Quantum Interferometry with Photon-Subtracted Twin Beams

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Physics relies on the experimental observation of Nature and the Universe, with ever more precise measurements. Improving the precision and sensitivity of physical measurement is therefore key to furthering the progress of science. This project will explore a novel method to improve measurements using quantum optics to advance the art of interferometry. Interferometric measurements, in which the phase difference between two waves is measured in order to gain information about the physical laws that cause such a phase difference, are among the most precise classes of measurements. A spectacular illustration of the power of interferometry is the Laser Interferometer Gravitational-wave Observatory (LIGO), which has revealed the minescule ripples of the fabric of space-time caused by the merging of black holes in the cosmos, with three such events observed since the fall of 2015. Other prevalent examples of interferometry include high-resolution phase masks in microlithography, crucial to semiconductor chip fabrication and Moore's law in the computer industry; interferometric techniques for microscopy, such as phase contrast imaging in biology; and Ramsey interferometry, used for precise atomic clocks. Thus the results of this project can have a significant impact in a number of critical areas of technology.The ultimate sensitivity of interferometers such as LIGO and atomic clocks is dictated by quantum mechanics. Indeed, any interferometer, for example an optical one, possesses intrisically quantum mechanical measurement noise floors which are consequences of the quantum duality between the undulatory (electromagnetic wave) and corpuscular (photon) nature of light. Classical interferometers, such as LIGO and atomic clocks in their current versions, are shot-noise limited, which means their phase noise floor is of the order of the reciprocal of the square root of the average particle number (photon or atom) used in a measurement. However, this is far from the ultimate, Heisenberg, limit, which is of the order of the reciprocal of the average particle number. This limit can be reached by "quantum engineering" the light waves used in the interferometer. This project puts forward a new type of Heisenberg-limited interferometer, in which twin optical beams, the photon-correlated beams emitted by a optical parametric amplifier, see one photon subtracted from them in an indistinguishable manner before they are used as input in a Mach-Zehnder interferometer. The project is unique among all previous proposals for Heisenberg-limited interferometry in that it features a Heisenberg-limited noise floor, a strong, direct interference fringe signal, and is experimentally feasible using state-of-the-art quantum optics technology that was previously demonstrated in the group of Prof. Pfister at the University of Virginia. If successful, this project could bring about multiple orders of magnitude measurement sensitivity improvement in phase-contrast imaging, which would allow nondestructive in vivo biological imaging by applying quantum light.