Cross-verification of independent quantum devices

Quantum computers are on the brink of surpassing the capabilities of even the most powerful classical computers. This naturally raises the question of how one can trust the results of a quantum computer when they cannot be compared to classical simulation. Here we present a verification technique that exploits the principles of measurement-based quantum computation to link quantum circuits of different input size, depth, and structure. Our approach enables consistency checks of quantum computations within a device, as well as between independent devices. We showcase our protocol by applying it to five state-of-the-art quantum processors, based on four distinct physical architectures: nuclear magnetic resonance, superconducting circuits, trapped ions, and photonics, with up to 6 qubits and 200 distinct circuits

Postharvest Quality of Kiwifruit (Actinidia chinensis) after X-ray Irradiation Quarantine Treatment

The quality of two kiwifruit varieties [Actinidia chinensis var. deliciosa, ‘Hayward’ (green-fleshed), and Actinidia chinensis var. chinensis, ‘Zesy002’ (gold-fleshed)] was determined after X-ray irradiation at doses suitable for disinfestation of quarantine pests. Fruit were treated with irradiation doses of 0, 200, 400, 600, or 800 Gy and stored for 14 days at 2 °C. Irradiation did not affect soluble solids content, respiration rate, or taste. Minimal softening occurred to ‘Zesy002’ treated with irradiation doses of 400 or 800 Gy. No visible radiation injury, scald, or discoloration was observed. Irradiation treatment of kiwifruit at doses ≤800 Gy would ensure visual, compositional, and sensory quality while providing quarantine security

Cryogenic setup for trapped ion quantum computing

We report on the design of a cryogenic setup for trapped ion quantum computing containing a segmented surface electrode trap. The heat shield of our cryostat is designed to attenuate alternating magnetic field noise, resulting in 120~dB reduction of 50~Hz noise along the magnetic field axis. We combine this efficient magnetic shielding with high optical access required for single ion addressing as well as for efficient state detection by placing two lenses each with numerical aperture 0.23 inside the inner heat shield. The cryostat design incorporates vibration isolation to avoid decoherence of optical qubits due to the motion of the cryostat. We measure vibrations of the cryostat of less than $\pm$20~nm over 2~s. In addition to the cryogenic apparatus, we describe the setup required for an operation with $^{\mathrm{40}}$Ca$^{\mathrm{+}}$ and $^{\mathrm{88}}$Sr$^{\mathrm{+}}$ ions. The instability of the laser manipulating the optical qubits in $^{\mathrm{40}}$Ca$^{\mathrm{+}}$ is characterized yielding a minimum of its Allan deviation of 2.4$\cdot$10$^{\mathrm{-15}}$ at 0.33~s. To evaluate the performance of the apparatus, we trapped $^{\mathrm{40}}$Ca$^{\mathrm{+}}$ ions, obtaining a heating rate of 2.14(16)~phonons/s and a Gaussian decay of the Ramsey contrast with a 1/e-time of 18.2(8)~ms

Characterizing large-scale quantum computers via cycle benchmarking

Quantum computers promise to solve certain problems more efficiently than their digital counterparts. A major challenge towards practically useful quantum computing is characterizing and reducing the various errors that accumulate during an algorithm running on large-scale processors. Current characterization techniques are unable to adequately account for the exponentially large set of potential errors, including cross-talk and other correlated noise sources. Here we develop cycle benchmarking, a rigorous and practically scalable protocol for characterizing local and global errors across multi-qubit quantum processors. We experimentally demonstrate its practicality by quantifying such errors in non-entangling and entangling operations on an ion-trap quantum computer with up to 10 qubits, with total process fidelities for multi-qubit entangling gates ranging from 99.6(1)% for 2 qubits to 86(2)% for 10 qubits. Furthermore, cycle benchmarking data validates that the error rate per single-qubit gate and per two-qubit coupling does not increase with increasing system size.Comment: The main text consists of 6 pages, 3 figures and 1 table. The supplementary information consists of 6 pages, 2 figures and 3 table

Atomically resolved phase transition of fullerene cations solvated in helium droplets

Helium has a unique phase diagram and below 25 bar it does not form a solid even at the lowest temperatures. Electrostriction leads to the formation of a solid layer of helium around charged impurities at much lower pressures in liquid and superfluid helium. These so-called ‘Atkins snowballs’ have been investigated for several simple ions. Here we form HenC60+ complexes with n exceeding 100 via electron ionization of helium nanodroplets doped with C60. Photofragmentation of these complexes is measured by merging a tunable narrow- bandwidth laser beam with the ions. A switch from red- to blueshift of the absorption frequency of HenC60+ on addition of He atoms at n=32 is associated with a phase transition in the attached helium layer from solid to partly liquid (melting of the Atkins snowball). Elaborate molecular dynamics simulations using a realistic force field and including quantum effects support this interpretation

Experimental deterministic correction of qubit loss

The successful operation of quantum computers relies on protecting qubits from decoherence and noise which, if uncorrected, will lead to erroneous results. These errors accumulate during an algorithm and thus correcting them becomes a key requirement for large-scale and fault-tolerant quantum information processors. Besides computational errors, which can be addressed by quantum error correction, the carrier of the information can also be completely lost or the information can leak out of the computational space. It is expected that such loss errors will occur at rates that are comparable to computational errors. Here we experimentally implement a full cycle of qubit loss detection and correction on a minimal instance of a topological surface code in a trapped-ion quantum processor. The key technique for this correction is a quantum non-demolition measurement via an ancillary qubit, which acts as a minimally invasive probe to detect absent qubits while only imparting the minimal quantum-mechanically possible disturbance on the remaining qubits. Upon detecting qubit loss, a recovery procedure is triggered in real-time, which maps the logical information onto a new encoding on the remaining qubits. Although the current demonstration is performed in a trapped-ion quantum processor, the protocol is applicable to other quantum computing architectures and error correcting codes, including leading 2D and 3D topological codes. These methods provide a complete toolbox for the correction of qubit loss that complements techniques to mitigate computational errors, which together constitute the building blocks for complete and scalable quantum error correction