HN Theater @HNTheaterMonth

⬐ ivan_ah

This is an excellent video, probably the best TL;DR on all things quantum computing I have ever seen (the first 1/4 of the video), making clear the limitations. A great example of what all science comms should be like.

some choice quotes:

> quantum initiatives differ somewhat from nation to nation but they usually contain research programs on four key topics: - (1) quantum computing - (2) the quantum internet - (3) quantum metrology - (4) and quantum simulations

## (1) Quantum computing

> quantum computing quantum computing is one of the most interesting developments in the foundations of physics right now. > [...] this speed up only works for certain types of calculations, so quantum computers are special purpose machines [...] > the problem with quantum computers is that for them to become commercially useful you need to be able to bring a large number of qubits into controllable quantum states and that's really really difficult [...] > estimates say the number we need to reach is roughly a million qubits [...] status of research is presently at about 50 qubits > [...] the major question for quantum computing is not does it work; the question is will it scale.

⬐ gallegojaime

When we're talking about exchanging information between two parties, like on Sabine's hacker interception case, how is that quantum information exchanged? If it can be done through an electric cable, what forbids the hacker from just using a high impedance tap in the cable?

Apart from that doubt I had, good video.

⬐ Ma8ee

One way to see it is that any measurement will collapse the wave function, which can be detected can by the communicating parties.

⬐ ivan_ah

> When we're talking about exchanging information between two parties, like on Sabine's hacker interception case, how is that quantum information exchanged?

The assumption is quantum info. is exchange in the form of the quantum state of a single qubit, which is of the form of a two-dimensional vector [alpha,beta] where alpha and beta are some numbers. Quantum states are often written as

   alpha*|0) + beta*|1>

where |0> = [1,0] = ihat and |1> = [0,1] = jhat are two basis elements for the space of quantum states (a two-dimensional vector space).

> If it can be done through an electric cable, ...

No. Usually we assume coherency-preserving optical fibre connects sender to receiver or there is a free-space link (some sort of "carrier" for single photons such that quantum state encoded in input photons roughly comes out the same at output side).

> ... what forbids the hacker from just using a high impedance tap in the cable?

I'm interpreting your question a little, but if I understand correctly what you are asking is:

Given Alice's (the sender) signal being v_A and Bobs' "receiver load" being R_B, why can't an eavesdropper Eve insert a "high impedance tap" (R_E >> R_B) in parallel with Bob, such that she receives a copy of the Alice-transmitted signal v_A, but since adding a high-impedence-load in parallel doesn't matter much, Bob will not be able to detect the tapping.

Bob's expected received current if no wiretap is: i_B = v_A/R_B = i_A (i.e. all signal (current) sent by Alice is received by Bob)

Bob's received current given Eve's tap: i_B = i_A * (R_E / R_E + R_B), which is approx. equal to i_A when R_E >> R_B (high impedance tap)

______

The answer is that because Alice is sending one photon at a time (quantum regime = such low energies that we're talking about individual systems state, like single photons being emitted and received). In the quantum regime each photon is "forced" to either be detected by the Eve detector OR the Bob detector (no such thing as 30% of a photon).

The only-one-receiver nature of single-photon coherent quantum communication gives us the eavesdropping-detection capability of quantum crypto systems, i.e., Eve's wiretapping cannot be done without detection.

To actually generate a secure protocol based on wiretap-detection requires additional clever hacks and conventions between Alice and Bob, which are described in the BB84 protocol, see https://en.wikipedia.org/wiki/BB84

⬐ ivan_ah

More info about quantum cryptography: https://www.youtube.com/watch?v=VigvP8cVZMQ&t=424

⬐ andreareina

Quantum information cannot be cloned. This forms the basis of quantum key exchange schemes that can say whether some communication (e.g. a keystream) was intercepted.

⬐ _Microft

If you want to know more about this, it's called the "no-cloning theorem": https://en.wikipedia.org/wiki/No-cloning_theorem