I understand that right now Qubits are physical entities in a Quantum Computer and I am playing around on the IBM Quantum Computer as well as the Q# language and dipping my toes into the Quantum world for the first time.

I have read a lot of the Alice and Bob style scenarios where it often mentions transporting a Qubit from Alice to Bob. I am inferring that as physically transporting it but I haven't found a discussion on what this looks like from a computing sense. As in how it could be theoretically achieved to "package" a Qubit, or even a representation of a Qubit (state or values) for transportation via a Classical or Quantum channel? I am assuming the only way this is possible is via entanglement and teleportation. Is it possible for non-entangled, ordinary Qubits, to be represented in some format and transferred logically between two points, whereby the receiving point can decode and interpret the information contained within. That receiving point could be a computer service in a Classical computer architecture or another Quantum machine.

I ask this in the sense of Classical Computing, where we can encode bits onto a chip but logically represent a bit (or series of bits) in numerous formats and transfer them for manipulation. As a software engineer that's where my thought process is coming from. This might not be a practical thing to want to do in Quantum but in theory, is it something that could be achieved? Any guidance would be welcome.

EDIT: Thank you for the really comprehensive answers, it has filled in a lot of gaps and I did not realise the strong link between photons & fiber which brings a potential bridge in theory. I'm working my way through the basic hello world applications and was trying to mentally bridge my software knowledge of Classical into this world at a basic transfer and representation level. I'm trying to build some small apps that bridge both worlds and my mental block right now is representing the characteristics of a Qubit in traditional programming notations. Have you any thoughts on what would need to be modelled to create a logical representation of a Qubit? What I am getting at is something similar to a specification that would allow a programmer represent a type (like a String e.g. https://en.wikipedia.org/wiki/String_(computer_science)). In the Quantum programming languages the Qubit is it's own type, drilling down a level can the characteristics be captured in a very basic manner so that potentially they could be represented in something like a vector array to capture key characteristics e.g. state (notwithstanding the difficulty of superposition!), spin etc.

up vote 10 down vote accepted

You are totally right in your assumption about transporting qubits from Alice to Bob implies something physical. Usually problems/situations that have this setup of a transmission between two parties are called quantum communications. These problems/situations sometimes disambiguate by calling their qubits "flying qubits" which are almost always photons. Single photons are also quantum systems that can be prepared in useful qubit states, they can be operated on with gates (but not all kinds of gates and not as easily as some other physical implementations of qubits), and can be measured just like any other qubit system. Alice and Bob would literally share these photos by either an optical fiber they are connected by or through free space (which could be literally to a satellite in space).

Photons are great for this application because we already use them for a large portion of our classical communication networks. "Fiber" internet or photonic networks send classical information in optical fiber with strong pulsed lasers. So if you wanted to have both a classical channel and a quantum communication channel you could do both with the same fiber (hard for some technical reasons but totally possible).

There are also may other physical systems that you cab make qubits out of for quantum computing (super conductors, ion traps, etc.). You are correct in that to connect different groups of these qubits one would not pick the chip up and move it, but instead they often create a(n) photon(s) that are entangled with the original system or have some information they want to share with the second system and then send the photon over.

  • Thank you for your answer, very helpful and has certainly bridged a mental gap to even visualise how these things could move about. It actually prompted me to edit my original question. This can be summarised as "Have you any thoughts on what would need to be modelled to create a logical representation of a Qubit?". In lieu of a fiber network and protons, I'm interested in seeing how I can 'capture' the essence of a Qubit and break down the characteristics of it. – Leigh Griffin Jul 27 at 7:12
  • As you allude to, qubits are already an abstraction. For a single qubit, it can be fully described by a 2*2 matrix of complex numbers. Some good resources to look up could be the bloch sphere, which is a common way to visualize single qubits. en.m.wikipedia.org/wiki/Bloch_sphere – Dr. Sarah Kaiser Jul 27 at 16:32

It’s worth stating from the start that “Alice and Bob” scenarios are very different from quantum computation scenarios. The Alice and Bob scenarios are very much that there are two distantly separated locations between which it is impossible to directly perform quantum gates. Meanwhile in the quantum computing architectures you’re talking about, two-qubit gates are readily available. Even if you can’t directly interact a pair of qubits, a bunch of swap gates are enough to move the qubits next to each other, and back again.

You also want to be careful with your classical software engineer interpretation, because classically it’s very easy to move things about, and make multiple copies. In quantum, you can’t make copies of your data.

So, how do you move a qubit? Entanglement and teleportation aren’t really an answer. They might help give you error-correction enhanced protocols, but the basic question is still how you share the entangled state to achieve the teleportation.

Probably the best way is to transfer the quantum information from one physical carrier, such as the qubits in a quantum computer, to a different one. We’d usually think about photons in this context - they’re really good at travelling long distances without interacting too much. In the same way that classical data can transfer via an optical fibre, it’s not too wild to imagine sending photons in superpositions through an optical fibre. You ‘just’ have to convert the photon at either end into the different storage/manipulation type of qubit. The technology certainly exists to do this, but I don’t know how reliably it happens.

  • Thank you for the great answer and you are absolutely correct, the classical software engineer interpretation is taking over due to the lack of understanding at the mechanics level! I'm playing around with some of the toolkits and basic apps and my thoughts immediately try and bridge this to things I know, as coming from a software background that's where my brain thinks the challenge could be. With that in mind I edited my original question to effectively ask "Have you any thoughts on what would need to be modelled to create a logical representation of a Qubit?". – Leigh Griffin Jul 27 at 7:15

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