing realm, quantum information realm. You’re right there in the middle. How has it been to be at the interface of these two different fields? Has it been challenging? Do people from one field struggle to understand the other, or is it the case that everyone’s working towards a common goal and everyone’s kind of speaking the same language?

🟣 Sumeet Khatri (35:13): Yeah, this is a great point, and I think it’s one of the strengths of quantum information science in general, that it’s so multidisciplinary. It allows people from so many different academic communities to come together towards a common goal. So for example, not just physics and computer science, but also mathematics, chemistry. I think this is really cool. And I mean, yeah, there are issues with the sort of language. We don’t always speak the same language, but when it comes, for example, for me specifically when it comes to physics and computer science, I’ve really enjoyed the opportunity to learn computer science and learn that language and learn the tools that come along with it. And I think it can only help if the two communities are coming together and really genuinely trying to understand each other as opposed to staying isolated. And I think having this common goal of building a quantum computer, building quantum communication networks, will allow us to do this very easily. I think it’s just a matter of time. I think it’s already happening. I mean, I already collaborate, for example, with some people who are more on the computer science side, and I think that’s that it’s extremely enjoyable. They enjoy it. I enjoy it. It’s a win-win.

🟢 Steven Thomson (36:35): Yeah, I mean, it sounds perfect, right? You both get to bring your own expertise and your own ways of solving problems and then combine them. Yeah, it feels like very much the best of both worlds.

🟣 Sumeet Khatri (36:45): Exactly.

🟢 Steven Thomson (36:47): So on the note of bringing people together, there’s one question that I always ask every guest on the podcast, and that’s to say that physics, historically speaking, has been a field dominated by white cisgender men for a very, very long time. And it feels like things are slowly changing, but there’s still a long way to go before we reach a level playing field. So in your experience over your career so far, having worked in several countries and worked between several disciplines, have you seen attitudes towards diversity changing either across the years or in the different places that you’ve worked?

🟣 Sumeet Khatri (37:18): Yeah. This is a very, very important point. And I do believe I’ve seen improvements. I’ve seen things change in the direction of more diversity, more women, people of color taking part in the activities of our field in various different ways. And I think we are moving forward in the right direction, but this still requires work. So it’s not something that will change drastically in a very short amount of time. It will take time and it will take effort from, I would say all of us in the community. It’s an important thing. And the community as a whole in quantum information science, and not just quantum information science, but in science in general, in all aspects of society, we would benefit from having more diversity. So with more diversity comes different viewpoints, different ideas, and in my opinion, that can only be a good thing, and I think we are moving in the right direction.

(38:18): But as I said, it requires time. And one of the things that it requires is a lot of work at the grassroots level, I like to say. So work at the sort of primary, high school level and to, in a way such that it causes like a trickle up effect. So that maybe not so much in the near future, but down the line far ahead, we will be at the point where there are people of various backgrounds in equal measure taking part in scientific activities. And I, I really do believe that these grassroots efforts, and in particular mentorship, mentorship is a very big part of this. Not just so that students from underrepresented groups can see people like them, but also so that they can see people who might not necessarily be like them, but that they can still see that certain misconceptions that have been imparted onto them by the culture in which they grew up or by their early teachers, they can actually get firsthand experience with the real life scientist, even though they might not look like them.

(39:25): They might get to see the kinds of things that they do, and it might still encourage them to pursue a career in science. And of course, it never hurts to actually emphasise more, during the classes that are taught, the role that people from underrepresented groups have played in pioneering various fields. I mean, there were admittedly few of them, but they did exist. There were women who made important contributions to science, computer science, mathematics, and also people of colour and their roles can and should be emphasised a little bit more. And I think that would be extremely helpful. It was actually helpful even for someone like me. So I’m a person of colour, and it was helpful for me to see, even when I studied astrophysics, to see that people from Indian backgrounds – and my ethnic background is Indian – and it was helpful to see people from that background having made contributions to that field. It is helpful. So yeah, it’s extremely important, and we are going in the right direction, but it still requires work. And I think the kind of work that we can do as postdoctoral researchers and even graduate students can play a role in mentoring younger students. It’s really about that. These types of outreach activities, mentorship activities, grassroots efforts.

🟢 Steven Thomson (40:51): I think you make a good point there about the erasure of contributions of a lot of people historically. And I think there are efforts now to go back and recognize the contributions of people from underrepresented backgrounds that were often attributed usually to white cisgender men. And yeah, I think there is a growing awareness of these contributions that have been minimised or ignored or forgotten about, and people are starting to go back and find them, which is a very good thing. There’s also a comment a few of our guests have made, which is that because quantum technology is such a new field and because it is kind of cherry-picking bits from other fields and then forming something new, this is kind of a unique time for the field because now is the time to get this stuff. And if quantum technologies and quantum information, it takes these lessons from physics and from other fields that have had discrimination for a long time, learns the lessons and gets it right now, it could set the fields on a really good path.

🟣 Sumeet Khatri (41:50): Right. Yeah, this is absolutely a great point. Our field is very young, so we can probably say that it really started sometime in the nineties, although one can actually say that maybe even started earlier. But the point is that it’s a very young field regardless, and this is exactly the point that we are sort of fresh and right at the starting point. And if we can get it right now, then, and really make a concerted effort to avoid mistakes of the past, then yes, down the line we will see a situation in which, yeah, there’s even representation in our field. I’m very optimistic about that.

🟢 Steven Thomson (42:35): I very much hope so. Okay, one final question to end with then. If you could go back in time and give yourself just one piece of advice, what would it be?

🟣 Sumeet Khatri (42:45): Oh, yeah, that’s a tough one. I talked about mentorship earlier, and I think that’s probably the advice that I would give. To younger students who are coming up and still trying to figure out what they would like to do: have a mentor, not just one maybe, but several, at least a couple people to talk to who you feel comfortable talking to and to whom you don’t, are not afraid to ask, “stupid” questions, so you can feel comfortable asking them whatever you want about anything. And I think this is probably the most important aspect. This is what would allow anyone who’s sort of coming up still young, trying to figure out what they want to do, a mentor can really help them to figure this out. When I was younger, I really was unsure and struggled to…I never really fully answered the question of what I really wanted to do.

(43:40): And I think I kind of alluded to this already earlier, and I feel like if I had sought out a little bit more mentorship, then I would’ve had some guidance on how to answer this question. Because once you know what you want, and of course what you want can change over time, but at every stage in life, it’s important to have some sense of what you want because it provides a proper guiding principle. It provides you a platform from which you can advocate for yourself. It provides a platform from which you can make very important decisions in your life, and it’s not always possible to do this on your own. So have people whom you trust, who you feel are close to you and talk to them, talk to them. Yeah.

🟢 Steven Thomson (44:25): I think that is excellent advice to end on. And yes, I very much agree with everything that you’ve said there. Okay, fantastic. So if our audience would like to learn a little bit more about you, is there anywhere that they can find you on the Internets, on social media, anything like that?

🟣 Sumeet Khatri (44:40): Yeah, I have. I’m not so active on social media admittedly, but I do have a website. It’s sumeetkhatri.com. It’s just that. And there they can find out more about me and what I do, and I even have some resources that they might find helpful on various topics related to quantum information, quantum communication, and so on.

🟢 Steven Thomson (45:03): Great. I can say I found some of your resources helpful, in fact.

🟣 Sumeet Khatri (45:07): Ah, that’s great, that’s perfect.

🟢 Steven Thomson (45:07): So I can highly recommend that. Okay. We will leave a link to your websites on our own website, insidequantum.org. Perfect. So thank you very much, Dr. Sumeet Khatri, for your time here today.

🟣 Sumeet Khatri (45:18): Yeah, thank you Steven. Thanks. Thanks so much for this opportunity. I really appreciate it.

🟢 Steven Thomson (45:21): Thanks also to the Unitary Fund for supporting this podcast. If you’ve enjoyed today’s episode, please consider liking, sharing and subscribing wherever you’d like to listen to your podcasts. It really helps us to get our guest stories out to as wide an audience as possible. I hope you’ll join us again for our next episode. And until then, this has been insideQuantum. I’ve been Dr. Steven Thomson, and thank you very much for listening. Goodbye.