Efficiently entangling pairs of qubits is essential to fully harness the power of quantum computing. Here, we devise an exact protocol that simultaneously entangles arbitrary pairs of qubits on a trapped-ion quantum computer. The protocol requires classical computational resources polynomial in the system size, and very little overhead in the quantum control compared to a single-pair case. We demonstrate an exponential improvement in both classical and quantum resources over the current state of the art. We implement the protocol on a software-defined trapped-ion quantum computer, where we reconfigure the quantum computer architecture on demand. Our protocol may also be extended to a wide variety of other quantum computing platforms.

In order to scale up quantum processors and achieve a quantum advantage, it is crucial to economize on the power requirement of two-qubit gates, make them robust to drift in experimental parameters, and shorten the gate times. Applicable to all quantum computer architectures whose two-qubit gates rely on phase-space closure, we present here a new gate-optimizing principle according to which negligible amounts of gate fidelity are traded for substantial savings in power, which, in turn, can be traded for substantial increases in gate speed and/or qubit connectivity. As a concrete example, we illustrate the method by constructing optimal pulses for entangling gates on a pair of ions within a trapped-ion chain, one of the leading quantum computing architectures. Our method is direct, noniterative, and linear, and, in some parameter regimes, constructs gate-steering pulses requiring up to an order of magnitude less power than the standard method. Additionally, our method provides increased robustness to mode drift. We verify the new trade-off principle experimentally on our trapped-ion quantum computer.

We performed high-resolution Fourier-transform infrared (FTIR) spectroscopy of a polymethyl methacrylate (PMMA) sphere of unknown size in the Mie scattering region. Apart from a slow, oscillatory structure (wiggles), which is due to an interference effect, the measured FTIR extinction spectrum exhibits a ripple structure, which is due to electromagnetic resonances. We fully characterize the underlying electromagnetic mode structure of the spectrum by assigning mode numbers to each of the ripples in the measured spectrum. We show that analyzing the ripple structure in the spectrum in the wavenumber region from about $3000{\mathrm{cm}}^{-1}$ to $8000{\mathrm{cm}}^{-1}$ allows us to determine both the unknown radius of the sphere and the PMMA index of refraction, which shows a strong frequency dependence in this infrared spectral region. While in this paper we focus on examining a PMMA sphere as an example, our method of determining the refractive index and its dispersion from infrared extinction spectra is generally applicable for the determination of the index of refraction of any transparent substance that can be shaped into micron-sized spheres.

With the help of analytical and numerical methods we analyze the nonlinear dynamics of a single ion stored in periodically driven dynamical traps of cylindrical and spherical design. Sinusoidal and impulsive drives are investigated. Both traps exhibit a mixed phase space for both drives. Additionally there is a route to chaos via period-doubling bifurcations of the fundamental stable trapping island. We demonstrate that the bifurcation scenarios of the kicked and cw-driven traps are quantitatively close and qualitatively identical.

Using Doppler-free Rb two-photon excitation measurements, we present support for the hypothesis that semiconductor-laser noise is characterized by pure phase-diffusion noise. We measured near-Lorentzian shapes for both the laser and the two-photon excitation spectra and a slope of 3.7±0.3 for the dependence of the two-photon excitation width on the laser width. These represent measurements of the second-order field statistics of a naturally operating laser where the noise is dominated by spontaneous emission. The measured spectral shape and slope are in excellent agreement with the Mollow model, which predicts a Lorentzian spectral shape and a slope of 4 for weak-field, two-photon excitation with a pure phase-diffusion field (Lorentzian spectral density). The dominance of phase-diffusion noise is further corroborated by an analytically solvable microscopic noise model that includes phase and amplitude noise.