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Superconducting quantum bits are considered promising candidates for constructing a solid-state quantum computer. They are based on the Josephson junction, formed by sandwiching a thin dielectric between two superconducting leads, through which Cooper pairs can tunnel. The Josephson junction provides a nearly dissipationless, highly nonlinear circuit element that allows the construction of a quantum bit. A particular implementation, the phase qubit, is realized by current-biasing a Josephson junction near the critical current of the junction. Besides demonstrating long coherence times, phase qubits can be tuned over a large frequency range. Furthermore, they provide all the advantages of solid-state electrical circuits, as they can be fabricated using fully scalable conventional integrated circuit technology. In addition, controlled coupling to other quantum circuits can be achieved, facilitating readout and gate implementation.

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 Andrew Cleland and John Martinis.

Freely suspended nanostructures with a thickness of the order of only 100 nm only permit a discrete set of lattice vibrations in the confined direction and are therefore also referred to as phonon cavities. The phonon spectrum of such a cavity is characterized by quantized subbands as well as the occurrence of so-called cavity modes. This allows to tailor the electron-phonon coupling and thus phonon-mediated dephasing processes. The interaction of single electrons with individual cavity phonon modes and the effects of phonon quantum confinement on charge transport across these nanostructures is investigated in lateral quantum dots embedded in a free-standing phonon cavity.

Collaboration with Robert Blick.

Electrons in a 2DEG under the influence of a 1D lateral superlattices with broken inversion symmetry behave much like the gas molecules in Feynman's ratchet and pawl: In the absence of a net force, an electron can perform directed motion across the potential given the system is in non-equilibrium.

Collaboration with Axel Lorke.