Colloquia 2005

Readout of Flux Qubits

Dr Hideaki Takayanagi
NTT Basic Research Laboratories
3-1 Morinosato-Wakamiya Atsugi 243-0198 Japan, NTT Corporation
CREST-JST, Hideaki Takayanagi


Noon – 1:00 p.m., Thursday, 24 March, 2005


School of Physics Common Room
Room 64 Old Main Building
The University of New South Wales

In this talk, we report the observation of the single-shot readout of a superconducting flux qubit comprising three Josephson junction in a superconducting loop]. The quantum state of the flux qubit (an Al loop with three Josephson junctions) can be readout by a dc-SQUID that surrounds the qubit. We find experimentally that the switching current distribution of the SQUID at f = 1.5 is narrow enough to observe the two energy eigenstates of the qubit with a single-shot measurement. Here the filling f is the external magnetic flux Fqubit penetrating the qubit loop normalized by the flux quantum F0 =h/2e. Good agreement between the experimental and calculated results shows that single-shot readout experiments are performed.

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 [5]. 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.

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