Entanglement in two superconducting qubits from UCSB: “Measurement of the Entanglement of Two Superconducting Qubits via State Tomography” Matthias Steffen, M. Ansmann, Radoslaw C. Bialczak, N. Katz, Erik Lucero, R. McDermott, Matthew Neeley, E. M. Weig, A. N. Cleland, and John M. Martinis, Science 313, 1423 (paper here, Science magazine summary here, physics web article here) Note that this is the first demonstration of entanglement in the sense that they have performed tomography on their states (previous results had shown level crossings consistent with entanglement of coupled superconducting circuits.) The authors show a fidelity of 0.87 with the state they were attempting to prepare.
O brave new quantum world!
Ahh the tyranny of the time domain rears its ugly head yet again. Entanglement in superconducting systems with up to four qubits can be found at cond-mat/0509557 and its successors, tracing all the way back to cond-mat/0312332. Don’t believe it? Try a pinch of quant-ph/0602143. Still don’t believe it? Do 50 fourier transforms between frequency and time domain spectra, 10 hail marys and 5 YMCAs.
Okay first of all pardon my ignorance, but here was my understanding…
Geordie, my understand of cond-mat/0312332 was that entanglement was not shown using process tomography but instead the spectrum was determined which was consistent with a quantum mechanical description having entangled states. My understanding for the reason why one can doubt this sort of demonstration of entanglement is that nothing is known about the fidelity of those states. I mean even in the experiment the coupling coefficients obtained are on the same order of magnitude of the temperature of the system, so I would suspect there is significant population in the higher energy levels?
As for quant-ph/0602143 my understanding is that the same critique applies. The ground state of the Hamiltonian certainly has entanglement, but the experiments don’t show that the fidelity of that state is unity.
Of course my ignorance of this is huge, so I’d love to understand why the above isn’t a valid line of thought. And I promise to do 10 hail marys upon a further enlightening of my brain.
I’m not gonna waste your time. I don’t know a damn thing about quantum physics. I’m just waiting for the promised post about Cormac McCarthy and Bell inequalities.