Readout of Flux Qubits
Noon – 1:00 p.m., Thursday, 24 March, 2005
School of Physics Common Room
We next report the first observation of multiphoton transition between superposition states of macroscopically distinct states . The observed distinct resonant peaks and dips are attributed to situations, in which the effective energy separation between the ground and the first excited states matches an integer multiple of the RF photon energy. We have detected up to three resonant peaks and dips for various fixed RF frequencies. It should be noted that at the resonant peaks and dips, the qubit is in a macroscopic quantum superposition of the two energy eigenstates.
Resonant microwave pulse methods induce coherent quantum oscillations between these macroscopic quantum states, e.g., Rabi oscillations or Ramsey fringes. We succeeded in observing Larmor precession (11.4 GHz) of a flux qubit with the phase shifted double pulse method. This new method provides an arbitrary unitary transformation of a single qubit with a rapid control (~10 GHz) of the flux qubit. Compared with the previous method (detuning one), the new method can save time for each quantum-gate operation and results in a 10-100 times faster gate operation than the previous one.
The operation of a single qubit is almost accomplished for many types of solid state qubit. The next target is of course to achieve entangled state using coupled two qubits. It is very promising to analogically apply the so-called cavity QED to a superconducting device coupled with a microwave cavity. It is because we can use many sophisticated methods established in atom physics. We have achieved the coupling between the flux qubit and a LC-resonator (microwave cavity) and observed red and blue sideband resonance. We also observed Rabi oscillations at the red and blue sidebands . This clearly indicates that entangle states are generated between two macroscopic quantum systems.
The audience, including graduate students, are invited to meet the speaker 15 minutes beforehand over wine and cheese in the Physics Common Room.