Quantum Many Body Control and Metrology with an Atom-Light Interface

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At its core, technology is concerned with the design and control of physical systems (devices) that can perform desired tasks. The more complex the device, the greater the likelihood that it will consist of many interconnected parts, as is the case, for example, with the hardware in a computer. Physicists have long known that the behavior of interconnected systems will change in fundamental ways when the entire network (nodes and interconnects) is governed by the laws of quantum mechanics, and more recent work has shown that this allows for dramatic improvements in the performance of computers, communication networks, and sensors. Building and operating quantum devices of this sort remains a grand challenge for modern science. This project will contribute by developing a quantum interface that can connect distant quantum systems. The main goal is to use laser light, first to connect and "entangle" the quantum states of atoms confined in a "trap" made of laser light, and subsequently to perform a quantum limited measurement on them. Ensembles of atoms are used in the most precise sensors of time (atomic clocks), of rotation and acceleration (inertial sensors), and of magnetic fields. The principal investigator and others have demonstrated that a quantum interface can be used to suppress the intrinsic quantum uncertainty of measurement (a phenomenon known as "squeezing"), and thus to improve the precision of such atomic sensors beyond the "standard" limit. This project seeks to increase the amount of squeezing that can be generated by making optimal use of the internal structure of the atoms, with the goal of boosting measurement sensitivity by an additional order of magnitude. A secondary goal is to explore a version of the atom-light quantum interface where the light is guided by an optical nanofiber and overlaps with atoms trapped close to its surface. This geometry is far more compact and has the potential to operate as a quantum node in an optical fiber based network.Quantum control on all scales, from single particles to complex many body systems, is a grand challenge for the second century of quantum mechanics. Over the past decade there has been substantial progress towards this goal, including significant advances in real-world technical capabilities. As a result, non-trivial quantum control has become routine in experiments ranging from quantum metrology to analog quantum simulation and rudimentary digital quantum computing. The goal of this project is to contribute new ideas and capabilities in the areas of quantum many body control and quantum metrology. The context is that of a Cs atomic ensemble, driven by magnetic fields and coupled to a quantized light field for the purpose of generating entanglement and performing quantum limited measurements. Two distinct versions will be studied: a "free-space" geometry with atoms in a dipole trap coupled to a paraxial probe beam, and a "nanofiber" geometry with atoms trapped and probed by evanescent fields around an optical nanofiber. The principal investigator brings expertise in internal state control of complex atoms that can significantly enhance the entangling power of the atom-light interface. A common measure of entangling power is the spin squeezing generated by quantum backaction when measuring the collective atomic spin. A preliminary exploration indicates that squeezing can be increased from a baseline of ~3dB to as much as 8dB through optimal control of the atomic internal state, reaching a level comparable to cavity enhanced experiments. The nanofiber platform is less developed, but short-circuits some of the limitations that diffraction imposes on atom-light coupling in free space, and has the potential to be more compact, less complex, and far more robust than cavity-enhanced setups.