Arrow of time and its reversal on IBM quantum computer (2018)
arxiv.org
arxiv.org
https://sciencedemonstrations.fas.harvard.edu/presentations/...
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.)
They didn't test for eg. 010 back and forth. There might be a 000 bias, etc.
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.
[0] - https://arxiv.org/abs/1405.1548
EDIT: See page 79 of the referenced paper for the detailed argument.
I believe you meant "quantum computing hasn't provided anything substantial yet". I first read it as "quantum theory hasn't provided anything substantial yet", and I was a bit shocked!
I mean, non casual filter (time travel filters!) are nothing more fancy than a post measurement analysis where we know y_n+1 at y_n, for example.
The "time reversal operator" and "time reversal symmetry" are usual technical terms in Quantum Physics, the authors didn't invent them. In an advanced quantum physics course you get 2 or 3 weeks about them, and the profesor will spend a long time explaining what they mean exactly.
The problem is that the name is too catchy and it generate a lot of misunderstanding when they are used in the press coverage of the technical articles.
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.
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.
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.
[1] https://en.wikipedia.org/wiki/Language_of_mathematics
[2] http://www.cut-the-knot.org/language/MathIsLanguage.shtml
[3] 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.
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.
If you return a chess piece to a previous position, it's possible that doing so loses you a tempo. Even if, strictly speaking, the game/board doesn't care about the direction you move, saying it's "just" another position isn't quite accurate, since turns matter, and using up a turn to walk back a piece may give your opponent a +1 turn advantage?
For a single electron, this would not be the case.
But tracking just the ball's position is meant to analogize the fact that we have all information about e.g. a fundamental particle's position.
The simple model of a ball shifting from one point to another is a fair macro analogy because, like the quantum system described here, we realistically have all information about the ball as it moved from point A to point B.
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.
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.
Of course, memory would be a problem.
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.
'"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.
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
If they can read a bit now that had itself set even one microsecond in the future, flash traders will arrive with truckloads of money.
If someone downvotes this can they reply to explain why?
Anyhow, both formulations of entropy - the thermodynamical one with the integral and the statistical physics one with multiplicity - are equivalent. At least the latter already contains statistics because of the combinatorial part.
And then there is one formulation of the 2nd law which states that on statistical average the entropy rises. So it doesn't break the law if it falls down. Given that this quantum computer doesn't involve many objects, the number of combinations isn't that high. But it's nice to observe Entropy falling! (And there are other situations in which this can happen anyway.)
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?
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."
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.
"Normally in a quantum computer, if we apply measurement function F to input x so y=F(x), it's impossible to reverse F and turn y into x. However, if we constrain y, we can make a special function G_Y such that x=G_Y(y)"
Whilst I don't have time to really dive through all the sections, this seems weirdly click-baity and IBM-promoting for an academic article.
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.
https://sydney.edu.au/news-opinion/news/2018/07/25/first-eve...
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.
Can anyone who knows this field describe what they did? I assume it's more exciting than applying a gate followed by its inverse.
For an accessible description of what is going on here see:
http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
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.