
Arrow of time and its reversal on IBM quantum computer (2018) - hownottowrite
https://arxiv.org/abs/1712.10057
======
delibes
I don't know enough quantum physics. Is this a qubit equivalent of reversing
the mixture of dyes in viscous liquids?

[https://sciencedemonstrations.fas.harvard.edu/presentations/...](https://sciencedemonstrations.fas.harvard.edu/presentations/reversible-
fluid-mixing)

~~~
fizx
Close enough for a popsci article!

~~~
gus_massa
I agree. There are a million of differences, but the main idea is the same.
You go from an very ordered state to an state that looks unordered, but it's
actually very specific state (with a weird order that is not evident).

Then in one case you reverse the mechanical part and the fluid movement is
reversed and you get back to the initial state. (Well, almost the initial
state because there is some diffusion and other non reversible effects.)

In the other case, you apply a transformation that reverse the "speed" of the
particles (even if they are entangled, without destroying the entanglement,
here a few technical details that is better to avoid), and you get back to the
initial state. (Well, almost because if you are unlucky soothing can affect
unexpectedly the system and you loose the control and you don't return to the
initial state. Errors like this are more probable when you use more qbits.)

------
panic
I'm not a physicist, but isn't this kind of like building a fridge and saying
it's reversing time because it makes it look like the usual process of heat
dissipation is running in reverse? Or is there something more fundamental
going on here?

~~~
lisper
That's not a bad analogy. The difference is that the fridge requires energy
input, while quantum "time reversal" does not. I'm putting "time reversal" in
scare quotes because it's not really reversing time, it's "simply" (again in
scare quotes because actually doing it is not so simple) reversing the normal
direction of the evolution of a quantum process.

~~~
rightbyte
How do we know that there are no work done by the computer on the states?

They didn't test for eg. 010 back and forth. There might be a 000 bias, etc.

~~~
lisper
I don't know what you mean by "010 back and forth" or "000 bias", but I'm
talking about theory, not practice. Even under ideal conditions a refrigerator
requires energy input in order to work. A quantum computer does not. I have no
idea whether this particular implementation was dissipative or not.

~~~
rightbyte
The autors write that the registers went from 000 to something and back
approx. 50% of the time when the random chance of it was really small.

An ideal 386 computer could be thought off as not requiring energy too, right?
A looping pseudo random number generator would then be time reversal. It's not
saying anything really.

------
typon
It's like saying picking up a ball from point A and putting in point B, then
taking it from point B and putting back in point A is "reversing time".

~~~
phkahler
Yeah, this is a poor use of words. I was actually hoping they had read some
quantum information "from the future" but in a way that precludes transmitting
useful information to the past - similar to how entangled pairs seem to
transmit information faster than light, but in a way that doesn't enable FLT
communication. But no, nothing about this suggests anything to me about
reversing time. Reversing decoherence (if that's a better description) is nice
but it's not about time reversal.

~~~
zwkrt
> similar to how entangled pairs seem to transmit information faster than
> light

If your lover sends you a left-half heart locket through the postal service,
when you get it you can assume he has the right-half of the locket "faster
than the speed of light". However this is cheating because to know that he has
the other half requires prior knowledge of the exchange--either he sent a
previous letter where he stated his intent, or you just made an assumption
about the state of the environment (namely, that your partner is the only one
who would send you presents in the mail). In either case, no laws of physics
need be broken.

~~~
ahelwer
This is absolutely not how entanglement works. This analogy requires the
locket half you receive to be defined at time of "entanglement" (when the
locket halves are separated & sent). Half a century of Bell experiments have
told us that the result of measuring an entangled system is defined at time of
measurement, not at time of entanglement. If it were defined at time of
entanglement, the particles would have to carry information with them to
"remember" how they decided to collapse (a local hidden variable). Bell
experiments disprove the existence of local hidden variables as a means to
explain entanglement.

I wish people would stop posting these third-hand metaphors about quantum
entanglement being like finding a single right-handed glove in your pocket or
whatever. They are irredeemably flawed and completely misrepresent how
entanglement works.

~~~
zwkrt
Yes my analogy had a hidden variable, but it is "less wrong" than saying that
entanglement allows FTL communication, for reasons analogous to those that I
provided at the end; entanglement can only convey _information_ as fast as
light because the two parties detecting the entangled particles aren't
communicating they are just on the receiving end of a shared experience. I
don't think I can explain entanglement to a lay person more succinctly than
the locket/glove analogy, even though it does hand wave the most quantum-ey
part of the experiment.

~~~
ahelwer
But I feel this harms more than it helps, like saying "quantum computers
compute with every possible value simultaneously in superposition" which gives
the audience a very flawed understanding of how QCs work (that they'll be able
to solve NP-complete problems in polynomial time for example). By phrasing
quantum phenomena in terms familiar to the classical audience, we do them a
disservice; we are lying to them.

In this case, the question "why doesn't quantum entanglement enable FTL
communication?" is properly that the reduced density operator of an EPR half
is the maximally mixed state. All we can do to simplify that is to tell them
that quantum concepts exist outside of classical language, and in order to
understand them they'll have to learn a new language - mathematics. The most
you can say is quantum entanglement enables FTL "correlations" which are
stronger than classical correlations but not strong enough to enable
information transfer.

~~~
dwringer
Not encountering this for the first time, I've given much thought to your
position over the years. But according to many mathematicians, mathematics is
within the purview of natural language[1][2]. And wasn't it Feynman himself
alleged to have said if we can't explain something in a lecture intended for
undergraduate freshmen, we don't really understand it? [3, p19].

[1]
[https://en.wikipedia.org/wiki/Language_of_mathematics](https://en.wikipedia.org/wiki/Language_of_mathematics)

[2] [http://www.cut-the-
knot.org/language/MathIsLanguage.shtml](http://www.cut-the-
knot.org/language/MathIsLanguage.shtml)

[3]
[http://calteches.library.caltech.edu/563/2/Goodstein.pdf](http://calteches.library.caltech.edu/563/2/Goodstein.pdf)

EDIT: This is not to challenge your specific assertions on this topic - on
which I cannot speak with any particular authority.

~~~
ahelwer
Undergraduate freshmen are perfectly capable of understanding basic linear
algebra, and thus quantum entanglement.

Regarding the nature of language and how it relates to mathematics, I don't
think it's correct to say math is a "subset" of language; there is no
satisfactory definition of language that encompasses all the way humans
communicate with one another, and it makes more sense to restrict the
definition of language to its use in specific interactions (the language of
interacting with a cashier to buy an item from a store, and the different
language of telling your team what you worked on during standup, for example).
In this sense the use of mathematics to describe quantum mechanics is a
language in itself.

~~~
dwringer
Sorry, my earlier use of the word subset was a mistake and I have edited the
sentence in question to say "within the purview" which is more what I had in
mind. I appreciate your reply and do agree with your final statement, though
I'm not really sure on the idea of restricting the definition of language as
you've described and personally have trouble finding such delineations.

~~~
ahelwer
You can read more on Wittgenstein's idea of "language games" here:
[https://plato.stanford.edu/entries/wittgenstein/#LangGameFam...](https://plato.stanford.edu/entries/wittgenstein/#LangGameFamiRese)

------
jakeinspace
I suppose this is interesting because unlike cleaning up one's room, (an
example of equivalent "time reversal" given by an obnoxious commenter on the
article site), this appears to be total time reversal. All properties of the
system are set in reverse (where the system obviously does not include the
outside world and apparatus which has been contrived to reverse the qubits).
Whether this has any real value seems debatable, but it's pretty neat to know
that we're even capable of such precise reversal of a basic quantum system.

~~~
throwawaymath
No, it's actually just like cleaning your room.

Imagine your room (system) has three objects (quantum state) whose respective
positions are (a, b), (x, y) and (p, q). You move the objects in your room to
positions (a + k_{1}, b + k_{2}), (x + j_{1}, y + j_{2}) and (p + m_{1}, q +
m_{2}), respectively. You know where the objects were before and you move them
by adding or scaling their coordinates within the room. Thus you know how to
move them back to precisely where they were before.

Likewise, this process is linear and preserves all information inherent to the
system. Therefore it's precisely invertible, and voila.

It probably has some kind of value and it's a neat result, but this doesn't
constitute time travel (in any meaningful sense) in the nonlinear world we
reside in.

EDIT: Come to think of it, this probably has value for debugging and auditing
the states, as a perfect rewind stepping function.

~~~
jakeinspace
It's only roughly equivalent to cleaning your room, in a hand-wavey way. Is
your cleaned room in an identical quantum state as it was a week ago? No?
Well, then the situations aren't the same.

~~~
throwawaymath
Yes of course it's hand wavey, all analogies are hand wavey. That's why
they're called analogies and not lectures. You don't devise an analogy to
deliver all the academic rigor of a topic, you devise it to ground something
back into the realm of the intuitive and familiar.

Not all analogies are useful, but the reason _this_ one is useful is because
it captures the heart of why this isn't meaningfully "time reversal." If you
have all information about a linear system, yes you can transform it back into
its previous state. That's not at all mysterious or unintuitive even if it's a
technical achievement.

Obviously reality is nonlinear, which is precisely the point the analogy is
trying to capture. We're not trying to teach quantum mechanics here, we're
trying to make sure people don't come away from the article thinking the
second law of thermodynamics is (non-locally) violated or that time travel at
the macro level is plausible.

The basic idea is that you did a reversible thing, then reversed it.

------
_Schizotypy
I don't think this breaks the second law, it sounds like they put some energy
into the system to reverse the process

~~~
gus_massa
This experiment doesn't break the second law, because they have perfect
knowledge of the intermediate states, so they can apply a transformation to
invert the evolution.

In a big macroscopic system this is imposible in practice, because you only
get a parcial knowledge of a few important properties, not of every detail.

Moreover, in most systems it is imposible to know everything about the system,
but in some specially build systems like a quantum computer you can know the
state of all the qbits.

~~~
_Schizotypy
The reason I made the statement was because the article said

'"This is one in a series of papers on the possibility of violating the second
law of thermodynamics. That law is closely related to the notion of the arrow
of time that posits the one-way direction of time from the past to the
future," said the study's lead author Gordey Lesovik, who heads the Laboratory
of the Physics of Quantum Information Technology at MIPT.'

When I don't see how violating the second law could even be a possibility.

~~~
gus_massa
I think we both agree that this is not violating the second law of
thermodynamics.

We disagree in that this time reversal operation need energy. It's possible to
do this experiment with a frequency doubler [1] crystal in reverse to split a
photon in two photons with less energy, then use mirrors to reverse the
photons, and then you will be able to "see" that the photons return to the
crystal and produce the original photon. [Good luck aligning all the optical
equipment perfectly. This is theoretically possible, but it would be very
difficult to make the experiment. Perhaps it's easier with other particles.]
Anyway, the time reversal operation in this experiment only use a few
perfectly aligned mirrors, so it doesn't need additional energy.

In the experiment in the article, they use a setup that is like a quantum
computer. It use energy to keep everything working, but the additional energy
is not necessary for the main part of the experiment. (The energy is important
to make the experiment possible, i.e. transform "perfect alignment" into "we
can build this".)

[1]
[https://en.wikipedia.org/wiki/Optical_frequency_multiplier](https://en.wikipedia.org/wiki/Optical_frequency_multiplier)

------
drinane
Sounds like they implemented a theoretical architecture called uncomputing.
Apparently useful for all sorts of theorized quantum algorithms.

[https://www.scottaaronson.com/democritus/lec10.html](https://www.scottaaronson.com/democritus/lec10.html)

------
cirgue
Is this the same quantum computer that had the weird promo video where they
talked at length about assembling it in Italy out of aircraft-grade aluminum
and didn't mention benchmarks/capabilities?

------
drinane
Sounds like they implemented a theoretical architecture called uncomputing.
Apparently useful for all sorts of theorized quantum algorithms.
[https://www.scottaaronson.com/democritus/lec10.html](https://www.scottaaronson.com/democritus/lec10.html)

~~~
anticensor
Uncomputation is a fundamental concept of quantum computing. You cannot
isolate a meaningful result without it.

~~~
drinane
So did I get it right... Is that what they are talking about as time travel?
It sounds a lot like it to me

------
nickpsecurity
Alternative theory: they reversed a quantum state while moving forward in
time. The researchers, the computer, etc.

~~~
ZeroFries
Time-travel as popularity conceived is the same: reverse the state of
everything outside the time-machine while everything inside progresses as
usual.

------
tannerc
Interesting that many of the comments here attempt to label this as false
research simply because it's taking one state and changing it to another.

Absent from the conversation seems to be agreement on what exactly "time" is.

Much has been studied and said about time, but if we think of time as being
something other than an observable change—or that of a recognizable, unified
state to one of chaos, aka entropy—what is it?

~~~
tannerc
I'd add here that the observations of this research aren't that striking.
We've known for some time that time may not be a single direction arrow but
rather one pointing both ways, dependent on what we're observing and our
classification of what's possible.

In his book Your Brain is a Time Machine, Dean Buonomano gives an excellent
and well-written example of how studies like the one reported here actually
work. A favorite quote of mine from that section sums the point as so:

"Decreases in entropy are improbable, not impossible, and given enough time
the improbable becomes probable."

~~~
tannerc
These were dumb comments for me to make, I wish HN supported deletion at times
like these. Oh well.

------
montalbano
Does anyone know if this has any implications for reversible computing? I
haven't the expertise to draw links between the two topics.

[https://en.wikipedia.org/wiki/Reversible_computing](https://en.wikipedia.org/wiki/Reversible_computing)

------
novaRom
Aside from the clickbait title, can anyone please ELI5, TL;DR this paper?

From abstract: Here we show that, while in nature the complex conjugation
needed for time reversal is exponentially improbable, one can design a quantum
algorithm that includes complex conjugation and thus reverses a given quantum
state.

~~~
krastanov
A physics simulation usually simulates the future, but there is nothing
stopping you from running the code "backwards" in order to study the past
state of the system. If we want to sound fancy we call this "reversing time".

In the case of a quantum computer simulating something (we can not really do
this yet, but we are hard at work building the hardware) that type of reversal
might require us to calculate complex conjugates of quantum states. For
various reasons this is nontrivial and this paper describes ways to do that.

~~~
otoburb
Another physics layperson question here: Can this technique of studying a past
state reversal apply to compromising one-way hash functions in some manner?

~~~
krastanov
No! Contrary to what the sibling comment says, there are plenty of one-way
hash functions that are resistant to quantum computers. Two comments though:

1\. This particular work is explicitly relying on the fact that many quantum
operations (including the type of time evolution they are considering) are a
one-to-one map (not the one-to-many "irreversible" hash functions). Hence this
work is explicitly not applicable.

2\. Quantum computers do break some form of public key encryption, but this is
a solved problem, as there are many other public key encryption protocols that
are can not be broken by a quantum computer. Quantum computers do provide
modest speedup in all types of brute force searches, like breaking symmetric
encryption, but this is trivial to defend against by using a slightly bigger
encryption key.

------
SquishyPanda23
Quantum computing is reversible right?

Can anyone who knows this field describe what they did? I assume it's more
exciting than applying a gate followed by its inverse.

~~~
ahelwer
It is not in fact more exciting than that.

------
edmack
The word IBM feels conspicuous.

------
HNLurker2
(Poetry) So does the time arrow marches forward unstoppably?

------
zitterbewegung
TLDR instead of traveling through time they just ┬──┬ ﾉ(° -°ﾉ). In a
simulation.

~~~
piker
This is the best TLDR ever written.

------
el_don_almighty
Does this mean we can make Pluto a planet again??

------
loblollyboy
*localized

------
steve76
Is this like Wheeler's Delayed Choice experiment?

~~~
lisper
No. This is a quantum system that evolves from a state A to a state B, then is
exposed to a very specifically designed potential field that causes it to
evolve back to state A. It's actually not that mysterious nor unexpected, just
an impressive technical achievement.

For an accessible description of what is going on here see:

[http://blog.rongarret.info/2014/10/parallel-universes-and-
ar...](http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-
time.html)

~~~
stevenwoo
I appreciate the link, but fair warning to others - it has a prerequisite of
reading three other postings for complete comprehension per the article.

~~~
lisper
Three? I only count one. (BTW, I'm the author.)

But yes, you do need to understand entanglement and how it relates to
measurement before you can understand time reversal. There's a reason QM has a
reputation for being a difficult topic.

