QUROPE Quantum Information Processing and Communication in Europe

Objectives: Specifically quantum phenomena such as coherence and entanglement can be exploited to develop new modes of measurements, sensing, and imaging that offer unprecedented levels of precision, spatial and temporal resolution, and possibly auto-compensation against certain environmental factors, such as dispersion. These promising applications require development of techniques that will be robust against noise and imperfections to be deployed in real-world scenarios. Quantum technologies will benefit in particular time and frequency standards, light-based calibration, gravitometry, magnetometry, accelerometry, including the prospects of offering new medical diagnostic tools.

State of the art: Reaching quantum-enhanced precision beyond standard quantum limits in metrology relies on generating non-classical collective states of atoms and non-classical multi-photon states of light. Extensive effort has been dedicated to these goals with proof-of-principle demonstrations in the atomic domain and the first squeezed-light-enhanced operation of a gravitational wave detector with practical suppression of vacuum fluctuations. Novel concepts, such as systems with an effective negative mass or negative frequency have been shown to be capable of providing magnetometry with virtually unlimited sensitivity. Possibilities to define new frequency standards have been explored with the readout based on quantum logic techniques borrowed directly from the field of quantum computing and with entangled atoms providing ultimate quantum sensitivity. Enormous progress has been made on single photon sources, both deterministic and heralded, that can be used for optical calibration as well as a building block for photonic quantum communication and computing. Artificial atoms (such as nitrogen vacancy centers) have been investigated as ultraprecise sensors e.g. in magnetometry.

Future directions: Original techniques are needed to make quantum-enhanced metrology and sensing deployable in non-laboratory environments. Because of the wide range of prospective applications and their specificity, a broad range of physical platforms needs to be considered, including (but not limited to) trapped ions, ultra-cold atoms and room-temperature atomic vapours, artificial systems such as quantum dots and defect centers, as well as all-optical set-ups based e.g. on nonlinear optical interactions. Thorough theoretical analysis of noise mechanisms is needed, leading to feasible proposals that will be subsequently implemented to realize quantum-enhanced strategies. In particular the following need to be addressed:

• Novel sources of non-classical radiation and methods to engineer quantum states of matter are required to attain quantum-enhanced operation.
• Develop detection schemes that are optimized with respect to extracting relevant information from physical systems, with optimization criteria selected for specific applications. These techniques may find applications in other photonic technologies, e.g. increasing transmission rates in optical communication.
• Micro- and nanofabrication of quantum sensors including integration with fiber networks.
• Development of hybrid quantum sensors that use optimal quantum interfaces for transduction of signals across the electro-magnetic radiation spectrum.
• Compact solutions for quantum imaging, allowing for the interconversion of detected frequencies including preservation of coherence, as well as quantum ranging and timing that can suppress the spatial/temporal spread of transmitted signals.
• Implementation of entanglement assisted atom clocks
• Study of the performance of quantum sensing protocols in realistic regimes including noise and losses.
• Extend the reach of quantum sensing and metrology into other fields of science to uncover novel natural phenomena, e.g. biology, fundamental physics, high-energy physics, quantum gravity.