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The future of quantum mechanics: Unraveling entanglement's secrets

A physicist explains the complexities of quantum entanglement and why scientists are so keen to understand and control this elusive phenomenon.
3D rendered illustration of an atom
Quantum mechanics helps us understand the behavior of atoms. | iStock/vchal

Guest Monika Schleier-Smith is a physicist who says that quantum principles, like entanglement, can make atoms do funny things, such as allowing two atoms to share secrets across great distances. While entanglement opens tantalizing possibilities like quantum computing, there’s still much we don’t know about quantum mechanics. She now uses lasers to “cool” atoms to near motionlessness as a starting point for controlling and proving entanglement, as she tells host Russ Altman on this episode of Stanford Engineering’s The Future of Everything podcast.

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[00:00:00] Monika Schleier Smith: The goal is to solve a problem that's inherently quantum mechanical. So, um, chemistry problems that involve the quantum mechanics of many interacting electrons. Those are hard to do on classical computers. You can do them better. That could have implications for, let's say, drug discovery, right? And so that's something where it's really natural to say, why don't we make the hardware natively be quantum mechanical?

[00:00:28] Russ Altman: This is Stanford Engineering's The [00:00:30] Future of Everything, and I'm your host, Russ Altman. If you enjoy the podcast, please follow or subscribe wherever you listen to your podcasts. It'll help us grow and it'll make sure that you never miss an episode.

Today, Monika Schleier Smith will tell us about quantum mechanics and a mysterious and amazing phenomenon called entanglement. It's the future of quantum mechanics.

Before we jump into this episode, I'd like to ask you to rate and review the podcast. It will help us [00:01:00] improve and it'll make sure that others learn about the podcast as well.

So there's physics. In the old days, we had classical physics where there were particles. Particles had locations, they could move and they would have momentum and velocity and we could measure all of those things. And that was all true until the beginning of the 20th century when physicists were compelled to invent quantum mechanics, which is a much [00:01:30] more complex version of physics that is necessary to understand and explain the behavior of atoms.

Quantum mechanics is filled with non intuitive phenomenon. For example, sometimes you can't measure both where a particle is as well as how fast it's moving. Sometimes two particles can talk to one another at a distance through unclear mechanisms.

Well, Monika Schleier Smith is a professor of physics at Stanford University, [00:02:00] and her lab studies quantum mechanical phenomenon, especially this amazing phenomenon of entanglement.

In entanglement, two atoms seem to be talking to one another and doing things in a correlated manner with no clear physical connection. Well, that's not right. Actually, the physical connection is through quantum mechanics. Monika Schleier Smith will help us understand a little bit about how to think about entanglement, how to measure it, and how to make sure that you can get your molecules into an [00:02:30] entangled state and make sure that they stay entangled.

So Monica, many people don't think about quantum mechanics and quantum systems every day. So I think it's important to start out for you to explain the key features of quantum mechanics and what makes it so fascinating to study to you?

[00:02:49] Monika Schleier Smith: Well, you know, the reason many of us don't think about quantum mechanics every day is that it doesn't, it's not the natural sort of theory to describe our interactions with the everyday [00:03:00] world. It describes really kind of microscopic systems: atoms, electrons, and their behavior. And it has a number of really remarkeble features that seems counterintuitive to sort of, uh, based on our everyday lives, such as quantum uncertainty.

[00:03:17] Russ Altman: Yes.

[00:03:17] Monika Schleier Smith: Um, this is something sort of that first got me interested in quantum mechanics. I learned in my high school chemistry classes that if I think about an electron, I shouldn't think of it as just a being at a point with a well defined position and a perfectly [00:03:30] defined, um, speed, but actually, um, there's some fundamental uncertainty in where it is. I should think of it as kind of smeared out in space, and I can never know precisely where it is and how fast it's moving at the same time. Um.

[00:03:41] Russ Altman: And that's quantum, that's quantum uncertainty, and that's a real thing for like all particles basically.

[00:03:47] Monika Schleier Smith: That's right. Yeah, that's right. Um, and that tells us more generally that, um, Uh, whenever we kind of measure the properties of a particle, um, there's some kind of random probabilistic [00:04:00] aspect of it. We can measure the position of an electron, um, but we fundamentally can't predict from the outset what that result of that measurement is going to be. So quantum mechanics, the laws of quantum mechanics are probabilistic, um, and that sort of, um, you know, blew 20th century physicists minds. Um, we always previously thought that the laws of nature are deterministic.

[00:04:21] Russ Altman: Right.

[00:04:22] Monika Schleier Smith: Um, but beyond that, there are, um, remarkable properties such as, really I would say the most remarkable feature of quantum mechanics is something [00:04:30] called entanglement.

[00:04:31] Russ Altman: Which is a great, I just have to say, it is a great word, whoever was the physicist doing marketing picked a great. Because entanglement, we kind of have a sense of what it might be, but we don't really, so please continue, I just got to say it's a great word.

[00:04:44] Monika Schleier Smith: Yeah, so I mean, entanglement essentially tells us that, it really tells us something fundamental about information in our universe, that information doesn't have to be stored locally. Um, in something like a single bit in your computer or, um, a single particle. But [00:05:00] rather information can be non local and can be somehow shared or distributed between particles. And that can show up in funny ways where if I measure, um, you know, the state of one electron, it looks random, and I measure the state of another electron and it looks random. But if I were to compare the measurement route, outcomes, I would always see some correlations in the randomness.

[00:05:19] Russ Altman: So that's really just to stop there for a minute. That means that even though they look random Something is being transferred between them to allow them to be correlated, right? They [00:05:30] must be getting information from one another in some way. Is that true?

[00:05:33] Monika Schleier Smith: Well, I would be a little bit careful

[00:05:35] Russ Altman: Okay.

[00:05:35] Monika Schleier Smith: And say something has been transferred between them. So and so I mean the type of I like to sort of use an analogy it's as if Um, I were here in my office tossing coins, and you were in your office tossing coins, and if we, you know, didn't compare notes, it would look totally, both coin tosses would look random.

But we could have a situation where I toss my coin a hundred times. And every time I get heads, you get heads, and every time I get tails, you get [00:06:00] tails. And that, that's weird, right?

[00:06:01] Russ Altman: Right.

[00:06:01] Monika Schleier Smith: That would never actually happen. Um, and that could happen. I don't, you're probably in another office at Stanford, I'm not sure. But, Um, even if you're somewhere else in the world, like, in principle, this can happen over very long distances.

[00:06:14] Russ Altman: That's the other thing, so the entanglement literally is that they're kind of, there's an interaction between them that kind of tangles their, I guess they're futures or they're present in a...

[00:06:25] Monika Schleier Smith: Exactly. And so initially there needs to be some interaction that establishes that [00:06:30] entanglement.

[00:06:30] Russ Altman: Okay.

[00:06:30] Monika Schleier Smith: Um, and that allows subsequently these two particles to be correlated. Um, so it's not a way to transmit information faster than the speed of light. It doesn't, you know, it doesn't break that law.

[00:06:42] Russ Altman: So do these particles generally need to be close to one another at some point in order to establish that relationship and then they can travel far apart or can that estab and I understand this is probabilistic, but in general are the most probable entangled atoms the ones that have been close to one another or is that not even true?[00:07:00]

[00:07:00] Monika Schleier Smith: So in general, um, you know, intuitively you would think they should be close to each other to establish that entanglement and it's true that some, at some stage, um, that's required, some kind of a local interaction. Um, but So if, for example, I can take light, uh, or photons, particles of light, and those can sort of easily travel over long distances. And so those could then be used as sort of a messenger to generate entanglement....

[00:07:23] Russ Altman: Okay.

[00:07:23] Monika Schleier Smith: ...between some systems that are further apart.

[00:07:26] Russ Altman: And what is the time persistence of these phenomenons? So [00:07:30] like, I know that if you and I talk and say, we're going to do a dance, but we're not going to look at each other, we'll start off looking pretty correlated. But unless we are really good timing, my dance is going to start to look different from your dance after a couple of minutes. So is there a similar phenomenon with this entanglement where as time goes on, the kind of level of communication or correlation degrades?

[00:07:53] Monika Schleier Smith: Um, yes, absolutely. And, um, In general, this is kind of, I would say, one of the big [00:08:00] challenges in kind of harnessing this phenomenon of entanglement for, you know, a variety of technologies where it's useful, um, is that it is difficult to preserve. And it's maybe not so surprising because essentially, this other sort of remarkable aspect of quantum mechanics is that when we measure a quantum system, um, you know, it has to sort of initially, it has some inherent uncertainty, but it has to decide when we do a measurement.

[00:08:23] Russ Altman: Right.

[00:08:23] Monika Schleier Smith: And so that process of that measurement actually affects the quantum state and it can destroy the entanglement. [00:08:30] So if initially, you know, you and I each had a particle and they were in an entangled state, but I measure mine, then after that, this sort of entanglement, this state has collapsed, one could say, into a particular configuration. Like, um, after I measure my coin to be heads, we know that yours is also heads. And um, from that point on it's, the randomness is gone. If we do another measurement, we'll actually get the same result.

[00:08:54] Russ Altman: Ah.

[00:08:55] Monika Schleier Smith: Um, and so because the measurement necessarily perturbs [00:09:00] the quantum state. That means that, that could be something that we intentionally do, that I intentionally, you know, measure the state of my electron.

But it could also be, um, something that happens unintentionally. Um, uh, some light bounces off this system that I'm trying to preserve in a fragile, entangled state. Um, and just that photon bouncing off of my atom, let's say. Um, perturbs its state and now, um, that, that can destroy the entanglement, even if I don't look at the...

[00:09:26] Russ Altman: Got it.

[00:09:26] Monika Schleier Smith: ...measurement outcome.

[00:09:27] Russ Altman: Got it. Okay.

[00:09:28] Monika Schleier Smith: But in terms of absolute [00:09:30] timescales, I would say it depends a lot on the physical system. It might be nanoseconds, it might be seconds, and now it depends on the details.

[00:09:36] Russ Altman: Okay. So, okay. So that's very helpful. So, and we started this part of the conversation where you're saying that this is one of the things that gets you most, you find the most fascinating in quantum mechanics is this entanglement phenomenon. And I believe you study it. And so maybe you can tell us, and it sounds like you've implied for sure, that you've been able to create some entangled atoms. And it sounds like from your most recent comment, that sometimes you work very hard, and that other [00:10:00] things are kind of messing up these systems that you've worked very hard.

So tell us what happens in the lab, and how do we actually study this? And then later on, we'll talk about what are the applications that might come out of this downstream.

[00:10:12] Monika Schleier Smith: Sure, yeah. Um, you know, there are a variety of physical systems where this has by now been studied, but the platform we work with in my lab, um, is one of, uh, laser cooled atoms.

[00:10:25] Russ Altman: Okay.

[00:10:25] Monika Schleier Smith: And so, um, maybe I should kind of break that down a little bit and say...

[00:10:28] Russ Altman: Yeah, I think of [00:10:30] lasers as heating things up, so right away I'm a little bit confused.

[00:10:33] Monika Schleier Smith: Exactly, yeah. So, I mean, and so first of all, you know, what do we, what do I even mean by cold? The idea, their goal is to have kind of atoms that are very well controlled that aren't just whizzing around as they are at room temperature. But that are essentially brought to a standstill, um, and kind of held in place in a vacuum chamber where there's no other atoms colliding with them.

[00:10:51] Russ Altman: Okay.

[00:10:52] Monika Schleier Smith: Um, uh, the lasers are essentially a trick for getting those well controlled atoms. And like you said, you know, you might [00:11:00] think a laser should heat things up. Um, uh, but it turns out there's a trick to use a laser to slow down, um, atoms with a, if I use basically a laser with a very specific wavelength, um, that interacts with a particular transition. Let's say one of the atoms we use is rubidium, uh, rubidium has a transition at 780 nanometers. And if I have a laser that's exactly at the right wavelength, um, it can be used to very precisely control that atom. Um, should I say a little bit about the trick for cooling them [00:11:30] down?

[00:11:30] Russ Altman: What the heck, absolutely. This is a podcast where time is a very ephemeral concept. So yes, please tell us about that.

[00:11:39] Monika Schleier Smith: Yeah, yeah. So, um, so there's a trick that involves the Doppler effect.

[00:11:43] Russ Altman: Okay.

[00:11:43] Monika Schleier Smith: And, um, the way you can kind of visualize it is imagine that I have an atom that's sort of whizzing around in my vacuum chamber and I have a pair of laser beams. We'll actually do this in all three dimensions, but let's think about a pair of laser beams that's sort of counter propagating.

[00:11:56] Russ Altman: Yes.

[00:11:56] Monika Schleier Smith: Um, and what we do is we tune the [00:12:00] lasers. Um, so that they're a little bit lower in energy than what they would need to be resonant with the atomic transition.

[00:12:07] Russ Altman: Right. So you were, you gave me a number before. You're going to be close to that number, but not the same number. 700, 780.

[00:12:15] Monika Schleier Smith: Far out decimal point.

[00:12:16] Russ Altman: Yeah.

[00:12:16] Monika Schleier Smith: We'll get a little bit. Less than that number and so um a little bit lower in frequency.

[00:12:22] Russ Altman: Yup.

[00:12:23] Monika Schleier Smith: And now if you the Doppler effect is going to help us so you know this phonomenon where basically like I'm driving [00:12:30] along there's a truck coming the other way and it's coming the other way it will sound kind of higher pitch.

[00:12:34] Russ Altman: I live next door to the train tracks here and in fact, I go on mute sometimes when we're talking so that you don't hear them, but it is nothing but Doppler effect all morning.

[00:12:45] Monika Schleier Smith: Right. Um, so that same effect means that this atom that's whizzing around. Um, if it is moving sort of opposite the direction of a laser beam.

[00:12:54] Russ Altman: Ahhh.

[00:12:55] Monika Schleier Smith: That laser beam, it'll sound like it has a, it'll seem like it has a higher frequency to the app.

[00:12:59] Russ Altman: And it gets [00:13:00] you into that magic zone.

[00:13:01] Monika Schleier Smith: Normally it wouldn't be resonant. It becomes resonant.

[00:13:03] Russ Altman: Yes.

[00:13:03] Monika Schleier Smith: The other laser beam is sort of pushed away from resonance. So, but the key point is the laser beam. That's pointing opposite the atom's motion. That's the one that we want it to interact with because it's going to give it a kick. That photon is going to give it a kick that slows it down.

[00:13:18] Russ Altman: Yep, yep. And just to make, uh, this is, forgive this very primitive analogy. But if I have a train coming towards me and if it sounds like an A flat note, if I [00:13:30] run fast enough towards that train, I could increase that A flat to a normal A by...

[00:13:34] Monika Schleier Smith: Exactly.

[00:13:35] Russ Altman: ...the Doppler effect. And then if that A leads to me being entrapped or whatever the right word is, then now that train is controlling me where it couldn't control me before, but I ran fast enough to hit that magical frequency.

[00:13:47] Monika Schleier Smith: Exactly. Yeah, I have this picture in mind now where once it hits an A, some wine glass starts to rain.

[00:13:52] Russ Altman: Exactly, exactly. Some beautiful thing happens. Okay. Okay, so that was great.

[00:13:56] Monika Schleier Smith: We just want to slow down. We just want to give this atom a little kick that slows it [00:14:00] down.

[00:14:00] Russ Altman: So once you've isolated these atoms, my guess is you have to get two of them or something to do this entanglement trick. But please go on about entanglement.

[00:14:08] Monika Schleier Smith: Yeah, exactly. So slowing them down, that's just the first step. And then we track them. You can also sort of track them at the focus of a laser beam. Um, and you know, there are some labs, a number of labs around the world where people have actually individual atoms, E1 trapped at the focus of each laser beam.

Um, that's, um, not exactly what we've been doing so far in my lab. We have maybe [00:14:30] some clouds of thousands of atoms at the focus of each laser beam, but, um, you know, different, the details might differ from one experiment to the next, but roughly speaking, we have these atoms, we track them, and now we want to actually get them to talk to each other.

[00:14:43] Russ Altman: Yes.

[00:14:43] Monika Schleier Smith: To generate this phenomenon of entanglement.

[00:14:46] Russ Altman: So how do we do that? Because now I'm all on board with this.

[00:14:49] Monika Schleier Smith: Exactly. And so there are actually a few different tricks you can use. Um, one is, um, you said before, maybe they should be close together if you want them to talk to each other. Um, [00:15:00] and... So what you mean by close together, you know, might depend on what system you're used to thinking about.

Um, you know, in a solid, the atoms are separated by kind of Angstrom scales. Um, in these, uh, systems where we use light to manipulate atoms, the kind of characteristic length scale on which we can control their motion is the micron scale, roughly the kind of wavelength scale.

[00:15:23] Russ Altman: Right. So a micron, for people who don't think about microns, there's a million of them in a meter.

[00:15:29] Monika Schleier Smith: [00:15:30] Yeah, yeah. Um, if you want to get interactions on the micron scale, one trick that you can play, and this is something we've done in our lab is take an atom and excite it so that the electron's cloud becomes sort of bigger and fuzzier. Um, and, uh, so you can excite the electron to something called a Rydberg state.

And then actually atoms that, you know, again, a micron is way further apart than atoms would be in a solid.

[00:15:53] Russ Altman: Right.

[00:15:53] Monika Schleier Smith: Um, but even on that length scale, they can start to interact and become entangled. That's one approach. [00:16:00]

[00:16:00] Russ Altman: Yeah.

[00:16:00] Monika Schleier Smith: Um, but another approach we take that sort of blows up the length scale even more, um, if we want to get interactions on the millimeter length scale, um, then we can play a trick where we use light to convey information between different atoms.

[00:16:12] Russ Altman: Yes, you hinted at that earlier. I noticed you were talking about light as a messenger.

[00:16:17] Monika Schleier Smith: Yeah, exactly. And so, uh, and that can happen on, you know, that can happen on, um, length scales of kilometers, but what, then you start to have less control. Um, we

[00:16:27] Russ Altman: And also in your lab you have to be very large, and I'm sure [00:16:30] the dean would be upset.

[00:16:31] Monika Schleier Smith: There are some trade offs depending what distance you're aiming for, but we can have sort of a, you know, a millimeter scale, um, system, atoms that are millimeters apart, um, and trap them between two mirrors, um, that forms an optical resonator or cavity. And light, basically, the point is, um, if the mirrors are good, then light can make many, many round trips, thousands, or even a hundred thousand round trips, um, between the mirrors before it leaves.

And that actually gives a way for the light to kind of carry information between...

[00:16:59] Russ Altman: Yeah.

[00:16:59] Monika Schleier Smith: ... these [00:17:00] atoms, and let them talk to each other, kind of secretly without that information, um, leaking into the outside world. Because if it were to leak to the outside world, in principle, somebody could measure it. And that could destroy this, um, this entanglement.

[00:17:12] Russ Altman: Gotcha.

Now, one final question before we go to a break, which is when you've done all these, you, you met, you mentioned two or three manipulations that can get these uh, atoms entangled. How do you know that they're entangled? What's the assay or the test where you all shake each other's [00:17:30] hands and have a party and say, these atoms are definitely entangled.

[00:17:33] Monika Schleier Smith: Right. Um, so in general, this is actually, this is a great question. Um, because Um, you know, what you need to be careful about is that you're not just seeing some classical correlation.

[00:17:46] Russ Altman: Right.

[00:17:47] Monika Schleier Smith: Um, and that there's really sort of no way that you see these correlations, but you need to really prove there's no way that information sort of in principle even could have been known in advance.

Um, and so actually there's a wonderful, just as a digression, there's this wonderful [00:18:00] paper by, um, the theorist John Bell. who had sort of seminal contributions to how you prove entanglement.

[00:18:06] Russ Altman: Huh.

[00:18:06] Monika Schleier Smith: Um, and it's called Bertlmann's Socks and the Nature of Reality. Um, and it describes a, um, professor, Professor Bertlmann, who sort of, you know, if you, um, see his right sock on any given day and it's blue, you know that the left sock is not going to be blue, because he always wears mismatched socks. Right?

And so, and it's random, you know, sometimes they'll be blue, sometimes the first one will be red, but you know they won't match. Okay, so that's not entanglement, [00:18:30] but it seems kind of similar, right? Like...

[00:18:32] Russ Altman: Right.

[00:18:32] Monika Schleier Smith: ...when it's blue, you know the other will be red. Um..

[00:18:34] Russ Altman: So That's what you referred to as classical correlation.

[00:18:37] Monika Schleier Smith: That's what I mean by classical correlation, exactly. Um, and so generally, um, to prove entanglement, you somehow need to do measurements. Um, in two, um, fundamentally kind of incompatible observables. So earlier I talked about like the position and the elec and the momentum, um, of a particle. Um, and, uh, one can essentially, you know, prove [00:19:00] that you can't know both of those to arbitrary precision.

With entanglement, you could set up a system where, um, if I measure the position of one particle, it's perfectly correlated with the position of the other. If I measure the momentum of one particle, it's perfectly correlated with the momentum of another. And so using the information, um, in one particle, you can perfectly predict either one of these two kind of incompatible observables in the other particle. And that's one example.

[00:19:26] Russ Altman: Yeah.

[00:19:26] Monika Schleier Smith: Um, of how one can prove entanglement. [00:19:30] Um, and this is something we've done kind of variants of this. Um, in our quantum systems of atoms in the lab, in our case, we're not working with position and momentum. It's something like the atom has some kind of a spin, um, that can point anywhere on a sphere, and there's no way you can measure both in the up down direction and the left right direction at the same time.

[00:19:48] Russ Altman: And is it true, so you mentioned before something about measurement affecting the system and therefore you...

[00:19:54] Monika Schleier Smith: Yeah.

[00:19:54] Russ Altman: ...really have to, you have to budget your measurements very carefully. When you do this measurement, is it [00:20:00] going to ruin the entanglement or do the molecules or the atoms live on to continue to be entangled or is it a destructive measurement?

[00:20:07] Monika Schleier Smith: In our experiments, in our recent experiments, we've been doing destructive measurements.

[00:20:12] Russ Altman: Okay.

[00:20:13] Monika Schleier Smith: There's also a class of measurements called quantum non demolition measurements.

[00:20:16] Russ Altman: Non demolition.

[00:20:17] Monika Schleier Smith: Where, sort of, this is actually kind of a, in a, if you read a textbook on quantum mechanics, it will tell you after the measurement, the system is in the state that corresponds to the measurement outcome, often that's not true. Like I detect a photon, [00:20:30] the photon crashes into the detector and the photon is gone.

[00:20:32] Russ Altman: Okay, okay.

[00:20:32] Monika Schleier Smith: There is this special class of non demolition measurements that can actually be used to generate entanglement.

[00:20:38] Russ Altman: Ah.

[00:20:38] Monika Schleier Smith: You look at the measurement outcome, and the measurement outcome tells you something like, if, um, you know, this, um, electron was moving this way, the other one had to be moving the other way because I measured that their total momentum was zero, or something like that.

[00:20:52] Russ Altman: This is the Future of Everything with Russ Altman. More with Monika Schleier Smith next.[00:21:00]

Welcome back to The Future of Everything. I'm Russ Altman, and I'm speaking with Monika Schleier Smith, a professor of physics at Stanford University.

In the last segment, Monika told us about quantum mechanics and especially about this fascinating phenomenon called entanglement, how you can get atoms to be entangled with one another, and how you can make sure that they're entangled.

In the next segment, Monika will tell us some of the applications of entanglement [00:21:30] once we can control it. This has wide ranging implications for very accurate clocks, sensing, communications, and even quantum computers. So Monika, I wanted to ask you now about the downstream applications. If you're successful at routinely being able to create entangled atoms, make sure they're entangled, uh, what are the kinds of things we're going to be able to use that for?

[00:21:55] Monika Schleier Smith: Yeah, so, um, broadly speaking, there are kind of a few different spheres of application. Um, one [00:22:00] is precision measurement. So I mentioned to you that there's some kind of randomness inherent in problem, in quantum mechanics. Uh, some probabilistic nature that can actually limit the precision of, um, things like clocks and magnetometers, accelerometers, um, and entanglement can be a resource for actually, um, getting to even more fundamental limits of precision. Um, another really exciting potential, um, direction is computation, and that's one that, you know, some, uh, you might have heard about.

[00:22:27] Russ Altman: Yes, these are the so called quantum computers [00:22:30] that, uh, I've been basically waiting for, for my entire professional career.

[00:22:32] Monika Schleier Smith: Right. And so, yeah, you've been waiting a while, but I would say there's been actually really a lot of progress in just the past sort of decade or two or so in the level of control we have over entanglement and the ability to kind of, um, not only entangle two particles, but scale that up to more in order to, uh, approach this vision of quantum.

[00:22:52] Russ Altman: Oh, this is a very important question. Let me interrupt you.

Yeah, we've been talking about two atoms being entangled. Can it be a threesome or a [00:23:00] foursome where there are three or four, or is it always a pairwise phenomenon?

[00:23:04] Monika Schleier Smith: Yeah, um, so there's actually a, um, a sort of a rule called monogamy of entanglement.

[00:23:08] Russ Altman: That's exactly what I was wondering about.

[00:23:11] Monika Schleier Smith: But, but you can have either two atoms, let's say, that are two particles that are maximally entangled, um, or you can have many atoms that are weakly entangled. Where there's a little bit of entanglement between each pair and actually on some of the states we've been creating in my lab which are partly motivated by these [00:23:30] sort of improving precision measurements. They actually feature this sort of phenomenon where there's maybe a thousand atoms and there's sort of weak correlations between different pairs of them.

[00:23:39] Russ Altman: Okay. Thank you.

I'm sorry that I interrupted you. You were talking about quantum computation.

[00:23:44] Monika Schleier Smith: Um, sure, yeah, I mean, so that's another exciting application, um, and maybe just to mention sort of one, third one, quantum communications in another direction. Can you sort of leverage entanglement and the fact that it can exist over long distances to really kind of securely, [00:24:00] um, uh, have, uh, secure communications where the laws of quantum mechanics, um, kind of prohibit eavesdropping. So those are, um, kind of three spheres of application, um, that, uh, are, uh, yeah, that are kind of...

[00:24:14] Russ Altman: So, so why don't you pick one and take us through how the kind of features that you've described for Entanglement would enable. I don't, it actually almost doesn't matter.

[00:24:23] Monika Schleier Smith: Yeah.

[00:24:23] Russ Altman: You talked about censoring, computation, other things, uh, communications. Take us through how the entanglement, [00:24:30] I think people would be interested in.

[00:24:31] Monika Schleier Smith: Yeah.

[00:24:31] Russ Altman: Is this going to be powerful, is this going to be very power hungry or does it have a possibility of being very green and efficient and also things like precision and speed, I think people care about as well.

[00:24:44] Monika Schleier Smith: Yeah, so, um, yeah, so just to give us a couple of examples. Maybe I'll give some examples from the sensing and the computation area.

[00:24:51] Russ Altman: Right, right.

[00:24:51] Monika Schleier Smith: Um, so in sensing, again, it's essentially, so one of the places where specifically the systems I talked about, these laser cooled atoms are used are in atomic clocks, [00:25:00] um, which are our, you know, our best measurements of time.

The second is defined in terms of some number of cesium atom. Um, and the best of these clocks, actually it's remarkable, if you'd started one when the universe began, it would be off by, um, some fraction of a second today. So they're really extraordinary devices already as they are. Um, but the limitation actually, um, in the best of these clocks is actually this randomness of quantum mechanics.

[00:25:27] Russ Altman: Yes.

[00:25:27] Monika Schleier Smith: The fact that somehow you need to measure whether an atom is in one of two [00:25:30] states and there's some randomness in that outcome. So it turns out with entanglement. Um, you can have this phenomenon where, you know, let's say I have these two atomic states, the up state and the down state. It's a little bit like the heads and the tails of the coin.

Um, if I measure a hundred atoms, I'll, you know, typically get some fluctuations on the order of plus or minus ten. Um, but now with entanglement, you can set up this situation where if I measure one atom to be up, another one is more likely to be down, and that can actually reduce those fluctuations.

[00:25:58] Russ Altman: Ah.

[00:25:58] Monika Schleier Smith: So we reduce those statistical [00:26:00] fluctuations, um, in the measurement and that can, um, enable a more precise, uh, clock. And, um, clocks in turn, you know, have a number of applications, for example, in navigation, GPS satellites, that can be done with clocks.

[00:26:11] Russ Altman: Yes, yes.

[00:26:11] Monika Schleier Smith: Um, and so, uh, this is something where... It's exciting to be able to really sort of push the fundamental limits of what precision you can get leveraging quantum mechanics.

[00:26:20] Russ Altman: Yes, so that actually is a great example, because I do know a little bit about the cesium oscillations, and they are always out to many, many significant digits. I think your point [00:26:30] is that there are more significant digits to be had if we can remove the noise, and by having a number of atoms that are linked, you can kind of average out their errors. I mean, that's my phrase, not yours, to kind of get increased accuracy of the measurement.

[00:26:44] Monika Schleier Smith: That's right. You can get increased accuracy. You can also get the same accuracy faster. Yeah, they're the same precision faster, um, which can also matter. Some of these clocks are so precise, they can measure how high up they are in a gravitational field.

[00:26:55] Russ Altman: Right, right.

[00:26:56] Monika Schleier Smith: A few centimeters difference will matter. And so then you can start to think of the clock as a [00:27:00] sensor. Um, and the faster it is, the better, right?

[00:27:02] Russ Altman: Right.

[00:27:02] Monika Schleier Smith: And so, yeah,

[00:27:03] Russ Altman: what about computation?

[00:27:05] Monika Schleier Smith: Yeah.

[00:27:05] Russ Altman: Because we do see, we do hear about breakthroughs. We know that some of the big tech companies have big investments in quantum computing because they obviously see an upside with success. So maybe take us through what the value proposition is of quantum computing.

[00:27:19] Monika Schleier Smith: Yeah. So, um, just at a, um, kind of fundamental level, um, this phenomenon, this entanglement means that information can be stored, not just in individual bits, [00:27:30] but in correlations between quantum bits or qubits. Um, and that means actually that because I need to keep track of not just the state of each bit, but all the correlations of if this one is more likely to be a one, this other one is more likely to be a zero, um, the description of a system of, you know, n quantum bits takes resources classically that grows exponentially with n. And so, um, if I just get to something on the order of 60 cubits, I cannot describe that system on even the world's [00:28:00] largest supercomputer. Um, and so that already kind of tells you that fundamentally, these, um, qubits should have some computational power that classical computers don't.

[00:28:10] Russ Altman: I see. That's interesting. So the proof is because our current computers can't even model it, it must be capable of doing things that our current computers can't do, basically.

[00:28:22] Monika Schleier Smith: Exactly. And so there's, you know, there's one space of applications. Where the goal is to solve a problem that's inherently quantum mechanical, right? [00:28:30] So, um, chemistry problems

[00:28:31] Russ Altman: Yes.

[00:28:32] Monika Schleier Smith: That involve the quantum mechanics of many interacting electrons. Those are hard to do on classical computers. If you can do them better, that could have implications for, let's say, drug discovery, right? And so, that's something where it's really natural to say, why don't we make the hardware natively be quantum mechanical? Um, also problems in material science. And this actually, there's a bit of a gradient from computation to also something we call quantum simulation. Can you build model systems where, [00:29:00] um, the components are quantum mechanical? Um, but you can maybe model the properties of the, um, the fundament model build a simple model that might capture some properties of, let's say, superconductivity.

[00:29:12] Russ Altman: Yes.

[00:29:12] Monika Schleier Smith: Um, and so you could ask, can I build a universal quantum computer that can answer any problem, or you can build some have something more purpose built to tackle a particular type of problem like simulating solid state systems to learn about superconductivity.

[00:29:23] Russ Altman: When I look at these papers, one of the things I'm struck by, um, in addition to my not fully understanding the paper, but one of the [00:29:30] things I'm struck by is that, uh, I don't have a good feel for the potential computational power of 60 qubits. You said 60 qubits is very hard to model.

[00:29:38] Monika Schleier Smith: Yes.

[00:29:39] Russ Altman: Would 60 qubits be a very non trivial computer or are we going to need to wait for the Moore's law of quantum computing to get us to millions or trillions of qubits in order to do really, uh, kind of non trivial things.

[00:29:52] Monika Schleier Smith: Yeah, great question. So at the level of, you know, 60, you can start to get into this regime where, um, you. [00:30:00] Can't calculate classically what will happen.

[00:30:03] Russ Altman: Right.

[00:30:03] Monika Schleier Smith: Or it would take an unreasonable amount of time. Um, but so far we're not quite at the point yet of being able to actually like solve a useful problem with those systems. And then I would say there's sort of like two, you know, two types of algorithms that, um, people are working on.

One is, um, what's called, uh, there's a term NISQ, which stands for Noisy Intermediate Scale Quantum Algorithm. So the question is, if you have some, you know, modest number of qubits in this regime, you [00:30:30] know, hundreds where you can't have a full description on the classical computer, can you already start, and maybe they're not perfect, we try really hard.

[00:30:37] Russ Altman: Right.

[00:30:37] Monika Schleier Smith: The gates might have 99. 9 percent fidelity, but it's not 100.

[00:30:41] Russ Altman: Right. You told us about those nasty photons that can mess up your measurement.

[00:30:45] Monika Schleier Smith: Exactly. Can you use that already to start solving some problems? And so the flavor problems there might be there are certain optimization problems, scheduling problems that you can actually map to minimizing the energy of some interacting system of spins, let's say.

[00:30:59] Russ Altman: Right, [00:31:00] right.

[00:31:00] Monika Schleier Smith: So, and if you can program in the interactions. Maybe you can sort of directly kind of take your problem, map it onto the hardware, and use the hardware to help solve it. And there's really an open question about to what extent quantum systems actually provide an advantage for this class of problem.

Um, it's to the best of our understanding, if they do, it's probably something that's a polynomial win and not an exponential win. Um, but I would say, you know, there's a lot of good science in actually just building the systems and starting to [00:31:30] explore and seeing how much they can help in that regime.

[00:31:32] Russ Altman: Right.

[00:31:32] Monika Schleier Smith: If you want to, but then there's this goal of having a full error corrected quantum computer. An error correction is subtle in quantum mechanics, because it's not enough to say is the qubit in a zero state or a one state. It can be in this sort of so called superposition of both, where there's, um.

[00:31:46] Russ Altman: Yes.

[00:31:47] Monika Schleier Smith: And so, um, So quantum error correction is another really active area of research, um, and having an error corrected quantum computer will require many physical qubits for each logical qubit and then the number that you need [00:32:00] is going to be, you know, Not just hundreds, but, uh, hundreds of thousands.

[00:32:08] Russ Altman: Yes, I mean I don't know if this is in right anology but even in your description of sensing, I could see that you, you were talking about multiple, uh, entangled things to kind of do an averaging and I can imagine that to get an error corrected computer that's quantum, you would also need to have a certain redundancy.

[00:32:21] Monika Schleier Smith: Exactly.

[00:32:21] Russ Altman: So that it could be like voting mechanisms and stuff where.

[00:32:24] Monika Schleier Smith: Exactly.

[00:32:25] Russ Altman: All the cubits get a vote to figure out if it's up or down or whatever.

[00:32:28] Monika Schleier Smith: Right. And the thing is, [00:32:30] so in quantum mechanics, you have to vote about is it up or down, but you also have to vote about is it left or right.

[00:32:34] Russ Altman: Right.

[00:32:34] Monika Schleier Smith: And there's some extra subtleties there. Yeah.

[00:32:39] Russ Altman: Thanks to Monika Schleier Smith. That was The Future of Quantum Mechanics. You have been listening to The Future of Everything and I'm your host, Russ Altman.

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