Why does measurement destroy superposition in qubits?

hbarhacker

I read a comment in a paper that described measurement as just projecting the qubit state, but it didn’t really explain why the superposition gets destroyed. When you measure, the detector has to interact with the qubit to register 0 or 1, and that interaction entangles the system so the original coherent state can’t hold. The phase information that kept it in both states at once gets scrambled by the environment, leaving a definite outcome with no way back. I ran a simple two-qubit case in Qiskit last week and saw the off-diagonal terms in the density matrix drop to zero right after the measurement gate, which matched what I’d read. How does the entanglement step specifically erase the possibility of reversing it?

hypecheck

That decoherence from measurement is the exact reason most near-term quantum advantage claims fall apart fast. https://arxiv.org/abs/2305.19141

scqubits

In transmon setups the readout resonator does exactly that scrambling through the dispersive shift, and you see the coherence vanish on the timescale of the resonator ringdown.

qmlscripts

the entanglement with the resonator doesn’t just scramble phase, it effectively traces out the qubit into a classical mixture once you condition on the readout signal, which is why you can’t rewind without full knowledge of the bath. ties right into why my kernel attempts on small datasets keep needing heavy error mitigation even before the SVM step.

anyonbraid

The no-cloning theorem already kills reversal once the resonator mixes in, even if you knew the full bath. Anyone seen a Majorana readout that dodges this?

garageiontrap

Majorana readouts might dodge some of the resonator damping but in my ion trap rig the laser scatter still couples the qubit to the vacuum chamber walls the same way, and once those photons escape there’s no rewinding the phase info even if you map the whole thing. Had a leak in the viewport seal last month that let in just enough stray light to kill coherence mid-experiment, took three days of hunting with a helium sniffer before I found it. Check your O-rings with a fresh coat of vacuum grease every pumpdown.