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Weak values in an electronic mach-zehnder interferometer

Alessandro Romito

Published Oct 26, 2016• 0 citations
OpenAlex

Abstract

In the Demonstration it is possible to choose whether to plot the charge's weak value, D2<Q2>S1(in units of the electron charge), as a function of the voltage bias (measured in μV), the phase difference, or the temperature (measured in mK). (The phase difference includes the Aharonov–Bohm flux and the phases of the transmission coefficients of both the quantum point contacts.) In all cases it is possible to control the other parameters, including the transparencies of both the quantum point contacts and the duration of the measurement (measured in nsec). In the upper panel of the Demonstration we also plot the upper bound for standard quantum average of the charge in arm 2 (the lower bound is obviously 0): the weak value is clearly beyond the marked bounds

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In the Demonstration it is possible to choose whether to plot the charge's weak value, D2<Q2>S1(in units of the electron charge), as a function of the voltage bias (measured in μV), the phase difference, or the temperature (measured in mK). (The phase difference includes the Aharonov–Bohm flux and the phases of the transmission coefficients of both the quantum point contacts.) In all cases it is possible to control the other parameters, including the transparencies of both the quantum point contacts and the duration of the measurement (measured in nsec). In the upper panel of the Demonstration we also plot the upper bound for standard quantum average of the charge in arm 2 (the lower bound is obviously 0): the weak value is clearly beyond the marked bounds
Throughout the "creation" of Quantum Mechanics, a theory has been built that is based on a single way of measuring quantum systems. These measurements have always taken place so that the act of measuring disturbs the system to the point of collapsing it, and then it acquires all the properties of a single state. However, in the late 1980s, a new way of measuring a quantum system emerged, called the Weak Measurement. With it, we were able to deal with experiments with a new approach, where the configuration of the measurement is changed by a factor that disturbs the system very little. With the way of measuring before this new concept, it would be impossible to detect small changes in a quantum system, but now, the weak measurement reaches new horizons. We developed a form of weak measurement using a general coupling in terms of the superposition of an initial state and a final state, in the case of a qubit. With that, it was possible to see in more detail how the process of this way of measuring occurs, where we will see how the coupling between the two states must be done to carry out such an experiment. We have also developed a weak measurement in a quantum system using a Mach-Zehnder interferometer for such a study. In it, with the help of a value that will determine how weak the weak measure is, we can carry out a measure in which we do not need to disturb a system to obtain information.
We consider dephasing in the electronic Mach-Zehnder interferometer strongly coupled to current noise created by a voltage biased quantum point contact (QPC). We find the visibility of Aharonov-Bohm oscillations as a function of voltage bias and express it via the cumulant generating function of noise. In the large-bias regime, high-order cumulants of current add up to cancel the dilution effect of a QPC. This leads to an abrupt change in the dependence of the visibility on voltage bias which occurs at the QPC's transparency T=1/2. Quantum fluctuations in the vicinity of this point smear out the sharp transition.
Quantum interference is typically detected through the dependence of the interference signal on certain parameters (path length, Aharonov-Bohm flux, etc.), which can be varied in a controlled manner. The destruction of interference by a which-path measurement is a paradigmatic manifestation of quantum effects. Here we report on a novel measurement protocol that realizes two objectives: (i) certifying that a measured signal is the result of interference avoiding the need to vary parameters of the underlying interferometer, and (ii) certifying that the interference signal at hand is of quantum nature. In particular, it yields a null outcome in the case of classical interference. Our protocol comprises measurements of cross-correlations between the readings of which-path weakly coupled detectors positioned at the respective interferometer's arms and the current in one of the interferometer's drains. We discuss its implementation with an experimentally available platform: an electronic Mach-Zehnder interferometer (MZI) coupled electrostatically to "detectors" (quantum point contacts).