*[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]*

Thanks for clicking on this video, by which you’ve dominated out a attainable world through which you by no means watched it. This various world has grow to be a “counterfactual” actuality. For us, counterfactuals are simply issues that might have occurred however didn’t, like my husband mowing the garden. In quantum mechanics, it’s harder. In quantum mechanics, occasions which might have occurred however didn’t nonetheless have an affect on what really occurs. Yeah, that’s bizarre. What does quantum mechanics should do with counterfactual actuality? That’s what we’ll discuss as we speak.

I’ve solely not too long ago begun working within the foundations of quantum mechanics. For the earlier decade I’ve principally labored on Basic Relativity, cosmology, darkish matter, and stuff like this. And I’ve to say I fairly like engaged on quantum mechanics as a result of it’s easy. I like easy issues. It’s why I’ve plastic crops as an alternative of a canine.

In case you had been laughing, this wasn’t a joke. I really do have plastic crops, and quantum mechanics is certainly a easy concept – if you happen to take a look at the arithmetic. The tough half is making sense of it. For Basic Relativity it’s the opposite manner spherical, and all of the maths on the planet gained’t enable you to make sense of canines.

Okay, I can see you’re not completely satisfied that quantum mechanics is in some sense easy, however please give me an opportunity to persuade you. In quantum mechanics we describe every part by a wave-function. It’s normally denoted psi, which is a Greek letter however perhaps not coincidentally additionally the response I get from my buddies after I go on about quantum mechanics.

We compute how the wave-function behaves from the Schrödinger equation, however for a lot of circumstances we don’t want this. For a lot of circumstances we simply have to know that the Schrödinger equation is a type of machine that takes in a wave-function and spits out one other wave-function. And the wave-function is a tool from which you calculate possibilities. To maintain issues easy, I’ll immediately discuss in regards to the possibilities. This doesn’t at all times work, so please don’t assume quantum mechanics is absolutely simply possibilities, nevertheless it’s ok for our functions.

Right here is an instance. Suppose we’ve a laser. The place did we get it from? Effectively, perhaps it was on sale? Or we borrowed it from the quantum optics lab? Perhaps the laser fairy introduced it? Look, that is theoretical physics, let’s simply assume we’ve a laser, and never ask the place we acquired it, okay?

So, suppose we’ve a laser. The laser hits a beam splitter. A beam splitter, nicely, splits a beam. I informed you, this isn’t rocket science! Within the easiest case, the splitter splits the beam into half, however this doesn’t should be the case. May be a 3rd and two thirds or a tenth and 9 tenth, as long as the fractions add as much as 1. You get the thought. For now, let’s simply take the case with a half-half break up.

Up to now we’ve been speaking a few laser beam, however the beam is made up of many quanta of sunshine. The quanta of sunshine are the photons. What occurs with the person quanta once they hit the beam splitter? The quanta are every described by a wave-function. Did I simply hear you sigh?

The Schrödinger equation tells you one thing difficult occurs to this wave-function, however let’s neglect about this and simply take a look at the result. So we are saying, the beam splitter is a machine that does one thing to this wave-function. What does it do?

It’s not that the photon which comes out of the beam splitter goes a technique half of the time and the opposite manner the opposite half. As an alternative, right here it comes, the photon itself is break up into half, type of. We will describe this by saying the photon goes in with a wave-function going on this route. And out comes a wave-function that may be a sum of each paths.

I already informed you that the wave-function is a tool from which we calculate possibilities. Extra exactly we do that by taking absolutely the sq. of the weights within the wave-function. For the reason that possibilities are ½ for every risk, this implies the load for every path within the wave-function is one over sq. root two. If the beam splitter doesn’t break up the beam half-half however, say 1/3 and a couple of/3, then the weights are sq. root of 1/3 and sq. root of two/3 and so forth.

We are saying that this sum of wave-functions is a “superposition” of each paths. That’s the easy half. The tough half is the query whether or not the photon actually is on each paths. I’ll not talk about this right here, as a result of we simply talked about this some weeks in the past, so take a look at my earlier video about this.

That the photon is now in a superposition of each paths tells you the chance to measure the particle on both path. However after all if you happen to do measure the particle, you realize for certain the place it’s. So this implies if you happen to measure the photon, it’s now not in a superposition of each paths; the wave-function has “collapsed” on one of many paths like me after an extended hike.

So long as you don’t measure the wave-function this beam splitter additionally works backwards. In case you flip across the instructions of these two photons, for instance with mirrors, they’ll mix to a photon on one path. You possibly can perceive this by remembering that the photon is a wave, and waves can intervene constructively and destructively. In order that they intervene constructively on this output route, however destructively on the opposite. Once more, the good factor right here is that you simply don’t really have to know this. The beam splitter is only a machine that converts some wave-functions into others.

Let’s take a look at one thing slightly extra helpful. We’ll flip this round once more, put two mirrors right here and mix the 2 paths at one other beam splitter. What occurs? Effectively, this decrease beam splitter is strictly the turned-around model of the higher beam splitter, which is what we simply checked out. The superposition will recombine to 1 path, and the photon at all times goes into detector 2.

Effectively, really, I ought to add that is solely the case if the paths are precisely the identical size. As a result of if you happen to change the size of 1 path, that may shift the phase-relations, and so the interference might now not be precisely damaging in detector 1. This implies a tool like that is extraordinarily delicate to modifications within the lengths of the paths. It’s referred to as an interferometer. In case you change the orientation of these mirrors and transfer the second beam splitter close to the primary, then that is mainly how gravitational wave interferometers work. If a gravitational wave comes by means of, this modifications the relative lengths of the paths and that modifications the interference sample.

Okay, so we’ve an interferometer. It’s referred to as a Mach-Zehnder interferometer by the way in which. Now let’s make this slightly extra difficult, which is what I stated final time after I purchased the 5000 piece puzzle that’s been sitting on the shelf for five years, however fortunately we don’t want fairly as many items.

We add two extra beam splitters and one other mirror. After which we want a 3rd detector right here. And, be careful, right here’s an added complication. These two outer beam splitters break up a beam into fractions 1/3 and a couple of/3, and people two interior ones 1/2 every. Yeah, sorry about that, however in any other case it gained’t work.

What occurs if you happen to ship a photon into this setup? Effectively, this half that we simply added right here, that’s simply one other interferometer. So if one thing goes in up right here, it’ll come out down right here. So 2/3 likelihood the photon leads to detector 3. And a 1/3 likelihood it goes down right here, after which by means of the second beam splitter. And bear in mind this splitter splits 2 to 1, so it’s 2/9 in detector 2 and 1/9 in detector 1.

Now comes the enjoyable half. Suppose we’ve a pc, a extremely easy one. It may possibly solely reply questions with “sure” or no”. It’s a programmable gadget with some interior working that doesn’t have to concern us. It informed me we are able to name it James, and it really would favor that we don’t ask any additional questions. Solely factor we have to know is that upon getting programmed your pc, I imply James, you run it by inputting a single photon. If the reply is “sure” the photon goes proper by means of, completely undisturbed. If the reply is “no”, the photon simply doesn’t come out. Keith Bowden urged one might do that by making a maze for the photon, the place the format of the maze encodes this system, although I’m unsure how James feels about this.

So let’s assume you’ve got programmed the pc to as soon as and for all settle the query whether or not it’s okay to place pineapple on pizza. You then put your pc… right here. What occurs if you happen to flip in your photon supply this time? If the reply to the query is “sure, pineapple is okay” then nothing occurs on the pc, and it’s simply the identical as we simply talked about. The photon goes to detector 3 2/3 of the time, and within the different circumstances it splits up between detector 1 and a couple of.

However now suppose the reply is “no”. What occurs then? Effectively, one factor that may occur is that the photon goes into the pc and doesn’t come out. Nothing ever seems in any detector, and you realize the reply is “no, pineapples are dangerous, don’t put them on pizza”. That is the boring case and it occurs 1/3 of the time, however a minimum of you now know what to consider individuals who put pineapple on pizza.

Right here is the extra fascinating case. If the photon is within the interior interferometer however doesn’t go into the pc and will get caught there, then it goes the higher path. However then when it reaches the following beam splitter, it has nothing to recombine with. So then, it will get break up up once more right into a superposition. It both goes into detector 3, this occurs 1/6 of the time, or it goes down right here after which it recombines with the decrease path from the outer interferometer. This occurs in half of the circumstances, and if it occurs, then the photon at all times goes to detector 2, and by no means to detector 1. This solely comes out appropriately if the beam splitters have the precise ratios, which is why we want this.

Okay, so we see the maths is simply including up fractions, that is the easy half. However now let’s take into consideration what this implies. We now have seen that the one manner we are able to measure a photon in detector 1 is that if the result of the computation is “sure”. However we’ve additionally seen that if the reply is “sure” and the photon really goes by means of this interior half the place the pc is positioned, it can’t attain detector 1. So we all know that the reply is “sure” with out ever having to run the pc. The photon that goes to detector 1 appears to know what would have occurred, had it gone the opposite path. It is aware of its personal counterfactual actuality. In different phrases, if we had a quantum garden then it nonetheless wouldn’t be mowed, nevertheless it’d know what my husband does when he doesn’t mow the garden. I hope this is smart now.

And no, this video isn’t a joke, a minimum of not all of it. It’s really true, you may compute with a pc that doesn’t compute. It’s referred to as “counterfactual computation”. The concept was introduced up within the late Nineties by Richard Josza and Graeme Mitchison. The instance which we simply mentioned isn’t significantly environment friendly as a result of it occurs so not often that you simply get your reply with out operating the pc that you simply’re higher off guessing. However if you happen to make the setup extra difficult you may improve the chance for locating out what the pc did with out operating it.

That this certainly works was demonstrated in 2006 the place the pc carried out a easy search algorithm referred to as Grover’s algorithm. This doesn’t inform you whether or not pineapple on pizza is okay, however in case you have an unsorted database with totally different entries, this algorithm will inform you which entry is identical as your enter worth.

Now, let me be clear, it is a table-top experiment that doesn’t calculate something of use to anyone. I imply, not until you wish to depend the usage of publishing a paper about it. The database they used for this experiment had 4 entries by way of polarized photons. You may argue that you simply don’t want a whole laboratory to seek for one among the many beautiful variety of 4 entries, and I might agree. However this experiment has demonstrated that counterfactual computation certainly works.

The concept has led to quite a lot of follow-up works, which embody counterfactual quantum cryptography, and how you can use counterfactual computation to hurry up quantum computer systems and so forth. There may be quite a lot of controversy within the literature about what this all means, however no disagreement on the way it works or that it really works. And that just about tells you what the present standing of quantum mechanics is. We agree on the way it works. We simply don’t agree on what all of it means.