Southwest Quantum Information and Technology

Sixteenth Annual Meeting, February 20-22, 2014
Santa Fe, New Mexico

Full Program | Thursday | Friday | Saturday | All Sessions

SESSION 13: Quantum Metrology and Control
4:15pm - 5:00pm	Ania Bleszynski-Jayich, University of California Santa Barbara (invited)
	Quantum assisted sensing with diamond spins
	Abstract. Nitrogen-vacancy (NV) centers in diamond are atomic-scale spin systems with remarkable quantum properties that persist to room temperature. They are highly sensitive to a wide variety of fields (magnetic, electric, thermal) and are easy to initialize, read-out, and manipulate on the individual spin level; thus they make excellent nanoscale sensors. The NV’s sensitivity is a double-edged sword however; environmental fluctuating fields are also a source of decoherence. We use the NV to probe these fluctuating fields, both their frequency spectrum and spatial character, and we mitigate their induced decoherence through engineered CVD diamond growth and quantum control of the NV. I will also present my group’s work on quantum assisted sensing of strain fields on the nanoscale. We demonstrate strain coupling of a single NV spin to a high quality factor mechanical mode of a single-crystal diamond mechanical resonator. This hybrid system has exciting prospects for a phonon-based approach to integrating NVs into quantum networks.
5:00pm - 5:30pm	Joshua Combes, University of New Mexico
	Probabilistic protocols in quantum information? Probably not.
	Abstract. Probabilistic protocols in quantum information are an attempt to improve performance by occasionally reporting a better result than could be expected from a deterministic protocol. Here we show that probabilistic protocols can never improve performance beyond the quantum limits on the corresponding deterministic protocol. To illustrate this result we examine three common probabilistic protocols: probabilistic amplification, weak value amplification, and probabilistic metrology. In each of these protocols we show explicitly that the optimal deterministic protocol is better than the corresponding probabilistic protocol when the probabilistic nature of the protocol is correctly accounted for.
5:30pm - 6:00pm	Sergey Knysh, NASA Ames Research Center
	True Quantum Precision and Unique Optimal Probes in presence of Decoherence.
	Abstract. Quantum instruments derived from composite systems allow greater measurement precision than their classical counterparts due to coherences maintained between the N component elements; spins, atoms or photons. Typical decoherence that plagues real-world devices can be dephasing, particle loss, thermal excitation and relaxation. All these adversely affect precision (mean squared error), whether one is measuring time or phase, or even the noise amplitude itself. We develop a novel technique that uncovers the uniquely optimal probe states of the N `qubits' alongside new tight bounds on precision under local and collective mechanisms of these noise types above. For large quantum ensembles (where numerical techniques fail), the problem reduces by analogy to finding the ground state of a 1-D particle in a potential well, with the shape of the well dictated by the type and strength of decoherence. Under collective dephasing alone we find that optimal estimation of phase and noise parameter can be effected simultaneously, utilizing the same optimal probe and measurement scheme. The formalism is applied to real-world devices such as the Mach-Zehnder interferometer and atom clocks.