[Show abstract][Hide abstract]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 $2k_BT/h \approx 1\,\rm{GHz}$, 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 $1\,{\rm{Hz}}$. The antisymmetric component displays a 1/T dependence below $100\,\rm{mK}$, 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.
[Show abstract][Hide abstract]ABSTRACT: Surface distributions of two level system (TLS) defects and magnetic vortices are limiting dissipation sources in superconducting quantum circuits. Arrays of flux-trapping holes are commonly used to eliminate loss due to magnetic vortices, but may increase dielectric TLS loss. We find that dielectric TLS loss increases by approximately $25\, \%$ for resonators with a hole array beginning 2 $\mu \text{m}$ from the resonator edge, while the dielectric loss added by holes further away was below measurement sensitivity. Other forms of loss were not affected by the holes. Additionally, we bound the loss tangent due to residual magnetic effects to $<9\times 10^{-11} /\text{mG}$ for resonators patterned with flux-traps and operated in magnetic fields up to $50\, \text{mG}$.
Article · Jul 2016 · Superconductor Science and Technology
[Show abstract][Hide abstract]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.
[Show abstract][Hide abstract]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.
[Show abstract][Hide abstract]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 and the potential for harnessing their unique properties. However, concurrently bringing together the main ingredients for realizing many-body phenomena in a single experimental platform is a major challenge. 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 (FQH) 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 via the creation of photon-vacancies, or "holes", which circulate in the opposite direction. The combination of these key elements results in chiral groundstate currents, the first direct measurement of persistent currents in low-lying eigenstates of strongly interacting bosons. The observation of chiral currents at such a small scale is interesting and suggests that the rich many-body physics could survive to smaller scales. We also motivate the feasibility of creating FQH states with near future superconducting technologies. Our work introduces an experimental platform for engineering quantum phases of strongly interacting photons and highlight a path toward realization of bosonic FQH states.
[Show abstract][Hide abstract]ABSTRACT: We present a method to optimize qubit control parameters during error detection which is compatible with large-scale qubit arrays. We demonstrate our method to optimize single or two-qubit gates in parallel on a nine-qubit system. Additionally, we show how parameter drift can be compensated for during computation by inserting a frequency drift and using our method to remove it. We remove both drift on a single qubit and independent drifts on all qubits simultaneously. We believe this method will be useful in keeping error rates low on all physical qubits throughout the course of a computation. Our method is O(1) scalable to systems of arbitrary size, providing a path towards controlling the large numbers of qubits needed for a fault-tolerant quantum computer
[Show abstract][Hide abstract]ABSTRACT: Weak measurement has provided new insight into the nature of quantum measurement, by demonstrating the ability to extract average state information without fully projecting the system. For single-qubit measurements, this partial projection has been demonstrated with violations of the Leggett–Garg inequality. Here we investigate the effects of weak measurement on a maximally entangled Bell state through application of the Hybrid Bell–Leggett–Garg inequality (BLGI) on a linear chain of four transmon qubits. By correlating the results of weak ancilla measurements with subsequent projective readout, we achieve a violation of the BLGI with 27 s.d.s. of certainty.
[Show abstract][Hide abstract]ABSTRACT: Statistical mechanics is founded on the assumption that all accessible
configurations of a system are equally likely. This requires dynamics that
explore all states over time, known as ergodic dynamics. In isolated quantum
systems, however, the occurrence of ergodic behavior has remained an
outstanding question. Here, we demonstrate ergodic dynamics in a small quantum
system consisting of only three superconducting qubits. The qubits undergo a
sequence of rotations and interactions and we measure the evolution of the
density matrix. Maps of the entanglement entropy show that the full system can
act like a reservoir for individual qubits, increasing their entropy through
entanglement. Surprisingly, these maps bear a strong resemblance to the phase
space dynamics in the classical limit; classically chaotic motion coincides
with higher entanglement entropy. We further show that in regions of high
entropy the full multi-qubit system undergoes ergodic dynamics. Our work
illustrates how controllable quantum systems can investigate fundamental
questions in non-equilibrium thermodynamics.
[Show abstract][Hide abstract]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. Next, 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, inspiring hope
that quantum simulation of classically intractable molecules may be viable in
the near future.
[Show abstract][Hide abstract]ABSTRACT: Leakage errors occur when a quantum system leaves the two-level qubit
subspace. Reducing these errors is critically important for quantum error
correction to be viable. To quantify leakage errors, we use randomized
benchmarking in conjunction with measurement of the leakage population. We
characterize single qubit gates in a superconducting qubit, and by refining our
use of Derivative Reduction by Adiabatic Gate (DRAG) pulse shaping along with
detuning of the pulses, we obtain gate errors consistently below $10^{-3}$ and
leakage rates at the $10^{-5}$ level. With the control optimized, we find that
a significant portion of the remaining leakage is due to incoherent heating of
the qubit.
[Show abstract][Hide abstract]ABSTRACT: Simulating quantum physics with a device which itself is quantum mechanical,
a notion Richard Feynman originated, would be an unparallelled computational
resource. However, the universal quantum simulation of fermionic systems is
daunting due to their particle statistics, and Feynman left as an open question
whether it could be done, because of the need for non-local control. Here, we
implement fermionic interactions with digital techniques in a superconducting
circuit. Focusing on the Hubbard model, we perform time evolution with constant
interactions as well as a dynamic phase transition with up to four fermionic
modes encoded in four qubits. The implemented digital approach is universal and
allows for the efficient simulation of fermions in arbitrary spatial
dimensions. We use in excess of 300 single-qubit and two-qubit gates, and reach
global fidelities which are limited by gate errors. This demonstration
highlights the feasibility of the digital approach and opens a viable route
towards analog-digital quantum simulation of interacting fermions and bosons in
large-scale solid state systems.
[Show abstract][Hide abstract]ABSTRACT: A precise measurement of dephasing over a range of time scales is critical for improving quantum gates beyond the error correction threshold. We present a metrological tool based on randomized benchmarking capable of greatly increasing the precision of Ramsey and spin-echo sequences by the repeated but incoherent addition of phase noise. We find our superconducting-quantum-interference-device-based qubit is not limited by 1/f flux noise at short time scales but instead observe a telegraph noise mechanism that is not amenable to study with standard measurement techniques.
[Show abstract][Hide abstract]ABSTRACT: Since the inception of quantum mechanics, its validity as a complete
description of reality has been challenged due to predictions that defy
classical intuition. For many years it was unclear whether predictions like
entanglement and projective measurement represented real phenomena or artifacts
of an incomplete model. Bell inequalities (BI) provided the first quantitative
test to distinguish between quantum entanglement and a yet undiscovered
classical hidden variable theory. The Leggett-Garg inequality (LGI) provides a
similar test for projective measurement, and more recently has been adapted to
include variable strength measurements to study the process of measurement
itself. Here we probe the intersection of both entanglement and measurement
through the lens of the hybrid Bell-Leggett-Garg inequality (BLGI). By
correlating data from ancilla-based weak measurements and direct projective
measurements, we for the first time quantify the effect of measurement strength
on entanglement collapse. Violation of the BLGI, which we achieve only at the
weakest measurement strengths, offers compelling evidence of the completeness
of quantum mechanics while avoiding several loopholes common to previous
experimental tests. This uniquely quantum result significantly constrains the
nature of any possible classical theory of reality. Additionally, we
demonstrate that with sufficient scale and fidelity, a universal quantum
processor can be used to study richer fundamental physics.
[Show abstract][Hide abstract]ABSTRACT: Josephson parametric amplifiers have become a critical tool in
superconducting device physics due to their high gain and quantum-limited
noise. Traveling wave parametric amplifiers (TWPAs) promise similar noise
performance while allowing for significant increases in both bandwidth and
dynamic range. We present a TWPA device based on an LC-ladder transmission line
of Josephson junctions and parallel plate capacitors using low-loss amorphous
silicon dielectric. Crucially, we have inserted $\lambda/4$ resonators at
regular intervals along the transmission line in order to maintain the phase
matching condition between pump, signal, and idler and increase gain. We
achieve an average gain of 12\,dB across a 4\,GHz span, along with an average
saturation power of -92\,dBm with noise approaching the quantum limit.
[Show abstract][Hide abstract]ABSTRACT: Quantum computing becomes viable when a quantum state can be preserved from
environmentally-induced error. If quantum bits (qubits) are sufficiently
reliable, errors are sparse and quantum error correction (QEC) is capable of
identifying and correcting them. Adding more qubits improves the preservation
by guaranteeing increasingly larger clusters of errors will not cause logical
failure - a key requirement for large-scale systems. Using QEC to extend the
qubit lifetime remains one of the outstanding experimental challenges in
quantum computing. Here, we report the protection of classical states from
environmental bit-flip errors and demonstrate the suppression of these errors
with increasing system size. We use a linear array of nine qubits, which is a
natural precursor of the two-dimensional surface code QEC scheme, and track
errors as they occur by repeatedly performing projective quantum non-demolition
(QND) parity measurements. Relative to a single physical qubit, we reduce the
failure rate in retrieving an input state by a factor of 2.7 for five qubits
and a factor of 8.5 for nine qubits after eight cycles. Additionally, we
tomographically verify preservation of the non-classical
Greenberger-Horne-Zeilinger (GHZ) state. The successful suppression of
environmentally-induced errors strongly motivates further research into the
many exciting challenges associated with building a large-scale superconducting
quantum computer.
[Show abstract][Hide abstract]ABSTRACT: A precise measurement of dephasing over a range of timescales is critical for
improving quantum gates beyond the error correction threshold. We present a
method for measuring dephasing in experimental quantum systems based on
randomized benchmarking that excels at measuring small levels of phase noise at
the timescales relevant to gates. We find our SQUID-based qubit is not limited
by 1/f flux noise, but instead observe a previously unreported telegraph noise
mechanism. We demonstrate that full understanding of dephasing allows for the
use of "mediocre clocks"--systems with correlated phase noise--as good qubits.
[Show abstract][Hide abstract]ABSTRACT: We show how capacitance can be calculated simply and efficiently for
electrodes cut in a 2-dimensional ground plane. These results are in good
agreement with exact formulas and numerical simulations.
[Show abstract][Hide abstract]ABSTRACT: Many superconducting qubits are highly sensitive to dielectric loss, making
the fabrication of coherent quantum circuits challenging. To elucidate this
issue, we characterize the interfaces and surfaces of superconducting coplanar
waveguide resonators and study the associated microwave loss. We show that
contamination induced by traditional qubit lift-off processing is particularly
detrimental to quality factors without proper substrate cleaning, while
roughness plays at most a small role. Aggressive surface treatment is shown to
damage the crystalline substrate and degrade resonator quality. We also
introduce methods to characterize and remove ultra-thin resist residue,
providing a way to quantify and minimize remnant sources of loss on device
surfaces.
[Show abstract][Hide abstract]ABSTRACT: The discovery of topological phases in condensed matter systems has changed
the modern conception of phases of matter. The global nature of topological
ordering makes these phases robust and hence promising for applications.
However, the non-locality of this ordering makes direct experimental studies an
outstanding challenge, even in the simplest model topological systems, and
interactions among the constituent particles adds to this challenge. Here we
demonstrate a novel dynamical method to explore topological phases in both
interacting and non-interacting systems, by employing the exquisite control
afforded by state-of-the-art superconducting quantum circuits. We utilize this
method to experimentally explore the well-known Haldane model of topological
phase transitions by directly measuring the topological invariants of the
system. We construct the topological phase diagram of this model and visualize
the microscopic evolution of states across the phase transition, tasks whose
experimental realizations have remained elusive. Furthermore, we developed a
new qubit architecture that allows simultaneous control over every term in a
two-qubit Hamiltonian, with which we extend our studies to an interacting
Hamiltonian and discover the emergence of an interaction-induced topological
phase. Our implementation, involving the measurement of both global and local
textures of quantum systems, is close to the original idea of quantum
simulation as envisioned by R. Feynman, where a controllable quantum system is
used to investigate otherwise inaccessible quantum phenomena. This approach
demonstrates the potential of superconducting qubits for quantum simulation and
establishes a powerful platform for the study of topological phases in quantum
systems.
[Show abstract][Hide abstract]ABSTRACT: We apply the method of compressed sensing (CS) quantum process tomography
(QPT) to characterize quantum gates based on superconducting Xmon and phase
qubits. Using experimental data for a two-qubit controlled-Z gate, we obtain an
estimate for the process matrix $\chi$ with reasonably high fidelity compared
to full QPT, but using a significantly reduced set of initial states and
measurement configurations. We show that the CS method still works when the
amount of used data is so small that the standard QPT would have an
underdetermined system of equations. We also apply the CS method to the
analysis of the three-qubit Toffoli gate with numerically added noise, and
similarly show that the method works well for a substantially reduced set of
data. For the CS calculations we use two different bases in which the process
matrix $\chi$ is approximately sparse, and show that the resulting estimates of
the process matrices match each ther with reasonably high fidelity. For both
two-qubit and three-qubit gates, we characterize the quantum process by not
only its process matrix and fidelity, but also by the corresponding standard
deviation, defined via variation of the state fidelity for different initial
states.