# Peter James Joyce O'Malley

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Physical qubit calibration on a directed acyclic graph

John Martinis

arXiv, possibly npj Quantum Information (2018)

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.
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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.
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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.
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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.
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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.
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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.
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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