We present a protocol for encoding $N$ real numbers stored in $N$ memory registers into the amplitudes of the quantum superposition that describes the state of ${log}_{2}N$ qubits. This task is one of the main steps in quantum machine learning algorithms applied to classical data. The protocol combines partial CNOT gate rotations with probabilistic projection onto the desired state. The number of additional ancilla qubits used during the implementation of the protocol, as well as the number of quantum gates, scales linearly with the number of qubits in the processing register and hence logarithmically with $N$. The average time needed to successfully perform the encoding scales logarithmically with the number of qubits, in addition to being inversely proportional to the acceptable error in the encoded amplitudes. It also depends on the structure of the dataset in such a way that the protocol is most efficient for nonsparse data.

The Axion Resonant Interaction Detection Experiment (ARIADNE) is a nuclear-magnetic-resonance-based experiment that will search for novel axion-induced spin-dependent interactions between an unpolarized source mass rotor and spin-polarized ${}^3\mathrm{He}$ nuclei placed nearby. To detect a feeble axion-mediated signal at the subattotesla level, the experiment relies on ultralow magnetic background and noise. We measure and characterize the magnetic field from a prototype tungsten rotor. We show that the field is dominantly caused by a few discrete magnetic dipoles, likely due to impurities in the rotor. This is done via a numerical optimization pipeline which fits for the locations and magnetic moments of each dipole. We find that under the current demagnetization procedure, the magnetic moment of the impurities is bounded at ${10}^{-9}$ A ${\mathrm{m}}^2$. We further show that a shielding factor of ${10}^9$ will support ARIADNE's design sensitivity with the current level of tungsten purity and demagnetization process.

Controlling the spread of correlations in quantum many-body systems is a key challenge at the heart of quantum science and technology. Correlations are usually destroyed by dissipation arising from coupling between a system and its environment. Here, we show that dissipation can instead be used to engineer a wide variety of spatiotemporal correlation profiles in an easily tunable manner. We describe how dissipation with any translationally invariant spatial profile can be realized in cold atoms trapped in an optical cavity. A uniform external field and the choice of spatial profile can be used to design when and how dissipation creates or destroys correlations. We demonstrate this control by generating entanglement preferentially sensitive to a desired spatial component of a magnetic field. We thus establish nonlocal dissipation as a route toward engineering the far-from-equilibrium dynamics of quantum information, with potential applications in quantum metrology, state preparation, and transport.

We show that corner Majorana zero modes in a two-dimensional $p+id$ topological superconductor can be controlled by the manipulation of the parent $p$-wave superconducting order. Assuming that the $p$-wave superconducting order is in either a chiral or helical phase, we find that when a $d_{x^2-y^2}$ wave superconducting order is induced, the system exhibits quite different behavior depending on the nature of the parent $p$-wave phase. In particular, we find that while in the helical phase, a localized Majorana mode appears at each of the four corners, in the chiral phase, it is localized along only two of the four edges. We furthermore demonstrate that the Majoranas can be directly controlled by the form of the edges, as we explicitly show in the case of circular edges. We argue that the application of strain may provide additional means of fine-tuning the Majorana zero modes in the system; in particular, it can partially gap them out. Our findings may be relevant for probing the topology in two-dimensional mixed-pairing superconductors.

Topological insulator (TI) nanowires in proximity to conventional superconductors have been proposed as a tunable platform to realize topological superconductivity and Majorana zero modes. The tuning is done using an axial magnetic flux $\varphi$ which allows transforming the system from trivial at $\varphi =0$ to topologically nontrivial when half a magnetic flux quantum ${\varphi }_0/2$ threads the cross-section of the wire. Here, we explore the expected topological transition in TI-wire-based Josephson junctions as a function of magnetic flux by probing the $4\pi$-periodic fraction of the supercurrent, which is considered an indicator of topological superconductivity. Our data suggest that this $4\pi$-periodic supercurrent is at lower magnetic field largely of trivial origin but that, at magnetic fields above $\sim {\varphi }_0/4$, topological $4\pi$-periodic supercurrents take over.

We investigate the novel density distributions acquired by a dipolar Bose-Einstein condensed gas confined in a box potential, with special focus on the effects of supersolidity. Different from the case of harmonic trapping, the ground-state density reveals a strong depletion in the bulk region and an accumulation of atoms near the walls, well separated from the bulk, as a consequence of the competition between the attractive and the repulsive nature of the dipolar force. In a quasi-two-dimensional geometry characterized by cylindrical box trapping, we observe the emergence of a ringlike configuration near the boundary of the box, revealing peculiar supersolid and crystal effects in a useful range of parameters. In the case of square box trapping, the density oscillations along the edges, caused by the enhanced accumulation of atoms near the vertices, exhibit interesting analogies with the case of box-trapped quasi-one-dimensional configurations. For sufficiently large values of the atom number, the bulk region can also exhibit supersolidity, the resulting geometry reflecting the symmetry of the confining potential even for large systems.

We have designed different honeycomb lattices for microwave photons in the 4 to 8 $\mathrm{G}\mathrm{Hz}$ band using superconducting spiral resonators. Each lattice comprises a few hundred sites. Two designs have been studied, one leading to two bands touching at the Dirac points and one where a gap opens at the Dirac points. Using a scanning laser technique to image the eigenmodes of these photonic lattices, we are able to reconstruct their band structure. The measured bands are in excellent agreement with ab initio models that combine numerical simulations of the electromagnetic properties of the spiral resonator and analytical calculations.

We propose a scheme to generate remote asymmetric Einstein-Podolsky-Rosen (EPR) steering of two magnon modes in a hybrid ferromagnet-superconductor system. In our scheme, the indirect coherent coupling between two distant magnons and a $\mathrm{\Delta }$-type superconducting atom is established via exchange of virtual photons in two dispersive cavities. As a consequence, the artificial atom acting as a reservoir can produce asymmetrical magnon-magnon steering via a single dissipation channel. Interestingly, we find that the steering directivity is determined by cooling either Bogoliubov mode selectively rather than increasing the extra noise as in conventional schemes. This provides a novel idea to prepare remote EPR steering in solid devices, which may find potential applications in quantum information processing.

The quantum Fisher information (QFI) quantifies the ultimate precision of estimating a parameter from a quantum state and can be regarded as a reliability measure of a quantum system as a quantum sensor. However, estimation of the QFI for a mixed state is in general a computationally demanding task. In this paper we present a variational quantum algorithm called variational quantum Fisher information estimation (VQFIE) to address this task. By estimating lower and upper bounds on the QFI, based on bounding the fidelity, VQFIE outputs a range in which the actual QFI lies. This result can then be used to variationally prepare the state that maximizes the QFI, for the application of quantum sensing. In contrast to previous approaches, VQFIE does not require knowledge of the explicit form of the sensor dynamics. We simulate the algorithm for a magnetometry setup and demonstrate the tightening of our bounds as the state purity increases. For this example, we compare our bounds with literature bounds and show that our bounds are tighter.

Stabilized cat codes can provide a biased noise channel with a set of bias-preserving (BP) gates, which can significantly reduce the resource overhead for fault-tolerant quantum computing. All existing schemes of BP gates, however, require adiabatic quantum evolution, with performance limited by excitation loss and nonadiabatic errors during the adiabatic gates. In this paper, we apply a derivative-based leakage-suppression technique to overcome nonadiabatic errors, so that we can implement fast BP gates on Kerr-cat qubits with improved gate fidelity while maintaining high noise bias. When applied to concatenated quantum error correction, the fast BP gates not only can improve the logical error rate but also can reduce resource overhead, which enables more efficient implementation of fault-tolerant quantum computing.

We present and validate a method for noise injection of arbitrary spectra in quantum circuits that can be applied to any system capable of executing arbitrary single qubit rotations, including cloud-based quantum processors. As the consequences of temporally correlated noise on the performance of quantum algorithms are not well understood, the capability to engineer and inject such noise in quantum systems is paramount. To date, noise injection capabilities have been limited and highly platform specific, requiring low-level access to control hardware. We experimentally validate our method by comparing to a direct hardware-based noise-injection scheme. Using a combination of quantum noise spectroscopy and classical signal analysis, we show that the two approaches agree. These results showcase a highly versatile method for noise injection that can be utilized by theoretical and experimental researchers to verify, evaluate, and improve quantum characterization protocols and quantum algorithms for sensing and computing.

We introduce a protocol for dynamical dispersion engineering in an atomic chain formed by an ordered array of multilevel atoms with subwavelength lattice constant. This chain supports dark states that are protected from dissipation and can be understood as spin waves traveling along the array. By using an external control field with a spatially varying elliptical polarization we correlate internal and external degrees of freedom of the array in a controllable way. The coherent control over atomic states translates into control over the group velocity of the spin waves. A traveling excitation can be stored and released without dissipation by adiabatically changing the control field amplitude. This protocol is an alternative to the more conventional electromagnetically induced transparency and exemplifies the rich physics born of the interplay between coherent control and correlated decay.

Trapping cold neutral atoms in close proximity to nanostructures has raised a large interest in recent years, pushing the frontiers of cavity-QED and boosting the emergence of the waveguide-QED field of research. The design of efficient dipole trapping schemes in evanescent fields is a crucial requirement and a difficult task. Here we present an open-source Python package for calculating optical trapping potentials for neutral atoms, especially in the vicinity of nanostructures. Given field distributions and for a variety of trap configurations, nanotrappy computes the three-dimensional trapping potentials as well as the trap properties, ranging from trap positions to trap frequencies and state-dependent light shifts. We demonstrate the versatility for various seminal structures in the field, e.g., optical nanofiber, alligator slow-mode photonic-crystal waveguide, and microtoroid. This versatile package facilitates the systematic design of structures and provides a full characterization of trapping potentials with applications to the coherent manipulation of atoms and quantum information science.

The ground-state magnetic structures of the ${\mathrm{Eu}}^{2+}$ spins in recently discovered $\mathrm{RbEu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_4{\mathrm{As}}_4$ superconductors have been investigated by neutron powder diffraction measurements. It is found that as the superconductivity gets suppressed with the increase in Ni doping, the magnetic propagation vector of the Eu sublattice diminishes, corresponding to the decrease in the rotation angle between the moments in neighboring Eu layers. The ferromagnetic Eu layers are helically modulated along the $\mathit{c}$ axis with an incommensurate magnetic propagation vector in both the ferromagnetic superconductor $\mathrm{RbEu}{\left({\mathrm{Fe}}_{0.95}{\mathrm{Ni}}_{0.05}\right)}_4{\mathrm{As}}_4$ and the superconducting ferromagnet $\mathrm{RbEu}{\left({\mathrm{Fe}}_{0.93}{\mathrm{Ni}}_{0.07}\right)}_4{\mathrm{As}}_4$. Such a helical structure transforms into a purely collinear ferromagnetic structure for nonsuperconducting $\mathrm{RbEu}{\left({\mathrm{Fe}}_{0.91}{\mathrm{Ni}}_{0.09}\right)}_4{\mathrm{As}}_4$, with all the ${\mathrm{Eu}}^{2+}$ spins lying along the tetragonal (1 1 0) direction. The evolution from helical to collinear ferromagnetic order of the ${\mathrm{Eu}}^{2+}$ spins with increasing Ni doping is supported by first-principles calculations. The variation of the rotation angle between adjacent ${\mathrm{Eu}}^{2+}$ layers can be well explained by considering the change of magnetic exchange couplings mediated by the indirect Ruderman-Kittel-Kasuya-Yosida interaction.