The stable operation of quantum computers will rely on error-correction, in which single quantum bits of information are stored redundantly in the Hilbert space of a larger system. Such encoded qubits are commonly based on arrays of many physical qubits, but can also be realized using a single higher-dimensional quantum system, such as a harmonic oscillator [1,2]. A powerful encoding is formed from a periodically spaced superposition of position eigenstates [3][4][5]. Various proposals have been made for realizing approximations to such states, but these have thus far remained out of reach [6][7][8][9][10]. Here, we demonstrate such an encoded qubit using a superposition of displaced squeezed states of the harmonic motion of a single trapped 40 Ca + ion, controlling and measuring the oscillator through coupling to an ancilliary internal-state qubit [11]. We prepare and reconstruct logical states with an average square fidelity of 87.3 ± 0.7%, and demonstrate a universal logical single qubit gate set which we analyze using process tomography. For Pauli gates we reach process fidelities of ≈ 97%, while for continuous rotations we use gate teleportation achieving fidelities of ≈ 89%. The control demonstrated opens a route for exploring continuous variable error-correction as well as hybrid quantum information schemes using both discrete and continuous variables [12]. The code states also have direct applications in quantum sensing, allowing simultaneous measurement of small displacements in both position and momentum [13,14].
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 photonics will play a key role in quantum systems as they grow from few-qubit prototypes to tens of thousands of qubits. The underlying optical quantum technologies can only be realized through the integration of these components onto quantum photonic integrated circuits (QPICs) with accompanying electronics. In the last decade, remarkable advances in quantum photonic integration have enabled table-top experiments to be scaled down to prototype chips with improvements in efficiency, robustness, and key performance metrics. These advances have enabled integrated quantum photonic technologies combining up to 650 optical and electrical components onto a single chip that are capable of programmable quantum information processing, chip-to-chip networking, hybrid quantum system integration, and high-speed communications. In this roadmap article, we highlight the status, current and future challenges, and emerging technologies in several key research areas in integrated quantum photonics, including photonic platforms, quantum and classical light sources, quantum frequency conversion, integrated detectors, and applications in computing, communications, and sensing. With advances in materials, photonic design architectures, fabrication and integration processes, packaging, and testing and benchmarking, in the next decade we can expect a transition from single- and few-function prototypes to large-scale integration of multi-functional and reconfigurable devices that will have a transformative impact on quantum information science and engineering.
The long coherence times and strong Coulomb interactions afforded by trapped ion qubits have enabled realizations of the necessary primitives for quantum information processing (QIP) 1 , and indeed the highest-fidelity quantum operations in any qubit to date 2-4 . But while light delivery to each individual ion in a system is essential for general quantum manipulations and readout, experiments so far have employed optical systems cumbersome to scale to even a few tens of qubits 5 . Here we demonstrate lithographically defined nanophotonic waveguide devices for light routing and ion addressing fully integrated within a surface-electrode ion trap chip 6 . 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 88 Sr + ions. The grating focuses the beam to a diffraction-limited spot near the ion position with a 2 µm 1/e 2 -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 center. Owing to the scalability of the planar fabrication employed, together with the tight focusing and stable alignment afforded by optics integration within the trap chip, this approach presents a path to creating the optical systems required for large-scale trapped-ion QIP.Individual trapped ions show great promise for quantum computing; however, the lack of a scalable optical interface to manipulate and measure the quantum states of ions has been a major limitation to the development of a large-scale system 5 . Our approach to this problem utilizes nanophotonic single-mode (SM) waveguides and grating couplers integrated within the trap chip. Light is routed on chip by the waveguides and coupled by the gratings to beams with designed amplitude and phase profiles emitting from the chip towards the ions. These gratings are compact compared to the optical fibers and Fresnel lenses (both with cross-sections ≥100 µm in diameter) previously integrated with planar traps for addressing 7 and fluorescence collection 8,9 , and most importantly the planar fabrication used here to define the optics for both routing and addressing lends itself to intimate integration with the trap electrodes. Furthermore, such waveguide systems have been demonstrated to be scalable to complex geometries of thousands of devices or more 10 . Though micro-electro-mechanical systems (MEMS) mirrors integrated with traps have been proposed as well 11 , experiments so far have utilized MEMS components external to the vacuum chamber and separate from the chip 12 , leaving full integration an essential outstanding challenge.Integrated waveguide devices bring several advantages for ion addressing in planar traps. The ability to fabricate, in the same lithographically defined waveguide layer, multiple splitters, waveguide crossings, and bends with radii less than 10 µm, would enable the realization of a v...
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