xQIT W. M. Keck Foundation Center For Extreme Quantum Information Theory at the Massachusetts Institute of Technology
News About People Research Publications Events Courses Positions Contact

< Back to Research


Problem Importance

  Research > Projects

xQIT: Fundamental Limits on Quantum Sensing and Control

xQIT Focus Area 3: Fundamental Limits on Quantum Sensing and Control


Prof. Seth Lloyd, Team Leader
Prof. Leonid Levitov
Prof. Sanjoy Mitter
Prof. Jeffrey Shapiro
Prof. Peter Shor
Prof. Jean-Jacques Slotine
Postdoctoral Fellow
Graduate Student
Visiting Scientists

Why it is important to attack the problem?

The exponential advance in computing power embodied in Moore’s law arises out of the longstanding and ongoing miniaturization process whereby the transistors on computer chips become smaller and smaller and the chips themselves grow more complicated and powerful (and power hungry). Continuation of this miniaturization process requires ever more precise instrumentation and lithography techniques. As the components of
computers press down to the quantum-mechanical scale, so, too, do the instruments used to construct those components.

The advance of computing power is just one example of the march of technologies towards the quantum scale. Other powerful technologies rely intrinsically on quantum technologies. Thus, for example, the Global Positioning System derives its astounding capability from atomic clocks whose accuracies are inherently limited by the laws of quantum mechanics. To construct more powerful computers and more accurate atomic clocks, and to continue the advance of quantum technologies in general, we must understand the fundamental physical limits to the processes of sensing, control, and measurement that are imposed by quantum mechanics.

The physical laws that govern how much information a measurement apparatus or sensor can obtain are closely related to the physics of quantum communication. As Shannon noted, when an apparatus obtains information about some system, the interaction between system and apparatus is essentially a noisy communication channel: the system effectively “sends” information to the measurement apparatus, and that information is corrupted by noise and distortion. Measurement and sensing are key components of the general problem of quantum control, in which information is obtained and fed back to drive a quantum system towards some desired state or dynamics.

Measurement, sensing, and control are fundamentally concerned with the gathering, processing, and application of information, so it should come as no surprise that quantum mechanics introduces new features to these procedures. In particular, measurement uncertainty is intrinsic to quantum mechanics. Thus, after all technical noises are eliminated, the fundamental limits to the accuracy of measurement devices and sensors are those posed by quantum mechanics. Just as in the case of computation and communication, however, state-of-the-art precision measurement systems—such as optical interferometers, including those used for lithography—are reaching quantum-limited performance. Once again, these are standard quantum limits associated with the specific architectures of, say, interferometry with laser light. But quantum mechanics also provides new ways in which systems can be configured to obtain, process, and apply information, and the capabilities of these techniques—which employ peculiar quantum effects such as squeezing or entanglement—are not bound by the standard quantum limits. For example, it has long been known that the quantum-limited performance of squeezed-state interferometry is far better than the standard quantum limit,and more recently it has been shown that entanglement offers similar performance gains for optical lithography.

The rapid miniaturization of computer circuitry over the past half century, embodied in Moore’s law, has given rise to huge advances in computational power. Moore’s law itself arose from the similarly rapid increase in the accuracy and precision of technologies for measurement, sensing, manufacturing, and control. What are the ultimate physical limits on this miniaturization progression? Just how accurate can measurements, sensors, and control systems become? And how can we use quantum mechanics to attain those limits of accuracy? Answering these questions is crucial to continuing Moore’s law to the atomic scale. As Feynman noted, "There’s plenty of room at the bottom." The frontier of the very small is in fact a huge place—as long as one possesses the skills to live there. In attempting to reach the ultimate limits of accuracy for measurement, sensing, and control, we are developing the skills to explore and inhabit this frontier.




"Thus, after all technical noises are eliminated, the fundamental limits to the accuracy of measurement devices and sensors are those posed by quantum mechanics."