*****NON-TECHNICAL ABSTRACT***** Quantum mechanics demands that the act of measuring the position of a particle such as an electron will influence the particle by changing its velocity. The famous statement that such a measurement acts back on the object being measured is known as Heisenberg's Uncertainly Principle. One often speaks of a measurement as having some "backaction" on the object being measured. This single-investigator award supports research that will directly investigate these fundamental issues. Recent technological advances in the PI's laboratory have led to ultrasensitive charge detectors for which the noisy forces that cause to backaction can be directly observed. These detectors will be used to observe the position of a single electron confined to a nanometer-sized region in semiconductor known as a quantum dot. The backaction forces that influence the electron position will independently be monitored. Measurements like these will allow a direct investigation of the physical processes underlying the Uncertainty Principle. Results from this research will also have implications for measurement of bits in quantum computers, which are expected to be able to solve problems classical computers cannot. Students involved in this project will receive training in a broad array of advanced fabrication and measurement techniques, leaving them well-prepared for a career in academia, industry or government.
There is a deep relationship between Heisenberg's Uncertainty Principle and noise in a quantum system. Since a detector necessarily influences a system it is measuring, the quantum mechanical noise produced by a system should be directly influenced by the noisy force (the backaction) exerted on it by a detector. This single-investigator award supports research that will directly investigate these fundamental issues. Recent technological advances in the PI's laboratory have led to ultrasensitive charge detectors for which the intrinsic noise that leads to backaction can be observed. By using these detectors to monitor the position of a single electron in a semiconductor device, this research will take a direct look at the physical processes underlying the Uncertainty Principle. Results from this work will also have implications for the field of quantum computation, in which a central problem is the measurement of the state of a quantum bit. Students involved in this project will receive training in a broad array of advanced fabrication and measurement techniques, leaving them well-prepared for a career in academia, industry or government.
In classical mechanics, it is possible to imagine a measurement that is completely passive, in that it only records information from whatever is being observed and does not influence it in any way. Quantum mechanics, however, states that the act of observing a system must influence that system in some way; this phenomenon is known in physics as backaction, since performing a measurement necessarily "acts back" on the system being measured. In the research funded by this grant, we have taken a direct look at the ways in which electrical devices based on superconductors or semiconductors exert backaction on objects they measure. The two particular devices we investigated, the single electron transistor and the quantum point contact, are widely used for measurements of very small amounts of electrical charge. Their ability to measure even the charge of a single electron makes these two devices very versatile for a whole host of applications, ranging from sensing very small magnetic moments to detecting extremely tiny mechanical motions. Our measurements led to two key outcomes. First, our measurements showed that superconducting single electron transistors operated as sensors of electrical charge can closely approach the limits on backaction set by quantum mechanics. In other words, these sensors measure charge very efficiently, in that they do so while having close to the minimum backaction allowed by the laws of quantum physics. Second, our measurements of quantum point contacts led to the discovery that the backaction of these microscopic electrical devices can have an effect even on a macroscopic object. Under the right circumstance, a quantum point contact made in a chip of a semiconductor called gallium arsenide can measure small motions of the chip itself. But measuring the motion of the chip leads to a backaction force that reinforces the same motion. The net result is that forces caused by a microscopic object can dominate the motion of a macroscopic object. The laws of quantum mechanics make themselves known in surprising ways, and can have unexpected effects on the classical world in which we live. In addition, this work has supported education and training of four graduate and two undergraduate students.
