| Abstract: | In this colloquium I will describe the implementation of a quantum analog to the classical lock-in amplifier. All the lock-in operations: modulation, detection and mixing, are performed via the application of non-commuting quantum operators on the electronic spin state of a single trapped Sr+ ion. We significantly increase its sensitivity to external fields while extending phase coherence by three orders of magnitude, to more than one second. With this technique we measure frequency shifts (magnetic fields) with sensitivity of 0.42 Hz/sqrt(Hz) (15 pT/sqrt(Hz)), obtaining an uncertainty below 10 mHz (350 fT) after 3720 seconds of averaging. These sensitivities are limited by quantum projection noise and, to our knowledge, are more than two orders of magnitude better than with other single-spin probe technologies. In fact, our reported sensitivity is sufficient for the measurement of parity non-conservation, as well as the detection of the magnetic field of a single electronic-spin one micrometer from an ion-detector with nanometer resolution.Figure1. Phase scan contrast vs. lock-in modulation period in the absence of any modulated signal. We observe contrast drops the modulation frequency approaches 200Hz, 100Hz, and 50Hz magnetic field noise components.As a first application we perform light shift spectroscopy of a narrow optical quadruple transition. Finally, we emphasize that the quantum lock-in technique is generic and can potentially enhance the sensitivity of any quantum sensor.Figure2. A measured lock-in phase vs. modulation frequency. A light shift of 9.7(4) Hz is measured (with 95% confidence).S. Kotler, N. Akerman, Y. Glickman, A. Kesselman and R. Ozeri, arXiv:quant-ph/0185372 (2010); accepted in Nature. |
