Condensed Matter Physics Seminar

Prof. M. Gershenson, Rutgers University, Dept. Physics and Astronomy

Friday 09 May 2014, 1500-1600
C1 Physics Building

"Parity-Protected Josephson Qubits"

We have developed two novel types of protected qubits that decouple logical states from the environment by encoding them in a parity of a large number. The first type, the so-called charge-pairing qubit, represents a chain of two Josephson elements characterised by the pi periodicity of the phase dependence of their Josephson energy. The second type, the flux-pairing qubit, consists of a 4.pi periodic Josephson element (a Cooper pair box with the e charge on the central island) shunted by a superinductor. The lowest-energy quantum states of the charge-pairing qubit are encoded in the parity of Cooper pairs on a superconducting island flanked by the Josephson rhombi. The flux-pairing qubit encodes its quantum states in the parity of magnetic flux quanta inside a superconducting loop. The design of these devices enforces, respectively, charge- and flux-pairing, prohibits the mixing of lowest-energy quantum states, and protects the qubits from both the decay and dephasing. We have studied the low-energy excitations in each of these qubits by microwave spectroscopy; the observed spectra are in agreement with theoretical expectations. We will present experimental evidence for the decay protection which resulted in the energy relaxation time T1 up to 100 µs and the quality factors w01T1 > 10^6, a 100-fold improvement vs. the unprotected state of each of these qubits.