CAN PHYSICS COMPUTE?

Is Quantum Computing Beyond Physics?

*More exactly, do we know, can we know, enough physics for (full) quantum computing?*

I have long suggested that the answer to this question was negative, and smirked at physicists sitting billions of universes on a pinhead, as if they had nothing better to do, the children they are. (Just as their Christian predecessors in the Middle Ages, their motives are not pure.)

Now an article in the American Mathematical Society Journal of May 2016 repeats (some) of the arguments I had in mind: ** The Quantum Computer Puzzle**. Here are some of the arguments. One often hears that Quantum Computers are a done deal. Here is the explanation from Justin Trudeau, Canada’s Prime Minister, which reflects perfectly the official scientific conventional wisdom on the subject: https://youtu.be/rRmv4uD2RQ4

(One wishes all our great leaders would be as knowledgeable… And I am not joking as I write this! Trudeau did engineering and ecological studies.)

Before some object that physicists are better qualified than mathematicians to talk about the Quantum, let me point towards someone who is perhaps the most qualified experimentalist in the world on the foundations of Quantum Physics. Serge Haroche is a French physicist who got the Nobel Prize for figuring out how to count photons without seeing them. It’s the most delicate Quantum Non-Demolition (QND) method I have heard of. It involved making the world’s most perfect mirrors. The punch line? Serge Haroche does not believe Quantum Computers are feasible. However Haroche does not suggest how he got there. The article in the AMS does make plenty of suggestions to that effect.

Let me hasten to add some form of Quantum Computing (or Quantum Simulation) called “annealing” is obviously feasible. D Wave, a Canadian company is selling such devices. In my view, Quantum Annealing is just the two slit experiment written large. Thus the counter-argument can be made that conventional computers can simulate annealing (and that has been the argument against D Wave’s machines).

Full Quantum Computing (also called “*Quantum Supremacy*”) would be something completely different. Gil Kalai, a famous mathematician, and a specialist of Quantum Computing, is skeptical:

*“Quantum computers are hypothetical devices, based on quantum physics, which would enable us to perform certain computations hundreds of orders of magnitude faster than digital computers. This feature is coined “quantum supremacy”, and one aspect or another of such quantum computational supremacy might be seen by experiments in the near future: by implementing quantum error-correction or by systems of noninteracting bosons or by exotic new phases of matter called anyons or by quantum annealing, or in various other ways… *

*A main reason for concern regarding the feasibility of quantum computers is that quantum systems are inherently noisy. We will describe an optimistic hypothesis regarding quantum noise that will allow quantum computing and a pessimistic hypothesis that won’t.”*

Gil Katai rolls out a couple of theorems which suggest that Quantum Computing is very sensitive to noise (those are similar to finding out which slit a photon went through). Moreover, he uses a philosophical argument against Quantum Computing:

“*It is often claimed that quantum computers can perform certain computations that even a classical computer of the size of the entire universe cannot perform! Indeed it is useful to examine not only things that were previously impossible and that are now made possible by a new technology but also the improvement in terms of orders of magnitude for tasks that could have been achieved by the old technology.*

**Quantum computers represent enormous, unprecedented order-of-magnitude improvement of controlled physical phenomena as well as of algorithms**. Nuclear weapons represent an improvement of 6–7 orders of magnitude over conventional ordnance: the first atomic bomb was *a million times stronger than the most powerful (single) conventional bomb at the time. The telegraph could deliver a transatlantic message in a few seconds compared to **the previous three-month period. This represents an (immense) improvement of 4–5 orders of magnitude. Memory and speed of computers were improved by 10–12 orders of magnitude over several decades. Breakthrough algorithms at the time of their discovery also represented **practical improvements of no more than a few orders of magnitude. Yet implementing Boson Sampling with a hundred bosons represents more than a hundred orders of magnitude improvement compared to digital computers.*”

In other words, it unrealistic to expect such a, well, quantum jump…

“Boson Sampling” is a hypothetical, and simplest way, proposed to implement a Quantum Computer. (It is neither known if it could be made nor if it would be good enough for Quantum Computing[ yet it’s intensely studied nevertheless.)

***

*Quantum Physics Is The Non-Local Engine Of Space, and Time Itself:*

Here is Gil Kalai again:

*“Locality, Space and Time*

*The decision between the optimistic and pessimistic hypotheses is, to a large extent, a question about modeling locality in quantum physics. Modeling natural quantum evolutions by quantum computers represents the important physical principle of “locality”: quantum interactions **are limited to a few particles. The quantum circuit model enforces local rules on quantum evolutions and still allows the creation of very nonlocal quantum states.*

*This remains true for noisy quantum circuits under the optimistic hypothesis. The pessimistic hypothesis suggests that **quantum supremacy is an artifact of incorrect modeling of locality**. We expect modeling based on the pessimistic hypothesis, which relates the laws of the **“noise” to the laws of the “signal”, to imply a strong form of locality for both. We can even propose that spacetime itself emerges from the absence of quantum fault tolerance. It is a familiar idea that since (noiseless) quantum systems are time reversible, time emerges from quantum noise (decoherence). However, also in the presence of noise, with quantum fault tolerance, every quantum evolution that can experimentally be created can be time-reversed, **and, in fact, we can time-permute the sequence of unitary operators describing the evolution in an arbitrary way. It is therefore both quantum noise and the absence of quantum fault tolerance that enable an arrow of time.”*

Just for future reference, let’s “*note that with quantum computers one can **emulate a quantum evolution on an arbitrary geometry. For example, a complicated quantum evolution representing the dynamics of a four-dimensional lattice model could be emulated on a one-dimensional chain of qubits.*

*This would be vastly different from today’s experimental quantum physics, and it is also in tension with insights from physics, where witnessing different geometries supporting the same physics is rare and important. Since a universal quantum computer allows the breaking of the **connection between physics and geometry, it is noise and the absence of quantum fault tolerance that distinguish physical processes based on different geometries and **enable geometry to emerge from the physics.”*

***

I have proposed a theory which explains the preceding features, including the emergence of space. Let’s call it Sub Quantum Physics (SQP). The theory breaks a lot of sacred cows. Besides, it brings an obvious explanation for Dark Matter. If I am correct the Dark matter Puzzle is directly tied in with the Quantum Puzzle.

In any case, it is a delight to see in print part of what I have been severely criticized for saying for all too many decades… The gist of it all is that** present day physics would be completely incomplete.**

**Patrice Ayme’ **