Sustained, high-fidelity quantum teleportation

Re:Sustained, high-fidelity quantum teleportation by fastball on Friday December 18, 2020 @03:05PM (#25471276)

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.

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Re:Sustained, high-fidelity quantum teleportation by jiofih on Friday December 18, 2020 @03:43PM (#25471700)

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

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Re:Sustained, high-fidelity quantum teleportation by fastball on Saturday December 19, 2020 @02:02AM (#25475700)

Sorry, I was mixing terms. You need to send classical information over the wire, but you're also sending photonic qubits thru fiber, just not as a communication method. The fiber is how you get the entangled qubit to the other side of the network (entangled qubits are generated together). But again, this is not how you are communicating information. Technically this doesn't need to happen over fiber, you just need to somehow have entangled qubits in two separate locations, and obviously if we're talking about entangled photons, fiber is probably the best way to separate them.

Information is communicated once the entangled photon has reached the other side of the network, at which point a measurement at one end of the network is made and communicated (classically). Once again though, the purpose of the classical bits is not actually to send information, but rather because any measurement of one part of the entangled system results in its collapse (Copenhagen interpretation), and thus a collapse of the entangled quantum system as a whole. So if you have a quantum computer doing some calculation with qubits and those qubits have been entangled with others at the other side of the network, you can perform measurements on the qubits at one end and this will collapse the qubits at the other end. This would not be possible to do as you'd expect from classic digital stuff, where you can just send the info over the wire.

The distance is limited because of fidelity loss in fiber - you actually need your single photonic qubit (entangled with another qubit at the source) to make it from one end of the network to the other. In order to extend the range you can use quantum repeaters, which are effectively just a sequence of entanglements.

As an aside, higher quality fiber made in microgravity (ZBLAN) could also help bring these transmission distances up.

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