Nine Days, Four Qubits

Stephen sent me a fun google query, discovered by one of his students:


Those are some pretty impressive four entangled qubits: sticking around for nine days without decohering :)!

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i read this as: "it takes 9 days to entangle 4 qubits" or something.

you are now the #1 hit for "four qubits can be entangled in 9 days". Congrats!

Google is funny but the standard explanation of the effects and implications of decoherence is teh unsatisfactory IMHO. It's widespread use as a pretense for solving collapse is a circular argument with many flaws. Decoherence involves e.g. pretending that the mixture definable for a set of multiple cases (like, various runs of an experiment with varying phases each time) can be bastardized onto a given single event - "A coherent superposition becomes an incoherent mixture."

And, another nostrum is "they don't interfere with each other anymore." Well, "interference" is a global way of talking about patterns created by the superposition principle. The SP is supposed to apply in any case, even if it makes nice patterns one time and messy combinations another time - and in any case, both wave contributions continue to at least exist. So what does decoherence say happens to the other, "lost" state that used to be in combination with the first? No good answer is given.

Very low temperature?

Congrats Neil B, you are entangled in even more ways than four qubits!

Dave, that's cute but do you have a good nugget to offer in reply on the subject? Really, even Penrose has made similar criticisms...


I do not believe the decoherence solves the measurement problem, no.

Ack, my ride just showed up so I have to run.

I thought that four qubits were entangled in 6 days, and it was good, and on the 7th day the Principal Investigator rested.

But that's Theophysics as applied to Quantum Cosmology.