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Fabrication and testing of flux-state two-qubit circuits with SQUID flux readout

The scalability of a qubit is a key condition for its potential suitability as a building block for a quantum computer. The first step is to couple two qubits. This should be done in such a way that the coupling can be chosen by design, allowing weak or strong interactions to be implemented as required from a systems design point of view. Flux qubits should be coupled employing the flux parameter. To augment the mutal inductance by the kinetic contribution we introduced an additional Josephson junction as the actual coupling element. The coupling energy can be chosen by the critical current of the junction. From the results may draw a few very important conclusions: we have been able to trace the full spectrum; the experimental diagram closely matches the predicted design; the qubit-qubit coupling strength is very close to the design value; the visibility of transitions between excited states indicates initially non-zero occupation of excited states.

The results were analysed by comparing with the standard simple model Hamiltonian. Remarkably good agreement was obtained between spectroscopic data and the model, indicating that indeed the flux qubits behave as simple two level systems.

In our first measurements on two-coupled flux qubits the qubits could not be tuned to optimal conditions independently. The applied flux could be set to a good value for inducing Rabi oscillations on one transition involving one of the qubits. At that same value of the external magnetic field the flux setting for the other qubit was not optimal and no Rabi oscillation could be induced there. The lesson from these first experiments is that future systems must necessarily have independent tuning. Until now the only good coherence is obtained when a qubit is biased at its symmetry point. In a system with multiple qubits therefore, each qubit must be tuned to its symmetry point with independent bias lines. Tunable coupling is possible with a dynamic procedure.

Rabi oscillations could be induced for some of the transitions at some of the flux values. Not at all positions was coherence high enough. Conditional spectroscopy was performed by first sending a pi-pulse to occupy one of the excited states and subsequently scanning a certain frequency range with a longer RF pulse. In doing this, it was found that the transitions that could be observed in the scan were consistent with a full transition to the excited state in question. No complete two-qubit operations could yet be performed due to limited coherence.

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Julianalaan 134
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