The nature of superposition. Mach-Zehnder interferometer

MIT 8.04 Quantum Physics I, Spring 2016
View the complete course: ocw.mit.edu/8-04S16
Instructor: Barton Zwiebach
License: Creative Commons BY-NC-SA
More information at ocw.mit.edu/terms
More courses at ocw.mit.edu

[@bigtimernow]

It's great how he presents the historical arguments about quantum mechanics and he gets so emphatic about an important point. Excellent!

[@wasabistreetfighter519]

I realized that the textbook I've been reading sounded familiar and it was written by THIS professor! Incredible!

[@minhanhtran4906]

Can i have the name of the book? You make me curious too

[@chanderbatti3849]

@@minhanhtran4906 Mastering Quantum Mechanics: Essentials, Theory, and Applications

[@ohidulislam5545]

@@minhanhtran4906 Mastering Quantum Mechanics: Essentials, Theory, and Applications

[@danopo]

@@minhanhtran4906 It might be Mastering Quantum Mechanics: Essentials, Theory, and Applications

[@TheChapiex]

Peru is proud to have produced such a good teacher

[@galaxy-wy9sd]

Be quiet

[@MikehMike01]

in an American school

[@magnumsxs]

From German minds.

[@TheChapiex]

@@MikehMike01 He studied in Peru 💀

[@MikehMike01]

@@TheChapiex and yet he’s at MIT in Massachusetts USA

[@afifakimih8823]

"That's how He fall you in love with Physics.." Excellent lecture as alwz!!

[@livelaughloveandmore]

what an amazing and effortless approach to the concept. loved it!

[@meet560]

Haha

[@BapiKAR]

Absolutely brilliant explanation. Wondering superposition being so easy a concept, lies the fact beneath it is not the case for QM. Must thank you Prof.

[@chaitanya.damu.01]

Brilliant! Loved how he made it so simple to understand the superposition of states

[@RalphDratman]

The reason quantum mechanics is so puzzling is that the experimental results themselves are intrinsically puzzling. Quantization is counter-intuitive. Turning everything into waves of probability works as a mathematical model of the physical phenomena, but waves of probability don't really make sense to us. We have perfectly good mathematical models that predict results to stunning accuracy, but we just can't fully accept what the math is telling us because the way it works seems fundamentally un-physical to our limited minds.

[@arjuncalidas2736]

That is true.. Maybe it seems counter-intuitive because of our perception of the physical reality.. Perhaps the un-physicality, if I may say so, is because we cannot perceive an electron.. However, an ammeter will give you the rate of flow of electrons pretty accurately.. The good mathematical models have been designed to achieve just that.. Amazing how the human mind finds a way to uncover truths..

[@dcamron46]

@@arjuncalidas2736 but I think Ralph's point goes beyond the simple inability to 'see' or perceive an electron, it is the nature of the electron (being described mathemetically as a particle and a wave in different cases) which is difficult to comprehend. I simply can't imagine or visualize much of the concepts of quantum physics, which are often relegated to rote memory or mathematical reasoning alone. Even Einstein referred to 'spooky action at a distance' . I agree that it is impressive what science has revealed, it is so far beyond our everyday experience, we truly do stand on the shoulders of our predecessors.

[@arjuncalidas2736]

@@dcamron46 This could very well be true if we haven't developed methods to experimentally verify treat concepts. But we have. The dual nature of particles are not only defined mathematically. We can very well project these mathematical models to yield results in physics and chemistry. This is why it is very important to not do science, especially quantum mechanics, based only on our physical perception. If we can theorise physical behaviour of a particle, we should be able to verify the said behaviour experimentally. And for the most part, we are able to.

[@danielackles4265]

Fascinating video thanks Barton Zwiebach!

[@alexandrugheorghe5610]

It depends. If you entangle the particles and get them through a detector at different angles, you'll observe that only specific angles will give you "right" proportions for detection (fractions) when you measure spin.

[@vinycole7800]

Great lecture that inspired and motivate teachers and students. Thanks MIT

[@zack_120]

6:51- mind blowing 😱 but so fascinating 😇

[@wojciechdojlido8235]

God bless you for this course! Thanks from Poland!

[@swloke1]

Brilliant lecture!

[@p.s.design4338]

really clear explanation!

[@physics_conceptmantra]

What a lecture sir. Just awesome.

[@surendrakverma555]

Excellent lecture Sir Thanks 🙏🙏🙏🙏🙏🙏🙏

[@awazii__9084]

Very well explained!

[@chinmaykrishna6485]

Hey MIT OCW and Prof. Barton Zweibach, your lectures are incredible. BTW I do not know why I laughed so hard at some photons can go both and down.

[@pinklady7184]

Thank you for mentioning his name. Now, I am going to check him out.

[@wanda5983]

Great camerawork

[@himanshu9145]

Really good course!

[@carly09et]

The problem here is the "existence" of the photon between the source and the detector. The source can infer the photons existence, the detector the transform of the photon. The path/paths are assumed.

[@eightysevenf]

Quantum mechanics is just insane lol. I’m so happy I live in a time that has found this nonsensical state of nature.

[@stephenanastasi748]

At 14:30 - wow. Each photon must interfere with itself... which makes sense given the set-up and the speed of light (different quanta could not interfere with each other as they arrive at different times). But this says something very deep about the underlying construction of the universe, and things like three polarising filters allowing light through, where two polarising filters do not, seems to be the same thing. But this implies directly that the holographic principle is correct - that the universe of our experience is actually a secondary phenomenon riding on the back of something more fundamental, and that Schrodinger's equation just catches that main idea.

[@alexandrugheorghe5610]

The photons do not experience time nor space. That's us that observe the phenomenon in that way.

[@scienceandstudy6431]

Excellent lecture ! But I m not able to understand if a photon is indivisible how it can go in two directional simultaneously as sir said just nearly 6:30

[@alexandrugheorghe5610]

They must be waves, right? Travelling in all directions at the same time.

[@speedracer6294]

It's like on the subatomic level reality is being constantly created. Every photon, electron,etc. is potential until it settles into definable state.

[@johnbruhling8018]

These lectures really are wonderful and I can't help but think about harmonics and waves when the Professor is erasing things. No, seriously. The audio capture and reproduction technologies nowadays are really impressive, you can really feel it! Squeaksqueaksqueaksqueak! 😬 It must be that dreaded 7th harmonic (maybe)

[@myna2mac]

About superposition : as I understood, a photon can be either in A or B state because thats the only way to explain results of experiments. How reliable these experiments were (blasphemy!), but curious to know what was done to validate experiments and design them. Also, are there alternate explanations.

[@AmitKumar-lp3fl]

Thank you sir Its good stuff

[@sayantaniroy1520]

how Mach Zehnder Interferometer gate represent universal gate library? plz help me

[@georgepp98]

Hi! How can a photon interfere with itself? Isn't also the energy conservation law being violated? Because we have something like: photon -> 0 (for destructive) and photon -> 2*photon (for constructive). In these equations photons are monochromatic.

[@AlexBlade27]

After watching this lecture, I got 2 questions in my mind. 1. Why we use States as a vector in Dirac notation? 2. How would Two photons interact with each other, because Photons are the interaction particle for electromagnetic waves, so how would they interact with themselves?

[@dons.9458]

I was lost after the professor said superposition.

[@roger_isaksson]

Isn’t it the experimental apparatus that produce the interference? From the photon frame of (time) reference the elapsed time of flight is instantaneous. Instantaneously looking down the optical axis of the photon emitter, the apparatus would form a probability surface of where the photon can exchange its energy.

[@Google_Censored_Commenter]

neat, can't wait for that to be proven. It doesn't matter if it's instant or not in this case. Whether it gains the information of the bomb's existence during travel or not. That it can gain the information without being detected while we know with certainty the detector is working, for if it did not it would have behaved as if it did not collect the information to begin with.

[@rezokobaidze8501]

thanks a lot

[@user-cl6rm4ie9x]

how phase shift happens in a beam splitter which is an half silvered mirror

[@blacklotusgym]

As someone who only did one year of college. This makes me want to go back.

[@zacharythatcher3267]

I think there is some information being lost in his description of what is interesting about the interferometer. When he says that it can be arranged such that one detector or the other sees all of the intensity it is easy to imagine a scenario where each beam splitter passes either everything or nothing. All of the outcomes to that setup (where each splitter passes all or nothing) are that all of the intensity lands on one detector. I think that what is unique to quantum mechanics is that you can separate the beams with both beam splitter and still get all of the light intensity on one detector or the other.

[@scienceandstudy6431]

But how's that possible u said at the end

[@dcamron46]

Quick question: how does the double slit experiment work with photons if supposedly they don't interact with one another? Just trying to understand the nature of the photon first before complicating it with mirrors and all the rest...

[@hectormartinpenapollastri8431]

They interfere with themselves

[@akshatsalhotra118]

Is quantum mechanics useful if we wanna go in aerospace propulsion research???? Please someone tell....I wanna make my career in it and I don't know whether to study it or not...

[@user-ij6fj3bk5e]

Inspiring....

[@mubbasilkhanpathan3665]

Its very hard to imagine a single photon in two different beams but they exist

[@9assahrasoum3asahboou87]

fathi medos fes said thank you so much

[@luisfelipegonzales980]

Can photons be sent one by one? If it were that way, the size of a photon could be determined by moving the array of mirrors and when no signal is found it is because the photon has not yet arrived.

[@kaushaljain5999]

2:01 Explain_ It may be possible that light entering in second beam splitter from both ways goes in different directions so will not interfere.

[@davidwilkie9551]

Words don't make sense. Senses.., interpret the interactive shaped occurrence of probability, for which we have (inadequate-incomplete) words, but this a good Methodology of illustration/teaching.

[@martindorrance8133]

Does this mean that if a quantum computer is to work many repeated ‘calculations’ must be done? If so, how is this any different to an analogue computer?

[@cleo7363]

we can use less storage to hold more data at an exponential scale in a QC

[@jorfoas]

It cannot be that each photon interacts with itself only. If that were the case, then slight differences in length of the two paths would prevent the photon to coincide in space with itself and no interference of any kind would be observed.

[@Andratos95]

Isn't that kind of what happens with interferometers though? If the two paths aren't the same length, you don't get interference

[@marialiyubman]

Wait, how would you differentiate superposition of a particle from the same particle being in two different places with such a tiny time difference that it would be undetectable to the human eye (or even available machinery)?

[@9_budimanapriutomo238]

well we should know that "superposition is work to single particle in 2 different place". of course we know the different from them

[@alexandrugheorghe5610]

You'd observe a wave as a result in your last detector and not a point particle pattern. Which means the photons are neither quanta, neither waves, but both or none at the same time .🤷🏻‍♂️ I know... It's.mind boggling. Richard Feynman said well when he stated: "If you think you understood Quantum Mechanics then you understood nothing of it".

[@manudehanoi]

I don't see what prevents superposition in QM to be explained with classical superposition of waves. (and by the way, cube beamsplitters are positionned so that the incoming beam is 90 deg to the cube surface)

[@benmustermann2045]

conservation of energy

[@manudehanoi]

how so ? both QM and classical waves haev conservation of energy

[@jyun3102]

watch the whole video lol

[@brightjoseph4480]

Svp puis je avoir cette vidéo en version française ?

[@Sa3vis]

How many times can the interferometer split the beams of light? Are there limits on superposition?

[@harshrajveermaran5792]

That's a nice thinking, but honestly it depends how much can you direct the beam of light to a particular direction as far it goes it's intensity will become less. And to your query how many times we actually can do it probably infinty just keep the energy conserved cause we thought of it as in a single photon travels through both the path, so perhaps we can even say it can also travel through 3 path or so on but honestly I don't know it. Well it's been 3 years I don't know how much knowledgeable you might be now.

[@johncgibson4720]

4:35 is the nail in the coffin for classical wave theory. "A bunch of energy canceling each out into nothing" is impossible by laws of conservation of energy that classical theory champions. Just shows how broken classical theory was.

[@MrBeen992]

Why he didnt use the double slit experiment to show the nature of superposition ? Does the Mach Zehnder interferometer shows it more explicitly. I didnt understand that.

[@alexandrugheorghe5610]

He wanted to explicitly point out the nature of superposition in QM when you've a split beam of photons. Since they are elementary particles and since you can easily build the story regarding the amplitude and frequency it's also easy to explain what it means to think of the beam as a quanta or a wave. The drawing also made it easy to understand. All of this to make it clear (how strange QM really is!): with two separate beams, the photons will interfere with themselves and not each other!

[@MrBeen992]

@Sam why are you stupid ?

[@MrBeen992]

@@alexandrugheorghe5610 the double slit experiment can be done without photons interfering wth each other too by decreasing the intensity of the beam

[@bernardmcgarvey4169]

Anyone know what year the students are that are taking this course?

[@mitocw]

This course was taught in 2016. The students are undergraduates who are probably in their second to third year of schooling.

[@bernardmcgarvey4169]

MIT OpenCourseWare just out of curiosity, when would a physics student at MIT be introduced to say the Dirac equation and quantum field theory?

[@adamfattal468]

Bernard McGarvey Depends. Usually in grad school but some unis are integrating QFT into the undergrad programs.

[@jackmaxwell3134]

So the measurement itself is NOT linear? measuring |A> or |B> is different of measuring alpha*|A> + beta*|B>

[@luisfabricio6439]

@5:04 it would not break conservation of energy

[@juliakim1208]

Is it true to say that |A> and |B> orthogonal states?

[@johnjobs3027]

Precisely. Each posible outcomeof the experiment corresponds to a unique basis vector, in your case |A> and |B> . They are not only orthogonal but also unit vectors which makes ortholinear.

[@hildebrandogarrosoto8433]

para cuanto una clase en español prof

[@praveenkumardhankar2716]

How much is the right answer with an experimental demonstration worth?

[@marialiyubman]

Superposition: when the sound of that eraser is EVERYWHERE

[@mohammadkaviany1159]

really annoying... I hate it ! :/

[@MM-1820]

The struggle of professors and teachers;They have to repeat the same thing over and over again for at least 30 years,and that comes with many challenges and risk of losing their jobs because their livelihood depends on it.Basically,every jobs are like this,you must repeat the same texts again and again,and they thought this is a career when its only a job,if it were a career they would go on progressing but its only a job so it gets boring as every job is.But quantum physics is different.

[@youchuanwang1738]

Since you brought up linearity in the beginning and claimed that the SE is linear. These wave functions are solutions to the equation, then how come you can have such superposition

[@jamesfullwood7788]

If you have two solutions of the SE, a superposition of them will solve the equation as well, as a consequence of the SE being linear.

[@georgiapsyrri716]

@@jamesfullwood7788 I am still having trouble understanding how we can have an either A or B solution and still call it a linear equation.

[@onderozenc4470]

It doesn't seem plausible to carry out the experiment with one single photon. I don't think there is difference between the quantum and EM superposition.

[@Airbag1010674]

They have something against white boards?

[@brainstormingsharing1309]

👍👍👍👍👍👍👍👍

[@spavithran997]

Sir 1.how energy is conserved when a photon superimposes with itself? 2. Why in QM only a or b and why not a+b ..because QM is probabilistic in nature? Please reply as soon as possible.

[@gabrieletorrini4913]

I am here.

[@mikiallen7733]

Do these two probabilities always sum to 1 or 0 !

[@MrMoro25]

0! = 1 so...

[@ennamnazir2151]

Love From Kashmir.

[@meri3815]

this is crazy

[@charankol]

if a photon is travelling in both beams, how does this not violate conservation of energy?

[@RanEncounter]

No because the energy of the photons going both ways combined is equal to the starting photon.

[@user-pt-au-hg]

how about if each is half the intensity, or energy, that is to say, both take half the energy along their individual paths and than recombine at the detector.

[@oneinabillion654]

Maybe it's can just multiply and whether it is single or in groups the energy is always constant? hahahaha. The lecturer did say it was weird.

[@maraudeuse952]

There's something I'm not getting here. I've heard different word choice, namely: "a photon is travellig in both beams with probability XXX for each beam", and it made sense. But this? How can a photon *just* be in two different beams, without further specification?

[@appaul7095]

​@@maraudeuse952 Not until you measure the photon directly, it is always in a superposition state. Therefore, a photon technically "looks like" it is traveling in two different beams since you are seeing the interference result.

[@leonhardeuler9839]

8:51

[@antonioluperini5684]

No one got it

[@antonioluperini5684]

In a physics class no one has the sense of humour

[@viksusername]

How exactly do you "send" only one photon?

[@adrianwright8685]

You simply reduce the beam intensity. The photons can be individually counted using a photo-electric detector and you can ensure that say one photon about every 10 seconds is passing through.

[@julianmunoz7836]

Guys top universities still use Blackboard and why?

[@spyrex3988]

8:51 he wrote Ass

[@9_budimanapriutomo238]

hahaha

[@donniedollarhidejr.2766]

MeASSure

[@hseedagaj886]

💌

[@gordhanjat4501]

you did a good job A lot of thanks from HINDUSTAN(INDIA)

[@iloveamerica1966]

6:20 Wait, if a photon is the smallest energy for light, and you're telling me you can split it, then it's not possible that a photon is the smallest Quantum of light, the half Photon must be. What are the energies of the to split halves? They can't both be one quanta of energy. Sounds to me like some physicists are not doing their proper due diligence.

[@user-ng2rw6cm8o]

Democrats killed America and I stood by & watched After that, he says the to reflected or transmitted photons are the same . Another words, when it hits the BS, it creates another copy and that’s why called Super position!

[@BiscuitZombies]

Similar to what the other guy said. When it hits the beam, it isn't literally splitting. It creates a copy of itself in either direction, and these represent possibilities of where it went. We represent the superposition as some probability of it reflecting plus some probability of it being transmitted. The notion of the superposition is one of probability.

[@alexandrugheorghe5610]

It doesn't create a copy of itself. 🤦🏻‍♂️ It means you'd have twice the energy and you'd violate law of conservation of energy/momentum (a.big no no). What it means is that the photons must be existing in both places at the same time. It means: they are a wave.

[@jacobvandijk6525]

TONOMURA-experiment: kzbin.info/www/bejne/oKeyYYNrYriArs0

[@adrianwright8685]

0:54. "Zehnder"

[@ugurtamer944]

ugur was here

[@denniscowdrick1255]

Arguments sure look deeply flawed to me. If a photon has a frequency (E=hf) and yet has a limited spatial extent in order to be particle like then the frequency becomes undefined! Frequency is events per time interval. To be a ‘particle’ the photon would be way to short to allow vibrations of its electric field so that it could be said to HAVE a frequency.

[@alexandrugheorghe5610]

Photons don't experience time nor space. That's just us. 🙂

[@antonioluperini5684]

0:18 electric fields in 7 seconds

[@Genshin_suit]

superposition of seaking electric fields

[@kazimierzbaran5586]

OMG, I know it's like the least important thing here, but the way he writes dots and dashes and how he (like in 1:02) can even mistake a dot and a dash is just so funny (good funny - don't you judge me, I so love these kind of things!)

[@harshrajveermaran5792]

Tbh when you teach something really you don't care about how much accurately you write, what's in your mind is you tell each and every thing to the other person in all the possible ways so that he can understand. After gaining a lot knowledge probably someday you might realise nothing will happen keeping all that in me, I'll die one day and so these ideas, understandings, clarity everthing and that's the moment where you might rush to teach everything to every possible mind that can understand it. Even I don't care what I am writing like the calculation in my head is going right suppose it's some answer in root and i just don't write root. Even when I explain any theory in my exam paper I something while writing in English switch to my mother tongue and I'm like what the hell I just wrote then I again start it.

[@juanmanuelpedrosa53]

PUT THAT COOKIE DOWN!

[@eganeshseshadri6441]

Great lecture, but unfortunately a lousy student. I think I'm condemned to only understand physics as it was till about the time of Rutherford and Maxwell.

[@conscienceaginBlackadder]

This is not about the nature of superposition, at all. It's about the nature of maths on measurements of superposition. No description of what superposition physically is. All the way from the school definition of momentum as "product of mass × velocity", as if that piece of abstract maths is what it physically is, this is always the con in pure physics. It's never actually a physical description of anything's physical nature.

[@-BuddyGuy]

The problem is that your perception of what is real is less objective than mathematics and your brain is not very scientific in its perceptions of what's around it. You look at a thing whacking another thing but this is not a detailed view of what's happening at a microscopic or atomic or subatomic level within the thing. You need maths for it and the concepts will all be abstract. To pursue understanding behind that you need mathematics and if you try to connect it to reality you have to do so through experimentation. This is why physicists can send things to the moon and you can't

[@conscienceaginBlackadder]

@@-BuddyGuy Xcyooz me: none of them acting alone can send things to the moon, only a team can. Sending things to the moon is applied physics, while my point was on pure physics. Whether it describes physical reality to the exclusion of other approaches to describing it. In Newton era physics, maths representing physical reality starts from a physical reasoning that what is being represented can be. In the century since quantum theory began, they have changed to willingness to believe surreal physical illogicalities if the maths leads to them. e.g. they believe in singularities in black holes, because the maths does not discover any force to halt the compression. But there is no way to express in the maths this bit of physical reason. Gravity is physically exerted through space by a mass, hence only be exerted by masses that still have any physical existence, onto what is spatially around them. Mass within a zero-sized point exerting it on other mass within the same point, across no space and with no space to exert it through, is nonsensical. So the compression must stop at a finite maximum level of compression of matter for matter still to exist with spatial extension hence to exert gravity. When you use an x^2 equation, that has 2 solutions, on a real situation, you don't say that both solutions somehow surreally must be true. You use physical reasoning to discard one solution, because it either is negative or is the wrong order of magnitude. The discarded solution was an artifice of the maths + it represented nothing real.

[@-BuddyGuy]

@@conscienceaginBlackadder No physicist says for sure what's in there, but you do

[@hadlevick]

It's the only possibility...:Fluid theory (Reproduction/Feed/Reasoning) decanted selfover hexagon...

[@lakshyabansal8615]

When my mom walked in I changed the screen to porn, cuz it was easier to explain

[@vamc294]

Idheto pothandhi rooooo

[@vallikencorvuskane4556]

Every reflection in some way is refraction! Everything passes through airborne particulates, frequencies, etc. therefore altered from its origination, no true rendering. The returned image, whether it be the eye or heat, light, etc. is casted, meaning the defused in this case, using your splitter, the primary chamber creates chaos and loses the fullness of the beam, this is refraction if you will, this furor is only partially retrieved and sent to the secondary! The primary continues to collect what was lost in the initial reflection "actually refraction*... This increases the strength of the primary output! Focusing to a desired point with disproportionate energy offsets your calculations, therefore your rendering is inconclusive. You yourself said you did not fully understand in your video! The manipulation of digits to statistics are no more than self proclaimed, not unlike the perimeters of an aquarium. Is this not a form of matrix?! Human "thrives" due to the ability to reverse engineer what is observed. I did not say "flourish"... The engineering is no more than claiming the understanding. One cannot re-create what is not understood this is why your practices simply unsustainable. You will never understand creation nor creator. Just because human can recreate life, you have overlooked the variables and your outcome is tribulation... We are electromagnetic ephemeral Tellurians, nothing more than synthesis to Genesis. The manipulation to fit the rendering is a farce. This is a matrix within a matrix within a matrix so on and so forth. It is time to step back, cogitate, ruminate on your direction. For your direction is our direction and forced fed! Many of us live with bird in the hand, proof and direction too... You take us away from reality in a already distorted world. What I have stated here is provable by every human being that is not disinclined, but your offer is limited to your thesis, your calculated outcome, "calculated" in every sense of the word is misconception due to leaving out the variables. This is why more than not, a creation of something is more harmful than anticipated. For example try to contain every heat wave that comes from the surface, roads, rocks, etc. you cannot contain all, meaning you have not accounted for all of the variables and the outcome, the rendering is false!!! Love to ALL

[@3karus]

And that's where statistics turn out to be very useful.

[@lsbrother]

hint - don't click 'read more'

[@kokeiro89]

Dude, you cannot replace Science with handwaving nonsense. Stop this KZbin trend, plenty of people like you thinking you understand something just because you can spew a chain of words like "chaos", "cogitate", and what the heck are Tellurians?? Admit your ignorance and study some science or shut up for good. You guys are embarrassing

[@tomgaylor]

Someone skipped their meds

[@bingbingsun6304]

中国人在看 好有吗？

[@ubcphysicsyangbo]

Wow so it’s been a decade+ since I’ve left the department of physics and it shows - now when I see the acronyms BS I just think of the word bullshit 😂l

[@cafe-tomate]

If some photons may "go both up and down" then why isn't it in the polarizer that some photons get both through and not through ?