Counterfactual Quantum Computation is Really Weird 

“counterfactual is like my husband mowing the lawn.” I love her dry sense of humor.

Sabine, even when I don’t completely understand every aspect of your videos, they always leave with this marvelous, almost childlike, sense of wonder and awe at this extraordinary universe in which we live. Thank you so much for this gift!

1:08 Math is simple, Meaning is difficult 1:25 A Psi Wave Function 2:34 Lasar hits Beam Spliter 3:30 Photon Split in Half 4:48 Collapse of Wave Function 5:51 Interferometer 7:10 More Beam Splitters 8:17 Yes/No , No - photon ends 9:45 Splitting Up 10:30 What does this mean? 11:17 Computing with Not Computing 12:37 It works

"Maybe the Laser Fairy brought it." Just the first of several perfect little jokes sprinkled through the learning. And *this* is why I love your videos, Sabine! Thanks for all you do, including your wit. You're very appreciated! ❤️❤️

The more I learn about this weird Quantum Mechanics thing, the more it sounds just like the rules for magic to me, thanks for the video.

Read the paper, “The wave-function is real but nonphysical: A view from counterfactual quantum cryptography.” Interesting read that’s backed by a couple of subsequent experiments.

This channel is a shining beacon in the quantum field of random disinformation! Please continue doing what you’re doing!

The first sentence in this video is the pinnacle of quantum comedy. Also "You can compute with a computer that doesn't compute". Priceless.

10:39 "if the photon goes through the inner part" Isn't the whole point of the double slit experiment that the photon isn't picking 1 path, it's taking every path at the same time and only decides where to pop up when the wave meets a detector? So even if the photon appears in D1, the wave still went through the computer?

This all seems inexorably tied with the Bell inequality. Or from the superdeterministic point of view, the lack of statistical independence between the computer and the detector. I feel like there is a deep symmetry at work here.

My jaw literally dropped when you got to the part where the photon that indicates the result of the computation must have taken the path that does not engage the computer. That is stunning. Thank you for this explanation and for the gentle humour you inject into each of the topics you cover

this kind of "paradox" only tells me we undertood something wrog or something else is missing in physics...

We cannot have a field equation and ignore it at the same time. One of the questions that we need to ask 'James' (the photon computer) is: How does the computation change the path-length (phase) of the photons that pass through? Will they all have the same delta? And we really do need to ask about the photons that do not pass through ... Are they absorbed? reflected? ... In any case, introducing James to the apparatus changes the wave-function because it introduces new boundary conditions.

It seems to me that the claim that if the photon doesn't pass through the computer, then no computation happens, is simply incorrect. The wavefunction still passes through the computer, and interacts with the computer's wavefunction, and this interaction performs the computation, not the photon.

I am glad Sabine is here. YT is getting full of click bait videos. She brings to light what others refuse to address.

3:23: unless I have completely forgotten my QM, Schroedinger's equation cannot describe the behaviour of photons.

I absolutely admire the way that you explain things. You never make me sigh.

I love this channel. You're one of my favorite science communicators and you cover the most fascinating concepts. Do you have any lectures on video?

Been catching your vids for a bit, but this one is da bomb. I have to view this a dozen times to get just a glimpse of what that heck you are explaining, but putting in the humor is just over the top, as the humor is sophisticated but creates a wonderfully bright juxtaposition of/for the overarching communication of quantum phenomenon. Please create more like this.

I love how you jumped into the topic directly. More RU-vidrs should do this

I feel like this video sparks more questions than it answers. What does it mean to "run" the computer? If the computer works by having a maze for the photon where the layout encodes the problem, then there is nothing to run. The solution already exists as described by the wave equation and we just need to measure it by sending a photon, and it "costs" us the same regardless of which path it takes. If light behaved purely as a wave with no quantum effects, it feels like the outcome would have been, on average, identical and nobody would find it weird. I guess the weirdness comes from the fact that what gets quantized are our measurements of where the particle ends up, rather than the particles themselves? But isn't that the same concept as the famous double slit experiment, just said in a different way?

"Like my husband mowing the lawn" Oh boy, someone's in trouble.

I love you Sabine. I don't have a degree in maths or sciences but have always had a love and fascination with them. Your channel helps break things down and make them understandable without stripping away too much to the necessary information needed to 'get it' without actually understanding it completely (which I've heard means you know less than you think you do). Keep up the amazing work!

I've been patiently waiting for sabine to make a video on this topic. It's about time

This is a great follow up to the quantum games video and totally new to me. QM has become so interesting to a non physicist like me, I look forward to hearing more from you.

I was going to do some counterfactual quantum computation.--but then things got really busy at work.

Hi Sabine. Thanks for your enlightening and light hearted videos! I am now reading your Lost in Math book. Very smooth reading!

Thanks for another great and funny video! BTW, my copy of your newest book has shipped to me, and I'm excited to read it! Thanks for all you do!

the "how does it know?" part reminded me of high school when we were learning about snell's law. the teacher said it turns out that the path the light takes is not the min distance but the min amount to time to reach the end spot and compared it to being on a beach and seeing someone drowning in the water, we don't run straight to the water (perpendicular to the shore) and then swim to the person but we run a ways on land and then swim the rest of the way because we know from swimming that it's slower. so the question was "how does light "know" that too?" so is there something similar going on? where the rules of reality seem to impart some skill or knowledge to otherwise brainless things? [oh! and in the first part where you describe counterfactual as something that doesn't happen affects what does happen made me think of the origins of "alternate facts" where things that aren't true definitely have an effect on reality, but i think that's more a phenomenon with society/psychology/misinformation 😞]

Thank-you for this. A new Sabine video always brightens my day, and this one - the wonderful humour and material - was exceptional for me.

I recall a Scientific American article describing a way to image something without actually hitting it with photons, a way to detect Medusa without actually looking at her.

Love the videos, and how effectively and efficiently you explain your subjects.

I think the video would benefit a lot by inserting, after the sentence about interference that begins at 6:06, a description of how the two recombining beams would interfere if one or both of the beam splitter's probabilities are something other than 1/2 + 1/2. This is needed to understand the outcome of the recombining in the case where food nazi James doesn't like pineapple on pizza.

Today I learned how the laser fairy carries a plastic plant on a long hike, and then collapses...or something like that. (What do I need to leave under my pillow so that the laser fairy will visit me?) And James has conditional Write-Only Memory. Maybe I should have some coffee before watching these videos.

How has it taken me so long to find Sabine’s RU-vid channel?? I’ve had an interest in science as a young kid & refined that down to physics by the time my GCSEs and A Levels came around. I am 39 now & still sit here fascinated as I read or watch videos on the absurdity that is the post-Einstein world of physics. Physics aside, Sabine has a great understanding of British humour. Very witty, often clever, generally dry humour, with a dash of dad jokes before mixing up. Only a recent subscriber, but keep up the good work Sabine. I find when it comes to difficult concepts, no one can break it down to such an easily digestible level like she does. This channel has been my recent go-to nighttime binge watch

Excellent video! Once again, I'm digging the humor in these videos!

Come for the science; stay for the jokes.

5 hours after being uploaded and this vid has almost 20 k views, and over 3 hundred comments. Now this is really a quantum leap for a science channel in YT, I wish you all the best for this amazing achievement, keep learning anf the teaching for the rest of us!!! Thanks

Fascinating! Thanks, Sabine!!! 😃 Stay safe there with your family! 🖖😊

This is your best video to date! BRAVO!

I believe “Counterfactual” anything may just be another word for Entanglement. Now I’m more of an amateur physicist (math PhD), but it seems to me that when we say a system is no longer entangled, it is only that there are two many entangled particles for us to keep track of, and so no longer a “pure” entanglement that we can parse or compute/validate. But does that mean it’s not there? I would really like a physicist to explain clearly why in complex systems the answer is “we cannot preserve entanglement” and not “everything is too entangled for us to make sense of anymore”. Perhaps our definition of Entanglement is too limited? It just seems as though all of these “counterfactual”, “spooky” non-local effects could be chalked up in simple terms to the facts that the summation of local quantum particle interactions creates a kind of non-local web of interactions/dependencies, which en masse and at the large limit appear to average out and give us the macro world we see. But as we go further from that limit, entanglement plays a bigger role. It seems like it’s always there in every interaction of every particle, but just like charge spreads on a sphere to cancel out internal field, entanglements are spread out an effects cancel… but where there are asymmetries I expect their effect is still present, although negligible. In this experiment in a highly isolated environment, I think the “counterfactual” piece is just a macro result of our lack of understanding/information about the entanglement that creates the effect. Entanglement is the non-local, faster-than-light effect that pushes our logic and reality beyond the classical. I do like to imagine a web, almost like a huge loom across space and time, of entanglement. Averaging limits the effects. I’d love to see Sabine or PBS Space Time do an episode.. “Is Entanglement Everywhere”? I would like to know in what sense this view is right or wrong. I don’t see it as magic, it’s an extension of our classical understanding of reality. But still an ever-present, shifting web of immediate interaction across time and space paints an incredible picture of the universe. Is it real????

Amazing and mindblowing. I'm inspired and frustrated at the same time.

This was awesome ‼️ love the easy to understand examples And James is pretty cool too 😎

Love this channel. Love Sabine’s sense of humor.

Non-reversible computing is supposed to increase heat (or more precisely entropy) by a particular minimum amount somewhere. Does this happen to the same amount with counterfactual computing in the case when you happen to get the answer in 1 (or in the aggregate?)? And which part of the system heats up? Is it "James" ?

Plants (-) Dogs = Plastic Plants / (5000 piece puzzle on shelf) Laser (+) Cats = {Plastic Plants (+) 5000 Puzzle Pieces (+) Coffee Cup} / Floor Damn Schrödinger's cat, should have bought a dog and replace your live plants every 3 months.

Thanks for the explanation Dr H. The second time I watched you depict the counterfactual result, I could conceptualize it clearly, for about 5 seconds. Nice feeling!

This is so much better than when I first learned about it from an hour long talk on youtube and then tried to code it up on IBM's quantum computer to see what happens.

11:01 you say the photon reaching D1 didn’t go the other path. But it did, it went through both paths and only collapsed at D1 this time. So it actually went through the computer as a wave function. I don’t get why this is so different from usual quantum computing. Please explain me where I am wrong ! 😊

Does counterfactual computation have anything to say about whether the Copenhagen or Many Worlds interpretation are true? Could the counterfactual computation be a sort of ‘signal’ from the other world? Asking because this was also an intriguing part of Neal Stephenson’s Anathem. Also, is it just me or was Sabine’s humor especially good this episode?

If the interference pattern is affected, then the wavefunction encountered (and was affected by) the computer. The wavefunction, being a wave, travels (and is influenced by) all paths. Wavefunction "collapse" on measurement doesn't erase all of those previous interactions. Those branches of the wavefunction are "regathered" to the "chosen" outcome, but only "after" all of those interactions.

I could listen for ages to you going on about quantum mechanics, so perhaps we should have a big fan meet in a Biergarten and give you a microphone. And never hold back on your humour, I love it!

The problem with these interferometer examples is they are always dependent on EXACTLY how the experiment is set up...and that level of detail is always missing...

This is my type of computer, one that does work even if you do not ask it too! its like the quantum cryptic unbreakable "key", you can make the "key" obsolete just by looking at it, this will let the receiver know you have peeked at the secret code " Alice n bob ", therefore making you continuously change the unbreakable code, even if you do not want too?

Ma'am you have a phenomenal intellect and your humor is spot on!!

I first encountered her today (Aug. 11) as I caught the tail end of the interview on NPR about her new book, "Existential Physics." I plan on purchasing it, not merely because the subject matter has long been of interest to me but also because in my research on her, I found we have the same birthday. Happy Birthday ... early, Ms. Hossenfelder.

I have a silly question; About the output of the computer, if beam hits D1 and we know the answer without computation, in that moment, entropy of the system is the same another with the working computer? I mean, output information is a result of change right? So this change is exactly the same output for the engineers, but system too? If it is, is that mean the "standby" computer effects surroundings exactly the "working" one? Thanks for the answers in advance.

OK, you finally persuaded me! I just signed up for my free seven days on Brilliant, and just watched your first lesson and it was really good! And I am a visual learner so the graphics really helped me to better visualize all this,. And even though I already knew the material it still helped to do that. For once I see the graphics I can then picture it in my mind. Then the math starts to make sense and is not just a manipulation of terms but a succinct way to describe what I can then see.

Great video showing some of the mind boggling QM. Could have been a plus to have the different valued beam splitters in individual colors.

My dear Sabine. I love the way you stretch my brain. I always look forward to your newest post. Stay strong, free and healthy.... yours 💛

You know that someone ate the cheese, if only holes left over. Counterfactual mouse-trap ^^ Maybe, with that, you also could proof the anthropic principle. The quantum mechanical version of it, is called participatory anthropic principle. Without us (human detectors), the universe never ran.

Sabine, your sense of humour is amazing, I had to go back to the beginning because I was laughing so much I missed some of the explanation. Thanks again for making these wonderful concepts so clearly understandable.

Most of these videos hurt my brain with smoke coming out my ears, this there again some smoke and I also had a few good chuckles. You again were brilliant👍. Thank You

I watch all your videos, been a fan for a long time. Thanks for what you do, I loved the book.

Thank you for a fun and insightful presentation on a fascinating topic! Question: Would the paths in Feynman's history integrals count as counterfactual reasoning? After all, a photon "knows" where to land at the back of your eye only because of the vast number of paths that it did _not_ take (did not leave any historical trace) as it passes through the lens of your eye.

I prefer these types of presentations rather than some you have done recently. This one is worth watching several times and worth keeping in the archives. Pineapple plus pizza equals yes no matter how you split the pizza.

This new video style is very entertaining!

My fave youtuber. The new comedic breaks are fun.

There are only apparently counterfactual events here. Complete collapse of the wave function is subsequent to detection at any of the detectors. That is, once the photon is absorbed in D1, D2 or D3, the photon decides which path it followed based on the set up of the experiment during the beginning and end of the experiment. In this way, at the end of the experiment a kind of retroactivity occurs since the processor computation happened, but was later "removed". From the photon's point of view, all the events from when the photon is emitted until it is absorbed occur simultaneously and immediately since their speed is infinite. It does not matter that the experiment set up is modified by free will during a test, since the result is already known to the photon because it has already occurred at the moment of photon emission. Why we humans interpret reality in a "relativistic" way, I mean, spatio-temporally, must have to do with how the mind works. But at the level of quantum systems, it makes no sense to talk about events or a timeline.

Physicist seem to have amazing requisition departments: here we have Sabine telling us to just assume we have a laser, while Simon Clark recently told us to just assume a spherical cow. Where do you get all this stuff? Must have something to do with the multiverse… 😉

I am glad I found this channel..I have lots of questions about stuff..

> counterfactual computation > computing without running the computer > no electricity for the computer > having a crypto mining warehouse counterfactually > stonks

Would be funny if counterfactual computation could solve the halting problem 😜

As always, great and interesting video. Thank you! But I do not get the excitment at 11:00. Of course the photon that hits D1 knows the answer is "yes". If the answer is "no", the photon aiming at D1 is hit by another photon from computer and both destroy each other (destructive interference). Since the "photon destroyer" does not arrive, the answet must be "yes". Where is magic here? Am I missing something? Actually the destructive interference in case of "no" is more magical. What happens to energy of the photons?

I love your sense of humor!

You don't need to "run" the computer, but to get a photon at detector 1 you do need to send photons. This is just a twist on the double slit experiment with delayed choice. That is, the "computer" is actually a polarizer that, when oriented to block a certain photon, that photon will "know" it is about to be blocked and thus "choose" to split when passing through the first splitter at a (higher than simple probalistic) rate that violates Bell's inequalities. I guess.

dear sabine, you made me laugh, while shredding me to parts with quantum brainfood. i bow in respect and keeps smiling, sabine, you got our back!

My experience: IBM offered access to their Quantum Computer. I started simple, 2 + 2. Every iteration, resulted in a different answer. None being 4. Not being a college professor, I quit before I started a trillion computation program. Seeing what the average answer was.

Joining the voices of "Thanks" for all your intelligent and excellent videos. (and the candor)

This is really strange. I'm gonna have to rewatch this tomorrow.

"How to compute with a computer that doesn't compute"? - Burn finite fossil fuels - like no tomorrow. "Energy, like time, flows from past to future". Wailing.

Sabine is really playing for the laughs in this episode. But I wonder if a universe in which her husband mows the lawn is logically consistent.

You said it in a previous video: light “knows” which path uses the less energy to do the work like your husband . It’s all about field and less energy paths. The light will arrive in the more energy saving medium, even if it has to split. Water in pipes function the same way. The light beam of light just colapses / resolves in the most energy efficient path in the medium among all the probable paths in the field. It is also posible that for any posible path there is a probability that the beam will resolve through that path, even if the probability is to low. When you put a barrier, the light will not pas through one point and the light will find others ways to pass, like water or your husband. Your husband know the loan will be done eventually in the right time to save the most energy to keep it short, that’s at the end of the somer.

Much of what you cover is over my head. But, i enjoy hearing about it and learning.

So let me get this straight, you could program the computer to perform a task you don't know will halt (as per the halting problem, such as calculate all the digits of PI), and use the counterfactual to find out the answer without even running the program? 🤯

If you can compute with a computer that doesn't compute, can you think with a brain that doesn't think? This might be how we got the Flat Earth theory.

I loved the video sabine, it was a lot of fun. You made a remark towards the beginning in passing, that I think is very profound. That all the mathematics in the world won't help you understand what a dog is, or something to that extent. I think people that struggle with perceiving what quantum interactions actually are would benefit greatly from pondering that. Call it a quantum koan if you will.

Absolutely love your sense of humor. Big fan for years. You have helped me learn much more than I could have without your videos. Thank you Sabine. Looking forward to reading your new book.

Plastic plants are so sad. Ancient plants became crude oil which gets refined into plastics which we use to make plants.

The best decision I ever made in my life was investing in financial market. Trust me guys, it pays!😊

You can make a similar experiment using electricity or even water, where if certain path is not taken if affects the outcome. You perform the work to set up the system this way and you get the result. So why is this something to be amazed by exactly?

In the universe I live in, I'll be attending Sabine's lecture in a couple of weeks. Looking forward to it 🙂

Please stop integrating too many silly jokes into the lecture. The old format worked better for me. Just scrap 50% of all the jokes and everything is ok!

First 🥇

Sabine, I love your light devilish smile (in a funny, tongue in cheek way) in the beginning of the video when you say "that is what we talk about today!". Always makes me chuckle. Thanks for your always really interesting videos for us interested layman or laywoman!

It reminds me of the joke: She: You need to do more chores around the house. He: Can we change the subject? She: Okay. More chores around the house need to be done by you. By the way: Quantum mechanics is wrong. Not the results, but the assumptions. Your instructor lied to you.

I just love the sheer absurdity of the setup, photon wave goes through the interferometer maze and with the power of calculated, dumb luck, it crashes head on with... itself, skipping the program path, and going straight to the correct output pathway as if it had calculated it. and concluding the program performs better when not running it But its true at face value, some discoveries were just made by negligence and good luck

Fascinating. It is not often that I still get surprised by the quantum mechanics

actually i LOOOVVVEEE your Quantum mechanics Videos

Picking that lawn photo makes me feel better about my own lawn. Thanks. Sometimes time and energy are busy somewhere else lol.