Session T4: Invited Session: Quantum Metrology

Quantum metrology: Past, present, and future

The ability to make high-precision measurements lies at the heart of physics and optical science. Ever since quantum mechanics was discovered and became the framework for physical law, it has been understood that quantum effects limit how well physical quantities can be measured. I discuss the development of ideas regarding quantum limits on measurement precision, the accomplishments in achieving quantum-limited measurement precision and in overcoming perceived quantum limits, and the maturation of the field into quantum metrology, a subfield of quantum information science in which researchers rigorously investigate and formulate quantum limits and strategies for achieving those limits.   

Optical atomic clocks and metrology

The atomic clock has long demonstrated the capability to measure time or frequency with very high precision. Consequently, these clocks are used extensively in technological applications such as advanced synchronization or communication and navigation networks. Optical atomic clocks are next- generation timekeepers which reference narrowband optical transitions between suitable atomic states. Many optical time/frequency standards utilize state-of-the-art quantum control and precision measurement. Combined with the ultrahigh quality factors of the atomic resonances at their heart, optical atomic clocks have promised new levels of timekeeping precision, orders of magnitude higher than conventional atomic clocks based on microwave transitions. Such measurement capability enables and/or enhances many of the most exciting applications of these clocks, including the study of fundamental laws of physics through the measurement of time evolution. Here, I will highlight optical atomic clocks and their utility, as well as review recent advances in their development and performance. In particular, I will describe in detail the optical lattice clock and the realization of frequency measurement at the level of one part in 10$^{18}$. To push the performance of these atomic timekeepers to such a level and beyond, several key advances are being explored worldwide. These will be discussed generally, with particular emphasis on our recent efforts at NIST in developing the optical lattice clock based on atomic ytterbium.   

True Limits to Precision via Unique Quantum Probe

Quantum instruments derived from composite systems allow greater measurement precision than their classical counterparts due to coherences maintained between N components; spins, atoms or photons. Decoherence that plagues real-world devices can be particle loss, or thermal excitation and relaxation, or dephasing due to external noise sources (and also due to prior parameter uncertainty). All these adversely affect precision estimation of time, phase or frequency. We develop a novel technique uncovering 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; the shape of the well is dictated by the type and strength of decoherence. The formalism is applied to prototypical Mach-Zehnder and Ramsey interferometers to discover the ultimate performance of real-world instruments.\\[4pt] In collaboration with Sergey Knysh, Quantum Artificial Intelligence Laboratory (QuAIL), NASA Ames Research Center, Moffett Field, California 94035, USA and Edward H. Chen, Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology (MIT), Cambridge, Massachusets 02139, USA.   

Quantum Limits in Gravitational Wave Detectors

The sensitivity of a new generation of laser interferometer gravitational wave detectors is expected to be predominantly limited by the quantum fluctuations of light. I will explore the origins and implications of this quantum limit, and describe experimental progress toward circumventing it using sub-quantum (squeezed) states of light.