Publications (95)333.57 Total impact
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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 multiqubit system undergoes ergodic dynamics. Our work illustrates how controllable quantum systems can investigate fundamental questions in nonequilibrium 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: 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 smallscale systems as well as future errorcorrected quantum computers.  [Show abstract] [Hide abstract]
ABSTRACT: Leakage errors occur when a quantum system leaves the twolevel 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 nonlocal 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 singlequbit and twoqubit 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 analogdigital quantum simulation of interacting fermions and bosons in largescale 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 spinecho sequences by the repeated but incoherent addition of phase noise. We find our superconductingquantuminterferencedevicebased 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 LeggettGarg 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 BellLeggettGarg inequality (BLGI). By correlating data from ancillabased 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. 
Article: Traveling wave parametric amplifier with Josephson junctions using minimal resonator phase matching
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ABSTRACT: Josephson parametric amplifiers have become a critical tool in superconducting device physics due to their high gain and quantumlimited 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 LCladder transmission line of Josephson junctions and parallel plate capacitors using lowloss 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 environmentallyinduced 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 largescale 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 bitflip 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 twodimensional surface code QEC scheme, and track errors as they occur by repeatedly performing projective quantum nondemolition (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 nonclassical GreenbergerHorneZeilinger (GHZ) state. The successful suppression of environmentallyinduced errors strongly motivates further research into the many exciting challenges associated with building a largescale 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 SQUIDbased 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 noiseas good qubits.  [Show abstract] [Hide abstract]
ABSTRACT: We show how capacitance can be calculated simply and efficiently for electrodes cut in a 2dimensional 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 liftoff 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 ultrathin 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 nonlocality 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 noninteracting systems, by employing the exquisite control afforded by stateoftheart superconducting quantum circuits. We utilize this method to experimentally explore the wellknown 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 twoqubit Hamiltonian, with which we extend our studies to an interacting Hamiltonian and discover the emergence of an interactioninduced 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 twoqubit controlledZ 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 threequbit 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 twoqubit and threequbit 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.  [Show abstract] [Hide abstract]
ABSTRACT: One of the key challenges in quantum information is coherently manipulating the quantum state. However, it is an outstanding question whether control can be realized with low error. Only gates from the Clifford group  containing $\pi$, $\pi/2$, and Hadamard gates  have been characterized with high accuracy. Here, we show how the Platonic solids enable implementing and characterizing larger gate sets. We find that all gates can be implemented with low error. The results fundamentally imply arbitrary manipulation of the quantum state can be realized with high precision, providing new practical possibilities for designing efficient quantum algorithms.  [Show abstract] [Hide abstract]
ABSTRACT: Faster and more accurate state measurement is required for progress in superconducting qubit experiments with greater numbers of qubits and advanced techniques such as feedback. We have designed a multiplexed measurement system with a bandpass filter that allows fast measurement without increasing environmental damping of the qubits. We use this to demonstrate simultaneous measurement of four qubits on a single superconducting integrated circuit, the fastest of which can be measured to 99.8% accuracy in 140 ns. This accuracy and speed is suitable for advanced multiqubit experiments including surfacecode error correction.  [Show abstract] [Hide abstract]
ABSTRACT: Progress in superconducting qubit experiments with greater numbers of qubits or advanced techniques such as feedback requires faster and more accurate state measurement. We have designed a multiplexed measurement system with a bandpass filter that allows fast measurement without increasing environmental damping of the qubits. We use this to demonstrate simultaneous measurement of four qubits on a single superconducting integrated circuit, the fastest of which can be measured to 99.8% accuracy in 140ns. This accuracy and speed is suitable for advanced multiqubit experiments including surface code error correction.  [Show abstract] [Hide abstract]
ABSTRACT: Accurate methods of assessing the performance of quantum gates are extremely important. Quantum process tomography and randomized benchmarking are the current favored methods. Quantum process tomography gives detailed information, but significant approximations must be made to reduce this information to a form quantum error correction simulations can use. Randomized benchmarking typically outputs just a single number, the fidelity, giving no information on the structure of errors during the gate. Neither method is optimized to assess gate performance within an error detection circuit, where gates will be actually used in a largescale quantum computer. Specifically, the important issues of error composition and error propagation lie outside the scope of both methods. We present a fast, simple, and scalable method of obtaining exactly the information required to perform effective quantum error correction from the output of continuously running error detection circuits, enabling accurate prediction of largescale behavior.  [Show abstract] [Hide abstract]
ABSTRACT: Accurate methods of assessing the performance of quantum gates are extremely important. Quantum process tomography and randomized benchmarking are the current favored methods. Quantum process tomography gives detailed information, but significant approximations must be made to reduce this information to a form quantum error correction simulations can use. Randomized benchmarking typically outputs just a single number, the fidelity, giving no information on the structure of errors during the gate. Neither method is optimized to assess gate performance within an error detection circuit, where gates will be actually used in a largescale quantum computer. Specifically, the important issues of error composition and error propagation lie outside the scope of both methods. We present a fast, simple, and scalable method of obtaining exactly the information required to perform effective quantum error correction from the output of continuously running error detection circuits, enabling accurate prediction of largescale behavior.  [Show abstract] [Hide abstract]
ABSTRACT: A quantum computer can solve hard problems, such as prime factoring, database searching and quantum simulation, at the cost of needing to protect fragile quantum states from error. Quantum error correction provides this protection by distributing a logical state among many physical quantum bits (qubits) by means of quantum entanglement. Superconductivity is a useful phenomenon in this regard, because it allows the construction of large quantum circuits and is compatible with microfabrication. For superconducting qubits, the surface code approach to quantum computing is a natural choice for error correction, because it uses only nearestneighbour coupling and rapidly cycled entangling gates. The gate fidelity requirements are modest: the perstep fidelity threshold is only about 99 per cent. Here we demonstrate a universal set of logic gates in a superconducting multiqubit processor, achieving an average singlequbit gate fidelity of 99.92 per cent and a twoqubit gate fidelity of up to 99.4 per cent. This places Josephson quantum computing at the faulttolerance threshold for surface code error correction. Our quantum processor is a first step towards the surface code, using five qubits arranged in a linear array with nearestneighbour coupling. As a further demonstration, we construct a fivequbit GreenbergerHorneZeilinger state using the complete circuit and full set of gates. The results demonstrate that Josephson quantum computing is a highfidelity technology, with a clear path to scaling up to largescale, faulttolerant quantum circuits.
Publication Stats
1k  Citations  
333.57  Total Impact Points  
Top Journals
Institutions

20112015

University of California, Santa Barbara
 Department of Physics
Santa Barbara, California, United States


20042011

Delft University of Technology
 Faculty of Applied Sciences (AS)
Delft, South Holland, Netherlands
