Publications (52)269.91 Total impact
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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.Nature Communications 07/2015; 6:7654. DOI:10.1038/ncomms8654 · 11.47 Impact Factor  [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.Applied Physics Letters 03/2015; 106(24). DOI:10.1063/1.4922348 · 3.30 Impact Factor  [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.Nature 11/2014; 519(7541). DOI:10.1038/nature14270 · 41.46 Impact Factor  [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 review progress at UCSB on understanding the physics of decoherence in superconducting qubits. Although many decoherence mechanisms were studied and fixed in the last 5 years, the most important ones are twolevel state defects in amorphous dielectrics, nonequilibrium quasiparticles generated from stray infrared light, and radiation to slotline modes. With improved design, the performance of integrated circuit transmons using the Xmon design are now close to world record performance: these devices have the advantage of retaining coherence when scaled up to 9 qubits.  [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.Applied Physics Letters 07/2014; 105(6). DOI:10.1063/1.4893297 · 3.30 Impact Factor  [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.Nature 07/2014; 515(7526). DOI:10.1038/nature13891 · 41.46 Impact Factor  [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.Physical Review A 06/2014; 90(3). DOI:10.1103/PhysRevA.90.030303 · 2.81 Impact Factor  [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.Physical Review Letters 05/2014; 112(19):190504. · 7.51 Impact Factor  [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.Physical Review Letters 05/2014; 112(19). DOI:10.1103/PhysRevLett.112.190504 · 7.51 Impact Factor  [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.Nature 04/2014; 508(7497):5003. DOI:10.1038/nature13171 · 41.46 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Quantum interference is one of the most fundamental physical effects found in nature. Recent advances in quantum computing now employ interference as a fundamental resource for computation and control. Quantum interference also lies at the heart of sophisticated condensed matter phenomena such as Anderson localization, phenomena that are difficult to reproduce in numerical simulations. Here, employing a multipleelement superconducting quantum circuit, with which we manipulate a single microwave photon, we demonstrate that we can emulate the basic effects of weak localization. By engineering the control sequence, we are able to reproduce the wellknown negative magnetoresistance of weak localization as well as its temperature dependence. Furthermore, we can use our circuit to continuously tune the level of disorder, a parameter that is not readily accessible in mesoscopic systems. Demonstrating a high level of control, our experiment shows the potential for employing superconducting quantum circuits as emulators for complex quantum phenomena.Nature Communications 03/2014; 5:5184. DOI:10.1038/ncomms6184 · 11.47 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We introduce a superconducting qubit architecture that combines highcoherence qubits and tunable qubitqubit coupling. With the ability to set the coupling to zero, we demonstrate that this architecture is protected from the frequency crowding problems that arise from fixed coupling. More importantly, the coupling can be tuned dynamically with nanosecond resolution, making this architecture a versatile platform with applications ranging from quantum logic gates to quantum simulation. We illustrate the advantages of dynamic coupling by implementing a novel adiabatic controlledZ gate, at a speed approaching that of singlequbit gates. Integrating coherence and scalable control, our "gmon" architecture is a promising path towards largescale quantum computation and simulation.Physical Review Letters 02/2014; 113(22). DOI:10.1103/PhysRevLett.113.220502 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present a method for optimizing quantum control in experimental systems, using a subset of randomized benchmarking measurements to rapidly infer error. This is demonstrated to improve single and twoqubit gates, minimize gate bleedthrough, where a gate mechanism can cause errors on subsequent gates, and identify control crosstalk in superconducting qubits. This method is able to correct parameters to where control errors no longer dominate, and is suitable for automated and closedloop optimization of experimental systems.Physical Review Letters 02/2014; 112(24). DOI:10.1103/PhysRevLett.112.240504 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present a lumpedelement Josephson parametric amplifier designed to operate with strong coupling to the environment. In this regime, we observe broadband frequency dependent amplification with multipeaked gain profiles. We account for this behaviour using the "pumpistor" model which allows for frequency dependent variation of the external impedance. Using this understanding, we demonstrate control over gain profiles through changes in the environment impedance at a given frequency. With strong coupling to a suitable external impedance we observe a significant increase in dynamic range, and large amplification bandwidth up to 700 MHz giving near quantumlimited performance.Applied Physics Letters 01/2014; 104(26). DOI:10.1063/1.4886408 · 3.30 Impact Factor 
Article: Room temperature deposition of sputtered TiN films for superconducting coplanar waveguide resonators
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ABSTRACT: We present a systematic study of the properties of room temperature deposited TiN films by varying the deposition conditions in an ultrahighvacuum reactive magnetron sputtering chamber. By increasing the deposition pressure from 2 to 9 mTorr while keeping a nearly stoichiometric composition of Ti1xNx (x = 0.5) without substrate heating, the film resistivity increases, the dominant crystal orientation changes from (100) to (111), grain boundaries become clearer, and the strong compressive inplane strain changes to weak tensile inplane strain. The TiN films absorb a high concentration of contaminants including hydrogen, carbon, and oxygen when they are exposed to air after deposition. With the targetsubstrate distance set to 88 mm the contaminant levels increase from ˜0.1% to ˜10% as the pressure is increased from 2 to 9 mTorr. The contaminant concentrations also correlate with inplane distance from the center of the substrate and increase by roughly two orders of magnitude as the targetsubstrate distance is increased from 88 to 266 mm. These contaminants are found to strongly influence the properties of TiN thin films. For instance, the resistivity of stoichiometric films increases by around a factor of 5 as the oxygen content increases from 0.1% to 11%. These results strongly suggest that the energy of the sputtered TiN particles plays a crucial role in determining the TiN film properties, and that it is important to precisely control the energy of these particles to obtain highquality TiN films. Superconducting coplanar waveguide resonators made from a series of nearly stoichiometric films grown at pressures from 2 to 7 mTorr show a substantial increase in intrinsic quality factor from ˜104 to ˜106 as the magnitude of the compressive strain decreases from nearly 3800 MPa to approximately 150 MPa and the oxygen content increases from 0.1% to 8%. Surprisingly, the films with a higher oxygen content exhibit lower loss, but care must be taken when depositing at room temperature to avoid nonuniform oxygen incorporation, which presents as a radially dependent resistivity and becomes a radially dependent surface inductance in the superconductor.Superconductor Science and Technology 01/2014; 27(1):5009. DOI:10.1088/09532048/27/1/015009 · 2.33 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Quantum information systems require high fidelity quantum operations. It is particularly challenging to convert flying qubits to stationary qubits for deterministic quantum networks, since absorbing naturally shaped emission has a maximum fidelity of only 54%. Theoretical protocols reaching 100% efficiency rely upon sculpting the time dependence of photon wavepackets and receiver coupling. Using these schemes, experimental fidelities have reached up to 20% for optical photons and 81% for microwave photons, although with drive pulses much longer than the cavity decay rate. Here, we demonstrate a particularly simple "time reversed" photon shape and gated receiver with an absorption fidelity of 99.4% and a receiver efficiency of 97.4% for microwave photons. We classically drive a superconducting coplanar waveguide resonator an order of magnitude shorter than the intrinsic decay time. With the fidelity now at the error threshold for fault tolerant quantum communication (96%) and computation (99.4%) and comparable to fidelities of good logic gates and measurements, new designs may be envisioned for quantum communication and computation systems.Physical Review Letters 11/2013; 112(21). DOI:10.1103/PhysRevLett.112.210501 · 7.51 Impact Factor 
Article: Fabrication and Characterization of Aluminum Airbridges for Superconducting Microwave Circuits
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ABSTRACT: Superconducting microwave circuits based on coplanar waveguides (CPW) are susceptible to parasitic slotline modes which can lead to loss and decoherence. We motivate the use of superconducting airbridges as a reliable method for preventing the propagation of these modes. We describe the fabrication of these airbridges on superconducting resonators, which we use to measure the loss due to placing airbridges over CPW lines. We find that the additional loss at single photon levels is small, and decreases at higher drive powers.Applied Physics Letters 10/2013; 104(5). DOI:10.1063/1.4863745 · 3.30 Impact Factor
Publication Stats
378  Citations  
269.91  Total Impact Points  
Top Journals
 Applied Physics Letters (9)
 Physical Review Letters (6)
 Physical Review Letters (3)
 Nature (2)
 Nature Communications (2)
Institutions

2011–2015

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