Sensing with many-level systems

LTL Quantum Physics Seminar (Nanotalo). Speaker: Dr. Andrey Lebedev (ETH Zurich, Switzerland).

Making use of coherence and entanglement as metrological quantum resources allows to improve the measurement precision from the shot-noise- or quantum limit to the Heisenberg limit. Quantum metrology relies on the availability of quantum engineered systems that involve controllable quantum degrees of freedom which are sensitive to the measured quantity. So far, this has been demonstrated for qubits, by using the phase coherence as a quantum resource.  Here we extend this result to many-level systems (d-dimensional Hilbert space) and we show that the Heisenberg scaling can be reached. We particularize these results for the case of the transmon: in the case of three levels we obtain a reduction in the number of iteration steps of the quantum Fourier transformation by a factor log 2/log 3 ~0.63 as compared to the qubit mode. We describe a method to implement this algorithm experimentally, by simultaneous coupling with microwave fields of the two transitions.