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