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Peter James Joyce O'Malley

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    Preview abstract High-fidelity control of qubits requires precisely tuned control parameters. Typically, these param- eters are found through a series of bootstrapped calibration experiments which successively acquire more accurate information about a physical qubit. However, optimal parameters are typically dif- ferent between devices and can also drift in time, which begets the need for an efficient calibration strategy. Here, we introduce a framework to understand the relationship between calibrations as a directed graph. With this approach, calibration is reduced to a graph traversal problem that is automatable and extensible. View details
    Chiral Ground-State Currents of Interacting Photons in a Synthetic Magnetic Field
    Charles Neill
    Anthony Megrant
    Yu Chen
    Rami Barends
    Brooks Campbell
    Zijun Chen
    Ben Chiaro
    Andrew Dunsworth
    Josh Mutus
    Amit Vainsencher
    Jim Wenner
    Eliot Kapit
    John Martinis
    Nature Physics, vol. 13 (2017), pp. 146-151
    Preview abstract The intriguing many-body phases of quantum matter arise from the interplay of particle interactions, spatial symmetries, and external fields. Generating these phases in an engineered system could provide deeper insight into their nature. Using superconducting qubits, we simultaneously realize synthetic magnetic fields and strong particle interactions, which are among the essential elements for studying quantum magnetism and fractional quantum Hall phenomena. The artificial magnetic fields are synthesized by sinusoidally modulating the qubit couplings. In a closed loop formed by the three qubits, we observe the directional circulation of photons, a signature of broken time-reversal symmetry. We demonstrate strong interactions through the creation of photon vacancies, or "holes", which circulate in the opposite direction. The combination of these key elements results in chiral ground-state currents. Our work introduces an experimental platform for engineering quantum phases of strongly interacting photons. View details
    Observation of classical-quantum crossover of 1/f flux noise and its paramagnetic temperature dependence
    Yu Chen
    Andre Petukhov
    Ben Chiaro
    Anthony Megrant
    Rami Barends
    Brooks Campbell
    Zijun Chen
    Andrew Dunsworth
    Rob Graff
    Josh Mutus
    Charles Neill
    Alireza Shabani
    Vadim Smelyanskiy
    Amit Vainsencher
    Jim Wenner
    John Martinis
    Phys. Rev. Lett., vol. 118 (2017), pp. 057702
    Preview abstract By analyzing the dissipative dynamics of a tunable gap flux qubit, we extract both sides of its two-sided environmental flux noise spectral density over a range of frequencies around 2kT/h ≈ 1GHz, allowing for the observation of a classical-quantum crossover. Below the crossover point, the symmetric noise component follows a 1/f power law that matches the magnitude of the 1/f noise near 1 Hz. The antisymmetric component displays a 1/T dependence below 100 mK, providing dynamical evidence for a paramagnetic environment. Extrapolating the two-sided spectrum predicts the linewidth and reorganization energy of incoherent resonant tunneling between flux qubit wells. View details
    Observation of classical-quantum crossover of 1/f flux noise and its paramagnetic temperature dependence
    Yu Chen
    Andre Petukhov
    Ben Chiaro
    Anthony Megrant
    Rami Barends
    Brooks Campbell
    Zijun Chen
    Andrew Dunsworth
    Rob Graff
    Josh Mutus
    Charles Neill
    Alireza Shabani
    Vadim Smelyanskiy
    Amit Vainsencher
    Jim Wenner
    John Martinis
    Physical Review Letter, vol. 118 (2017), pp. 057702
    Preview abstract By analyzing the dissipative dynamics of a tunable gap flux qubit, we extract both sides of its two-sided environmental flux noise spectral density over a range of frequencies around 2kBT /h ≈ 1 GHz, allowing for the observation of a classical-quantum crossover. Below the crossover point, the symmetric noise component follows a 1/f power law that matches the magnitude of the 1/f noise near 1 Hz. The antisymmetric component displays a 1/T dependence below 100 mK, providing dynamical evidence for a paramagnetic environment. Extrapolating the two-sided spectrum predicts the linewidth and reorganization energy of incoherent resonant tunneling between flux qubit wells. View details
    Scalable Quantum Simulation of Molecular Energies
    Ian Kivlichan
    Jonathan Romero
    Rami Barends
    Andrew Tranter
    Brooks Campbell
    Yu Chen
    Zijun Chen
    Ben Chiaro
    Andrew Dunsworth
    Anthony Megrant
    Josh Mutus
    Charles Neil
    Jim Wenner
    Amit Vainsencher
    Peter Coveney
    Peter Love
    Alán Aspuru-Guzik
    John Martinis
    Physical Review X, vol. 6 (2016), pp. 031007
    Preview abstract We report the first electronic structure calculation performed on a quantum computer without exponentially costly precompilation. We use a programmable array of superconducting qubits to compute the energy surface of molecular hydrogen using two distinct quantum algorithms. First, we experimentally execute the unitary coupled cluster method using the variational quantum eigensolver. Our efficient implementation predicts the correct dissociation energy to within chemical accuracy of the numerically exact result. Second, we experimentally demonstrate the canonical quantum algorithm for chemistry, which consists of Trotterization and quantum phase estimation. We compare the experimental performance of these approaches to show clear evidence that the variational quantum eigensolver is robust to certain errors. This error tolerance inspires hope that variational quantum simulations of classically intractable molecules may be viable in the near future. View details
    Digitized Adiabatic Quantum Computing with a Superconducting Circuit
    Rami Barends
    Alireza Shabani
    Lucas Lamata
    Antonio Mezzacapo
    Urtzi Las Heras
    Brooks Campbell
    Yu Chen
    Zijun Chen
    Ben Chiaro
    Andrew Dunsworth
    Anthony Megrant
    Josh Mutus
    Charles Neill
    Enrique Solano
    Jim Wenner
    Amit Vainsencher
    John Martinis
    Nature, vol. 534 (2016), pp. 222-226
    Preview abstract A major challenge in quantum computing is to solve general problems with limited physical hardware. Here, we implement digitized adiabatic quantum computing, combining the generality of the adiabatic algorithm with the universality of the digital approach, using a superconducting circuit with nine qubits. We probe the adiabatic evolutions, and quantify the success of the algorithm for random spin problems. We find that the system can approximate the solutions to both frustrated Ising problems and problems with more complex interactions, with a performance that is comparable. The presented approach is compatible with small-scale systems as well as future error-corrected quantum computers. View details
    Measurement-induced state transitions in a superconducting qubit: Beyond the rotating wave approximation
    Alexander Korotkov
    Amit Vainsencher
    Andrew Dunsworth
    Anthony Megrant
    Ben Chiaro
    Brooks Campbell
    Charles Neill
    Jim Wenner
    John Martinis
    Josh Mutus
    Mostafa Khezri
    Rami Barends
    Yu Chen
    Zijun Chen
    Physical Review Letters (2016)
    Preview abstract Many superconducting qubit systems use the dispersive interaction between the qubit and a coupled harmonic resonator to perform quantum state measurement. Previous works have found that such measurements can induce state transitions in the qubit if the number of photons in the resonator is too high. We investigate these transitions and find that they can push the qubit out of the two-level subspace. Furthermore, these transitions show resonant behavior as a function of photon number. We develop a theory for these observations based on level crossings within the Jaynes-Cummings ladder, with transitions mediated by terms in the Hamiltonian which are typically ignored by the rotating wave approximation. We confirm the theory by measuring the photon occupation of the resonator when transitions occur while varying the detuning between the qubit and resonator. View details
    Characterization and reduction of microfabrication-induced decoherence in superconducting quantum circuits
    Anthony Megrant
    Zijun Chen
    Andrew Dunsworth
    Ben Chiaro
    Rami Barends
    Brooks Campbell
    Yu Chen
    I.-C. Hoi
    Josh Mutus
    Charles Neill
    Amit Vainsencher
    Jim Wenner
    Andrew Cleland
    John Martinis
    Appl. Phys. Lett., vol. 105 (2014), pp. 062601