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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.


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.

S. Kotler, N. Akerman, Y. Glickman, A. Kesselman and R. Ozeri, Nature 473, 61 (2011)

Retinal degenerative diseases lead to blindness due to loss of the “image capturing” photoreceptors, while neurons in the “image processing” inner retinal layers are relatively well preserved. Electronic retinal prostheses seek to restore sight by electrically stimulating surviving neurons. Current devices are powered through inductive coils, requiring complex surgical methods to implant the coil-decoder-cable-array systems, which deliver energy to retinal stimulating electrodes via intraocular cables.
We developed a photovoltaic retinal prosthesis where each pixel in the subretinal array directly converts light into stimulation current, avoiding the use of bulky power supplies, decoding electronics, and wiring, and thereby reducing surgical complexity. A processed video stream is projected onto retina by video goggles using pulsed near infrared (~900 nm) light. Each pixel contains 3 photodiodes in series connected between the central active electrode and a concentric return electrode. Implants with three pixel sizes: 280, 140 and 70 m have being fabricated.
In-vitro electrophysiological recordings from rat retinas demonstrated retinal stimulation with peak irradiance threshold of 0.3 mW/mm2 using 4 ms pulses – more than 2 orders of magnitude below the ocular safety limits. Retinal responses were detected even upon illumination of a single 70 m pixel (having 20 m active electrode). Elicited retinal responses disappeared upon addition of synaptic blockers, indicating that stimulation is mediated by retinal network, and raising hopes that prosthetic vision will preserve some of the retina’s natural signal processing. Retinal stimulation was also detected in rats in-vivo by Visual Evoked Potentials at irradiance of 0.5 mW/mm2 with 10 ms pulses, confirming the possibility of a fully-integrated high-resolution photovoltaic retinal prosthesis.