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Quantum field theory is completely characterized by the field correlations between spacetime points. In turn, some of these can be accessed by locally coupling to the field simple quantum systems, a.k.a. particle detectors. In this work, we consider what happens when a quantum- controlled superposition of detectors at different space-time points is used to probe the correlations of the field. We show that, due to quantum interference effects, two detectors can gain information on field correlations which would not be otherwise accessible. This has relevant consequences for information theoretic quantities, like entanglement and mutual information harvested from the field. In particular, the quantum control allows for extraction of entanglement in scenarios where this is otherwise provably impossible.

CAP Virtual Congress 2021

Quantum Temporal Superposition: the case of QFT

indefinite causal order

relativistic quantum information

entanglement harvesting

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technical paper

DarkSide-20k is a planned two-phase liquid Argon time-projection-chamber (LAr-TPC) for direct WIMP search. The S1 and S2 light from the 20 ton fiducial volume is detected with Silicon photomultipliers and digitized. Due to the expected data rate of hundreds of MB/s it is impractical to record full waveform data continuously like in the smaller DarkSide-50 experiment, instead complex filtering and processing is required to select meaningful data. Since this processing needs to take into account all (above threshold) data channels at once, it cannot happen directly in the acquisition module, but all data has to be transferred to a processing farm, where processing time will likely exceed acquisition time. In order to address this, data will be compiled into time slices (on the order of a second), which get distributed to individual processing machines. The process is somewhat complicated by the fact that due to the long drift time in the TPC signals from the same physical event can be spread out over milliseconds, it is thus necessary for time slices to overlap enough to ensure each physical event is contained completely in one slice.
This presentation will give an overview of the DarkSide vertical slice scheme and show how it is implemented within a MIDAS-based data acquisition system.

The Vertical Slice Data Processing Scheme for DarkSide

Attosecond science is a relatively new research field founded on high-harmonic generation (HHG) in atomic and molecular gases, which has recently transitioned to experiment in 3-dimensional solids. Most of the physics underlying attosecond pulse emission from 2-dimensional semiconductors is unknown. Following years of research focusing on HHG in bulk semiconductors, this project focuses on the mostly unstudied van der Waals materials ReS$_2$ and MoS$_2$. We measure the power of the emitted high harmonics for varying orientation of the crystallographic axis with respect to the linear polarization of the incident driving laser beam. Both crystals emit high harmonics in a very distinctive pattern which reveals the symmetry of the crystal as well as microscopic details about the orbitals involved in the emission. Our demonstration inches us closer to understanding high-field phenomena in van der Waals materials, and controlling high-harmonic emission with unprecedented precision thanks to the high degree of tunability of 2-dimensional materials and their heterostructures.

An exploration for new phenomena using high-harmonic generation in two-dimensional van der Waals materials

Graphene is among the most promising materials considered for next-generation gas sensing due to its properties including high mechanical strength and flexibility, high surface-to-volume ratio, large conductivity, and low electrical noise. While gas sensors based on graphene devices have already demonstrated high sensitivity, one of the most important figures of merit, selectivity, remains a challenge. In the last few years, however, surface functionalization emerged as a potential route to achieve selectivity. In this talk, we focus on experiments where we functionalized the surface of CVD graphene field-effect transistors (GFET) through thermal evaporation of metal-free phthalocyanines and copper phthalocyanines. We present and discuss sensitivity and selectivity results obtained when such sensors are exposed to volatile organic compounds such as ethanol, toluene, formaldehyde, and acetone. In general, the functionalized GFET presented enhanced selectivity for oxygen-containing molecules (formaldehyde and acetone).

Functionalized surfaces of graphene field effect transistors for gas sensing

Quantum confinement and manipulation of charge carriers are critical for achieving devices practical for various quantum technologies. Atomically thin transition metal dichalcogenides (TMDCs) have attractive properties such as spin-valley locking, large spin-orbit coupling and high confinement energies which provide a promising platform for novel quantum technologies. In this talk, we present the design and fabrication of electrostatically gated quantum structures based on fully encapsulated monolayer tungsten diselenide (WSe2) aimed at probing the confined electron states in these structures. Furthermore, we show that laterally gated quantum point contacts successfully pinch-off the current across the device with gate voltages consistent with their lithographic widths. Finally, we discuss the origins of the observed mesoscopic transport features related to the emergence of the quantum dots through the WSe2 channel.

Electrostatically gated quantum dots in van der Waals materials

Approximation of the ground state wave function and energy of a quantum system is often achieved using perturbation theory or the variational method. The former approach suffers from the requirement that the Hamiltonian perturbation be small enough for the series to converge while the variational method is only as good as the choice of functions used in the expansion, providing only an approximate ground state whose mean energy is an upper bound to the ground state energy. The method of gradient descent, typically used for optimizing functions over finite dimensions, may be generalized to the infinite-dimensional Hilbert space of a quantum mechanical system to avoid both previous limitations. One may find the ground state of a quantum system by minimizing the energy functional of the wave function of the system via iterative calculation of better approximations using the gradient of the functional. This can be achieved in an infinite-dimensional setting by careful bookkeeping of only the non-zero components of the state vector in the chosen basis of expansion and those matrix elements of the Hamiltonian in that basis required to calculate the next iterative approximation via gradient descent. The sparse nature of the Hamiltonian matrix elements in a chosen basis for many quantum systems ensures that subsequent iterations of the state vector have only a finite number of non-zero components, thereby making the method computationally tractable.

The gradient descent method will be formulated for a quantum system with a time-independent Hamiltonian. The algorithm will be illustrated with a simple quantum system. The impact of symmetry in the choice of initial state vector will be discussed. Limitations of the method and potential improvements (e.g. conjugate gradient) will be considered.

Implementing the Gradient Descent Method in an Infinite Dimensional Hilbert Space

This talk will focus on an overview of Eliashberg theory, a formalism that Jules was very well known for. But I will also discuss some potential shortcomings of this framework, as time permits, from the weak coupling to the strong coupling limit, with polaron physics, and applicability to the hydride and superhydride materials.

Eliashberg Theory and Jules Carbotte

In this work we explore the use of multi-layer graphene (MLG) films grown by chemical vapor deposition for adaptive thermal camouflage. Using different ionic liquids, we tune the opto-electronic properties of MLG (150 – 200 layers) and investigate changes in optical reflectivity and emissivity in the infrared region (IR). We fabricate devices having a metallic back electrode supporting a porous membrane onto which we deposit the MLG. We use both non-stretchable polyethylene (PE), and stretchable polydimethylsiloxane (PDMS) as porous membranes. Using a thermal imaging system, we demonstrate that even when the device temperature is maintained higher than the environment, the MLG emissivity can be electrically controlled such that the device appears indistinguishable from the environment [1]. Moreover, we evaluate the performance of such devices based on flexible textiles towards developing a new material platform for defense applications.

Multilayer Graphene as Adaptive Thermal Camouflage

Supercontinuum generation in optical fiber is the result of an interplay between multiple nonlinear processes. Simulations to reproduce experimental observations are well documented: a differential equation known as the generalized nonlinear Schrödinger equation (GNLSE) is solved to determine the effect of propagation on the spectral and temporal profiles of the slowly-varying amplitude of a laser pulse. For the scalar case, the GNLSE has been shown to yield good results. In a more accurate case, it becomes necessary to take into account the polarization evolution of light as it travels through the fiber, for instance for a fiber with two orthogonal principal axes. This is typically added into the GNLSE to form coupled equations including the birefringence of the fiber and additional nonlinear terms coupling the slowly-varying amplitudes of two polarization components. Upon studying the experimental spectra obtained from a germania-doped photonic crystal fiber, we observe that the polarization state varies significantly across the spectrum, leaving the spectral edges with a greater degree of polarization than the areas near the input pump wavelength. We also observe that the spectral region around the pump maintains a low degree of polarization. These trends lead us to believe that a portion of the energy transfer from input polarization state to an orthogonal output state is proportional to the propagation length, consistent with a linear depolarization contribution that cannot be reproduced using only nonlinear equations. We therefore demonstrate the effect of an additional depolarization term to account for the redistribution of energy between polarization modes. Furthermore, the interaction between linear and nonlinear depolarization mechanisms leads to a more realistic estimate of the polarization maintaining capability of an optical fiber over a broadband spectrum.

Adding a linear contribution to depolarization in simulations of supercontinuum generation

Spectral purity in laser emission is key for several applications such as remote sensing, non-linear optics and laser spectroscopy. However, producing single mode emission at high power in free-space, standing-wave resonators is challenging. Nanostructured laser mirrors can be used to achieve that in compact microchip monolithic laser resonators without using any additional intra-cavity element [1,2]. For instance, using a pair of mirrors displaying a π phase shift between orthogonal polarization states at reflection produces the twisted mode operation, which allows one to eliminate the contrast of the standing wave inside the cavity and makes it possible to eliminate multi-longitudinal mode operation originating from axial spatial hole burning. However, dual-polarization emission remains an issue if there is no mechanism to suppress competing polarization states, and this may give rise to unstable emission, in addition to producing dual frequency emission. In this presentation, we propose a resonator made of anisotropic mirrors displaying both birefringence and diattenuation as a mean to eliminate both spatial hole burning and dual polarization emission. The relative angle of the two mirrors’ principal axes is adjusted such that the laser operates just at the transition between broken and unbroken parity-time symmetric polarization states. At this transition point, the two eigen-polarization states are found to merge to a single state, called an exceptional point, while the counter-propagating waves are orthogonal, and no spatial hole burning takes place [3]. Hence, microchip PT-symmetric states are an attractive option to achieve single mode operation from a miniature microchip laser without the need to introduce any additional intracavity element.

[1] J.-F. Bisson, K. N. Amouzou, Controlling spatial hole burning in lasers using anisotropic laser mirrors J. Opt. Soc. Amer. B 36(12), 3322-
3332, (2019).
[2] J.-F. Bisson, K. N. Amouzou, Elimination of spatial hole burning in solid-state lasers using nanostructured thin films Appl. Opt. 59(5), A83-
A91, (2020).
[3] J.-F. Bisson, Y. C. Nonguierma, Single-mode lasers using parity-time-symmetric polarization eigenstates, Phys. Rev. A, 102, 043522, (2020).

Eliminating dual-polarization laser emission and spatial hole burning by using parity-time-symmetric eigenstates

A programmable quantum simulator can simulate models of quantum many-body systems that may otherwise be impossible to be modeled with conventional computers. Simulation of such many-body quantum systems may further advance our understanding of exotic quantum materials, fundamental forces of nature, molecules for drug discovery, etc. Laser-cooled and trapped atomic ions serve as an ideal platform for the simulation of interacting quantum spin models. In this talk, we describe the development of a large-scale quantum simulator based on a multi-segmented blade electrode ion trap capable of trapping a large (>50) chain of Ytterbium ions in a near uniformly spaced configuration. A high numerical aperture (NA) holographic optical addressing system will be used for aberration-corrected addressing of the trapped ions with minimal ‘crosstalk error’ [1]. This would usher the capability to engineer dynamic many-body Hamiltonians with control over the individual ion-spins and the interactions between them. A high NA imaging system would allow us to detect the spin states of individual ions simultaneously with high precision, including the capability to perform partial measurements without necessarily decohering rest of the system. The trap will be housed inside an XHV vacuum system to keep the ions free from background collisions. Optimal vacuum system engineering has allowed us to design a vacuum vessel with simulated pressures of at least one order of magnitude lower than current room temperature trapped ion quantum simulators.
[1] C.-Y. Shih, S. Motlakunta, N. Kotibhaskar, M.Sajjan, R. Hablützel, R. Islam npj Quantum Information (in press, 2021)

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Quantum Temporal Superposition: the case of QFT

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