April 2012

Abstracts of the QSIT Lunch Seminar, April 5, 2012

A hybrid on-chip optonanomechanical transducer for ultra-sensitive force measurements

by Emanuel Gavartin, Laboratory of Photonics and Quantum Measurements, EPFL

Nanomechanical oscillators serve as ultrasensitive detectors of force, mass and charge. They are also used in opto- or electromechanical experiments with the goal of quantum control of mechanical systems. Here, we report the realization and operation of a hybrid monolithically integrated transducer system having a high force sensitivity of 74 aN Hz-1/2 at room temperature. Our system is particularly suited for the detection of small incoherent forces, which generally suffers from slow convergence with the fourth root of the averaging time. Using dissipative feedback  based on radiation pressure enabled control, we explicitly demonstrate by detecting a weak incoherent force that this constraint can be significantly relaxed. We achieve a more than 30-fold reduction in averaging time with our hybrid transducer and are able to detect an incoherent force having a force spectral density as small as 15 aN Hz-1/2 within 35 s of averaging. This corresponds to a signal, which is 25 times smaller than the thermal noise and would otherwise remain out of reach. The reported monolithic platform is thus an enabling step towards hybrid nanomechanical transducers relying on the light-mechanics interface.

Co-authors: P. Verlot, T. J. Kippenberg, EPFL; Ref.: arXiv:1112.0797 (2011)

Multi-qubit gates in Circuit –QED

by Arkady Fedorov, Quantum Device Lab, ETH Zurich

The essential ingredient of a circuit Quantum Electrodynamics (circuit QED) system is strong controllable interaction between two-level quantum electronic circuits (qubits) with single photons stored in high quality on-chip cavities.  In this talk I will present recent experimental results obtained with the three-quantum bit register based on circuit QED architecture. First I discuss implementation of the Toffoli gate with superconducting circuits. By exploiting the third energy level of the transmon qubits, we have significantly reduced the number of elementary gates needed for the implementation of the Toffoli gate, relative to that required in theoretical proposals using only two-level systems. In the second half of the talk I will present first experimental realization of the Monte Carlo process certification.  This method allows for an efficient evaluation of fidelities for multi-qubit quantum gates without performing full process tomography.