Thoughts from reading “Decoherence of floating qubits due to capacitive coupling”

Published on: Tue Jan 11 2011

Link: Decoherence of floating qubits due to capacitive coupling.

I decided to read this paper because I saw a link to a different paper by the same author on the Caltech course website. I looked up the author and chose this paper because the qubit is floating.

Electrically floating qubits (floating flux qubits)  are one of the possible superconducting qubits for constructing gates in a quantum computer. The decoherence time of flux qubits is not as high as was expected. A higher decoherence time was expected because since the qubit is floating it is not in contact with ground.  The reason for the shorter decoherence time is that the reactance of the capacitance to the ground increases for frequencies in the microwave range, even though it is not touching the ground. To limit the short decoherence time, the qubit’s should be symmetrically coupled to bias leads. Then the admittance of the bias leads should be adjusted. Admittance is the inverse of the opposition to alternating current.

To show symmetrically coupled qubit’s are a better option a simulation of a RF SQUID is created. An RF SQUID is a “Radio Frequency Superconducting Quantum Interference Device”. The first example is of an RF SQUID using a capitance connected to ground and to a bias lead. The decoherence time is short for this example. The second example is connected to two bias leads. Then in a thought experiment one of the lead’s is removed and a current loops in either one direction or the other, depending on which lead was removed. Meaning when both lead’s are connected the current is zero.

Josephson Junction: Two superconductors separated by a very thin non-conducting film. A current still manages to cross, and it is called the Josephson current.