Said in my best Marvin the Martian voice: Delays, delays! #QuantumInternet thread coming up. This was posted first on the bird site to contact the creator of the video that prompted it, so some of it is written second person to her.
Hi Sabine, let's talk #QuantumInternet. We did this a few years ago, when I generated a tweetstorm about #QuantumComputerArchitecture, and I had been thinking it was about time to do something similar for quantum communications and Quantum Internet.
Let's talk about potential uses for a network that distributes entanglement, then about the distinction of different types of networks, then I want to expand on your cookie analogy. We'll end with some references. (I'll toss a few in here and there in the thread, too.)
You described the #QuantumInternet as "a solution in search of a problem", and while that's rather glib, it's not completely unjustified. The shift analog-->digital-->quantum information is so deep and profound, we will be exploring it for decades.
For quite some years, most of us in the field (including me) have been dividing use cases for quantum networks into three categories: cryptographic functions, sensor networks, and distributed quantum computing.
The only one you discussed in your video is quantum key distribution (QKD), the canonical example of a quantum crypto function. It would replace the classical Diffie-Hellman key exchange portion of an encrypted classical communication session.
(An encrypted conversation consists of, roughly, three phases: authentication, key generation, and encryption of the message, or bulk data encryption.)
QKD is a surprising and important concept, but just replacing D-H is unlikely to be sufficient incentive to build an entire new communication infrastructure. The other crypto functions, such as leader election, would fall into that same category.
So what about the other two? First we need to know about the difference between entangled and unentangled quantum networks. QKD can run either with or without entanglement.
I'm lumping several different things together in the category of sensor networks. Examples include high-precision clock synchronization, improved resolution in arrays of telescopes, and the like.
So the sensor networks are attractive but difficult, especially when doing the engineering of the classical interface and control and worrying about noise.
Especially, we need to understand that there are similarities and differences between system interconnects or data center networks and wide-area networks. https://www.osti.gov/biblio/1900586
It's critical to know that scaling up quantum computers requires entanglement between quantum processors. If we want to use two small quantum computers to solve one larger problem, we MUST be able to create inter-node entanglement.
Beyond that, what about wide-area entangling networks? One of my favorite ideas of the last two decades is blind quantum computation, by Broadbent, Kashefi and Fitzsimons. https://arxiv.org/abs/0807.4154
Maybe the most intriguing thing I have seen in recent years is the combination of quantum computing and quantum sensing, from the group surrounding John Preskill at Caltech, especially the astounding Robert Huang. https://scholar.google.com/citations?user=2y5YF-gAAAAJ&hl=en
They have shown how coupling a quantum sensor to a quantum computer dramatically reduces the number of times you need to actually run the quantum experiment. https://arxiv.org/abs/2112.00778
This is one of the most important ideas in recent years, IMO, and it will take us years to figure out all of its implications. It might be used in either a lab+data center configuration, or wide-area setup, it's not clear yet.
All of this work is supported on what, by my estimate, is about 1% of what's being spent on #QuantumComputing. It's a small but critical part of an entire ecosystem.
Okay, I think that covers what I wanted to say about the utility of entangled networks, both data center and wide-area. Let me also comment on the cookie:
The cookie analogy for entanglement is incomplete, and simplified to the point where it's misleading. Let me see if I can do a little better, but this makes it a LOT longer and murkier, so stick with me...
A cookie has two properties, perhaps flavor (chocolate and vanilla) and shape (round and square), but when you are given a cookie you can't learn about both. You have to pick one.
You can either feel the shape with your hand or you can taste it, but not both, and if you try to look at it instead it just crumbles before you can learn anything about it.
You and I ask the Quantum Internet to make a special pair for us. Then we each, independently, decide whether to taste our cookie or feel it, then we share what we found.
Of course, if we do this just once, it doesn't tell us much. In fact, we have to repeat this a bunch of times, and what we get is just this weird statistical result.
Until the advent of quantum computing, that's all this was, a weird statistical anomaly, with the profound but esoteric suggestion that quantum mechanics and relativity don't mix.
Nowadays it's critical to the operation of a quantum computer on the inside, but the question at hand is whether it is useful over longer distances. I hope the discussion above gives you some idea of the things we are working to realize.
If you're with me so far, that's roughly the way we use entanglement; taking some combination of the shape and flavor and mixing it with the other qubits at each end to entangle larger sets of qubits. But there's actually a catch in entanglement:
If we just talk about the flavor or shape, well, the cookies might have secretly been shaped & flavored, but we just don't what they are. That would just be classical correlation, not entanglement.
THEN it turns out that there is still a statistical correlation between the outcomes of the measurements (shape, flavor, flape) at the two ends...in some way that CANNOT be just a hidden characteristic of the already-shared cookies.
Since that discovery in the 1960s, it has been proven with increasing rigor in experiments. I won't go into them here, but search for "loophole-free Bell inequality" if you want to know more.
Yeah, it's weird, and it's not as easy to understand as a broken cookie. Sorry, this is the best I can do without skipping some critical facet of entanglement.
By now, hopefully you can see that there is an ABSOLUTELY CRITICAL NEED for data center-scale #QuantumNetworks. There are also Big Science things you can do with a wide-area #QuantumInternet or with related technology such as LIGO's squeezing.
Whether wide-area entanglement has truly compelling economic uses that will warrant deployment of a new global information infrastructure, well, that's a little harder to see. But would I bet on it? I already have. I've spent a good chunk of the last two decades working on this.
Which is based on the first of three online courses, all uploaded to YouTube, in both English and Japanese: Overview of Quantum Communications, From Classical to Quantum Light, and Quantum Internet. https://www.youtube.com/@QuantumCommEdu/playlists
And thanks to the brilliant team of several dozen people I work with, most especially Shota Nagayama (who leads the Quantum Internet Task Force) and Michal Hajdušek (who does much of the educational materials and most of the physicsing).
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