In order to achieve the maximum coherence time, to allow for multiple, complex quantum operations, the dissipation induced by the environment has to be minimized. Our research has therefore focused in part on minimizing environment-induced decoherence, and we have recently achieved a significant and important breakthrough, due to the realization that losses in the dielectrics, both in the Josephson junction itself and in associated circuitry, dominate the overall loss and limit the coherence time. Our demonstration of high visibility, long decoherence time qubits has been achieved by carefully redesigning and engineering our qubit circuit and the materials employed. Recent experiments have demonstrated qubit state preparation, manipulation and probing with a measurement fidelity of 95%. Rabi oscillations with a relaxation time T1 of 500 ns have been observed. Ramsey fringe as well as spin echo measurements yield decoherence times of T2 = 2 T1 and T2*= 150 ns. We have also begun to develop full quantum tomography of the qubit state. This experimental technique allows the reconstruction of the density matrix from a complete set of observables measured on an ensemble of identically prepared copies of the system.
The detailed understanding of a single qubit enables us to start focusing on coupled qubit systems. We are working on two coupled qubits to demonstrate the implementation of a CNOT gate. This is an essential milestone for quantum information processing, because the 2-qubit CNOT gate together with arbitrary 1-qubit rotations on the Bloch sphere form a universal set of gates with which all other unitary transformations can be performed. Furthermore, entanglement between the two qubits can be established and detected via the violation of Bell's inequality. By coupling a qubit to a harmonic oscillator such as an LC or a nanomechanical resonator, quantum information can be transferred and stored tuning the qubit in and out of resonance. This might ultimately be employed as a high fidelity bus in a more complex qubit architecturs.
Collaboration with Robert Blick.
Collaboration with Axel Lorke.