Homoepitaxial growth of single crystal diamond membranes is demonstrated employing a microwave plasma chemical vapor deposition technique. The membranes possess excellent structural, optical, and ...spin properties, which make them suitable for fabrication of optical microcavities for applications in quantum information processing, photonics, spintronics, and sensing.
We present a proof-of-concept eight-channel wavelength-division-multiplexed (WDM) source for future terabit interconnects based on a highly efficient laser array. The array is composed of novel, ...high-power DFB lasers with record electro-optical efficiency, operating at 1280 nm, with >250-mW laser output power and laser efficiencies of up to 36%. The eight-laser array, with ~100-GHz channel spacing is optically butt-coupled to a planar lightwave circuit that consists of low-loss silicon nitride waveguides clad with silicon oxide for appropriate optical routing. Two types of optical routing are explored: 1) all 8 laser wavelengths are mixed and output into 10 channels via a star coupler and 2) the laser wavelengths are combined into a single output via an arrayed waveguide grating router. The mixed/combined light is then butt-coupled to a fiber array to output the WDM source signal into polarization preserving single-mode fibers. Improvements in insertion loss, particularly from optical butt coupling, will make this approach a viable option for efficient WDM light sources.
Practical quantum computing will require error rates well below those achievable with physical qubits. Quantum error correction
offers a path to algorithmically relevant error rates by encoding ...logical qubits within many physical qubits, for which increasing the number of physical qubits enhances protection against physical errors. However, introducing more qubits also increases the number of error sources, so the density of errors must be sufficiently low for logical performance to improve with increasing code size. Here we report the measurement of logical qubit performance scaling across several code sizes, and demonstrate that our system of superconducting qubits has sufficient performance to overcome the additional errors from increasing qubit number. We find that our distance-5 surface code logical qubit modestly outperforms an ensemble of distance-3 logical qubits on average, in terms of both logical error probability over 25 cycles and logical error per cycle ((2.914 ± 0.016)% compared to (3.028 ± 0.023)%). To investigate damaging, low-probability error sources, we run a distance-25 repetition code and observe a 1.7 × 10
logical error per cycle floor set by a single high-energy event (1.6 × 10
excluding this event). We accurately model our experiment, extracting error budgets that highlight the biggest challenges for future systems. These results mark an experimental demonstration in which quantum error correction begins to improve performance with increasing qubit number, illuminating the path to reaching the logical error rates required for computation.
Faster algorithms for combinatorial optimization could prove transformative for diverse areas such as logistics, finance and machine learning. Accordingly, the possibility of quantum enhanced ...optimization has driven much interest in quantum technologies. Here we demonstrate the application of the Google Sycamore superconducting qubit quantum processor to combinatorial optimization problems with the quantum approximate optimization algorithm (QAOA). Like past QAOA experiments, we study performance for problems defined on the planar connectivity graph native to our hardware; however, we also apply the QAOA to the Sherrington–Kirkpatrick model and MaxCut, non-native problems that require extensive compilation to implement. For hardware-native problems, which are classically efficient to solve on average, we obtain an approximation ratio that is independent of problem size and observe that performance increases with circuit depth. For problems requiring compilation, performance decreases with problem size. Circuits involving several thousand gates still present an advantage over random guessing but not over some efficient classical algorithms. Our results suggest that it will be challenging to scale near-term implementations of the QAOA for problems on non-native graphs. As these graphs are closer to real-world instances, we suggest more emphasis should be placed on such problems when using the QAOA to benchmark quantum processors.It is hoped that quantum computers may be faster than classical ones at solving optimization problems. Here the authors implement a quantum optimization algorithm over 23 qubits but find more limited performance when an optimization problem structure does not match the underlying hardware.
The nitrogen-vacancy (NV) center in diamond offers the opportunity to develop quantum technologies that leverage the defect’s atom-like properties using established engineering techniques from the ...semiconductor industry. While many NV center applications are motivated by the remarkable properties of isolated NV centers in bulk diamond, realizing these technologies requires addressing a number of device and materials engineering challenges unique to creating and controlling individual semiconductor spins. We review recent advances in interfacing NV centers with on-chip electronics that enable control over the defect’s spin and orbital degrees of freedom and review fabrication techniques for creating single NV centers with nanometer-scale placement accuracies. We also discuss efforts, motivated by the success of diamond NV center applications, to identify defect spins with similar properties to the NV center in more technologically mature semiconductors such as SiC.
Quantum many-body systems display rich phase structure in their low-temperature equilibrium states
. However, much of nature is not in thermal equilibrium. Remarkably, it was recently predicted that ...out-of-equilibrium systems can exhibit novel dynamical phases
that may otherwise be forbidden by equilibrium thermodynamics, a paradigmatic example being the discrete time crystal (DTC)
. Concretely, dynamical phases can be defined in periodically driven many-body-localized (MBL) systems via the concept of eigenstate order
. In eigenstate-ordered MBL phases, the entire many-body spectrum exhibits quantum correlations and long-range order, with characteristic signatures in late-time dynamics from all initial states. It is, however, challenging to experimentally distinguish such stable phases from transient phenomena, or from regimes in which the dynamics of a few select states can mask typical behaviour. Here we implement tunable controlled-phase (CPHASE) gates on an array of superconducting qubits to experimentally observe an MBL-DTC and demonstrate its characteristic spatiotemporal response for generic initial states
. Our work employs a time-reversal protocol to quantify the impact of external decoherence, and leverages quantum typicality to circumvent the exponential cost of densely sampling the eigenspectrum. Furthermore, we locate the phase transition out of the DTC with an experimental finite-size analysis. These results establish a scalable approach to studying non-equilibrium phases of matter on quantum processors.
Information scrambling in quantum circuits Mi, Xiao; Roushan, Pedram; Quintana, Chris ...
Science,
2021-Dec-17, 2021-12-17, 20211217, Letnik:
374, Številka:
6574
Journal Article
Recenzirano
Odprti dostop
Interactions in quantum systems can spread initially localized quantum information into the exponentially many degrees of freedom of the entire system. Understanding this process, known as quantum ...scrambling, is key to resolving several open questions in physics. Here, by measuring the time-dependent evolution and fluctuation of out-of-time-order correlators, we experimentally investigate the dynamics of quantum scrambling on a 53-qubit quantum processor. We engineer quantum circuits that distinguish operator spreading and operator entanglement and experimentally observe their respective signatures. We show that whereas operator spreading is captured by an efficient classical model, operator entanglement in idealized circuits requires exponentially scaled computational resources to simulate. These results open the path to studying complex and practically relevant physical observables with near-term quantum processors.
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