New link between quantum computing and black hole may solve information loss problem

[image source: IBT]

If you leave the city limits of Established Knowledge and pass the Fields of Extrapolation, you enter the Forest of Speculations. As you get deeper into the forest, larger and larger trees impinge on the road, strangely deformed, knotted onto themselves, bent over backwards. They eventually grow so close that they block out the sunlight. It must be somewhere here, just before you cross over from speculation to insanity, that Gia Dvali looks for new ideas and drags them into the sunlight.

Dvali’s newest idea is that every black hole is a quantum computer. And not just any quantum computer, but a quantum computer made of a Bose-Einstein condensate that self-tunes to the quantum critical point. In one sweep, he has combined everything that is cool in physics at the moment.

This link between black holes and Bose-Einstein condensates is based on simple premises. Dvali set out to find some stuff that would share properties with black holes, notably the relation between entropy and mass (BH entropy), the decrease in entropy during evaporation (Page time), and the ability to scramble information quickly (scrambling time). What he found was that certain condensates do exactly this.

Consequently he went and conjectured that this is more than a coincidence, and that black holes themselves are condensates – condensates of gravitons, whose quantum criticality allows the fast scrambling. The gravitons equip black holes with quantum hair on horizon scale, and hence provide a solution to the black hole information loss problem by first storing information and then slowly leaking it out.

Bose-Einstein condensates on the other hand contain long-range quantum effects that make them good candidates for quantum computers. The individual q-bits that have been proposed for use in these condensates are normally correlated atoms trapped in optical lattices. Based on his analogy with black holes however, Dvali suggests to use a different type of state for information storage, which would optimize the storage capacity.

I had the opportunity to speak with Immanuel Bloch from the Max Planck Institute for Quantum Optics about Dvali’s idea, and I learned that while it seems possible to create a self-tuned condensate to mimic the black hole, addressing the states that Dvali has identified is difficult and, at least presently, not practical. You can read more about this in my recent Aeon essay.

But really, you may ask, what isn’t a quantum computer? Doesn’t anything that changes in time according to the equations of quantum mechanics process information and compute something? Doesn’t every piece of chalk execute the laws of nature and evaluate its own fate, doing a computation that somehow implies something with quantum?

That’s right. But when physicists speak of quantum computers, they mean a particularly powerful collection of entangled states, assemblies that allow to hold and manipulate much more information than a largely classical state. It’s this property of quantum computers specifically that Dvali claims black holes must also possess. The chalk just won’t do.

If it is correct what Dvali says, a real black hole out there in space doesn’t compute anything in particular. It merely stores the information of what fell in and spits it back out again. But a better understanding of how to initialize a state might allow us one day – give it some hundred years – to make use of nature’s ability to distribute information enormously quickly.

The relevant question is of course, can you test that it’s true?

I first heard of Dvali’s idea on a conference I attended last year in July. In his talk, Dvali spoke about possible observational evidence for the quantum hair due to modifications of orbits nearby the black hole. At least that’s my dim recollection almost a year later. He showed some preliminary results of this, but the paper hasn’t gotten published and the slides aren’t online. Instead, together with some collaborators, he published a paper arguing that the idea is compatible with the Hawking, Perry, Strominger proposal to solve the black hole information loss, which also relies on black hole hair.

In November then, I heard another talk by Stefan Hofmann, who had also worked on some aspects of the idea that black holes are Bose-Einstein condensates. He told the audience that one might see a modification in the gravitational wave signal of black hole merger ringdowns. Which have since indeed been detected. Again though, there is no paper.

So I am tentatively hopeful that we can look for evidence of this idea in the soon future, but so far there aren’t any predictions. I have an own proposal to add for observational consequences of this approach, which is to look at the scattering cross-section of the graviton condensate with photons in the wave-length regime of the horizon-size (ie radio-waves). I don’t have time to really work on this, but if you’re looking for one-year project in quantum gravity phenomenology, this one seems interesting.

Dvali’s idea has some loose ends of course. Notably it isn’t clear how the condensate escapes collapse, at least it isn’t clear to me and not clear to anyone I talked to. The general argument is that for the condensate the semi-classical limit is a bad approximation, and thus the singularity theorems are rather meaningless. While that might be, it’s too vague for my comfort. The idea also seems superficially similar to the fuzzball proposal, and it would be good to know the relation or differences.

After these words of caution, let me add that this link between condensed matter, quantum information, and black holes isn’t as crazy as it seems at first. In the last years, a lot of research has piled up that tightens the connections between these fields. Indeed, a recent paper by Brown et al hypothesizes that black holes are not only the most efficient storage devices but indeed the fastest computers.

It’s amazing just how much we have learned from a single solution to Einstein’s field equations, and not even a particularly difficult one. “Black hole physics” really should be a research field on its own right.