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Photon juggling: One big quantum processor from 100 little ones


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The processing assembly for Google’s Sycamore quantum computer.

In the not-terribly-distant past, the goal of quantum computing research was to achieve a milestone called quantum supremacy: the point in time when a quantum computer can, in practical terms, be considered superior to a classical, semiconductor-based computer for processing any task you give it. Certainly Google already made a big enough fuss about it. This is no longer true.  Engineers and scholars have since conceded that this is not possible — that a quantum device cannot supersede a classical device.  (Of course, it may seem a little too convenient that they should make this declaration now.)

The principal reason for this is not that a quantum computer (QC), once the plans for its development are fully realized, would somehow be inferior. A quantum computer is, and because of the nature of physics always will be, a quantum processor maintained and marshaled by a classical control system. Despite that title, “control” may be an imprecise word in this context. Although such devices may yet become the foundation for a new industry, they don’t really control quantum processing any more than a barbed wire fence controls a prison riot. More accurately, they control the gateway leading to and from the prison, with the guards making sure to watch only the gateway and nothing else (because watching something requires photons, and photons will make the qubit stack — the core processing element — decohere.)

No, the reason is because a quantum system includes, and depends upon, a classical computer. It’s tempting to say the two rely upon each other, but that would misinterpret their working relationship. Tell a QC it’s dependent upon anything else, and it’s liable to throw a qubit and fall apart.

What engineers and programmers are seeking now is a kinder, gentler position of achievement and authority. Some have opted for the phrase quantum advantage, which would imply that the QC has a clearly measurable virtue, in terms of performance, speed, or quality, over a classical supercomputer. Others prefer quantum practicality, which is softer still, implying that the QC would be the device one would rationally choose to perform a task, given a rational analysis of the alternatives.

“You might think, ‘Well, we’ve achieved quantum advantage last year at Google. So it’s probably a few years’ worth of work to get to quantum practicality, isn’t it?”http://www.zdnet.com/” said Prof. Lieven Vandersypen, the scientific director of Dutch public/academic partnership QuTech, speaking at the recent IQT Europe 2020 conference. Google’s supremacy claim was made after having provably maintained the execution of a task with a 53-qubit register. So perhaps the road to 100 qubits is paved, smooth, and unobstructed, if one takes this point of view. Prof. Vandersypen continued:

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Prof.  Lieven Vandersypen

A few hundred qubits comes not out of nowhere… This is the point where useful problems could be addressed. On the other hand, perhaps millions of qubits are needed… or maybe a miracle in designing new quantum algorithms. So which of these applies, and how do I look at it? Certainly I don’t believe for a minute that it is just a few years’ worth of work to achieve real quantum practicality. If we look at the projections, indeed, we are going to achieve as a community a few hundred qubits. But these will be not perfect qubits. Then what you need are a hundred perfect qubits that will run indefinitely without error, and can carry through as many operations as are needed to really enter this quantum practicality regime. Okay, they don’t need to be really perfect, but they have to be, let’s say, between 1,000 and 10,000 times higher quality than any of the qubits that we can operate today. That is not completely out of the question, but for sure, not going to happen in a few years’ time.

Vandersypen makes multiple references to “a few years,” and not by coincidence. In the midst of a global pandemic, and an ongoing shift in the global order, a few years’ worth of government and institutional funding may be all that institutions like QuTech can hope for.

What would render the entire question of supremacy, advantage, or “edginess” somewhat moot is if there were some force somewhere, perhaps a force of physics, that could make multiple QCs, and perhaps all QCs on Earth, simultaneously interoperable. This is what quantum entanglement actually is. A complete understanding of the underlying principles of a quantum information network (QIN) requires explanations that don’t just border on the philosophical, but plunge head-first into the ocean of the metaphysical.

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Entanglement-as-a-service

Generally speaking, the laws of physics have thus far referred mainly to the explicate order. Indeed, it may be said that the principle function of Cartesian coordinates is just to give a clear and precise description of explicate order. Now, we are proposing that in the formulation of the laws of physics, primary relevance is to be given to the implicate order, while the explicate order is to have a secondary kind of significance (e.g., as happened with Aristotle’s notion of movement, after the development of classical physics). Thus, it may be expected that a description in terms of Cartesian coordinates can no longer be given a primary emphasis, and that a new kind of description will indeed have to be developed for discussing the laws of physics.

                                                         -David Bohm
                                                          Wholeness and the Implicate Order, 1980

A quantum information network (QIN), if it can be built, would accomplish something that can’t be done in physical reality. Not even science fiction has manifest a contraption such as this. Had Isaac Asimov any clue that such a thing might be feasible, the Robot series would ultimately not have been about robots.

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  Mathias van den Bossche

“You don’t send information on a quantum information network,” explained Mathias van den Bossche, who directs telecommunications and navigation systems research for Italy-based satellite consortium Thales Alenia Space.  “You weave entanglement correlations from one end user to the other end user. When this is available, everything in the middle disappears, and the end users discuss directly. This means you have actually nothing that is being repeated along the network, apart from the entanglement that swaps from link to link — there is no information that is repeated.”

The only way to adequately convey the function of a QIN is with a ridiculous metaphor: Imagine if the state of being connected, of working together as a cohesive unit, were something you could take with you, as though a dealer in a poker game handed it to you. Own this card, and someone else’s poker hand at the other end of the table is part of yours. If he has two kings and so do you, you now have four-of-a-kind.  (And so does he, but at least you know that.)

Now imagine you were playing a variant of the game where players could trade cards. Connectedness with one player’s hand could be something you could trade, perhaps for a card granting you connectedness with another player’s hand. To complete this metaphor, imagine you were playing this game using a kind of networking where trading the value of the card would be exactly the same as trading the card itself.