[HN Gopher] Sustained, high-fidelity quantum teleportation
___________________________________________________________________
Sustained, high-fidelity quantum teleportation
Author : spurgu
Score : 163 points
Date : 2020-12-18 13:29 UTC (9 hours ago)
(HTM) web link (news.fnal.gov)
(TXT) w3m dump (news.fnal.gov)
| up6w6 wrote:
| Quantum teleportation cant transport data faster than light, but
| something very similar which looks more useful in terms of daily
| needs is superdense coding [0]. Basically you could encode two
| bits as one qubit, which looks like some type of "physical
| compression" and the algorithm is almost the same as
| teleportation.
|
| [0]: https://en.wikipedia.org/wiki/Superdense_coding
| ajuc wrote:
| Isn't that the same as just using 4 voltages (or any other
| analog property) instead of 2 to encode 2 bits instead of 1?
|
| Qbits are analog so it's no surprise it can be done with them
| as well.
|
| Flash memory already does that in classical chips. There's
| nothing preventing you from going further - 8 voltages for 3
| bits etc.
| tsimionescu wrote:
| The problem with analog encodings is that it often doesn't
| scale at all - the energy required to measure your analog
| state precisely enough to send large numbers of bits becomes
| too impractical.
| waterhouse wrote:
| And classical bits are presumably much easier to transport
| intact, by several orders of magnitude in terms of cost.
| Seems the superdense coding would only help if sending qubits
| were less than 2x as expensive as sending classical bits.
| nynx wrote:
| I remember reading a paper somewhere (can't find it now) on how
| it should be possible to compress N bits of data into log2(N)
| qubits. This would be utterly transformative since you could
| then compress basically any amount of data into just a few
| qubits (e.g. 1e15 bits would fit in ~50 qubits).
| whatshisface wrote:
| You're not going to get that data back out, though. When you
| measure 50 qbits, you get 50 bits, and you destroy the state
| irrecoverably.
| tsimionescu wrote:
| Per the wikipedia page, the protocol provides both encoding
| and decoding, and there have been experimental realizations
| that successfully retrieved the original message.
|
| Is the article inaccurate?
| whatshisface wrote:
| The parent comment was talking about data compression,
| the article is talking about copying a quantum state.
| Data compression will not work because you can't read a
| quantum state into classical information.
| nynx wrote:
| Yeah, I think whether extraction was possible was up in the
| air. I believe it mentioned that quantum RAM could let you
| store that data and operate upon it.
| [deleted]
| [deleted]
| Rooster61 wrote:
| So, something that always bothered me, and my admittedly ignorant
| understanding of QM as a non-physicist. I understand that the
| quantum state of a particle can be teleported, and that classical
| information must be passed to observe the particle and collapse
| the state correctly.
|
| My question is, if we can't observe the particle, how do we know
| that that particle is entangled with the original one?
| tsimionescu wrote:
| We take a process that produces entangled pairs, and send the 2
| particles to different places. As long as we don't measure
| their state (whatever that means - TBD :) ), they remain
| entangled.
| Rooster61 wrote:
| But that begs the question of how we know that we have
| produced an entangled pair. Do we not need a metric to use to
| make sure they are entangled?
| jaggirs wrote:
| Your question boils down to whether or not the quantum
| circuit that entangles the qubits works correctly. You can
| simply test the reliability of the quantum circuit by
| running it multiple times and checking that the outcomes
| behave as you expect. For example, if your circuit is
| supposed to generate the entagled pair [00 or 11 with equal
| probability] you would expect measurements to either be 00
| or 11 and never 01 or 10.
| stelfer wrote:
| The act of measurement on the second particle destroys the
| entagled state of the pair. That effect can be measured on
| the first particle's side (which may be far away... on the
| other side of the galaxy maybe).
| jiofih wrote:
| But to see/interpret the effect on the first particle,
| you need to know what the measurement on the second was,
| and that information needs to be transferred across the
| galaxy by conventional means.
|
| Honesty quantum mechanics sounds more like a bug in the
| universe or some quirk we just don't understand yet.
| Technically wrote:
| The particle can observed, but this collapses the entanglement.
| If the key derived from entangled particles is bad, and
| communication does not work with the observed key, you know
| with high probability the particle collapsed pre-entanglement,
| and and you resync to agree on a new key. There is no way to
| tell without comparing (classical) communication whether or not
| your observation collapsed the key.
|
| Most of the engineering effort is preserving the entanglement
| for farther travel from the source of entanglement.
|
| Note: I'm using "key" here because it's presumed the shared
| state would refer to an encryption key, but you could use it
| for other types of communication (eg agreeing on a novel
| primary key for some common database).
| peter_d_sherman wrote:
| You know, something that I think would be interesting in the
| future would be a detailed comparison between the simplest of
| radio waves -- and quantum teleportation.
|
| On the one hand, we have the simplest of radio waves -- which
| moves a little bit of information (depending on how it is
| encoded, how the radio wave is modulated) over space, and on the
| other, we have quantum teleportation, which also moves
| information over space.
|
| On the one hand, the simple radio wave dissipates according to
| the square of the distance -- the "inverse square law"
| (https://en.wikipedia.org/wiki/Inverse-square_law), whereas
| presumably (I do not know), the quantum teleportation does not.
|
| Which leads to a side question -- is there a physical limit to
| the distance that information can be transported via quantum
| teleportation?
|
| If not, then there's yet another example of light speed
| violation, and if so, then maybe the quantum teleportation isn't
| really quantum teleportation -- but rather some form of
| radio/wavelike communication at a distance, albeit, one physics
| has yet to describe...
|
| My point is simply this -- when you have a new fangled, not well
| understood phenomena (in this case Quantum Teleportation), and
| you have an old well understood phenomena (for example, radio
| waves -- but it could be any wave or member of the EM spectrum
| that conveys information over distance, for example, light),
| well, when you have two apparently disparate phenomena like that
| in Physics, you want to understand _WHAT AND HOW ARE THEY
| UNIFIED_ -- rather than the endless set of attributes that are
| different between them...
|
| You see, an advanced Physics would be able to
| express/quantify/explain both radio waves on the low-end of
| understanding, and _every single possible form_ of quantum
| teleportation on the high end... _IF IT WERE CORRECT_...
|
| Observation: What science at this point in time in earth's
| history needs to do is the following:
|
| 1) Construct FTL waves out of Slower-Than-Light waves...
|
| 2) Construct Wormholes (Wormhole = Black Hole = Quantum
| Telportation = Portal, use whatever language you wish!) out of
| the FTL waves.
|
| Of course, we'll leave #2 for the future.
|
| Goal #1 should be to construct a single, solitary, FTL wave (and
| be able to detect it over a distance, otherwise what's the
| point?) from Slower-Than-Light waves.
|
| By default, if you're moving Faster-Than-Light, then the
| path/space you are moving in _has similar characteristics_ to #2,
| which, over time, should yield a greater understanding of that
| phenomenon... (is the space compressed, or the wave expanded, or
| both?).
|
| In fact, maybe it would make sense to go back to Zeno's paradox
| of Achilles and the Tortoise:
| https://en.wikipedia.org/wiki/Zeno%27s_paradoxes#Achilles_an...
|
| That is, if you think about each, each EM wave that exists in
| space -- must first cross half of that space, but before that, it
| must cross half of that space, etc., ad infinitum.
|
| So, what happens if an EM wave _is made of nothing but space_ ,
| but _space that is twisting, space that is vibrating_?
|
| _If_ (and it 's a big _IF_!) that 's how EM waves work -- then
| you already have part of all of the above -- _BECAUSE AT THE
| SMALLEST SCALE IN AN EM WAVE -- SPACE ITSELF IS BEING COMPRESSED
| AND EXPANDED_...
|
| You see, conventional Physics uses words like "Electric Field"
| and "Magnetic Field" -- rather than such words as "Twist Vector
| Of Space", "Compression Vector Of Space", "Expansion Vector Of
| Space", and "Oscillation/Vibration Vector Of Space" (and of
| course, those are relative to size/wavelength/frequency, etc....
| "relative to scale", as I like to say...)
|
| But, I think todays Physics -- would get so much more out of
| itself -- if we stopped using words like "Electricity" and
| "Magnetism" -- and instead replaced those with "Something is
| happening to SPACE here!" (where the something was a more
| accurate description of what was actually going on!)
|
| But anyway, we need to know/understand/grok the root of all
| phenomena -- in the simplest of terms -- otherwise Physics will
| keep inventing a plethora of words and phrases to describe what
| can be in essence, described by a few simple fundamental
| understandings in the tersest of ways...
|
| Anyway, feel free to call me a crackpot <g>... I don't claim to
| be right -- I only claim that researchers may wish to investigate
| these subjects further!
|
| Phrased another way "Here be dragons!" -- in programmer parlance!
| <g>
| nabla9 wrote:
| context: Using term teleportation makes sense in the quantum
| realm.
|
| No-cloning theorem in quantum physics says that it's impossible
| to do exact copy of unknown quantum state. But it turns out that
| you can teleport it when the original state is destroyed.
|
| You can't have exact `cp` command for the quantum state, only
| `mv`. If this makes you think linear logic, you are right.
| Yajirobe wrote:
| what is non-linear logic?
| whatshisface wrote:
| Non-linear logic would be regular old logic. Linear logic is
| logic where every operation permanently consumes its input.
| mrfusion wrote:
| So that could neatly solve the philosophical teleportation
| problem?
|
| Ie if you teleport and the original isn't destroyed. Which one
| are you?
| ahelwer wrote:
| This problem is only a problem if you assume you exist in
| some persistent sense, rather than being a continuously-
| arising emergent phenomenon of your physiology.
|
| As for whether the no-cloning theorem informs this, there
| isn't any reason to believe consciousness is related to
| quantum mechanics.
| Avtomatk wrote:
| But if they destroy you and recreate you, are you still alive
| or die in the process?
| c22 wrote:
| If you're entangled before you're destroyed have you really
| been destroyed?
| phyalow wrote:
| https://en.wikipedia.org/wiki/Ship_of_Theseus
| jiofih wrote:
| Not the same as entanglement means you can't have both
| parts at the same time, you just need to decide which one
| you're gonna "use" while the other is destroyed.
| mrfusion wrote:
| Not really any different than going under general
| anesthesia.
| pvarangot wrote:
| Or sleeping.
| spurgu wrote:
| Or a rough Friday night out.
| shredprez wrote:
| For the recreated you, sure.
| sabellito wrote:
| I'm confused, as I'm an enthusiast but definitely no physicist.
|
| The article states that quantum information is being teleported
| via entanglement. However, it was my understanding that one
| cannot transfer information in such a way as the "information" is
| only revealed when interacting with the particle.
|
| Could someone perhaps clarify what's going on?
| [deleted]
| fastball wrote:
| We had digital data storage before we had the internet. Back
| then, if you wanted to share digital information with someone
| else, you had to save that information to a hard disk (or
| similar) and physically transport it to their location.
|
| Then we invented the internet and now you can transmit digital
| information between digital computers over wires / EM signals.
|
| Quantum teleportation / entanglement does the same but for
| quantum state rather than just digital state, which allows
| quantum computers to communicate with each other. Without this
| technology, quantum computers would not be able to communicate
| with each other over a network. So you still need to use wires
| / EM waves in order to transmit the data, but the data you are
| transmitting is quantum.
|
| Technically you're not actually sending qubits over the wire
| though, you're sending 2 classical bits per quibit which are
| able to tell the other system what each qubit in the target
| system is supposed to look like, but effectively you can
| pretend you're sending qubits over the wire.
| jiofih wrote:
| If sending a pair of bits over the wire accurately describes
| this, then what is the achievement of transmission over 44km?
| Wouldn't it simply be limited to our existing fiber coverage?
| Feels like something is missing here
| pontus wrote:
| You entangle two systems but in order to actually complete the
| teleportation you need to measure one system and then convey
| the outcome of that measurement to the other party. This
| information is needed by the second party in order for them to
| be able to correctly collapse the state of their system into
| one that is identical to the original system being teleported.
| The information that the first party must convey to the
| receiving party must be sent in a classical way (e.g. a phone
| call).
| raspasov wrote:
| This is a very high quality discussion on QM and explanations
| from pontus, keep it up!
| [deleted]
| junon wrote:
| This makes zero sense. If the information must be conveyed
| classically anyway, what's the point?
| marcosdumay wrote:
| The point is to test physics models.
| abdullahkhalids wrote:
| Physical qubits are much more sensitive to transmission
| noise than physical bits. If you want to transmit qubits,
| teleportation allows you to do it with higher fidelity than
| by physical transmission.
|
| Why do you want to transmit qubits? For various emerging
| quantum information technologies, each with different
| potential economic impact.
| spurgu wrote:
| This simple explanation clicked for me, thank you!
| pueblito wrote:
| Does this mean quantum gizmos will 'teleport' the data
| over space instead of using wires? Like, will a quantum
| processor pull the data in the quantum ram using
| teleportation?
| colechristensen wrote:
| It means quantum "networking" will be transmitted over
| classical communications channels.
| abdullahkhalids wrote:
| Well, not that short range! We are talking about where
| you have to transmit the qubits at least a few dozen
| meters.
|
| So perhaps distributed quantum computers spread across a
| university, city or country.
|
| Also, remember, you still need wired or wireless
| transmission of bits to do the teleportation protocol.
| 8note wrote:
| Supposing we get enough qubits to support teleporting you,
| we can regularly entangle qubits and ship them around the
| world, then when you want to travel, we can teleport you,
| and the trip time will be how long it takes to send the
| message. This cuts your trip time down from hours long
| flights to seconds long trips
| pontus wrote:
| Great question! This gets to the heart of why quantum
| teleportation has any value at all.
|
| So, before QM there was already a sense in which you could
| teleport an object: simply measure its state perfectly and
| send that information to another location and have them
| reconstruct that state particle by particle. In principle
| the new system would be indistinguishable from the original
| system and you could claim that you've teleported it. Now,
| with the discovery of quantum mechanics, this process no
| longer works because there is no way to measure the
| complete state of a quantum system. For example, you could
| measure the position of each particle to arbitrary accuracy
| or you can measure the speed of every particle to arbitrary
| position, but you can't do both (Heisenberg's uncertainty
| principle). So, it would seem like one could not construct
| a perfect replica of a quantum system in a new location by
| measuring its state in the original position.
|
| The cleverness of quantum teleportation is that you use
| entanglement to sort of short circuit this limitation. You
| let entanglement do the heavy lifting to sort of "copy" the
| state from one location to another and then perform a
| measurement in the original location to uncover just enough
| information so that the person in the second location can
| manipulate its system to reconstruct the original state.
|
| It's sort of like reconstructing the state without actually
| knowing what that state is.
|
| Now, an interesting side effect of the quantum version is
| that the first measurement of the original system is
| necessarily destructive. As such, it's not like you'll end
| up with two copies of the same thing (which is what would
| happen in the classical version) so there's no discussion
| necessary around the distinction between teleportation and
| cloning. Classically you'd be cloning the system but
| quantum mechanically you'd really truly be teleporting it
| (in fact, there's a result in quantum mechanics called "the
| no cloning theorem" that proves that cloning in QM is
| impossible).
| sebmellen wrote:
| Very interesting. If I understand correctly, does this
| mean cloning is a functionally impossible task?
|
| I've always been entertained by the paradoxes where
| someone is teleported ala Michael Crichton's _Timeline_ ,
| and is then (due to some glitch) "duplicated", leading to
| interesting quandaries about "who is who".
|
| If Penrose is right about consciousness [0], this means
| all these fantastical paradoxes are just fantasy, right?
|
| [0]: https://bigthink.com/paul-ratner/why-a-genius-
| scientist-thin...
| comfyinnernet wrote:
| I don't think you would need exact duplication for these
| situations to arise.
| chrisweekly wrote:
| Hey yeah I remember reading Penrose's "The Emperor's New
| Mind" in maybe 1998?, an interesting take on the nature
| of consciousness... fascinating stuff.
| pontus wrote:
| Correct, cloning in even the simplest sense of a single
| particle state (let alone a whole human) is impossible in
| quantum mechanics.
| tomrod wrote:
| It sounds like state send is partial using the "phone"
| analogy.
|
| Can there be triplicates of entanglement?
| vez- wrote:
| Yup https://en.wikipedia.org/wiki/W_state
| pontus wrote:
| There are limits to how much three systems can be
| entangled with each other. It turns out that one system
| cannot be maximally entangled with two different systems.
| In fact, these types of arguments are often used in
| studying information paradoxes in black holes.
| FeepingCreature wrote:
| Though cloning a human may be possible if the brain does
| not rely on quantum state. Obligatory "warm and wet"
| objection.
| drdeca wrote:
| Indeed there is a "no cloning theorem".
|
| Iirc it essentially comes from the fact that time
| evolution is linear, and a function that sends
| stateOfIntetest \otimes constantState to stateOfInterest
| \otimes stateOfInterest for all values of stateOfInterest
| would be quadratic?
| pontus wrote:
| Yes, it's a direct result from the linearity of time
| evolution in QM. Like you say, if state1 x raw_material
| -> state1 x state1 (i.e. we are able to take some raw
| material and clone state1) and also state2 x raw_material
| -> state2 x state2, then linearity forces us to also have
| (state1 + state2) x raw_material -> state1 x state1 +
| state2 x state2. This is not the same as (state1 +
| state2) x (state1 + state2) which is what we'd want in
| order to clone arbitrary states.
| mrtesthah wrote:
| It sounds like start trek's transporter beams may be more
| accurately described than we thought.
| freeflight wrote:
| It's also where the meme comes from how Star Trek
| transporers are actually suicide booths [0].
|
| [0] https://arstechnica.com/gaming/2017/09/is-beaming-
| down-in-st...
| maxerickson wrote:
| They repeatedly treat the pattern like data.
|
| Riker splits, Tuvix!, the one where the other doctor ages
| and gets de-aged in the transporter, the one where Scotty
| is stored in it for decades, etc.
| Snitch-Thursday wrote:
| That's exactly what I was thinking! Running far too much
| into fandon, this holds also for the fictional 'pattern
| buffer'. It has to examine the original AND communicate
| that state back to the duplicate quick enough or the
| 'clone' will be inaccurate, maybe even enough to kill
| someone a la Star Trek: The Motion Picture.
| edge17 wrote:
| Maybe I'm dense, but I still don't understand. The
| cloning explanation made sense to me, but to the original
| question - how does the recipient know the message is
| done being sent without the sender picking up the phone
| and calling the recipient...?
|
| i.e. Is there some equivalent of a termination
| code/header sort of thing that the recipient is looking
| for in the 'bit stream' or whatever? Or am I not even
| thinking about this in terms of the right analogy?
|
| edit: Thank btw, this comment was fascinating to me.
| pontus wrote:
| The receiver does not know that a message has been sent
| until the first person contacts them classically. It's a
| common mistake to think that quantum teleportation is a
| new way of sending information. It's really a way to use
| classical communication in order to leverage entanglement
| to bypass various limitations of quantum mechanics.
|
| So, the two people communicating would e.g. start out
| together and create a pair of entangled systems A and B.
| The person in possession of system B would then travel
| far away. The person in possession of system A then
| decides that they want to teleport a new system C to the
| person far away. They do this by placing system C next to
| system A and then performing a measurement on the
| combined system A+C causing these two states to become
| entangled. We now have an implicit entanglement between
| system C and system B that is far away. The person in
| possession of system A+C now picks up the phone and calls
| the other person to tell them what the outcome of their
| measurement on A+C was. The person far away then uses
| this information to determine a way to manipulate their
| state B in a certain way (the particular way in which
| they need to do this depends on the outcome of the
| measurement of A+C). Once that manipulation is complete,
| the system they have in their possession (B) is now in
| the quantum state that C was originally in. The system C,
| unfortunately has been destroyed in the process.
| ksec wrote:
| Thank You.
|
| That destroy my hopes of having (Close to ) Zero Latency
| Communication with Quantum Teleportation / Entanglement.
| We are still bound by the speed of light!
| tsimionescu wrote:
| Yes, all of our physics only works if we assume that
| there is a maximum physical speed, which only massless
| particles like light can even reach. QM is perfectly
| consistent with this well-confirmed observation.
| metadaemon wrote:
| I know this is probably a stupid question, but would a
| text message be considered classical in terms of
| communication?
| WFHRenaissance wrote:
| Yes
| stakkur wrote:
| _It 's a common mistake to think that quantum
| teleportation is a new way of sending information. It's
| really a way to use classical communication in order to
| leverage entanglement to bypass various limitations of
| quantum mechanics._
|
| Yes. This is the key point, I think, and it didn't seem
| well-communicated in the article.
| davidhyde wrote:
| Thank you for all your comments, very illuminating! Is
| the following classical analogy flawed? Say you have two
| pendulums and you set them in motion together so that
| they swing in perfect synchrony. Then you move the one
| (still swinging) pendulum to another location without
| disturbing it. Would it be reasonable to say that the
| physical pendulums are the "medium" and evolving
| information about the exact position and velocity of the
| pendulums the "system"? Because this is a classical
| system you can measure the position and velocity of the
| one pendulum and know that the other pendulum is at the
| exact same position and velocity. They are "coherent" in
| a way. However, in a quantum system, the medium (say a
| photon of light) is so fragile that measuring it removes
| its coherence to its entangled twin. This decoherence
| does not destroy the photon but the future information it
| carries. It now carries new information unrelated to the
| originally entangled photon. Kind of like having to stop
| a pendulum to figure out it's position and velocity. You
| haven't destroyed the pendulum but you have destroyed the
| potential for it to give you information about the other
| pendulum in the future. Following on from this analogy,
| if you crashed pendulum c into your one pendulum and
| destructively measured the resulting position and
| velocity you could send this information to the second
| pendulum to get that pendulum to set another pendulum in
| motion that would have an identical future to pendulum c
| before its system was destroyed. Thus, no information is
| really flowing between the two entangled photons because
| they are just "vibrating" identically until one is
| disturbed.
| pontus wrote:
| It sounds like there's some stuff in your analogy that is
| similar to the QM situation. I would caution against
| placing too much emphases on these analogies though since
| a very important aspect of all of this is not just that
| the two systems are correlated but rather that they are
| entangled.
|
| There's a classic analogy to this when we talk about
| entanglement: imagine taking a pair of gloves and mixing
| them up. Put one in one box and the other in another box.
| Send one of the boxes far away. When you look down at the
| box that you kept, there is no way of knowing if it
| contains a left handed or a right handed glove; it's a
| 50/50 shot either way. Similarly you have no idea what
| the other box contains. You then decide to open your box
| and find a right handed glove. You then immediately know
| that the other box contains a left handed glove. In some
| sense this feels similar to what we see in entanglement
| but I don't think most people would claim that you
| opening your box somehow compelled the other glove to
| pick left/right. They were just always that way, you just
| didn't know which glove was where.
|
| The claim, however, is that in QM it's not like this.
| Instead, your act of measuring your system actually does
| compell the other system to change.
|
| For a long time there were a lot of heated arguments
| around all of this (most prominently between Einstein and
| Bohr) trying to figure out if the state of either box was
| truly undecided until you opened it or if there could
| have been some type of "hidden variable" that we had yet
| not discovered that nonetheless dictated what the state
| was (i.e. could it be more like the glove example or was
| it truly a new "spooky action at a distance"?)
|
| For a long time physicists believed that this was an
| unanswerable question and should be relegated to
| philosophy. It wasn't until Bell discovered his
| inequality that this was dispelled. He designed an
| experiment that could be conducted to tell the two
| stories apart. When it was carried out, it was determined
| that nature is not like the glove example but rather
| consistent with the truly quantum story around
| entanglement. In other words, your measurement of your
| system actually does compelled the other system to
| change.
| davidhyde wrote:
| The glove explanation is excellent, thank you. I don't
| think I'll ever be able to bend my mind enough to leave
| Einstein's spooky action camp. For now I'm just going to
| start believing that we live in a simulation and that
| entangled things are just structures that share memory ;)
| pontus wrote:
| There is a fairly recent development in theoretical
| physics called ER=EPR that attempts to clarify
| entanglement by conjecturing that two entangled particles
| are equivalent to two particles that have a worm-hole
| connecting them.
|
| To me, this is a very elegant way of addressing this
| weird action at a distance.
|
| https://en.wikipedia.org/wiki/ER%3DEPR
| edge17 wrote:
| Got it, thanks for the explanation.
|
| Also, not to get ahead of ourselves (understanding this
| is research), but what is the use/benefit of this method?
| We can already send and receive information over great
| distances with and without wires at seemingly high
| speeds. Is this a new level of speed? Are there some
| previous limitations of distances that are now
| surmountable? Is the power or cost envelope required
| somehow reduced in some obvious way (not today, but in
| some future commercial implementation)?
| CreepGin wrote:
| I think you missed his remarks:
|
| > The receiver does not know that a message has been sent
| until the first person contacts them classically. It's a
| common mistake to think that quantum teleportation is a
| new way of sending information. It's really a way to use
| classical communication in order to leverage entanglement
| to bypass various limitations of quantum mechanics.
| edge17 wrote:
| I meant more like, wireless and fiber are both classical
| ways to send data but each clearly has a benefit. In the
| same vein, does this new method have some clear benefit?
| terminalcommand wrote:
| Maybe it may be useful for achieving truly one-way
| communication? Could it also be a stepping stone for
| transmitting state-heavy data? For example a human being
| with a consciousness :).
| [deleted]
| throwaway888abc wrote:
| Thanks for all your explanations here
| [deleted]
| rapht wrote:
| Thanks for all the explanations.
|
| I have to say I am always at a loss when quantum
| physicists start talking about "measurement".
|
| In the classical world, measuring means looking at a
| particular variable x in a system S at time t, S(t) and
| via some process specific to x (which we want to
| measure), Mx, obtain the value of Mx(S(t)).
|
| In QM by contrast, it seems that measurement itself has
| an action upon the system so that measuring in fact means
| looking at some Mx(Z(S,t)) where you actually never know
| S but only some kind of end product Z that is believed to
| reflect S but is itself the result of an unknown
| operation on S that QM people call "collapse".
|
| So you seek Mx(S) but in fact spend your time looking at
| Mx(Z(S)) and draw conclusions on S... but I have yet to
| hear anyone explain to me, physically what is Z, how it
| works, etc. Lots of statistics, but no real understanding
| of that "collapse" process.
| pontus wrote:
| You've hit the nail on the head. This is what's called
| the measurement problem in quantum mechanics and it's
| arguably the biggest open question in foundational
| quantum theory. Nobody knows what a measurement actually
| is nor does anyone know what happens during a
| measurement.
|
| There are some modified versions of QM that tries to
| place this on a more rigorous footing, but none of them
| have convinced everyone that they do. My personal
| favorite is the many world's approach that in many ways
| is simpler than traditional QM because it says that
| there's no such thing as a measurement. Instead, when you
| think you're measuring something what you're really doing
| is entangling yourself with the system you're measuring
| which means that your state is no longer separate from
| the state of the system. There's a part of you that sees
| each outcome.
|
| This is actually already how microscopic systems work: if
| two particles collide and get entangled, the state of
| each particle sort of splits in two. The only thing that
| MWI says is that this dynamics also applies to
| macroscopic objects.
| Twirrim wrote:
| If classical communication is still needed to this
| degree, what value does this approach bring vs classical
| communication?
|
| The dependency on classical communication would imply
| that it's not lower latency or higher throughput, and
| will remain subject to signal loss or degradation.
| ahelwer wrote:
| It's the only way to reliably communicate a quantum
| state. Want to network quantum computers? This is how you
| do it.
| motoboi wrote:
| When you entangle the particles, you have a copy of it.
| You then send _the clone particle_ over a mean (like
| fiber optics, if its a photon). Please note that you send
| the actual cloned particle, not information about it.
| Think about a cloned Heisenberg cat. You have to send the
| actual box with the cat inside.
|
| So now you have two copies of the same particle in two
| different locations.
|
| Now the tricky (and useless part): you DESTROY the first
| box. Was the cat dead or alive before you destroy it?
|
| If it was dead, you kill the cloned cat. If it was alive,
| you let the cloned cat live.
|
| So now, congratulations, you have teleported the cat just
| as it really was when you first cloned it.
|
| Obviously this is a gross approximation, but the central
| idea is that quantum teleportation let you clone and
| transport a particle, but you have to find a way to
| capture it's state and send it encoded in light or
| whatever method you prefer (fax?).
|
| UPDATE: The cat belonged to Schrodinger, actually.
| darau1 wrote:
| So we'd still be limited by the speed of classical
| communication in sending the object's state to the new
| location? I hope I'm understanding this correctly.
| pontus wrote:
| Yes, that's correct
| colechristensen wrote:
| Yes, to achieve entanglement in the first place you have
| to either do something to two objects at a distance and
| that action can't exceed the speed of light (and often is
| light) or you have to entangle two objects and then move
| them to their destination.
|
| Teleportation is achieved by doing a specific action on
| one and then communicating to the other to do a specific
| action to the other.
|
| In no case can information be transmitted faster than
| light.
| tgb wrote:
| If you want to transmit 1 quantum bit (qubit), then you
| need to transmit 2 classical bits. Why is this useful?
| Because otherwise you have to carry the qubit over by hand.
| It's really just "quantum ethernet" not "quantum
| teleportation".
|
| There are actually other uses, too, about error tolerance,
| allowing you to quality-control some steps of the
| computation and repeat them if necessary without risking
| damaging the results of other steps.
| 8note wrote:
| You still have to carry the qubit over by hand, but you
| can do it asynchronously to the change
| tgb wrote:
| I think that undersells the advantage. You have to carry
| _a_ qubit over, but it could be done even before know
| what you will need to send.
| ChrisLomont wrote:
| Superdense coding allows sending two bits of classical
| information by only sending one qubit physically. Ideally
| this will double transfer rates. And in any case it
| provides another "modality" for data communication, which
| may provide tradeoffs that have benefits in other
| directions.
|
| I'm pretty sure (haven't been in the field a while) that
| superdense coding the densest coding that one can gain
| using quantum entanglement.
|
| https://en.wikipedia.org/wiki/Superdense_coding
| ed25519FUUU wrote:
| It clearly makes no _practical_ sense at the moment outside
| of research.
| lscharen wrote:
| There are quantum communication links for satellites that
| guarantee that the data streams are not tampered with.
|
| https://directory.eoportal.org/web/eoportal/satellite-
| missio...
|
| https://spectrum.ieee.org/tech-
| talk/computing/networks/quant...
| foobiekr wrote:
| Less "tampered with" (because they can be trivially
| blocked or corrupted) and more "uninspected."
| dheera wrote:
| The point is actually in even transferring an arbitrary
| quantum state from one quantum particle to another, and
| given an entangled pair existing in advance. Transferring a
| quantum state from one particle to another isn't an easy or
| obvious task, because you can't measure it or you would
| collapse it.
|
| Note that "classical" is just stating that a classical
| channel is _good enough_ to serve that purpose. A channel
| that preserves quantum state can of course be used, it 's
| just that that isn't required for this to work.
|
| All classical phenomena are quantum, we just use the word
| "classical" to describe subsets of quantum phenomena acting
| qualitatively in ways that are consistent with macroscopic
| phenomena.
| sreejithr wrote:
| Does this mean information can be transmitted faster than light
| speed?
| pif wrote:
| Yes and no.
|
| Yes, information can travel with infinite speed.
|
| No, cause-effect relationship cannot travel faster than light.
|
| https://en.wikipedia.org/wiki/Quantum_nonlocality
| kazinator wrote:
| > _Yes and no._
|
| Could a QM answer be otherwise?
| mhh__ wrote:
| The idea that QM means anything can happen is a popular
| misconception, at a very basic level the probaility P(X) of
| X happening can quite happily be zero.
| rodiger wrote:
| I think this was more a joke about superpositions than
| "anything can happen"
| moron4hire wrote:
| No. Long range quantum networking means that guarantees on the
| security of communication between quantum computers in the
| network can be made. Short range means that quantum compute
| clusters can be made, to make quantum computers that can
| process more qubits.
|
| https://en.m.wikipedia.org/wiki/Quantum_network
| pontus wrote:
| No, while entanglement acts instantaneously across a large
| distance, there's no way for that "signal" to carry any
| information. In order to complete the teleportation the two
| parties must somehow communicate in order to convey an
| additional piece of information. This communication would be
| classical and slower than light.
| boie0025 wrote:
| This reveals my severe ignorance about quantum mechanics; but
| I've always wondered if entanglement could be used to
| transmit binary data by way of timing and presence or lack of
| presence of a transmission. So maybe every 1ms is a position,
| and either something is sent or not.
| roywiggins wrote:
| You can't. There's a theorem and everything. If there is
| some way to do it, QM must be wrong.
|
| https://en.m.wikipedia.org/wiki/No-communication_theorem
| walkerbrown wrote:
| Thank you. For me, this clears up a long held
| misconception.
| boie0025 wrote:
| Thanks for the link; I had no idea what to even search
| for to understand this.
| tgb wrote:
| Quantum teleportation is a process that both sender and
| receiver have to coordinate. Part of that coordination is
| that the sender measures 2 classical bits of information on
| their entangled qubit and transmits those two classical
| bits to the receiver. Then the receiver uses those two bits
| to perform certain operations on _their_ half of the
| entangled qubit pair in such a way that their qubit is now
| exactly the same as sender 's original qubit. As you can
| see, there's no way to use timing in this, other than the
| timing of the classical bits being transferred.
| [deleted]
| Mizza wrote:
| Any chance that communication could happen _before_ the
| measurement?
|
| Entangle some matter, give half of it to the space team and
| then have inter-stellar walkie talkies?
| magicalhippo wrote:
| The whole point is the other party needs to know the result
| of your measurement, and you don't know that before you
| measured (otherwise the particles wouldn't be entangled).
| djxfade wrote:
| So what can this really be useful for?
| dodobirdlord wrote:
| Another response already mentioned one aspect of how this
| can be used for secure communication, for detecting
| signal interception. But there's a second aspect as well.
| Since the signaling is broken up into two parts (send the
| entangled state, send the measurement result), both parts
| have to be intercepted to decode the communication. The
| entangled state can be sent over a secure channel in
| advance, and the measurement sent over an insecure
| channel at the time of information transmission. This is
| analogous to sharing a one-time pad in advance, but the
| key distinction is that the no-cloning theorem guarantees
| that it's impossible for someone to have stolen a copy of
| your one-time pad. They can only have stolen your one
| time pad, in which case you would notice.
| magicalhippo wrote:
| I'm no expert, but AFAIK one thing is secure
| communication, in the sense that the recipient can detect
| if anyone is eavesdropping.
|
| As I've understood it, to "listen in" the eavesdropper
| has to destroy the entangled state by measuring it, and
| there's no way to perfectly clone the entangled state
| before doing that.
|
| The recipient can compare the entangled data with the
| measurement results (sent via classical means) and detect
| statistical inconsistencies if there is an eavesdropper.
|
| edit: I see their page[1] mentions quantum metrology,
| which I found reference to in a page[2] describing work
| to improve GPS and similar detection using quantum
| entanglement. Not sure if it's directly related but seems
| like there should be room for some interesting work using
| this quantum network in this area.
|
| [1]: https://ieqnet.fnal.gov/
|
| [2]: https://news.engineering.arizona.edu/news/quantum-
| entangleme...
| lemonspat wrote:
| I think OP is asking, can you entangle, measure, discuss,
| and then go travel to another galaxy with real time
| communications?
|
| Edit: or can the communication only happen once and then
| you need to remeasure over classic communication?
| magicalhippo wrote:
| > travel to another galaxy with real time communications?
|
| As I said, no. How would you relay your measurement
| results in real-time? Without those measurement results,
| the receiver would in essence just hear white noise.
|
| That's the perplexing part of entanglement. Somehow it
| feels like information is instantaneously transferred
| from A to B, yet the information content is somehow zero
| so can't be used for proper communication.
| lemonspat wrote:
| So you have to measure every time you want to transmit?
| magicalhippo wrote:
| No, you have to transmit every time you want to transmit.
|
| When measuring one particle of an entangled pair and you
| get say "spin up", you know immediately that if someone
| measures the other particle (with the same measurement
| settings) they'll get "spin down", and vice versa.
|
| The chance that you get "spin up" or "spin down" is 50/50
| and, as far as we know, cannot be affected or determined
| in advance.
|
| So, on the receiving end, they measure some random
| combination of "spin up" and "spin down". Without
| anything else, this information is for all intents and
| purposes noise.
|
| What you can do however, is to send a message using
| regular means with what you measured: "up, down, down,
| up, down". Ok, at least now they can check that what they
| got was the exact opposite. However that still doesn't
| tell them anything.
|
| So instead what you do is that you change the measurement
| settings, and send via regular means not just your
| measurement results but also your measurement settings.
| So you'll send "H left, V down, V up, H left, H right".
| The recipient will then take the measurement settings (H
| or V) and measure the entangled particles in the same way
| you did, and then note down that they get the opposite.
|
| Note now that you suddenly got a way to communicate some
| actual information. By agreeing in advance that a
| Vertical measurement means 0 and a Horizontal measurement
| means 1, you can send information to the recipient.
|
| However also note that you had to make a measurement of
| your particles and then send the results using regular
| means, limited by the speed of light. So why bother with
| this complicated setup? Why not just send the data
| without all this entangled stuff?
|
| And indeed, for just sending plain messages it makes no
| sense to use entangled pairs.
|
| However as I noted in my other post, the inability to
| clone entangled states means an eavesdropper can be
| detected using the entangled setup.
| lemonspat wrote:
| That was a very clear explanation, thank you. So all my
| grand ideas about how quantum communication might work
| are now sadly dead
| rodiger wrote:
| There is no "transmission." When you measure you learn
| the state of one of the particles, and you can use that
| information to deduce the state of the other (which until
| measurement is indeterminate). You can see that this
| information is useless unless communicated classically.
| cambalache wrote:
| No communication is possible BUT, I have this toy scenario
| where a "quasi-communication" may be possible in FTL speed.
|
| "2 generals command 2 armies hundreds of km apart. They
| want to attack a common enemy, there are 2 options, A) And
| all-front attack. B) From the flanks. The generals want the
| plans to remain uncertain until the last second before the
| attack. So from an intermediate point, they send two
| "entangled coins" , one to each general, the coins will
| arrive at both sites at the exact start of the battle.Both
| will show the same face when "measured". The generals have
| agreed previously that if they turn out "heads" they will
| both attack from the flank, in the other case, they will do
| a front-attack.
|
| Of course you dont need a quantum system for this, you
| could have agreed on other stuff (like it if it is raining
| that day at certain place or sending a framed coin by
| regular mail) but I think the quantum solution is the more
| elegant, assuming no 3rd party snooping.
| bluesign wrote:
| Sorry but this example doesn't make sense, how it is
| faster than light?
| Poc wrote:
| It is not faster than light. I think what he wanted to
| show was a situation, in which entanglement and quantum
| mechanics, are superior to classical physics. i.e someone
| using quantum mechanics would have an advantage over
| someone who don't.
|
| However, as he said, in this situation the two generals
| could have agreed on something else, like if it was
| raining or not.
|
| In fact, something they could have done is to flip a
| coin, and split with a copy of the result, which they
| only look at when they launch the attack. It would have
| the same effect.
|
| With a random variable (the coin flip), that is hidden
| until revealed, we achieve the same results as quantum
| mechanics.
|
| Scientist call them "hidden values" and Einstein hopped
| we could explain the "spooky action at a distance" with
| such hidden variables. But we can't, Bell proposed an
| experiment with entangle state which measurement could
| not be explained by such hidden variables, and Aspect did
| the experiment and obtained the predicted results.
|
| So there exist situation where we can use entanglement to
| achieve better results, for example in "non-local games",
| where players sharing entangled state can win with
| probability 1, when "classical player" with probability <
| 1
| bluesign wrote:
| " So there exist situation where we can use entanglement
| to achieve better results, for example in "non-local
| games", where players sharing entangled state can win
| with probability 1, when "classical player" with
| probability < 1"
|
| Do you have an example for this? This is really
| interesting
| _underfl0w_ wrote:
| I think key is that both generals have already agreed on
| a predetermined "meaning" of each coin flip outcome,
| rather than having to communicate it on a different,
| slower medium after the coin has been flipped.
|
| So really it just front-loads that portion (I.e. removes
| it from consideration as part of the proposed solution)
| instead of allowing it to slow down solving the overall
| problem. It requires precomputation/agreement beforehand,
| rather than during the time allotted to the problem.
|
| The speed of that precomputation would still be
| unchanged, and still be the slowest portion.
| roywiggins wrote:
| There are game theoretical proofs that show that you can
| use entanglement to enable better-than-classical
| performance on games that reward cooperation but disallow
| actual communication.
|
| https://en.m.wikipedia.org/wiki/Quantum_pseudo-telepathy
| roywiggins wrote:
| You can't use entanglement (on its own) to communicate _at
| all_.
|
| https://en.wikipedia.org/wiki/No-communication_theorem
| pontus wrote:
| In some sense the two systems that are entangled are
| connected instantaneously. The problem is that the
| information doesn't reside in either system but rather in
| them as a whole. So, if you were in space with one part of
| your "walkie talkies" you wouldn't be able to make sense of
| the information unless you have access to the other system.
| This access would usually occur through some slower than
| light channel.
|
| Imagine that you have 2 coins and give one to someone on
| earth to flip and one to someone on the moon to flip. The
| outcomes in either location is random 50/50, but
| interestingly they are perfectly correlated (H<->H, T<->T).
| When you flip a coin you don't have the ability to pick
| it's outcome. The only thing you can pick is whether or not
| to flip it at all. So, imagine you want to communicate one
| bit of information from earth to the moon and decide that a
| 1 will be encoded as "flip the coin" and 0 as "don't flip
| the coin". When you're standing on the moon and want to
| reveal the information you flip the coin and see e.g. H.
| There are two ways this could have happened: either the
| earth coin had already been flipped and showed H or the
| earth coin had not yet been flipped and you just randomly
| got a H. In other words, the outcome by the flip is useless
| by itself.
| jv22222 wrote:
| Whey can't they just have an agreed upon stop point?
|
| Like, keep parsing the incoming information until you see a
| period (for example).
|
| (I know it's not using actual characters, just using the idea
| to illustrate the point).
| lemonspat wrote:
| that page links to the "Illinois Express Quantum Network", and is
| fascinating. They're trying to build a Q-MAN from three different
| Q-LANs in metro Chicago as an experiment??! This is cool!
|
| https://ieqnet.fnal.gov/
| wannabag wrote:
| Can anyone with a better understand than mine compare this
| experience to the one brought up by a Chinese team in nature in
| 2017[1]?
|
| I know close to nothing about this domain but these two
| experiments sounds very similar except for the setup (fiber vs.
| communication with satellite).
|
| [1] https://www.nature.com/articles/nature23675
| buggycoder wrote:
| Nice!!
| rd11235 wrote:
| This article says this result may transform communication, but
| seems to make no mention of _why_.
|
| What improvements does this have over classical communication?
| tacon wrote:
| One advantage of quantum communication systems is that tapping
| the flow is rendered essentially impossible.
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