[Regic]

The entangled photons can't detect the interruption alone, you need the message too. If you are interested how I suggest watching Veritasium's video about Bell's theorem as a start. The cryptographic method is called Ekert protocol and it uses Bell's theorem to detect if the quantum spin was measured in an incorrect "direction" (called base). Just write down the expected outcome in 60 degrees tilted base and you will see how it changes compared to being measured in the correct direction. Add enough photons and possible directions and you can detect eavesdropping with a very high certainty.