Quantum Simulation of the Bosonic Creutz Ladder with a Parametric Cavity.

There has been a growing interest in realizing quantum simulators for physical systems where perturbative methods are ineffective. The scalability and flexibility of circuit quantum electrodynamics make it a promising platform for implementing various types of simulators, including lattice models of strongly coupled field theories. Here, we use a multimode superconducting parametric cavity as a hardware-efficient analog quantum simulator, realizing a lattice in synthetic dimensions with complex hopping interactions. The coupling graph, i.e., the realized model, can be programmed in situ. The complex-valued hopping interaction further allows us to simulate, for instance, gauge potentials and topological models. As a demonstration, we simulate a plaquette of the bosonic Creutz ladder. We characterize the lattice with scattering measurements, reconstructing the experimental Hamiltonian and observing important precursors of topological features including nonreciprocal transport and Aharonov-Bohm caging. This platform can be easily extended to larger lattices and different models involving other interactions.

Tripartite Genuine Non-Gaussian Entanglement in Three-Mode Spontaneous Parametric Down-Conversion.

We show that the states generated by a three-mode spontaneous parametric down-conversion (SPDC) interaction Hamiltonian possess tripartite entanglement of a different nature to other paradigmatic three-mode entangled states generated by the combination of two-mode SPDC interactions. While two-mode SPDC generates Gaussian states whose entanglement can be characterized by standard criteria based on two-mode quantum correlations, these criteria fail to capture the entanglement generated by three-mode SPDC. We use criteria built from three-mode correlation functions to show that the class of states recently generated in a superconducting-circuit implementation of three-mode SPDC ideally have tripartite entanglement, contrary to recent claims in the literature. These criteria are suitable for triple SPDC but we show that they fail to detect tripartite entanglement in other states which are known to possess it, which illustrates the existence of two fundamentally different notions of tripartite entanglement in three-mode continuous-variable systems.

Antiferromagnetic vs. non-magnetic ε phase of solid oxygen. Periodic density functional theory studies using a localized atomic basis set and the role of exact exchange.

The question of the non-magnetic (NM) vs. antiferromagnetic (AF) nature of the ε phase of solid oxygen is a matter of great interest and continuing debate. In particular, it has been proposed that the ε phase is actually composed of two phases, a low-pressure AF ε1 phase and a higher pressure NM ε0 phase [Crespo et al., Proc. Natl. Acad. Sci. U. S. A., 2014, 111, 10427]. We address this problem through periodic spin-restricted and spin-polarized Kohn-Sham density functional theory calculations at pressures from 10 to 50 GPa using calibrated GGA and hybrid exchange-correlation functionals with Gaussian atomic basis sets. The two possible configurations for the antiferromagnetic (AF1 and AF2) coupling of the 0 ≤ S ≤ 1 O2 molecules in the (O2)4 unit cell were studied. Full enthalpy-driven geometry optimizations of the (O2)4 unit cells were done to study the pressure evolution of the enthalpy difference between the non-magnetic and both antiferromagnetic structures. We also address the evolution of structural parameters and the spin-per-molecule vs. pressure. We find that the spin-less solution becomes more stable than both AF structures above 50 GPa and, crucially, the spin-less solution yields lattice parameters in much better agreement with experimental data at all pressures than the AF structures. The optimized AF2 broken-symmetry structures lead to large errors of the a and b lattice parameters when compared with experiments. The results for the NM model are in much better agreement with the experimental data than those found for both AF models and are consistent with a completely non-magnetic (O2)4 unit cell for the low-pressure regime of the ε phase.

Observation of the dynamical Casimir effect in a superconducting circuit.

One of the most surprising predictions of modern quantum theory is that the vacuum of space is not empty. In fact, quantum theory predicts that it teems with virtual particles flitting in and out of existence. Although initially a curiosity, it was quickly realized that these vacuum fluctuations had measurable consequences--for instance, producing the Lamb shift of atomic spectra and modifying the magnetic moment of the electron. This type of renormalization due to vacuum fluctuations is now central to our understanding of nature. However, these effects provide indirect evidence for the existence of vacuum fluctuations. From early on, it was discussed whether it might be possible to more directly observe the virtual particles that compose the quantum vacuum. Forty years ago, it was suggested that a mirror undergoing relativistic motion could convert virtual photons into directly observable real photons. The phenomenon, later termed the dynamical Casimir effect, has not been demonstrated previously. Here we observe the dynamical Casimir effect in a superconducting circuit consisting of a coplanar transmission line with a tunable electrical length. The rate of change of the electrical length can be made very fast (a substantial fraction of the speed of light) by modulating the inductance of a superconducting quantum interference device at high frequencies (>10 gigahertz). In addition to observing the creation of real photons, we detect two-mode squeezing in the emitted radiation, which is a signature of the quantum character of the generation process.

The V + I defects in diamond: An ab initio investigation of the electronic structure, of the Raman and IR spectra, and of their possible recombination.

The double defect in diamond, vacancy (V) plus 〈100〉 self-split-interstitial (V+I), is investigated at the ab initio quantum mechanical level, by considering the vicinal case VI1 (V is one of the first neighbors of one of the two C atoms constituting the I defect) and the two possible "second neighbors" cases, VI2D, VI2S, in which a carbon atom is a first neighbor of both V and I. The case in which the two defects are at a larger distance is simulated by considering the two isolated defects separately (VI∞). A 6-21G local Gaussian-type basis set and the B3LYP hybrid functional are used for most of the calculations; richer basis sets and other functionals (a global hybrid as PBE0, a range-separated hybrid as HSE06, LDA, PBE, and Hartree-Fock) have also been used for comparison. With this computational approach we evaluate the energy difference between the various spin states, the location of the corresponding bands in the energy gap of pristine diamond, as well as the defect formation energy of the four defects. The path for the recombination of V and I is explored for the vicinal case, by using the distinguished reaction coordinate strategy. A barrier as high as 0.75 eV is found with B3LYP between VI1 and the perfect diamond recombined structure; when other hybrids are used, as PBE0 or HSE06, the barrier increases up to 1.01 eV (pure density functional theory produces lower barriers: 0.62 and 0.67 for PBE and LDA, respectively). Such a barrier is lower than the one estimated in a very indirect way through experimental data, ranging from 1.3 to 1.7 eV. It confirms however the evidence of the extremely low recombination rate also at high temperature. The Raman (and IR) spectra of the various defects are generated, which permit one to unambiguously attribute to these defects (thanks also to the graphical animation of the modes) many of the peaks observed in damaged diamond above the dominant peak of perfect bulk. For the residual non-attributed peaks, more complicated aggregations of defects should be explored.

Quantum nondemolition detection of a propagating microwave photon.

The ability to nondestructively detect the presence of a single, traveling photon has been a long-standing goal in optics, with applications in quantum information and measurement. Realizing such a detector is complicated by the fact that photon-photon interactions are typically very weak. At microwave frequencies, very strong effective photon-photon interactions in a waveguide have recently been demonstrated. Here we show how this type of interaction can be used to realize a quantum nondemolition measurement of a single propagating microwave photon. The scheme we propose uses a chain of solid-state three-level systems (transmons) cascaded through circulators which suppress photon backscattering. Our theoretical analysis shows that microwave-photon detection with fidelity around 90% can be realized with existing technologies.

Localisation of vibrational modes in high-entropy oxides.

The recently-discovered high-entropy oxides (HEO's) offer a paradoxical combination of crystalline arrangement and high disorder. They differ qualitatively from established paradigms for disordered solids such as glasses and alloys. In these latter systems, it is well known that disorder induces localised vibrational excitations. In this article, we explore the possibility of disorder-induced localisation in Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O, the prototypical HEO with rock-salt structure. To describe phononic excitations, we model the interatomic potentials for the cation-oxygen interactions by fitting to the physical properties of the parent binary oxides. We validate our model against the experimentally determined crystal structure and optical conductivity. The resulting phonon spectrum shows wave-like propagating modes at low energies and localised modes at high energies. Localisation is reflected in signatures such as participation ratio and correlation amplitude. Finally, we argue that mass disorder can be increased to enhance localisation. We consider a hypothetical material, high-entropy telluride-oxide, where tellurium atoms are admixed into the anion sublattice. This shows a larger localised fraction, with additional localised modes appearing in the middle of the spectrum. Our results demonstrate that HEO's are a promising platform to study Anderson localisation of phonons.

Higher-order combinatorial chromatin perturbations by engineered CRISPR-Cas12a for functional genomics.

Multiplexed genetic perturbations are critical for testing functional interactions among coding or non-coding genetic elements. Compared to double-stranded DNA cutting, repressive chromatin formation using CRISPR interference (CRISPRi) avoids genotoxicity and is more effective for perturbing non-coding regulatory elements in pooled assays. However, current CRISPRi pooled screening approaches are limited to targeting 1-3 genomic sites per cell. To develop a tool for higher-order ( > 3) combinatorial targeting of genomic sites with CRISPRi in functional genomics screens, we engineered an Acidaminococcus Cas12a variant -- referred to as mul tiplexed transcriptional interference AsCas12a (multiAsCas12a). multiAsCas12a incorporates a key mutation, R1226A, motivated by the hypothesis of nicking-induced stabilization of the ribonucleoprotein:DNA complex for improving CRISPRi activity. multiAsCas12a significantly outperforms prior state-of-the-art Cas12a variants in combinatorial CRISPRi targeting using high-order multiplexed arrays of lentivirally transduced CRISPR RNAs (crRNA), including in high-throughput pooled screens using 6-plex crRNA array libraries. Using multiAsCas12a CRISPRi, we discover new enhancer elements and dissect the combinatorial function of cis-regulatory elements. These results instantiate a group testing framework for efficiently surveying potentially numerous combinations of chromatin perturbations for biological discovery and engineering.

Engineered CRISPR-Cas12a for higher-order combinatorial chromatin perturbations.

Multiplexed genetic perturbations are critical for testing functional interactions among coding or non-coding genetic elements. Compared to double-stranded DNA cutting, repressive chromatin formation using CRISPR interference (CRISPRi) avoids genotoxicity and is more effective for perturbing non-coding regulatory elements in pooled assays. However, current CRISPRi pooled screening approaches are limited to targeting one to three genomic sites per cell. We engineer an Acidaminococcus Cas12a (AsCas12a) variant, multiplexed transcriptional interference AsCas12a (multiAsCas12a), that incorporates R1226A, a mutation that stabilizes the ribonucleoprotein-DNA complex via DNA nicking. The multiAsCas12a-KRAB fusion improves CRISPRi activity over DNase-dead AsCas12a-KRAB fusions, often rescuing the activities of lentivirally delivered CRISPR RNAs (crRNA) that are inactive when used with the latter. multiAsCas12a-KRAB supports CRISPRi using 6-plex crRNA arrays in high-throughput pooled screens. Using multiAsCas12a-KRAB, we discover enhancer elements and dissect the combinatorial function of cis-regulatory elements in human cells. These results instantiate a group testing framework for efficiently surveying numerous combinations of chromatin perturbations for biological discovery and engineering.

Breakdown of the cross-Kerr scheme for photon counting.

We show, in the context of single-photon detection, that an atomic three-level model for a transmon in a transmission line does not support the predictions of the nonlinear polarizability model known as the cross-Kerr effect. We show that the induced displacement of a probe in the presence or absence of a single photon in the signal field, cannot be resolved above the quantum noise in the probe. This strongly suggests that cross-Kerr media are not suitable for photon counting or related single-photon applications. Our results are presented in the context of a transmon in a one-dimensional microwave waveguide, but the conclusions also apply to optical systems.

Giant cross-Kerr effect for propagating microwaves induced by an artificial atom.

We investigate the effective interaction between two microwave fields, mediated by a transmon-type superconducting artificial atom which is strongly coupled to a coplanar transmission line. The interaction between the fields and atom produces an effective cross-Kerr coupling. We demonstrate average cross-Kerr phase shifts of up to 20 degrees per photon with both coherent microwave fields at the single-photon level. Our results provide an important step toward quantum applications with propagating microwave photons.

High critical-current superconductor-InAs nanowire-superconductor junctions.

We report on the fabrication of InAs nanowires coupled to superconducting leads with high critical current and widely tunable conductance. We implemented a double lift-off nanofabrication method to get very short nanowire devices with Ohmic contacts. We observe very high critical currents of up to 800 nA in a wire with a diameter of 80 nm. The current-voltage characteristics of longer and suspended nanowires display either Coulomb blockade or supercurrent depending on a local gate voltage, combining different regimes of transport in a single device.

Dirofilariasis mouse models for heartworm preclinical research.

Introduction: Dirofilariasis, including heartworm disease, is a major emergent veterinary parasitic infection and a human zoonosis. Currently, experimental infections of cats and dogs are used in veterinary heartworm preclinical drug research. Methods: As a refined alternative in vivo heartworm preventative drug screen, we assessed lymphopenic mouse strains with ablation of the interleukin-2/7 common gamma chain (γc) as susceptible to the larval development phase of Dirofilaria immitis. Results: Non-obese diabetic (NOD) severe combined immunodeficiency (SCID)γc-/- (NSG and NXG) and recombination-activating gene (RAG)2-/-γc-/- mouse strains yielded viable D. immitis larvae at 2-4 weeks post-infection, including the use of different batches of D. immitis infectious larvae, different D. immitis isolates, and at different laboratories. Mice did not display any clinical signs associated with infection for up to 4 weeks. Developing larvae were found in subcutaneous and muscle fascia tissues, which is the natural site of this stage of heartworm in dogs. Compared with in vitro-propagated larvae at day 14, in vivo-derived larvae had completed the L4 molt, were significantly larger, and contained expanded Wolbachia endobacteria titres. We established an ex vivo L4 paralytic screening system whereby assays with moxidectin or levamisole highlighted discrepancies in relative drug sensitivities in comparison with in vitro-reared L4 D. immitis. We demonstrated effective depletion of Wolbachia by 70%-90% in D. immitis L4 following 2- to 7-day oral in vivo exposures of NSG- or NXG-infected mice with doxycycline or the rapid-acting investigational drug, AWZ1066S. We validated NSG and NXG D. immitis mouse models as a filaricide screen by in vivo treatments with single injections of moxidectin, which mediated a 60%-88% reduction in L4 larvae at 14-28 days. Discussion: Future adoption of these mouse models will benefit end-user laboratories conducting research and development of novel heartworm preventatives via increased access, rapid turnaround, and reduced costs and may simultaneously decrease the need for experimental cat or dog use.

Generation of nonclassical microwave states using an artificial atom in 1D open space.

We have embedded an artificial atom, a superconducting transmon qubit, in a 1D open space and investigated the scattering properties of an incident microwave coherent state. By studying the statistics of the reflected and transmitted fields, we demonstrate that the scattered states can be nonclassical. In particular, by measuring the second-order correlation function, g((2)), we show photon antibunching in the reflected field and superbunching in the transmitted field. We also compare the elastically and inelastically scattered fields using both phase-sensitive and phase-insensitive measurements.

Demonstration of a single-photon router in the microwave regime.

We have embedded an artificial atom, a superconducting transmon qubit, in an open transmission line and investigated the strong scattering of incident microwave photons (∼6 GHz). When an input coherent state, with an average photon number N≪1 is on resonance with the artificial atom, we observe extinction of up to 99.6% in the forward propagating field. We use two-tone spectroscopy to study scattering from excited states and we observe electromagnetically induced transparency (EIT). We then use EIT to make a single-photon router, where we can control to what output port an incoming signal is delivered. The maximum on-off ratio is around 99% with a rise and fall time on the order of nanoseconds, consistent with theoretical expectations. The router can easily be extended to have multiple output ports and it can be viewed as a rudimentary quantum node, an important step towards building quantum information networks.

Are "pinholes" the cause of excess current in superconducting tunnel junctions? A study of Andreev current in highly resistive junctions.

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic pinholes in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlO(x)/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlO(x)/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement with experiment over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.

Excess dissipation in a single-electron box: the Sisyphus resistance.

We present measurements of the ac response of a single-electron box (SEB). We apply a radio frequency signal with a frequency larger than the tunneling rate and drive the system out of equilibrium. We observe much more dissipation in the SEB then one would expect from a simple circuit model. We can explain this in terms of a mechanism that we call the Sisyphus resistance. The Sisyphus resistance has a strong gate dependence which can be used for electrometery applications.

Photon generation in an electromagnetic cavity with a time-dependent boundary.

We report the observation of photon generation in a microwave cavity with a time-dependent boundary condition. Our system is a microfabricated quarter-wave coplanar waveguide cavity. The electrical length of the cavity is varied by using the tunable inductance of a superconducting quantum interference device. It is measured at a temperature significantly less than the resonance frequency. When the length is modulated at approximately twice the static resonance frequency, spontaneous parametric oscillations of the cavity field are observed. Time-resolved measurements of the dynamical state of the cavity show multiple stable states. The behavior is well described by theory. Our results may be considered a preliminary step towards demonstrating the dynamical Casimir effect.

Periodic density functional theory calculations for 3-dimensional polyacetylene with empirical dispersion terms.

We report periodic B3LYP density functional theory calculations for three-dimensional (3D) trans-polyacetylene (t-PA) fibers. Empirical dispersion terms, as proposed by Grimme, are included with an appropriate re-scaling to yield the B3LYP+D* method implemented in CRYSTAL06. The dispersion corrections are critical for obtaining correct unit cell parameters. In our calculations the out-of-phase P2(1)/n structure turns out to be a transition state for the interchain relative translational motion, which lies about 0.35 kcal mol(-1) above the two symmetrically located in-phase P2(1)/a minima. These results provide a possible new explanation for the observed XRD intensities. Our calculations should also be useful for comparison with more costly non-empirical treatments of 3D PA and other pi-conjugated polymers.

Dynamical Casimir effect in a superconducting coplanar waveguide.

We investigate the dynamical Casimir effect in a coplanar waveguide (CPW) terminated by a superconducting quantum interference device (SQUID). Changing the magnetic flux through the SQUID parametrically modulates the boundary condition of the CPW, and thereby, its effective length. Effective boundary velocities comparable to the speed of light in the CPW result in broadband photon generation which is identical to the one calculated in the dynamical Casimir effect for a single oscillating mirror. We estimate the power of the radiation for realistic parameters and show that it is experimentally feasible to directly detect this nonclassical broadband radiation.