8 ms·
Arrow of time and its reversal on IBM quantum computer (2018)
- loblollyboy 8y ago*localized
- _Schizotypy 8y agoI don't think this breaks the second law, it sounds like they put some energy into the system to reverse the process
- gus_massa 8y agoThis 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.
- sandworm101 8y agoThat was also my understanding. This looks less like a reversal than a simulation of a reversal. They are using perfect knowledge to engineer a system that operated in reverse to the norm. Actual reversal would reverse everything, including the unknown/unknowable. 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.
- blablabla123 8y agoOf course the entropy also only rises necessarily in the statistical average - which is quite strong for any system with a large amount of object.
- zimablue 8y agoI don't think this should be getting downvoted, in my undergrad we treated entropy as a measure of the number of possible "equivalent" states, multiplicity or probability equivalently. So if you had a small enough system you might see entropy go down, by chance. If someone downvotes this can they reply to explain why?
- blablabla123 8y agoLate answer, I didn't downvote this though. 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.)
- _Schizotypy 8y agoThe 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 8y agoI 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
- wwarner 8y agoI didn't read it as breaking the second law, but rather a quantum theory of the second law.
- empath75 8y agoIt violates the law locally.
- Allower 8y agoWow talk about misrepresenting research..
- zitterbewegung 8y agoTLDR instead of traveling through time they just ┬──┬ ﾉ(° -°ﾉ). In a simulation.
- piker 8y agoThis is the best TLDR ever written.
- nickpsecurity 8y agoAlternative theory: they reversed a quantum state while moving forward in time. The researchers, the computer, etc.
- ZeroFries 8y agoTime-travel as popularity conceived is the same: reverse the state of everything outside the time-machine while everything inside progresses as usual.
- steve76 8y agoIs this like Wheeler's Delayed Choice experiment?
- lisper 8y agoNo. 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-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
- stevenwoo 8y agoI appreciate the link, but fair warning to others - it has a prerequisite of reading three other postings for complete comprehension per the article.
- lisper 8y agoThree? 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.
- delibes 8y agoI 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/reversible-fluid-mixing https://sciencedemonstrations.fas.harvard.edu/presentations/...
- fizx 8y agoClose enough for a popsci article!
- gus_massa 8y agoI 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.)
- cgriswald 8y agoThis is more like turning in a single direction to mix, moving some dye around manually, then turning in the same direction again and it un-mixes.
- smeyer 8y agoAs a bit of a side note, that was the most striking of the demos I've ever seen from their catalog.
- panic 8y agoI'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 8y agoThat'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 8y agoHow 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 8y agoI 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 8y agoThe 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.
- 8y ago
- curiousjorge 8y agobefore i invest any emotion in to this, is this clickbait or is there actual merit with the major implication that it is possible for highly developed intelligent entities to travel back in ti-
- typon 8y agoIt'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".
- adamc 8y agoI don't think that is true. When you pick up the ball and move it, you are part of the system... and it takes work (and produces heat). No second law violation, right?
- krastanov 8y agoIndeed, no 2nd law violation. And this work does not have anything to do with the 2nd law. OP summary is great: they are just doing an operation and then doing it in reverse (albeit in a technically interesting way).
- _Schizotypy 8y agoThere is also no violation in the proposed experiment because they put energy into the system to do their "reversal"
- phkahler 8y agoYeah, 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 8y ago> 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.
- deleted 8y ago[deleted]
- jakeinspace 8y agoI 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.
- mises 8y agoIt does: reverse a human to the state assumed 30 years earlier. Fountain of youth? Of course, memory would be a problem.
- throwawaymath 8y agoNo, 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.
- empath75 8y agoCleaning your room increases entropy.
- throwawaymath 8y agoYeah, though I don't think you're supposed to litigate an analogy that far. The analogy fits because it grounds what's happening as something mundane rather than mysterious. This isn't really "time reversal" in the meaningful sense of the word, because a linear system can always be reversed if you have sufficient information.
- novaRom 8y agoAside 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 8y agoA 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.
- harias 8y agoHasn't quantum chemistry calculations already been simulated using quantum computers? https://sydney.edu.au/news-opinion/news/2018/07/25/first-ever-quantum-simulation-of-chemical-bonds-with-trapped-ion.html https://sydney.edu.au/news-opinion/news/2018/07/25/first-eve...
- krastanov 8y agoA bunch of non-scalable examples do exist, and they are very exciting, but we have nothing that can yet reliably simulate molecules of practical interest (we have only "toy" examples that are small enough to be already solvable with a classical computer).
- otoburb 8y agoAnother physics layperson question here: Can this technique of studying a past state reversal apply to compromising one-way hash functions in some manner?
- edmack 8y agoThe word IBM feels conspicuous.
- cirgue 8y agoIs 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?
- deleted 8y ago[deleted]
- markkm 8y agoYes, the "universal" complex-conjugate operator cannot be unitary. And yes, you can conjugate a vector or two using a unitary operator. Coming up with an algorithm to conjugate a specific configuration of qubits is just linear algebra. I am not a physicist, and so I don't have a good grasp of the physical significance of the size and existential probability of supersystems mentioned in the paper. But I have to say I'm just a bit skeptical about the main claim of the paper.
- SquishyPanda23 8y agoQuantum 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 8y agoIt is not in fact more exciting than that.
- el_don_almighty 8y agoDoes this mean we can make Pluto a planet again??
- drinane 8y agoSounds 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
- drinane 8y agoSounds 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 8y agoUncomputation is a fundamental concept of quantum computing. You cannot isolate a meaningful result without it.
- drinane 8y agoSo did I get it right... Is that what they are talking about as time travel? It sounds a lot like it to me
- montalbano 8y agoDoes 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
- tannerc 8y agoInteresting 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 8y agoI'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 8y agoThese were dumb comments for me to make, I wish HN supported deletion at times like these. Oh well.
- HNLurker2 8y ago(Poetry) So does the time arrow marches forward unstoppably?