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Quantum chemistry is interested in calculating ground and excited states of molecular systems by solving the electronic Schrödinger equation. The exact numerical solution of this equation, frequently represented as an eigenvalue problem, remains unfeasible for most molecules and requires approximate methods. In this paper we introduce the use of Quantum Community Detection performed using the D-Wave quantum annealer to reduce the molecular Hamiltonian matrix in Slater determinant basis without chemical knowledge. Given a molecule represented by a matrix of Slater determinants, the connectivity between Slater determinants (as off-diagonal elements) is viewed as a graph adjacency matrix for determining multiple communities based on modularity maximization. A gauge metric based on perturbation theory is used to determine the lowest energy cluster. This cluster or sub-matrix of Slater determinants is used to calculate approximate ground state and excited state energies within chemical accuracy. The details of this method are described along with demonstrating its performance across multiple molecules of interest and bond dissociation cases. These examples provide proof-of-principle results for approximate solution of the electronic structure problem using quantum computing. This approach is general and shows potential to reduce the computational complexity of post-Hartree–Fock methods as future advances in quantum hardware become available.

Symmetries offer simple solutions to complex problems: A permutation scheme allows solving high complexity problems in quantum chemistry with simple and short quantum circuits.

The variational quantum eigensolver (VQE) is one of the most promising algorithms to find eigenvalues and eigenvectors of a given Hamiltonian on noisy intermediate-scale quantum (NISQ) devices. A particular application is to obtain ground or excited states of molecules. The practical realization is currently limited by the complexity of quantum circuits. Here we present a novel approach to reduce quantum circuit complexity in VQE for electronic structure calculations. Our algorithm, called ClusterVQE, splits the initial qubit space into subspaces (qubit clusters) which are further distributed on individual (shallower) quantum circuits. The clusters are obtained based on quantum mutual information reflecting maximal entanglement between qubits, whereas entanglement between different clusters is taken into account via a new "dressed" Hamiltonian. ClusterVQE therefore allows exact simulation of the problem by using fewer qubits and shallower circuit depth compared to standard VQE at the cost of additional classical resources. In addition, a new gradient measurement method without using an ancillary qubit is also developed in this work. Proof-of-principle demonstrations are presented for several molecular systems based on quantum simulators as well as an IBM quantum device with accompanying error mitigation. The efficiency of the new algorithm is comparable to or even improved over qubit-ADAPT-VQE and iterative Qubit Coupled Cluster (iQCC), state-of-the-art circuit-efficient VQE methods to obtain variational ground state energies of molecules on NISQ hardware. Above all, the new ClusterVQE algorithm simultaneously reduces the number of qubits and circuit depth, making it a potential leader for quantum chemistry simulations on NISQ devices.

The possibility of using quantum computers for electronic structure calculations has opened up a promising avenue for computational chemistry. Towards this direction, numerous algorithmic advances have been made in the last five years. The potential of quantum annealers, which are the prototypes of adiabatic quantum computers, is yet to be fully explored. In this work, we demonstrate the use of a D-Wave quantum annealer for the calculation of excited electronic states of molecular systems. These simulations play an important role in a number of areas, such as photovoltaics, semiconductor technology and nanoscience. The excited states are treated using two methods, time-dependent Hartree-Fock (TDHF) and time-dependent density-functional theory (TDDFT), both within a commonly used Tamm-Dancoff approximation (TDA). The resulting TDA eigenvalue equations are solved on a D-Wave quantum annealer using the Quantum Annealer Eigensolver (QAE), developed previously. The method is shown to reproduce a typical basis set convergence on the example H$_2$ molecule and is also applied to several other molecular species. Characteristic properties such as transition dipole moments and oscillator strengths are computed as well. Three potential energy profiles for excited states are computed for NH$_3$ as a function of the molecular geometry. Similar to previous studies, the accuracy of the method is dependent on the accuracy of the intermediate meta-heuristic software called qbsolv., Comment: 17 pages, 2 figures, 14 pages of Supplemental Material

Electromagnetic fields induced by the space charge in relativistic beams play an important role in Accelerator Physics. They lead to emittance growth, slice energy change, and the microbunching instability. Typically, these effects are modeled numerically since simple description exists only in the limits of large- or small-scale current variations. In this paper we consider an axially symmetric charged beam inside a round pipe and find the solution of the space charge problem that is valid in the full range of current variations. We express the solution for the field components in terms of Green's functions, which are fully determined by just a single function. We then find that this function is an on-axis potential from a charged disk in a round pipe, with transverse charge density $\rho_\perp(r)$, and it has a compact analytical expression. We finally provide an integrated Green's function based approach for efficient numerical evaluation in the case when the transverse charge density stays the same along the beam.

In the field of beam physics, two frontier topics have taken center stage due to their potential to enable new approaches to discovery in a wide swath of science. These areas are: advanced, high gradient acceleration techniques, and x-ray free electron lasers (XFELs). Further, there is intense interest in the marriage of these two fields, with the goal of producing a very compact XFEL. In this context, recent advances in high gradient radio-frequency cryogenic copper structure research have opened the door to the use of surface electric fields between 250 and 500 MV/m. Such an approach is foreseen to enable a new generation of photoinjectors with six-dimensional beam brightness beyond the current state-of-the-art by well over an order of magnitude. This advance is an essential ingredient enabling an ultra-compact XFEL (UC-XFEL). In addition, one may accelerate these bright beams to GeV scale in less than 10 meters. Such an injector, when combined with inverse free electron laser-based bunching techniques can produce multi-kA beams with unprecedented beam quality, quantified by ~50 nm-rad normalized emittances. These beams, when injected into innovative, short-period (1-10 mm) undulators uniquely enable UC-XFELs having footprints consistent with university-scale laboratories. We describe the architecture and predicted performance of this novel light source, which promises photon production per pulse of a few percent of existing XFEL sources. We review implementation issues including collective beam effects, compact x-ray optics systems, and other relevant technical challenges. To illustrate the potential of such a light source to fundamentally change the current paradigm of XFELs with their limited access, we examine possible applications in biology, chemistry, materials, atomic physics, industry, and medicine which may profit from this new model of performing XFEL science., 80 pages, 24 figures

A future capability in dynamic mesoscale materials science is needed to study the limitations of materials under irreversible and extreme conditions, where these limitations are caused by nonuniformities and defects in the mesoscale. This capability gap could potentially be closed with an X-ray free-electron laser (XFEL), producing 5 &times, 1010 photons with an energy of 42 keV, known as the Matter&ndash, Radiation Interactions in Extremes (MaRIE) XFEL. Over the last few years, researchers at the Los Alamos National Laboratory have developed a preconceptual design for a MaRIE-class XFEL based on existing high-brightness beam technologies, including superconducting L-band cryomodules. However, the performance of a MaRIE-class XFEL can be improved and the risk of its operation reduced by investing in emerging high-brightness beam technologies, such as the development of high-gradient normal conducting radio frequency (RF) structures. Additionally, an alternative XFEL architecture, which generates a series of high-current microbunches instead of a single bunch with uniformly high current along it, may suppress the most important emittance degradation effects in the accelerator and in the XFEL undulator. In this paper, we describe the needed dynamic mesoscale materials science capability, a MaRIE-class XFEL, and the proposed microbunched XFEL accelerator architecture in detail.

Accurate transition probabilities of La ii ion are calculated using configuration-interaction many-body perturbation theory with 10 adjustable parameters, seven of which are evaluated from energies of the La iii single-valence electron ion. Comparison is given for transitions probabilities and lifetimes with experiments and theories. Close agreement with experiment is observed for most transitions. The theoretical approach can be extended to other divalent atoms and ions with strong valence-core interactions and to more complex atoms. The theory will be useful for opacity evaluation and astrophysical applications.

Design methodologies for high-performance photocathodes become increasingly important, driven by the needs of advanced x-ray light sources and ultrafast electron diffraction and microscopy. Cathode design has been hampered by interdependent performance metrics, where improving one compromises another. Rapid developments in modeling and nanomaterial fabrication allow tailoring of electronic structure in ways that influence the fundamental mechanisms of photoemission: absorption, transport, and emission. This survey of recent efforts and results in each of these areas proposes research priorities that are likely to yield significant gains in cathode design.

A fully quantized analysis is presented of induced magneto-electric rectification in individual diatomic molecules in the steady-state regime. Good agreement is obtained between this quantum theory and a classical model that includes the same key kinematic elements but predicts temporal dynamics as well. At the molecular level, an enhanced magneto-electric optical interaction driven by dual optical fields E and H* is shown to give rise to a static electric dipole (ED) moment oriented along the propagation direction of linearly-polarized light in dielectric materials. This longitudinal Hall effect or “charge separation” interaction is quadratic with respect to the incident field strength and exhibits an induced moment that is limited by the ED transition moment of the molecular resonance. Overall, the two-photon dynamics can be described as first establishing an electric polarization and imparting orbital angular momentum on which the magnetic field exerts torque in the excited state of the molecule. Magnetic torque mediates an exchange of orbital and rotational angular momenta that de-excites the molecule and simultaneously enhances magneto-electric rectification. Material properties that affect magneto-electric response at the molecular level are identified.

Here, we present recent developments in the field of quantum imaging focusing on the high dimensionality aspects of single and biphoton imaging. We discuss some systems that have a "quantum advantage" over classical counterparts. We highlight some recent experiments in single-photon image discrimination, large alphabet quantum key distribution and image buffering.

We introduce decaying-dressed states for a three-level Λ system driven at one atomic transition and probed at the adjacent one. These states allow for a simple interpretation and classification of various coherent effects in such a system.

A proposed phase-estimation protocol based on measuring the parity of a two-mode squeezed-vacuum state at the output of a Mach-Zehnder interferometer shows that the Cram\'{e}r-Rao sensitivity is sub-Heisenberg [Phys.\ Rev.\ Lett.\ {\bf104}, 103602 (2010)]. However, these measurements are problematic, making it unclear if this sensitivity can be obtained with a finite number of measurements. This sensitivity is only for phase near zero, and in this region there is a problem with ambiguity because measurements cannot distinguish the sign of the phase. Here, we consider a finite number of parity measurements, and show that an adaptive technique gives a highly accurate phase estimate regardless of the phase. We show that the Heisenberg limit is reachable, where the number of trials needed for mean photon number $\bar{n}=1$ is approximately one hundred. We show that the Cram\'{e}r-Rao sensitivity can be achieved approximately, and the estimation is unambiguous in the interval ($-\pi/2, \pi/2$)., Comment: 7 pages, 8 figures

We show that the interference effects (similar to electromagnetically induced transparency, which was widely studied earlier in electronic transitions in optics) may appear in γ-radiation at nuclear transitions under the condition of nuclear level anticrossing. We demonstrate it also experimentally in optically thin samples of FeCO3.

We describe a new FEL line-narrowing technique called distributed seeding (DS), using Si(111) Bragg crystal monochromators to enhance the spectral brightness of the MaRIE hard X-ray freeelectron laser. DS differs from self-seeding in three important aspects. First, DS relies on spectral filtering of the radiation at multiple locations along the undulator, with a monochromator located every few power gain lengths. Second, DS performs filtering early in the exponential gain region before SASE spikes start to appear in the radiation longitudinal profile. Third, DS provides the option to select a wavelength longer than the peak of the SASE gain curve, which leads to improved spectral contrast of the seeded FEL over the SASE background. Timedependent Genesis simulations show the power-vs-z growth curves for DS exhibit behaviors of a seeded FEL amplifier, such as exponential growth region immediately after the filters. Of the seeding approaches considered, the two-stage DS spectra produce the highest contrast of seeded FEL over the SASE background and that the three-stage DS provides the narrowest linewidth with a relative spectral FWHM of 8 X 10-5 .

We study nuclear elastic forward scattering of synchrotron radiation. In a simple model, we show that suppression of nuclear excitation by synchrotron radiation as well as nuclear elastic forward scattering are possible due to the formation of nuclear coherent population trapping. We consider a nucleus with ground- and excited-state nuclear momenta , , respectively. We demonstrate that nuclear coherent population trapping can be achieved for random phases of pulses in synchrotron radiation.

We conduct a numerical study of the possibility of using the `magic-angle' technique to suppress inhomogeneous broadening of Mossbauer spectra. We use the Floquet-state perturbation theory for the rf modulation of the Mossbauer resonance, which was adjusted to suit our purpose. A system of two nuclei of 57Fe coupled by the dipole–dipole interaction is considered. The analysis here completes studies started in 2004, by including the dipole–dipole interaction between one nucleus in the ground state and the other in the excited state. We show that partial suppression of inhomogeneous broadening remains possible.

Mossbauer spectra under the presence of inhomogeneous broadening due to dipole-dipole interaction have been studied. Floquet state perturbation theory has been used for the density matrix analysis of a simple model. The parameters of 57Fe are used for simulations. We have shown that a spinning magnetic field can eliminate inhomogeneous broadening. Application to Mossbauer spectroscopy is discussed.

Optical coherent states can be interpreted as d-dimensional quantum systems, or qudits of even superposition of pseudo-number states. Cross-Kerr nonlinear interaction can generate the maximal entanglements of pseudo- phase and pseudo-number states from two opticl coherent states. Extended network of these entangled coherent states is a qudit cluster state and can be used as qudit communication network for d-dimensional teleportation or multi-user quantum cryptographic network.

One of the major challenges in quantum computation has been to preserve the coherence of a quantum system against dephasing effects of the environment. The information stored in photon polarization, for example, is quickly lost due to such dephasing, and it is crucial to preserve the input states when one tries to transmit quantum information encoded in the photons through a communication channel. We propose a dynamical decoupling sequence to protect photonic qubits from dephasing by integrating wave plates into optical fiber at prescribed locations. We simulate random birefringent noise along realistic lengths of optical fiber and study preservation of polarization qubits through such fibers enhanced with Carr-Purcell-Meiboom-Gill (CPMG) dynamical decoupling. This technique can maintain photonic qubit coherence at high fidelity, making a step towards achieving scalable and useful quantum communication with photonic qubits., 8 pages, 5 figures

Mach-Zehnder interferometry based on mixing the coherent and the squeezed vacuum states of light has Heisenberg limited capabilities for phase estimation. This is also, because the quantum Cramer-Rao bound on sensitivity of phase estimation with the above interferometric scheme reaches the Heisenberg limit when the inputs are mixed in hear equal proportions. We show that a detection strategy based on the measurement of parity of photon number in one of the output modes of the interferometer saturates the quantum Cramer-Rao bound of the interferometric scheme, and therefore- as a consequence- hits the Heisenberg limit when the inputs are mixed in equal intensities.

We develop an improvement to the weak laser pulse BB84 scheme for quantum key distribution, which utilizes entanglement to improve the security of the scheme and enhance its resilience to the photon-number-splitting attack. This protocol relies on the non-commutation of photon phase and number to detect an eavesdropper performing quantum non-demolition measurement on number. The potential advantages and disadvantages of this scheme are compared to the coherent decoy state protocol., Comment: 10 pages, 4 figures

Article discussing research on the coherent control of refractive index in far-detuned Λ systems.

We present a numerical study of the properties of optical forces on moving atoms derived from purely stimulated processes produced by multiple adiabatic rapid-passage sequences. The optical Bloch equations are solved for a carefully timed sequence of frequency-swept pulses that can produce a force much larger than the ordinary radiative force. We describe the effects of the sweep range, peak intensity, sweep direction, number of pulses, atomic velocity, and spontaneous emission. Since the momentum of thermal atoms is much larger than that transferred by a single absorption-stimulated emission cycle, multiple repetitions are needed to make a significant velocity change.

We present a parity-measurement-based phase estimation protocol with two-mode squeezed vacuum states; effects of loss and excess noise in squeezed vacuum are discussed; and a parity detection scheme without number-resolving detectors is proposed.

We study the two-level atom interaction with bichromatic field to understand the entropy flow during this interaction in the absence of spontaneous emission. This flow is characterized by the Carnot cycle that demonstrates efficient cooling.

We study preservation of polarization qubits in the polarization-maintaining fibers enhanced with dynamical decoupling sequence implemented in space instead of time. Such fibers maintain high fidelity with scalable waveplate implementations for specific input states.

Autler-Townes splitting (ATS) and electromagnetically-induced transparency (EIT) both yield transparency in an absorption profile, but only EIT yields strong transparency for a weak pump field due to Fano interference. Empirically discriminating EIT from ATS is important but so far has been subjective. We introduce an objective method, based on Akaike's information criterion, to test ATS vs. EIT from experimental data and determine which pertains. We apply our method to a recently reported induced-transparency experiment in superconducting circuit quantum electrodynamics., Comment: Extended discussion of ATS vs. EIT testing in realistic systems is given. Two figures were combined and one new was added

Using Bayesian analysis we characterized the performance of phase estimation in the Mach-Zehnder interferometer with two-mode squeezed vacuum input. Phase uncertainty, averaged over many trials, is examined and the dependence on photon number is found.

We study preservation of polarization qubits in birefringent fibers with CPMG dynamical decoupling. Very high fidelity can be maintained by implementing such sequence with halfwave plates for given length of fiber-optical communication channel.