
Reversing cause and effect is no trouble for quantum computers - jonbaer
https://phys.org/news/2018-07-reversing-effect-quantum.html
======
lisper
The headline is, as popular articles on QM often are, misleading. One of the
central mysteries of QM has always been: how does the classical world with its
arrow of time emerge from the quantum world where the governing equation is
time-symmetric? And the answer is: the classical world emerges by a process of
decoherence, which is to say, by the creation of large (O(10^23)) networks of
entanglements which (it can be shown mathematically) have behavior that is
indistinguishable from classical systems. It is very similar to how
thermodynamics and the time-irreversibility of the second law emerge from
time-reversible Newtonian mechanics (except that the time-asymmetry that
emerges from QM is even more fundamental).

Time-reversibility is part-and-parcel of quantum computation because the
_whole point_ of quantum computation is to keep the system operating in the
regime where its behavior _is_ distinguishable from classical. As soon as you
lose coherence, you simultaneously lose _all_ of the quantum behavior,
including time-reversibility. Time-reversibility isn't anything special in
this regard. It's all part of the same weirdness.

It's all very interesting, but none of it is news.

~~~
posix_me_less
> _And the answer is: the classical world emerges by a process of decoherence,
> which is to say, by the creation of large (O(10^23)) networks of
> entanglements which (it can be shown mathematically) have behavior that is
> indistinguishable from classical systems._

This is the motivation for decoherence and for the Everettian view where the
psi function is "the only real thing" and everything else is derived. It is
often talked about as if actually accomplished. But it was not; the behaviour
of quantum systems, even with decoherence, was not yet shown to be
indistinguishable from the classical systems. Neither the fact that
measurements have one outcome, nor Born's rules for probability were derived
from this decoherence idea yet. One thing that follows from decoherence
hypothesis is decay of coherence elements in density matrices, but this is not
enough to get explanation for single outcomes or for Born's rules.

~~~
lisper
Ironically, I am just in the process of preparing a talk on the history of the
atomic theory. Even as late as 1905 when Einstein published his work on
brownian motion it was still highly controversial whether atoms were "really
real" or just a mathematical contrivance. Some people are able to maintain
skepticism even in the face of overwhelming evidence.

> Neither the fact that measurements have one outcome, nor Born's rules for
> probability were derived from this decoherence idea yet.

That is because it is not the case that measurements have one outcome.
Measurements _appear_ to have one outcome from the perspective of a given
classical observer, and the outcome observed by any given observer will be
classically consistent with all the other observations that observer makes
[1]. The Born rule (which, it must be stressed, is _not_ fundamental, but only
_apparent_ from the point of view of any given observer) follows from that,
not mathematically but _logically_ [2].

[1] [https://arxiv.org/pdf/quant-ph/9605002.pdf](https://arxiv.org/pdf/quant-
ph/9605002.pdf)

[2]
[https://www.scottaaronson.com/papers/island.pdf](https://www.scottaaronson.com/papers/island.pdf)

~~~
posix_me_less
Thanks for the links.

> "That is because it is not the case that measurements have one outcome.
> Measurements _appear_ to have one outcome from the perspective of a given
> classical observer [but actually have none]"

is a big claim, but it lacks "overwhelming evidence". Quantum theory self-
consistency is a problem worthy of investigation, but achieving self-
consistency is only one of the many possible valid results. This result is not
needed as much as many people seem to think; certainly not as much as to reify
psi function above objective results of measurements and claim the quoted
part. It is a methodological error; instead of shaping the theory around the
measurement results, it is rejecting the fact that those measurement results
exist.

> The Born rule (which, it must be stressed, is not fundamental, but only
> apparent from the point of view of any given observer) follows from that,
> not mathematically but logically

I skimmed the paper [2] but found no such thing there. Could you locate it in
more precisely or elaborate on the idea? All these kinds of proofs from
algebraic formalism seem to achieve is something like "if Born's rule would
not be true, we would have problems with the theory" which is not very
interesting; we have some big problems with the theory even without
decoherence.

~~~
lisper
> it is rejecting the fact that those measurement results exist

Well, sort of. I prefer to think of it as replacing one explanatory hypothesis
with a superior one, one that is actually consistent with the evidence. To
wit:

The observation that requires explaining is that classical measurements are
consistent across space and time.

The usual explanation for this is that a classical objective reality exists,
and that measurement faithfully reflects the state of this reality. This is a
plausible explanation. Einstein believed it. But Bell's theorem and subsequent
experiments show it to be false.

The correct explanation is that classical correlation emerges from the
Shroedinger equation alone. There is no objective classical reality. If there
were, it would be possible to construct a local hidden variable theory
consistent with all observations. But it's not so there isn't.

It is only because our subjective experience is (necessarily) classical that
we find all this hard to accept.

> I skimmed the paper [2] but found no such thing there. Could you locate it
> in more precisely or elaborate on the idea?

It's the part about Gleason's theorem:

[https://en.wikipedia.org/wiki/Gleason%27s_theorem](https://en.wikipedia.org/wiki/Gleason%27s_theorem)

See also:

[https://www.scottaaronson.com/democritus/lec9.html](https://www.scottaaronson.com/democritus/lec9.html)

~~~
posix_me_less
> But Bell's theorem and subsequent experiments show it to be false.

I think this is an unfounded conclusion. Sure, Bell's theorem and the
experiments it inspired did make lots of contribution to investigation of
these foundational problems.

But Bell's theorem is about a specific class of probabilistic descriptions. It
does not rule out "classical objective reality" unless you restrict meaning of
this term so much as to achieve that. Bell himself and many other people who
studied the theorem insisted that the theorem is about local hidden variable
models, as opposed to nonlocal models. Nothing general is implied about
"objective reality".

On the experimental side, I am not aware of any experiment that had ruled out
objective reality. How would you even test such a nebulous concept
experimentally? We can test only specific predictions.

~~~
lisper
> Nothing general is implied about "objective reality".

I disagree. The fact that nature cannot be described by any local hidden
variable theory tells you a lot about the nature of reality. There is a real
difference between quantum randomness and classical ignorance, and so there is
a real difference -- one which you can experimentally measure -- between a
particle whose position you don't know because you haven't looked at the
result of a measurement, and a particle whose position you don't know because
it hasn't been measured at all. It is still, perhaps, debatable whether that
difference lies in the particle or in you. But if you want to argue that the
difference lies in the particle then the burden is on you to provide an
account of _when and how_ the transition happens. No one has succeeded in
this, and it's not for want of trying.

~~~
posix_me_less
> The fact that nature cannot be described by any local hidden variable theory
> tells you a lot about the nature of reality.

If assumed to be true, it means one has to use non-local models such as the
quantum theory. Those do not explicitly deny objective reality. Perhaps by
"objective reality" you actually mean non-contextual hidden variable theory (a
theory that assigns well-defined values to all quantum observables at all
times). Then yes, those are incompatible with quantum theory predictions. But
I would not use the term objective reality. It is unclear and too general.

> there is a real difference -- one which you can experimentally measure --
> between a particle whose position you don't know because you haven't looked
> at the result of a measurement, and a particle whose position you don't know
> because it hasn't been measured at all.

Indeed, but this difference has nothing to do with objective reality, and
everything to do with the actions of the experimenter or apparatus. In the
first option, there is interaction with the particle but experimenter ignores
it, and in the second, there is no interaction with the particle. Of course
the two situations are different and can result in different results of other
measurements.

~~~
lisper
I am not denying objective reality, I am denying objective _classical_
reality. There is an objective reality, but it is quantum.

> the actions of the experimenter or apparatus

You are missing the point. Before you can talk about "the actions of the
experimenter or the apparatus" you have to tell me what an experimenter or an
apparatus _is_. AFAICT, these are classical objects. But theory tells us, and
experiment confirms, that classical objects do not actually exist. So why do
they appear to exist? And the answer to that is: decoherence/QIT. The
classical world is an illusion, an approximation. A very good approximation,
but an approximation nonetheless.

If you have a better theory, you should publish it.

~~~
posix_me_less
> Before you can talk about "the actions of the experimenter or the apparatus"
> you have to tell me what an experimenter or an apparatus is.

This is a very common error of armchair worldview building. Experiment-based
sciences such as physics do not work exclusively that way; we can make
progress without being so ambitious as to describe and exhaust the whole world
in a single scheme. There are things in the _real world_ that have no useful
definition or model in physics, such as matter, or experimenter, or apparatus,
or measurement. And this is fine, because we know them by experience, and the
subject of the investigation is usually something else and rather more
specific.

> But theory tells us, and experiment confirms, that classical objects do not
> actually exist.

Which theory, which experiment? The studies in quantum foundations showed that
explaining expensive experiments with light and particles in the classical
terms is hard, and some naive intuitive models, surprisingly, are not
consistent with quantum theory. This is very far from "classical objects do
not exist". They can easily exist, just with properties that are contextual
and interactions that are non-local. And those are categories we are aware of;
the possibilities are endless.

If you're thinking about the Everettian viewpoint where indeed no classical
objects exist, this is just one possible theoretical scheme of thinking, not
an experiment-based physical theory. There is Bohmian mechanics, where
particles do exist. There are other theories where particles do exist.

> If you have a better theory, you should publish it.

A better theory in place of "classical objects don't exist" is "don't try to
explain the whole world with a single scheme". There is the statistical
interpretation due to Einstein, Ballentine and the Bohmian viewpoint which get
us useful models and predictions in the realm of atomic physics. There is the
classical theory, which gets us understanding of civil engineering, Earth-
scale events and celestial mechanics. There is the general theory of
relativity, which gets us accurate description of gravity. None of these
overlap very well. Neither is explained by Everettian viewpoint.

~~~
lisper
> Experiment-based sciences such as physics do not work exclusively that way;
> we can make progress without being so ambitious as to describe and exhaust
> the whole world in a single scheme.

Nope The object of the game in the physical sciences is reductionism. Reducing
the number of explanations required to account for observations is the
_definition_ of progress.

> "don't try to explain the whole world with a single scheme"

That's not a theory. That's throwing in the towel.

------
jacobwilliamroy
Microsoft told me that any operation a quantum computer performs HAS to be
reversible: something like XOR doesn't work because there're four possible
inputs and only two possible outputs, so the state of the inputs is destroyed.
However, something like NOT works fine because the input-output mapping is
1:1.

It seems like cheating to say that "reversing cause and effect is no trouble"
when the computer was never allowed to perform irreversible operations to
begin with. I think I am missing some important information or maybe I am
misunderstanding the words used in the article?

~~~
emiraga
You have the right idea, but not the right operation. XOR is reversible.

To give a better example: COPY is not reversible. Imagine that you have two
qubits [A, B] as an input and you want to output [A, A]. That is not possible
with quantum computers.

~~~
mistaken
Isn't everything reversible if enough information is stored? Instead of COPY
[A, B] -> [A, A]; you could have [A, B, 0] -> [A, A, B] which saves the value
of B.

~~~
ahakki
IIRC this is how you implement stuff, you always keep the inputs, so
everything stays reversible.

~~~
lisper
Yes, but "keeping the inputs" is non-trivial in quantum-land because of the
no-cloning theorem. You can't just make a copy of your inputs and stash them.
You have to be more clever than that.

------
Severian
Forgive me for this being off topic: if you use a script blocker this site
will blow up your CPU unless you have ajax.googleapis.com allowed. My browser
became completely unresponsive. When I checked dev. console there were 50,000+
errors (and quickly climbing) on a video JS import. (2018-07-reversing-effect-
quantum.html:1314). Pretty buggy.

~~~
pmontra
Confirmed. It must be some recent change because we've got plenty of
submissions from phys.org:

[https://news.ycombinator.com/from?site=phys.org](https://news.ycombinator.com/from?site=phys.org)

Does anybody here work for them and can get in touch with the people that can
fix it?

------
jchook
Do I read this article in the future because I found it on HN today?

Or did I find this article on HN today because I read it in the future?

------
joe_the_user
If quantum computers can maintain a viewpoint where cause and effect don't
matter, it seems like they will be at a point very different from the human
viewpoint - where a person inherently has more difficulty predicting the
future than determining the past.

~~~
olliej
It hasn’t got anything to do with that, what the exceedingly clickbaity
headline is actually talking about is that QCs are more able to reverse
computation.

I’m unclear /exactly/ what they are trying to say as all QC gates are required
to be reversible, but it seems that they’re trying to say that to make a
computation reversible on a classical computer you in general (eg worst case)
need to store all the state (massive memory cost) or recompute from the
beginning, or somewhere in between (eg when skipping back in a video, a player
is generally going to compute relative to the nearest iframe).

In QC this extra memory isn’t needed as every computation /must/ be reversible
so you just run the program backwards, and rely on the increased state that
can be stored in each qubit (a lot of the exponential => polynomial
performance improvements basically seem to boil down to each individual qubit
being able to represent exponentially more state than a single bit).

Maybe that’s helpful? I’m not super sure as the places where QC is faster than
classical computation is pretty much only places where you can trade off
exponential time for exponential state. E.g. problems that you can’t plausibly
use a classical computer in the first place.

