<story><title>Sustained, high-fidelity quantum teleportation</title><url>https://news.fnal.gov/2020/12/fermilab-and-partners-achieve-sustained-high-fidelity-quantum-teleportation/</url></story><parent_chain><item><author>pontus</author><text>The receiver does not know that a message has been sent until the first person contacts them classically. It&amp;#x27;s a common mistake to think that quantum teleportation is a new way of sending information. It&amp;#x27;s really a way to use classical communication in order to leverage entanglement to bypass various limitations of quantum mechanics.&lt;p&gt;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.</text></item><item><author>edge17</author><text>Maybe I&amp;#x27;m dense, but I still don&amp;#x27;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...?&lt;p&gt;i.e. Is there some equivalent of a termination code&amp;#x2F;header sort of thing that the recipient is looking for in the &amp;#x27;bit stream&amp;#x27; or whatever? Or am I not even thinking about this in terms of the right analogy?&lt;p&gt;edit: Thank btw, this comment was fascinating to me.</text></item><item><author>pontus</author><text>Great question! This gets to the heart of why quantum teleportation has any value at all.&lt;p&gt;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&amp;#x27;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&amp;#x27;t do both (Heisenberg&amp;#x27;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.&lt;p&gt;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 &amp;quot;copy&amp;quot; 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.&lt;p&gt;It&amp;#x27;s sort of like reconstructing the state without actually knowing what that state is.&lt;p&gt;Now, an interesting side effect of the quantum version is that the first measurement of the original system is necessarily destructive. As such, it&amp;#x27;s not like you&amp;#x27;ll end up with two copies of the same thing (which is what would happen in the classical version) so there&amp;#x27;s no discussion necessary around the distinction between teleportation and cloning. Classically you&amp;#x27;d be cloning the system but quantum mechanically you&amp;#x27;d really truly be teleporting it (in fact, there&amp;#x27;s a result in quantum mechanics called &amp;quot;the no cloning theorem&amp;quot; that proves that cloning in QM is impossible).</text></item><item><author>junon</author><text>This makes zero sense. If the information must be conveyed classically anyway, what&amp;#x27;s the point?</text></item><item><author>pontus</author><text>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).</text></item><item><author>sabellito</author><text>I&amp;#x27;m confused, as I&amp;#x27;m an enthusiast but definitely no physicist.&lt;p&gt;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 &amp;quot;information&amp;quot; is only revealed when interacting with the particle.&lt;p&gt;Could someone perhaps clarify what&amp;#x27;s going on?</text></item></parent_chain><comment><author>davidhyde</author><text>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.</text></comment>
<story><title>Sustained, high-fidelity quantum teleportation</title><url>https://news.fnal.gov/2020/12/fermilab-and-partners-achieve-sustained-high-fidelity-quantum-teleportation/</url></story><parent_chain><item><author>pontus</author><text>The receiver does not know that a message has been sent until the first person contacts them classically. It&amp;#x27;s a common mistake to think that quantum teleportation is a new way of sending information. It&amp;#x27;s really a way to use classical communication in order to leverage entanglement to bypass various limitations of quantum mechanics.&lt;p&gt;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.</text></item><item><author>edge17</author><text>Maybe I&amp;#x27;m dense, but I still don&amp;#x27;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...?&lt;p&gt;i.e. Is there some equivalent of a termination code&amp;#x2F;header sort of thing that the recipient is looking for in the &amp;#x27;bit stream&amp;#x27; or whatever? Or am I not even thinking about this in terms of the right analogy?&lt;p&gt;edit: Thank btw, this comment was fascinating to me.</text></item><item><author>pontus</author><text>Great question! This gets to the heart of why quantum teleportation has any value at all.&lt;p&gt;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&amp;#x27;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&amp;#x27;t do both (Heisenberg&amp;#x27;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.&lt;p&gt;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 &amp;quot;copy&amp;quot; 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.&lt;p&gt;It&amp;#x27;s sort of like reconstructing the state without actually knowing what that state is.&lt;p&gt;Now, an interesting side effect of the quantum version is that the first measurement of the original system is necessarily destructive. As such, it&amp;#x27;s not like you&amp;#x27;ll end up with two copies of the same thing (which is what would happen in the classical version) so there&amp;#x27;s no discussion necessary around the distinction between teleportation and cloning. Classically you&amp;#x27;d be cloning the system but quantum mechanically you&amp;#x27;d really truly be teleporting it (in fact, there&amp;#x27;s a result in quantum mechanics called &amp;quot;the no cloning theorem&amp;quot; that proves that cloning in QM is impossible).</text></item><item><author>junon</author><text>This makes zero sense. If the information must be conveyed classically anyway, what&amp;#x27;s the point?</text></item><item><author>pontus</author><text>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).</text></item><item><author>sabellito</author><text>I&amp;#x27;m confused, as I&amp;#x27;m an enthusiast but definitely no physicist.&lt;p&gt;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 &amp;quot;information&amp;quot; is only revealed when interacting with the particle.&lt;p&gt;Could someone perhaps clarify what&amp;#x27;s going on?</text></item></parent_chain><comment><author>rapht</author><text>Thanks for all the explanations.&lt;p&gt;I have to say I am always at a loss when quantum physicists start talking about &amp;quot;measurement&amp;quot;.&lt;p&gt;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)).&lt;p&gt;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 &amp;quot;collapse&amp;quot;.&lt;p&gt;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 &amp;quot;collapse&amp;quot; process.</text></comment>