“Don't fall for quantum hype” video by Sabine Hossenfelder

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.

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

> 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)

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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