Researchers from Saarland University (UdS) have achieved an important breakthrough in Quantum Communication by demonstrating Quantum Entanglement and Teleportation over a 14 km long fiber link, the “Saarbrücken Quantum Communication Fiber Testbed”.
FYI, this was published September of 2024: https://www.nature.com/articles/s41534-024-00886-x
I get that they are creating a pair of entangled particles and sending one on a 14 km trip. When they measure the 1st particle at the lab, the wave form of the other also collapses 14 km away. My question is, how do they know when the second particle’s wave function collapses without observing it which would collapse both particle’s waves prematurely?
I’m no expert but I think this is how it works. We’ll call the particle pairs A, B, C, etc.
So when the researchers measure Local particle A (LA for short), the nature of entanglement means that Remote particle A (RA) must collapse into a specific known state. So they measure RA after that happens. The same is true for pairs LB and RB, LC and RC, and so on. Then they check statistically if the remote particles are all in the predicted states (should be 100% if this works flawlessly).
With enough repetition they can have very high confidence in the results. Of course those results must be communicated over traditional (non-entangled) channels that we already trust for reliability.
edit: typo
So they measure RA after that happens.
This is my problem though. How do they know when it happens unless they are observing RA? And if so, how are they making an observation of RA without collapsing the wave function for LA and RA.
They observe/measure the local particle first (LA) which causes the entangled system to collapse into definite states. Then they measure the remote particle (RA) afterward to confirm that it matches the expectation.
They know the “when” that the wavefunction collapse occurs for RA – it’s the moment they chose to observe LA. The “magic” of entanglement is that it’s not bound by the speed of light and effectively instantaneous.
I think what he is trying to say is its not useful for communication if you need to know that the other placed tried to communicate before measuring. So the measurement could be one that sets the state or one just reading a state set afar.
its not useful for communication if you need to know that the other placed tried to communicate before measuring.
But I don’t think you do. The classic slower-than-light communication here is just to verify the results. Once this system is operational, then by measuring the remote particles, you know exactly what information was sent.
This of course assumes very good transmission fidelity (or error correction), and that the local sending side has some way to control the state their particle wavefunctions collapse into (otherwise they’re just sending random noise).
the local sending side has some way to control the state their particle wavefunctions collapse into (otherwise they’re just sending random noise).
Do they? My impression is that, like the article says, “their states are random but always correlated”. I think they are in fact measuring random values on each side, it’s just that they correlate following Schroedinger’s equation.
I believe the intention is not “sending” specific data faster than light… but rather to “create Quantum Keys for secure information transmission”. The information between the quantum particles is correlated in both sides, so they can try to use this random data to generate keys on each side in a way that they can be used to create a secure encryption for communication (a “Quantum Network that will be used for secure communication and data exchange between Quantum Computers”), but the encrypted data wouldn’t travel faster than light.
Ah yeah I bet you’re right. I’m probably conflating the more serious articles I’ve read with aspirational (near-future sci-fi) material. Yes, quantum encryption should be much more practical and achievable.
