**单位：**MIT

研究方向：The Pressing Need for Advances in Extreme Quantum Infomation Theory

**xQIT: Introduction**

At the W. M. Keck Foundation Center for Extreme Quantum Information Theory (xQIT) at the Massachusetts Institute of Technology (MIT), we investigate information processing at the extreme limits posed by the laws of physics. Our objective is to devise the new quantum algorithms and protocols needed to approach the ultimate performance bounds of quantum information systems.

Existing technologies for these applications are currently reaching bounds set by quantum mechanics, that is, the standard quantum limits that constrain the performance of conventional systems. Theory and some proof of principle experiments have already shown that standard quantum limits do not represent the ultimate capabilities that might be achieved with novel system designs which make informed use of quantum-mechanical properties.

If we succeed, we will have uncovered the fundamental physical limits to computation, communication, and precision measurement.

**xQIT: Objectives and Goals**

The overarching objective of xQIT is to solve—or make significant progress toward solving—three interrelated theoretical problems in extreme quantum information:

Solving average-case NP-complete problems.

Deriving capacities and coding techniques for quantum communication channels.

Obtaining fundamental physical limits to quantum sensing and control.

Each of these problems has the potential for significant impact on society:

NP-complete problems encompass the bulk of outstanding problems in optimization. Examples include optimization tasks arising in communication networks, computer design, financial structures and portfolio management, drug design, and allocation of public resources, to name just a few. Enormous societal benefits would accrue, should solutions to these problems become possible with quantum computers.

All communication channels—including those associated with fiber-optic, wireless, and satellite technologies—are, at bottom, quantum mechanical, and existing optical communication systems are already pushing up against quantum-mechanical bounds, i.e., the standard quantum limits. Standard quantum limits, however, do not represent the ultimate capabilities that might be achieved, because conventional communication systems do not exploit "weird" quantum features like entanglement. So, if we are to reap the full harvest of the ongoing information processing revolution based on digital computation and communication, we must derive the ultimate quantum-limited capacities of such channels and develop coding techniques for attaining those capacities.

The rapid increase in computer power and the development of precision measurement technologies such as GPS rely crucially on technologies for fabrication, sensing, and control, and these too are reaching their standard quantum limits. Whether it be to improve the lithographic techniques used to make microprocessors or to enhance the atomic clocks on which GPS relies, we need to break the bonds of conventional thinking to reach truly fundamental limits on fabrication, sensing, and control by using all the special properties that quantum mechanics affords. For example, theory has already shown that entanglement can, in principle, greatly increase the accuracy of a variety of precision measurement systems. So, if we are to maintain the rapid advance of precision measurement technologies for a wealth of sensing and control applications, we must identify the fundamental performance bounds that quantum mechanics imposes on such systems, and we must develop procedures for attaining those bounds.

The preceding three problems lie at the core of quantum information theory. If one or more of them can be solved, then quantum information will have made a great societal contribution. Note that these problems are theoretical, not experimental. If theorists can solve one or more of them, it will make the need for quantum computing and quantum communication technology far more compelling than it is today, and will therefore spur government and industry to major investments in developing these technologies.