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arXiv:1612.09381 · APS March Meeting Abstracts

Towards Measuring Vacuum Polarization of Quantum Electrodynamics with Superconducting Junctions

Ali Izadi Rad

Published Dec 30, 2016• 0 citations
arXivHTMLPDF

Abstract

In this proposal, we present an experimental setup based on superconducting circuits and Josephson junctions to explore the modification of Josephson coefficient in the presence of external magnetic field due to vacuum polarization of quantum electrodynamics. This robust experiment can be considered as one of the few possible chances to observe the fine quantum field theory corrections in the low energy regimes in condensed matter systems. It can also be a new check for the universality of Josephson constant which is important in metrology. We will expect the signal to noise ratio of the read-out signal to increases quadratically by running time of the experiment. This characteristic of the output signal the will guarantee the feasibility of measurements with desired precision

DOI
10.48550/arxiv.1612.09381

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In this proposal, we present an experimental setup based on superconducting circuits and Josephson junctions to explore the modification of Josephson coefficient in the presence of external magnetic field due to vacuum polarization of quantum electrodynamics. This robust experiment can be considered as one of the few possible chances to observe the fine quantum field theory corrections in the low energy regimes in condensed matter systems. It can also be a new check for the universality of Josephson constant which is important in metrology. We will expect the signal to noise ratio of the read-out signal to increases quadratically by running time of the experiment. This characteristic of the output signal the will guarantee the feasibility of measurements with desired precision
The concept of coupled robust field induced Josephson junctions placed in complex electromagnetic environments is presented. Such structures are made by polarization of superconducting nanostructures by magnetic fields. The methodology of modeling of structures and possible implementations is introduced. The presented scheme is expected to implement and describe both classical and quantum computers with use of Josephson junction and artificial evolution. In such cases one can obtain unexpected new topologies of circuits that can contribute in enhancement of known and used circuit schemes. Furthermore one can obtain the non-invasive detectors of moving charged particles from robust field induced Josephson junctions which are illustrated by examples.
We investigate the possibility to employ magnetic Josephson junctions as magnetic noise detectors. To illustrate our idea, we consider a system consisting of a quantum dot coupled to superconducting leads in the presence of an external magnetic field. Under appropriate assumptions, we relate the noise in the Josephson current to magnetization noise. At the magnetic field driven 0-$\ensuremath{\pi}$ transition, the junction sensitivity as magnetic noise detector is strongly enhanced and it diverges in the zero temperature limit. Moreover, we demonstrate that, if also dot energy is affected by fluctuations, only the magnetic noise channel contributes to Josephson current noise response when the quantum dot is tuned in resonance with superconducting leads.
Circuit quantum electrodynamics has proven to be a powerful tool to probe mesoscopic effects in hybrid systems and is used in several quantum computing (QC) proposals that require a transmon qubit able to operate in strong magnetic fields. To address this we integrate monolayer graphene Josephson junctions into microwave frequency superconducting circuits to create graphene based transmons. Using dispersive microwave spectroscopy we resolve graphene's characteristic band dispersion and observe coherent electronic interference effects confirming the ballistic nature of our graphene Josephson junctions. We show that the monoatomic thickness of graphene renders the device insensitive to an applied magnetic field, allowing us to perform energy level spectroscopy of the circuit in a parallel magnetic field of 1 T, an order of magnitude higher than previous studies. These results establish graphene based superconducting circuits as a promising platform for QC and the study of mesoscopic quantum effects that appear in strong magnetic fields.
We investigate the entanglement properties of a standard circuit-QED setup that consists of a Cooper pair box coupled to a long chain of Josephson junctions. We calculate the static charge polarization at finite distances along the device. Our calculations reveal a deep connection to the Kondo screening cloud, together with robust correlations that are difficult to measure in a condensed matter context. We also find weak sensitivity of these Kondo signatures to the actual parameters and design of the device, demonstrating the universality of the Josephson entanglement cloud.
We have performed a quantitative electrodynamic experiment on a Josephson oxide tunneling junction. Using a current source at 30 Mc/sec, we have verified the time dependence of the Josephson equation, j=j0j={j}_{0}j=j0​ $sin [{\ensuremath{\gamma}}_{l}\ensuremath{-}{\ensuremath{\gamma}}_{r}\ensuremath{-}(\frac{2e}{\ensuremath{\hbar}})\ensuremath{\int}\mathrm{A}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{dl}]$, where $(\frac{\ensuremath{\partial}}{\ensuremath{\partial}t})[{\ensuremath{\gamma}}_{l}\ensuremath{-}{\ensuremath{\gamma}}_{r}\ensuremath{-}(\frac{2e}{\ensuremath{\hbar}})\ensuremath{\int}{l}^{r}\mathrm{A}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{dl}]=\ensuremath{-}(\frac{2eV}{\ensuremath{\hbar}})$. Thus the limiting voltage is shown to obey the quantum relationship relationship, $2eV=\ensuremath{\hbar}\ensuremath{\omega}$. Measurements were made completely in the coherent state showing the diffraction nature of the junction as a function of applied magnetic field. The Josephson (parametric) inductance of the junction was directly observed and is in good agreement with the theory.
With the recent technical breakthroughs, physicists fully play with the beauty of quantum mechanics. In this theoretical work on the mesoscopic Josephson effect, we take advantage of the collaborations with experimentalists as well as the exchanges between the cold atoms and condensed matter communities. We consider various systems based on the Josephson junction, beginning with its quantum description in the underdamped regime. Using the Keldysh formalism, we obtain the current-voltage characteristics from the classical to the zero temperature regime and the quantum Smoluchowski equation in the quasi-classical limit. We then study the quantum dynamics of a phase qubit realized with a SQUID in a novel configuration where tunneling events occur through two quartic barriers. The escape rate in this camel-back potential, calculated with the instantons technique, allows us to describe the experiments. Circuit quantum electrodynamics predicts that a lasing effect appears when a qubit is coupled to a resonant cavity. We consider the case of a charge qubit and also that of a transmon which exploits the Purcell effect. With the Lindbladian we obtain the density matrix from which we derive the spectrum of the output field. Finally, we study a gas of cold atoms in a ring shape trap comprising a barrier, thus creating a Bose Josephson junction. The low energy physics is described through the correlation functions with the Luttinger liquid theory. We show that the quantum fluctuations in the ring induce a renormalization of the Josephson energy.