Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B28: Focus Session: Superconducting Qubits: Error Correction & Validation Methods |
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Sponsoring Units: GQI Chair: Alexandre Blais, Sherbrooke University Room: 601 |
Monday, March 3, 2014 11:15AM - 11:51AM |
B28.00001: Controlling Schr\"{o}dinger cat states using qubit-photon entanglement Invited Speaker: Brian Vlastakis With a toolset of conditional qubit-photon logic, we manipulate the quantum state of a cavity resonator. We map a quantum bit to a superposition of coherent states, also known as a Schr\"{o}dinger cat state. We achieve this using a superconducting transmon qubit coupled to a microwave waveguide cavity with an ideal off-resonant coupling. This dispersive interaction is much greater than decoherence rates and higher-order nonlinearities which allows simultaneous control of over one hundred photons. We extend this protocol to create superpositions of up to four coherent states. Furthermore, we explore the conditional logic used in this procedure by performing state tomography on the joint qubit-cavity system. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B28.00002: Parity check operation in a surface code plaquette segment with superconducting qubits Jerry Chow, Jay Gambetta, Srikanth Srinivasan, Easwar Magesan, Andrew Cross, David Abraham, Nicholas Masluk, Blake Johnson, Colm Ryan, Christopher Lirakis, Matthias Steffen An essential part of the two-dimensional surface code is the ability to perform $X$ and $Z$-stabilizer parity checks of code qubits via the measurement of ancilla qubits. We benchmark a complete set of high-fidelity single- and two-qubit gates on a three-qubit sub-section of a surface code layout comprised of superconducting resonators and transmons. Combining these gates with high-fidelity individual single-shot readouts, we show a parity check operation, deterministically entangling two qubits which are non-nearest neighbors. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B28.00003: Demonstrating a four-qubit network for the surface code with superconducting qubits Srikanth Srinivasan, Easwar Magesan, Jerry Chow, Jay Gambetta, Andrew Cross, Nicholas Masluk, David Abraham, Nicholas Bronn, Christopher Lirakis, Matthias Steffen In the skew-symmetric layout of superconducting qubits and resonators for the surface-code error correction protocol, studying an inner ring structure of four qubits is a critical step towards demonstrating the core operations of a full plaquette tile. We show results for quantum devices consisting of twelve quantum degrees of freedom: four transmon qubits, coupled via four bus resonators, with four independent readout resonators. We discuss engineering challenges of such devices as well as benchmarked results for control and readout. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B28.00004: Tracking Quantum Jumps of Light with Repeated Single-Shot Parity Measurements Luyan Sun, Andrei Petrenko, Zaki Leghtas, Brian Vlastakis, Gerhard Kirchmair, Katrina Sliwa, Anirudh Narla, Michael Hatridge, Shyam Shankar, Jacob Blumoff, Luigi Frunzio, Mazyar Mirrahimi, Michel Devoret, Robert Schoelkopf Quantum error correction (QEC) is required for a practical quantum computer because of the fragile nature of quantum information. A measurement-based QEC requires the measurement of error syndromes in a quantum non-demolition way and at a rate which is faster than errors occur. In a 3D circuit quantum electrodynamics architecture, we realize a parity measurement of a microwave field with about 90{\%} fidelity by mapping its parity onto an ancilla qubit. The projective nature of the parity measurement onto a degenerate parity eigenspace, the cat states, is confirmed by Wigner tomography after a single parity measurement, showing 84{\%} fidelity to ideal cats. The parity can therefore serve as an error syndrome for a recently proposed QEC scheme [Leghtas et.al. PRL (2013)]. We then demonstrate a tracking of quantum jumps of this error syndrome by repeated parity measurements. We will also discuss a quantum filter developed to mitigate the imperfections during the parity measurement for a best estimate of the photon state parity. The demonstrated extraction of error syndromes without perturbing the encoded information is essential for QEC. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B28.00005: Quantum Non-Demolition Singleshot Parity Measurements for a Proposed Quantum Error Correction Scheme Andrei Petrenko, Luyan Sun, Zaki Leghtas, Brian Vlastakis, Gerhard Kirchmair, Katrina Sliwa, Anirudh Narla, Michael Hatridge, Shyam Shankar, Jacob Blumoff, Luigi Frunzio, Mazyar Mirrahimi, Michel Devoret, Robert Schoelkopf In order to be effective, a quantum error correction scheme(QEC) requires measurements of an error syndrome to be Quantum Non-Demolition (QND) and fast compared to the rate at which errors occur. Employing a superconducting circuit QED architecture, the parity of a superposition of coherent states in a cavity, or cat states, is the error syndrome for a recently proposed QEC scheme. We demonstrate the tracking of parity of cat states in a cavity and observe individual jumps of party in real-time with singleshot measurements that are much faster than the lifetime of the cavity. The projective nature of these measurements is evident when inspecting individual singleshot traces, yet when averaging the traces as an ensemble the average parity decays as predicted for a coherent state. We find our protocol to be 99.8\% QND per measurement, and our sensitivity to parity jumps to be very high at 96\% for an average photon number $\bar{n}=1$ in the cavity (85\% for $\bar{n} = 4$). Such levels of performance can already increase the lifetime of a quantum bit of information, and thereby present a promising step towards realizing a viable QEC scheme. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B28.00006: Deterministic entanglement of two transmon qubits by parity measurement and digital feedback Diego Rist\`e, Marcin Dukalski, Christopher Watson, Gijs de Lange, Marijn Tiggelman, Yaroslav Blanter, Konrad Lehnert, Raymond Schouten, Leonardo DiCarlo While quantum measurement typically collapses quantum superpositions into a basis state, a special type of joint measurement, detecting the parity of qubit excitations, can create entanglement. Building on recent developments in quantum nondemolition measurement and feedback control in circuit QED, we realize a continuous-time parity meter for two 3D-transmon qubits using a dispersively coupled cavity and Josephson parametric amplification. Starting from a maximal superposition, we first generate entanglement with up to $88\%$ fidelity to the closest Bell state by postselecting on the odd-parity result. The infidelity is due to measurement-induced dephasing, arising from imperfect cavity resonance matching in the odd-parity subspace and finite transmission in the even. We then incorporate the parity meter into a digital qubit feedback loop to turn the generation of entanglement from probabilistic to deterministic, achieving $66\%$ fidelity to the targeted Bell state. This combination of parity measurement and conditional qubit control is at the basis of modern error correction protocols. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B28.00007: Understanding the effects of leakage in superconducting quantum error detection circuits Joydip Ghosh, Austin Fowler, John Martinis, Michael Geller The majority of quantum error detection and correction protocols assume that the population in a qubit does not leak outside of its computational subspace. For many existing approaches, however, the physical qubits do possess more than two energy levels and consequently are prone to such leakage events. Analyzing the effects of leakage is therefore essential to devise optimal protocols for quantum gates, measurement, and error correction. In this talk, I discuss the role of leakage in a two-qubit superconducting quantum error detection circuit. We simulate the repeated ancilla-assisted measurement of a single $\sigma^{z}$ operator for a data qubit, record the outcome at the end of each measurement cycle, and explore the signature of leakage events in the obtained readout statistics. An analytic model is also developed that closely approximates the results of our numerical simulations. We find that leakage leads to destructive features in the quantum error detection scheme, making additional hardware and software protocols necessary. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B28.00008: Verification methods for surface code implementations in superconducting systems Easwar Magesan, Jay M. Gambetta, Jerry M. Chow, Srikanth J. Srinivasan, Andrew W. Cross, David W. Abraham, Nicholas A. Masluk, Matthias Steffen, Chris Lirakis The surface code is a promising error-correction protocol for realizing large-scale quantum computation in superconducting qubit systems. Multi-qubit states and processes have recently been implemented with high enough fidelities in these systems to realize small building blocks of the surface code. We will discuss various metrics and tomographic protocols that can be used to characterize the performance of these building blocks and present recent experimental results that demonstrate high fidelity implementations in superconducting systems. Moving forward, these techniques will be useful for characterizing more complex surface code implementations, as well as more general error-correction strategies. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B28.00009: Reducing the impact of intrinsic dissipation in a superconducting circuit by quantum error detection Youpeng Zhong, Zongli Wang, Haohua Wang, John M. Martinis, Andrew N. Cleland, Alexander N. Korotkov A fundamental challenge for quantum information processing is reducing the impact of environmentally-induced errors. Quantum error detection and rejection (QEDR) provides one approach to handling such errors, in which errors are rejected when they are detected. Here we demonstrate a QEDR protocol based on the idea of quantum un-collapsing, using this protocol to suppress energy relaxation due to the environment in a three-qubit superconducting circuit. We encode quantum information in a target qubit, and use the other two qubits to detect and reject errors caused by energy relaxation. This protocol improves the storage time of a quantum state by a factor of roughly three, at the cost of a reduced probability of success. This constitutes the first demonstration of the extension of the effective lifetime of a quantum state using a quantum protocol. Using a similar protocol and a four-qubit superconducting circuit, we further demonstrate the protection of Bell-state entanglement against energy relaxation. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B28.00010: Tomography of microwave states based on parametric interactions Manuel Castellanos-Beltran, Michael Defeo, Adam Sirois, Leonardo Ranzani, Florent Lecocq, Raymond Simmonds, John Teufel, Jose Aumentado Due to recent innovations, Josephson junction-based superconducting circuits have emerged as a platform for performing quantum optics experiments in the microwave regime. These circuits have given us the ability to manipulate the quantum states of microwave light fields in ways that have only been possible in theory. One crucial step in our efforts to build a flexible platform for implementing these experiments in superconducting circuits is the development of a protocol for efficient measurement of the state of light in our systems. In recent years, several approaches have been developed in order to accomplish this, either by performing tomography of states within the cavity or on the itinerant photons escaping from it. In this talk, I will discuss our progress toward the goal of efficient measurement of the quantum state of light, in particular, as part of our goal to implement an ``on-chip'' optical table which utilizes parametric interactions for state preparation and measurement. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B28.00011: Characterizing measurement and feedback processes in superconducting qubit systems N. Ofek, Y. Liu, M. Hatridge, S. Shankar, M.H. Devoret, R.J. Schoelkopf New strategies for quantum control have been enabled by integrating nearly quantum-limited amplifiers with long-lived superconducting qubits. We now record high fidelity~single shot measurements that are also QND. We rely on these properties of our measurement to apply an active feedback on a quantum system. Understanding the degree to which they are QND is desirable. For example, if measurements are perfectly QND yet have finite fidelity, repeated measurements can improve the overall fidelity. In this talk, we present a formalism to quantify a number of important independent measurement parameters including fidelity and the QND degree. We then apply this formalism to characterize and optimize a feedback experiment. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B28.00012: Implementing fast sideband-modulated ``wah-wah'' pulses for driving transmon qubits with tight frequency separation V. Vesterinen, O.-P. Saira, A. Bruno, D.J. Egger, F.K. Wilhelm, L. DiCarlo Packing multiple transmon qubits in a narrow frequency band is challenging due to the limited transmon anharmonicity: control drives targeting one qubit may drive the leakage transition of another. This cross-driving effect grows with decreasing gate time, potentially imposing a quantum speed limit. The widely used DRAG (derivative removal by adiabatic gate) technique only suppresses leakage in the targeted transmon. Adding a modulation tone to a Gaussian pulse envelope in one quadrature, and complementing with DRAG in the other, has been predicted [1] to reduce both intrinsic and cross-driving leakage. We have experimentally verified the performance of this new pulse-shaping method, termed ``wah-wah,'' with two transmons in a 2D circuit QED architecture. We optimize the modulation frequency and amplitude, and characterize the gate fidelity using randomized benchmarking (RB) and quantum process tomography. Pulses on the two qubits are characterized separately and simultaneously by interleaving the RB sequences. Wah-wah pulses show decoherence-limited fidelity at gate speeds where DRAG pulses add significant error.\\[4pt] [1] R. Schutjens \emph{et al.} arXiv:1306.2279 [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B28.00013: Improving Fidelity in Superconducting Xmon Qubits: Decreasing 1/f Flux Noise Peter O'Malley, Rami Barends, Ben Chiaro, Yu Chen, Evan Jeffrey, Julian Kelley, Anthony Megrant, Josh Mutus, Charles Neill, Pedram Roushan, Daniel Sank, James Wenner, Theodore White, Andrew Cleland, John Martinis Two qubit CZ gate fidelity in our superconducting Xmon qubits is currently 99.4$\%$. To achieve 99.9$\%$ fidelity, experiments indicate that we need to reduce 1/f flux noise. We present measurements of 1/f flux noise on the Xmon from sub-Hz to MHz frequencies. At low frequencies we measure an f$^{-1.0}$ frequency dependence, which is in agreement with previous phase qubit measurements but significantly different from the f$^{-0.7}$ dependence seen in SQUIDs. We also see a dependence on geometry that agrees with a theory of magnetic defects; this points toward a qubit design that will minimize dephasing. [Preview Abstract] |
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