Superposition Of Coherent States Research Articles

Gauge-invariant absorption of light from a coherent superposition of states

Absorption and emission of light is studied theoretically for excited atoms in coherent superposition of states subjected to isolated attosecond pulses in the extreme ultraviolet range. A gauge invariant formulation of transient absorption theory is motivated using the energy operator from Yang's gauge theory. The interaction, which simultaneously couples both bound and continuum states, is simulated by solving the time dependent Schr\"odinger equation for hydrogen and neon atoms. A strong dependence on the angular momentum and the relative phase of the states in the superposition is observed. Perturbation theory is used to disentangle the fundamental absorption processes and a rule is established to interpret the complex absorption behaviour. It is found that non-resonant transitions are the source of asymmetry in energy and phase, while resonant transitions to the continuum contribute symmetrically to absorption of light from coherent superpositions of states.

Quantum routing of information using chiral quantum walks

We address routing of classical and quantum information over quantum network and show how to exploit chirality (directionality) to achieve nearly optimal and robust transport. In particular, we prove how continuous-time chiral quantum walks over a minimal graph are able to model directional transfer of information over a network. At first, we show how classical information, encoded onto an excitation localized at one vertex of a simple graph, may be sent to any other chosen location with nearly unit fidelity by tuning a single phase. Then, we prove that high-fidelity transport is also possible for coherent superpositions of states, i.e., for routing of quantum information. Furthermore, we show that by tuning the phase parameter, one obtains universal quantum routing, i.e., independent on the input state. In our scheme, chirality is governed by a single phase, and the routing probability is robust against fluctuations of this parameter. Finally, we address characterization of quantum routers and show how to exploit the self-energies of the graph to achieve high precision in estimating the phase parameter.

Optical Manipulation of Layer–Valley Coherence via Strong Exciton–Photon Coupling in Microcavities

AbstractCoherent control and manipulation of quantum degrees of freedom such as spins forms the basis of emerging quantum technologies. In this context, the robust valley degree of freedom and the associated valley pseudospin found in two‐dimensional transition metal dichalcogenides is a highly attractive platform. Valley polarization and coherent superposition of valley states have been observed in these systems even up to room temperature. Control of valley coherence is an important building block for the implementation of valley qubit. Large magnetic fields or high‐power lasers have been used in the past to demonstrate the control (initialization and rotation) of the valley coherent states. Here, the control of layer–valley coherence via strong coupling of valley excitons in bilayer WS2 to microcavity photons is demonstrated by exploiting the pseudomagnetic field arising in optical cavities owing to the transverse electric–transverse magnetic (TE–TM)mode splitting. The use of photonic structures to generate pseudomagnetic fields which can be used to manipulate exciton‐polaritons presents an attractive approach to control optical responses without the need for large magnets or high‐intensity optical pump powers.

Simultaneous Preparation of Two Optical Cat States Based on a Nondegenerate Optical Parametric Amplifier

AbstractThe cat state, known as the superposition of coherent states, has broad applications in quantum computation and quantum metrology. Increasing the number of optical cat states is crucial to implement complex quantum information tasks based on them. Here, two optical cat states are prepared simultaneously based on a nondegenerate optical parametric amplifier. By subtracting one photon from each of two squeezed vacuum states, two odd cat states with orthogonal superposition direction in phase space are prepared simultaneously, which have similar fidelity of 60% and amplitude of 1.2. Compared with the traditional method to generate two odd optical cat states based on two degenerate optical parametric amplifiers, only one nondegenerate optical parametric amplifier is applied in this experiment, which saves half of the quantum resources of nonlinear cavities. The presented results make a step toward preparing the four‐component cat state, which has potential applications in fault‐tolerant quantum computation.

Multiphoton non-local quantum interference controlled by an undetected photon

The interference of quanta lies at the heart of quantum physics. The multipartite generalization of single-quanta interference creates entanglement, the coherent superposition of states shared by several quanta. Entanglement allows non-local correlations between many quanta and hence is a key resource for quantum information technology. Entanglement is typically considered to be essential for creating non-local quantum interference. Here, we show that this is not the case and demonstrate multiphoton non-local quantum interference that does not require entanglement of any intrinsic properties of the photons. We harness the superposition of the physical origin of a four-photon product state, which leads to constructive and destructive interference with the photons’ mere existence. With the intrinsic indistinguishability in the generation process of photons, we realize four-photon frustrated quantum interference. This allows us to observe the following noteworthy difference to quantum entanglement: We control the non-local multipartite quantum interference with a photon that we never detect, which does not require quantum entanglement. These non-local properties pave the way for the studies of foundations of quantum physics and potential applications in quantum technologies.

The role of symmetric vibrational modes in the decoherence of correlation-driven charge migration.

Due to the electron correlation, the fast removal of an electron from a molecule may create a coherent superposition of cationic states and in this way initiate pure electronic dynamics in which the hole-charge left by the ionization migrates throughout the system on an ultrashort time scale. The coupling to the nuclear motion introduces a decoherence that eventually traps the charge, and crucial questions in the field of attochemistry include how long the electronic coherence lasts and which nuclear degrees of freedom are mostly responsible for the decoherence. Here, we report full-dimensional quantum calculations of the concerted electron-nuclear dynamics following outer-valence ionization of propynamide, which reveal that the pure electronic coherences last only 2-3fs before being destroyed by the nuclear motion. Our analysis shows that the normal modes that are mostly responsible for the fast electronic decoherence are the symmetric in-plane modes. All other modes have little or no effect on the charge migration. This information can be useful to guide the development of reduced dimensionality models for larger systems or the search for molecules with long coherence times.

Classical analogous quantum coherent superposition through the Hertz-type nonlinearity of elastic granules

The theory of quantum superposition of states is one of the fundamental concepts related to quantum information science and technology (QIST). Two or more pure quantum states can be superposed to get a distinct quantum state, and one of the examples is the quantum bit, or qubit, state. But decoherence makes it easy for the current qubits, the most critical part of QIST platforms, to lose their superposition of states. Therefore, we theoretically propose and experimentally realize a classical analogous to the coherent superposition of energy states through the Hertz-type nonlinearity of elastic granules driven externally. The granules’ nonlinear vibrations depend mutually through phase and form a coherent superposition when projected into linear modes of vibration. We demonstrate how the state vector components can emerge from the amplitudes of coherent states, spanning to the two-dimensional time-dependent parametric Hilbert space. These amplitudes represent an actual amplitude rather than a probability amplitude, which opens the possibility of exploring two-state quantum-like computations through the superposition of states without decoherence and wave function collapse. These characteristics make it possible to understand the material-based quantum analogous information technology through experimentation of elastic bit.

Discriminating between optical coherent superposition states with imperfect detection

Discriminating between optical coherent superposition states with imperfect detection

Nonresonant coherent amplitude transfer in attosecond four-wave-mixing spectroscopy

The authors use attosecond four-wave-mixing spectroscopy to measure coherent superpositions of Rydberg states resulting from extreme ultraviolet excitation of a 3$s$ electron in gas phase atomic argon to a series of autoionizing 3$s$-1$n\phantom{\rule{0}{0ex}}p$ Rydberg states. The experimental observations are reproduced by electronic-structure and transition-moment calculations.

Femtosecond Time-Resolved Observation of Relaxation and Wave Packet Dynamics of the S1 State in Electronically Excited o-Fluoroaniline

Quantum beat frequency is the basis for understanding interference effects and vibrational wave packet dynamics and has important applications. Using femtosecond time-resolved mass spectrometry and femtosecond time-resolved photoelectron image combined with theoretical calculations, we study the electronic excited-state relaxation of o-fluoraniline molecule and the time-dependent evolution of vibrational wave packets between different eigenstates. After the molecule absorbs a photon of 288.3 nm and is excited to the S1 state, intramolecular vibrational redistribution first occurs on the time scale τ1 = 349 fs, and then the transition to the triplet state occurs through the intersystem crossing on the time scale τ2 = 583 ps, and finally, the triplet state occurs decays slowly through the time scale τ3 = 2074 ps. We find the intramolecular vibrational redistribution is caused by the 00, 10b1 and 16a1 vibrational modes of the Sl state origin. That is, the 288.3 nm femtosecond laser excites the molecule to the S1 state, and the continuous flow of the vibrational wave packet prepares a coherent superposition state of three vibrational modes. Through extracting the oscillation of different peak intensities in the photoelectron spectrum, we observe reversible changes caused by mutual interference of the S1 00, S1 10b1 and S1 16a1 states when the wave packets flow. When the pump pulse is 280 nm, the beat frequency disappears completely. This is explained in terms of increases in the vibrational field density and characteristic period of oscillation, and statistical averaging makes the quantum effect smooth and indistinguishable. In addition, the Rydberg component of the S1 state is more clearly resolved by combining experiment and theory.

Generation of spectrally uncorrelated biphotons via fiber nonlinear quantum interference

Spectrally uncorrelated biphotons are the essential resources for achieving various quantum information processing protocols. We theoretically investigate the generation of spectrally uncorrelated biphotons emitted by spontaneous four-wave mixing from a fiber nonlinear interferometer which consists of an N-stage nonlinear gain fiber and an (N-1)-stage dispersion modulation fiber. The output biphoton states of nonlinear interference are the coherent superposition of various biphoton states born in each nonlinear fiber, and thus the interference fringe will reshape the biphoton joint spectra. As a result, resorting to Taylor expansion to first order for phase mismatching, we theoretically verify that the orientation of phase matching contours will rotate in a specific way with only varying the length of dispersion modulation fiber. The rotation in orientation of phase matching contours may result in spectrally uncorrelated biphotons and even arbitrary correlation biphotons. Further, we choose micro/nanofiber as the nonlinear gain fiber and single-mode communication fiber as dispersion modulation fiber to numerically simulate the generation of spectrally uncorrelated biphotons from spontaneous fourwave mixing. Here, due to significant frequency detuning (hundreds of THz), Raman background noise can be considerably suppressed, even at room temperature, and photons with largely tunable wavelengths can be achieved, indicating a practicability in many quantum fields. A photon mode purity of 97.2% will be theoretically attained without weakening the heralding nature of biphoton sources. We think that this fiber nonlinear interference with the flexibly engineered quantum state can be an excellent practical source for quantum information processing.

Nonlinear Conformal Transformation for In Situ IR‐Visible Detection of Orbital Angular Momentum

AbstractLinear conformal transformation provides an effective way to detect orbital angular momentum (OAM) of photons, and particularly, their coherent superposition states that are significant for OAM‐based technologies. However, these methods have limited applicability—they are applied to single or few separated wavelengths and cannot achieve nondestructive detection—although those features are attractive for practical applications. Here, the second‐harmonic spiral transformation is theoretically described and experimentally demonstrated through IR‐visible detection of OAM states from 900 to 1400 nm, with a low energy loss of ≈10−6. Remarkably, a record high optical finesse of ≈5.52 is predicted and observed, indicating that a nonlinear enhancement factor resulting from OAM conservation significantly improves separation efficiency. Additionally, this scheme allows flexibility to achieve lower energy losses or higher sensitivity by adjusting phase‐matching conditions. These results can be applicable to classical and quantum areas and promote conformal transformation into nonlinear regions.

Experimental Limit on Nonlinear State-Dependent Terms in Quantum Theory.

Linear time evolution is one of the fundamental postulates of quantum theory. Past theoretical attempts to introduce nonlinearity into quantum evolution have violated causality. However, a recent theory has introduced nonlinear state-dependent terms in quantum field theory, preserving causality [D. E. Kaplan and S. Rajendran, Phys. Rev. D 105, 055002 (2022)PRVDAQ2470-001010.1103/PhysRevD.105.055002]. We report the results of an experiment that searches for such terms. Our approach, inspired by the Everett many-worlds interpretation of quantum theory, correlates a binary macroscopic classical voltage with the outcome of a projective measurement of a quantum bit, prepared in a coherent superposition state. Measurement results are recorded in a bit string, which is used to control a voltage switch. Presence of a nonzero voltage reading in cases of no applied voltage is the experimental signature of a nonlinear state-dependent shift of the electromagnetic field operator. We implement blinded measurement and data analysis with three control bit strings. Control of systematic effects is realized by producing one of the control bit strings with a classical random-bit generator. The other two bit strings are generated by measurements performed on a superconducting qubit in an IBM Quantum processor and on a ^{15}N nuclear spin in a nitrogen-vacancy center in diamond. Our measurements find no evidence for electromagnetic quantum state-dependent nonlinearity. We set a bound on the parameter that quantifies this nonlinearity |ε_{γ}|<4.7×10^{-11}, at 90%confidence level.

Quantum Electrodynamics of Intense Laser-Matter Interactions: A Tool for Quantum State Engineering

Intense laser-matter interactions are at the center of interest in research and technology since the development of high-power lasers. They have been widely used for fundamental studies in atomic, molecular, and optical physics, and they are at the core of attosecond physics and ultrafast optoelectronics. Although the majority of these studies have been successfully described using classical electromagnetic fields, recent investigations based on fully quantized approaches have shown that intense laser-atom interactions can be used for the generation of controllable high-photon-number entangled coherent states and coherent state superpositions. In this tutorial, we provide a comprehensive fully quantized description of intense laser-atom interactions. We elaborate on the processes of high-harmonic generation, above-threshold ionization, and we discuss new phenomena that cannot be revealed within the context of semiclassical theories. We provide the description for conditioning the light field on different electronic processes, and their consequences for quantum state engineering of light. Finally, we discuss the extension of the approach to more complex materials, and the impact to quantum technologies for a new photonic platform composed of the symbiosis of attosecond physics and quantum information science.12 MoreReceived 30 June 2022DOI:https://doi.org/10.1103/PRXQuantum.4.010201Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum description of light-matter interactionQuantum engineeringAtomic, Molecular & OpticalQuantum Information

Optimal control for maximally creating and maintaining a superposition state of a two‐level system under the influence of Markovian decoherence

AbstractReducing decoherence is an essential step toward realizing general‐purpose quantum computers beyond the present noisy intermediate‐scale quantum (NISQ) computers. To this end, dynamical decoupling (DD) approaches in which external fields are applied to qubits are often adopted. We numerically study DD using a two‐level model system (qubit) under the influence of Markovian decoherence by using quantum optimal control theory with slightly modified settings, in which the physical objective is to maximally create and maintain a specified superposition state in a specified control period. An optimal pulse is numerically designed while systematically varying the values of dephasing, population decay, pulse fluence, and control period as well as using two kinds of objective functionals. The decrease in purity due to the decoherence limits the ability to maintain a coherent superposition state; we refer to the state of maximal purity that can be maintained as the saturated value. The optimally shaped pulse minimizes the negative effect of decoherence by gradually populating and continuously replenishing the state of saturated purity.

Realizing acoustic qubit analogues with nonlinearly tunable phi-bits in externally driven coupled acoustic waveguides

Using experiments and theory, we investigate the behavior of nonlinear acoustic modes in a physical system composed of an array of three coupled acoustic waveguides, two of which are externally driven with two different frequencies. Nonlinear modes with frequency given by linear combinations of the driving frequencies are realizations of so-called logical phi-bits. A phi-bit is a two-state degree of freedom of an acoustic wave, which can be in a coherent superposition of states with complex amplitude coefficients, i.e., a qubit analogue. We demonstrate experimentally that phi-bit modes are supported in the array of waveguides. Using perturbation theory, we show that phi-bits may result from the intrinsic nonlinearity of the material used to couple the waveguides. We have also isolated possible effects on phi-bit states associated with the systems’ electronics, transducers and ultrasonic coupling agents used to probe the array and that may introduce extrinsic nonlinearities. These extrinsic effects are shown to be easily separable from the intrinsic behavior. The intrinsic behavior includes sharp jumps in phi-bit phases occurring over very narrow ranges of driving frequency. These jumps may also exhibit hysteretic behavior dependent on the direction of driving frequency tuning. The intrinsic states of phi-bits and multiple nonlinearly correlated phi-bits may serve as foundation for robust and practical quantum-analogue information technologies.

Experimental demonstration of classical analogous time-dependent superposition of states

One of the quantum theory concepts on which quantum information processing stands is superposition. Here we provide experimental evidence for the existence of classical analogues to the coherent superposition of energy states, which is made possible by the Hertz-type nonlinearity of the granules together with the external driving field. The granules’ nonlinear vibrations are projected into the linear modes of vibration, which depend on one another through the phase and form a coherent superposition. We show that the amplitudes of the coherent states form the components of a state vector that spans a two-dimensional Hilbert space, and time enables the system to span its Hilbert space parametrically. Thus, the superposition of states can be exploited in two-state quantum-like computations without decoherence and wave function collapse. Finally, we demonstrate the experimental realization of applying a reversible Hadamard gate to a pure base state that brings the state into a superposition.

Controlling quantum effects in enhanced strong-field ionisation with machine-learning techniques

We study non-classical pathways and quantum interference in enhanced ionisation of diatomic molecules in strong laser fields using machine learning techniques. Quantum interference provides a ‘bridge’, which facilitates intramolecular population transfer. Its frequency is higher than that of the field, intrinsic to the system and depends on several factors, for instance the state of the initial wavepacket or the internuclear separation. Using dimensionality reduction techniques, namely t-distributed stochastic neighbour embedding and principal component analysis, we investigate the effect of multiple parameters at once and find optimal conditions for enhanced ionisation in static fields, and controlled ionisation release for two-colour driving fields. This controlled ionisation manifests itself as a step-like behaviour in the time-dependent autocorrelation function. We explain the features encountered with phase-space arguments, and also establish a hierarchy of parameters for controlling ionisation via phase-space Wigner quasiprobability flows, such as specific coherent superpositions of states, electron localisation and internuclear-distance ranges.

Homodyne measurement with a Schrödinger cat state as a local oscillator

Homodyne measurements are a widely used quantum measurement. Using a coherent state of large amplitude as the local oscillator, it can be shown that the quantum homodyne measurement limits to a field quadrature measurement. In this work, we give an example of a general idea: injecting nonclassical states as a local oscillator can lead to nonclassical measurements. Specifically, we consider injecting a superposition of coherent states, a Schr\"odinger cat state, as a local oscillator. We derive the Kraus operators and the positive operator-valued measure in this situation.

Experimental generation of coherent-state superpositions with a quantum memory

We implement an iterative scheme for the generation of coherent states superpositions, using a quantum memory to take advantage of the iterative nature of the protocol. The generation rate has thus been increased by one order of magnitude, reaching 100 (Hz). Furthermore, the generated states were stored in the quantum memory during 184 (ns) before their characterization, paving the way toward the implementation of more complex iterative protocols.