Control of an Atomic Quadrupole Transition in a Phase-Stable Standing Wave.

Using a single calcium ion confined in a surface-electrode trap, we study the interaction of electric quadrupole transitions with a passively phase-stable optical standing wave field sourced by photonics integrated within the trap. We characterize the optical fields through spatial mapping of the Rabi frequencies of both carrier and motional sideband transitions as well as ac Stark shifts. Our measurements demonstrate the ability to engineer favorable combinations of sideband and carrier Rabi frequency as well as ac Stark shifts for specific tasks in quantum state control and metrology.

Materials challenges and opportunities for quantum computing hardware.

Quantum computing hardware technologies have advanced during the past two decades, with the goal of building systems that can solve problems that are intractable on classical computers. The ability to realize large-scale systems depends on major advances in materials science, materials engineering, and new fabrication techniques. We identify key materials challenges that currently limit progress in five quantum computing hardware platforms, propose how to tackle these problems, and discuss some new areas for exploration. Addressing these materials challenges will require scientists and engineers to work together to create new, interdisciplinary approaches beyond the current boundaries of the quantum computing field.

Integrated optical multi-ion quantum logic.

Practical and useful quantum information processing requires substantial improvements with respect to current systems, both in the error rates of basic operations and in scale. The fundamental qualities of individual trapped-ion1 qubits are promising for long-term systems2, but the optics involved in their precise control are a barrier to scaling3. Planar-fabricated optics integrated within ion-trap devices can make such systems simultaneously more robust and parallelizable, as suggested by previous work with single ions4. Here we use scalable optics co-fabricated with a surface-electrode ion trap to achieve high-fidelity multi-ion quantum logic gates, which are often the limiting elements in building up the precise, large-scale entanglement that is essential to quantum computation. Light is efficiently delivered to a trap chip in a cryogenic environment via direct fibre coupling on multiple channels, eliminating the need for beam alignment into vacuum systems and cryostats and lending robustness to vibrations and beam-pointing drifts. This allows us to perform ground-state laser cooling of ion motion and to implement gates generating two-ion entangled states with fidelities greater than 99.3(2) per cent. This work demonstrates hardware that reduces noise and drifts in sensitive quantum logic, and simultaneously offers a route to practical parallelization for high-fidelity quantum processors5. Similar devices may also find applications in atom- and ion-based quantum sensing and timekeeping6.

Precise and diffraction-limited waveguide-to-free-space focusing gratings.

We present the design and characterization of waveguide grating devices that couple visible-wavelength light at λ = 674 nm from single-mode, high index-contrast dielectric waveguides to free-space beams forming micron-scale diffraction-limited spots a designed distance and angle from the grating. With a view to application in spatially-selective optical addressing, and in contrast to previous work on similar devices, deviations from the main Gaussian lobe up to 25 microns from the focus and down to the 5 × 10-6 level in relative intensity are characterized as well; we show that along one dimension the intensity of these weak sidelobes approaches the limit imposed by diffraction from the finite field extent in the grating region. Additionally, we characterize the polarization purity in the focal region, observing at the center of the focus a low impurity <3 × 10-4 in relative intensity. Our approach allows quick, intuitive design of devices with such performance, which may be applied in trapped-ion quantum information processing and generally in any systems requiring optical routing to or from objects 10 s-100 s of microns from a chip surface, but benefitting from the parallelism and density of planar-fabricated dielectric integrated optics.

Integrated optical addressing of an ion qubit.

The long coherence times and strong Coulomb interactions afforded by trapped ion qubits have enabled realizations of the necessary primitives for quantum information processing and the highest-fidelity quantum operations in any qubit to date. Although light delivery to each individual ion in a system is essential for general quantum manipulations and readout, experiments so far have employed optical systems that are cumbersome to scale to even a few tens of qubits. Here we demonstrate lithographically defined nanophotonic waveguide devices for light routing and ion addressing that are fully integrated within a surface-electrode ion trap chip. Ion qubits are addressed at multiple locations via focusing grating couplers emitting through openings in the trap electrodes to ions trapped 50 µm above the chip; using this light, we perform quantum coherent operations on the optical qubit transition in individual 88Sr+ ions. The grating focuses the beam to a diffraction-limited spot near the ion position with 2 µm 1/e2 radius along the trap axis, and we measure crosstalk errors between 10-2 and 4 × 10-4 at distances 7.5-15 µm from the beam centre. Owing to the scalability of the planar fabrication technique employed, together with the tight focusing and stable alignment afforded by the integration of the optics within the trap chip, this approach presents a path to creating the optical systems required for large-scale trapped-ion quantum information processing.

High-Q CMOS-integrated photonic crystal microcavity devices.

Integrated optical resonators are necessary or beneficial in realizations of various functions in scaled photonic platforms, including filtering, modulation, and detection in classical communication systems, optical sensing, as well as addressing and control of solid state emitters for quantum technologies. Although photonic crystal (PhC) microresonators can be advantageous to the more commonly used microring devices due to the former's low mode volumes, fabrication of PhC cavities has typically relied on electron-beam lithography, which precludes integration with large-scale and reproducible CMOS fabrication. Here, we demonstrate wavelength-scale polycrystalline silicon (pSi) PhC microresonators with Qs up to 60,000 fabricated within a bulk CMOS process. Quasi-1D resonators in lateral p-i-n structures allow for resonant defect-state photodetection in all-silicon devices, exhibiting voltage-dependent quantum efficiencies in the range of a few 10 s of %, few-GHz bandwidths, and low dark currents, in devices with loaded Qs in the range of 4,300-9,300; one device, for example, exhibited a loaded Q of 4,300, 25% quantum efficiency (corresponding to a responsivity of 0.31 A/W), 3 GHz bandwidth, and 30 nA dark current at a reverse bias of 30 V. This work demonstrates the possibility for practical integration of PhC microresonators with active electro-optic capability into large-scale silicon photonic systems.

Polycrystalline silicon ring resonator photodiodes in a bulk complementary metal-oxide-semiconductor process.

We present measurements on resonant photodetectors utilizing sub-bandgap absorption in polycrystalline silicon ring resonators, in which light is localized in the intrinsic region of a p+/p/i/n/n+ diode. The devices, operating both at λ=1280 and λ=1550 nm and fabricated in a complementary metal-oxide-semiconductor (CMOS) dynamic random-access memory emulation process, exhibit detection quantum efficiencies around 20% and few-gigahertz response bandwidths. We observe this performance at low reverse biases in the range of a few volts and in devices with dark currents below 50 pA at 10 V. These results demonstrate that such photodetector behavior, previously reported by Preston et al. [Opt. Lett. 36, 52 (2011)], is achievable in bulk CMOS processes, with significant improvements with respect to the previous work in quantum efficiency, dark current, linearity, bandwidth, and operating bias due to additional midlevel doping implants and different material deposition. The present work thus offers a robust realization of a fully CMOS-fabricated all-silicon photodetector functional across a wide wavelength range.