Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session G26: Quantum Characterization, Verification, and Validation II |
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Sponsoring Units: GQI Chair: Robin Blume-Kohout, Sandia National Laboratories Room: 328 |
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G26.00001: Quantum process verification methods and applications to superconducting qubits Invited Speaker: Jay Gambetta Determining how well a quantum gate is implemented on a quantum device is of fundamental importance. Such a characterization allows a direct comparison between different architectures for computation as well as an understanding of the performance of the building blocks of a quantum computer. In this talk I will show that the standard approach of process tomography is grossly inaccurate in the case where the states and measurement operators used to interrogate the system are generated by gates that have some systematic error, a situation all but unavoidable in any practical setting. I will then present some recent proposals with experimental implementations that are resilient to this type of noise. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G26.00002: Quantum State Tomography of Spin Qubits Invited Speaker: Oliver Dial Quantitative and accurate state tomography is becoming increasingly necessary to establish gate fidelities, entanglement measures, and optimize the increasingly complex gate sequences needed to perform experiments. In singlet-triplet spin qubits, to perform state tomography single-qubit rotations are used to map different axes of the Bloch sphere to the singlet-triplet axis, followed by projective measurement onto the singlet-triplet axis. The two nominally orthogonal rotations needed are provided by two physically distinct mechanisms: magnetic field gradients and exchange rotations. The complex interplay between these mechanisms, noise sources, and pulse distortions make it difficult to accurately predict the angle and axis of rotations from first principles, leading to a circular problem: how can one calibrate tomographic rotations without any calibrated tomography? We describe and experimentally demonstrate a method which, using minimal assumptions, makes it possible to detect and correct for both axis errors in tomography and losses during rotations associated with state tomography. Unlike conventional tomography tuning schemes, this technique is not iterative, allowing it be used to post-correct data with minimal overhead and effort. The technique is easily adaptable to other implementations of qubits, and should be of value wherever accurate tomography is needed but tuning up a complete set of ideal rotations is unnecessary. Finally, we will discuss the influence of non-Markovian noise on state tomography and possible approaches to circumvent state estimation errors arising thereof. Together these techniques allow us to perform state tomography with unprecedented precision in spin qubits. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G26.00003: Compressed Sensing Quantum Process Tomography of Superconducting Qubit Gates Andrey Rodionov, Alexander N. Korotkov, Robert L. Kosut, Matteo Mariantoni, Daniel Sank, James Wenner, John M. Martinis We characterize the quantum gates based on superconducting phase qubits using the Quantum Process Tomography (QPT) with strongly reduced set of initial states and/or measurement configurations. This is done by applying the Compressed Sensing (CS) method to estimate the process matrix $\chi$. Using experimental data for 2-qubit controlled-Z gate, we show that the CS-QPT method gives an estimate of the $\chi$-matrix with reasonably high fidelity, compared with full QPT. The method works well even when the amount of used data is so small, that the standard QPT would have an underdetermined system of equations. The CS-QPT is also applied to the analysis of a three-qubit Toffoli gate with numerically added noise. Similarly, we show that the method works reasonably well for a strongly reduced set of data, including the underdetermined case. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G26.00004: Xmons: Transmon qubits for a scalable architecture Rami Barends, J. Kelly, D. Sank, J. Bochmann, B. Campbell, Y. Chen, B. Chiaro, E. Jeffrey, M. Mariantoni, A. Megrant, J. Mutus, C. Neill, P. O'Malley, S. Ohya, P. Roushan, A. Vainsencher, J. Wenner, T. White, A.N. Cleland, J.M. Martinis We have developed a new type of transmon qubit, the Xmon, which shows long coherence, allows for straightforward coupling to multiple elements, and has a low parasitic coupling. The Xmon is UCSB's building block for a superconducting multiqubit processor. The Xmon easily couples to four elements and is dispersively read out, making it compatible for use in a surface code quantum processor. At present, we are experimentally testing multiqubit chips for demonstrating single and two qubit state preparation and gates with high fidelity. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G26.00005: Benchmarking gates in a qubit-bus-qubit tunable transmon architecture Julian Kelly, R. Barends, J. Bochmann, B. Campbell, Y. Chen, B. Chiaro, E. Jeffrey, M. Mariantoni, A. Megrant, J. Mutus, C. Neill, P. O'Malley, S. Ohya, P. Roushan, D. Sank, A. Vainsencher, J. Wenner, T. White, A.N. Cleland, J.M. Martinis Using a newly developed frequency tunable transmon qubit (``Xmon''), we are beginning to construct the fundamental gates and architecture for a quantum computer. We show experimental data for gates in a qubit-bus-qubit configuration. We quantify the fidelity of a set of single qubit gates with both randomized benchmarking and tomography. We also investigate the fast swap style cPhase gate [Strauch PRL 2003], where the control qubit is swapped into the bus and interacts dispersively with the target qubit, as a fundamental two-qubit interaction. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G26.00006: Characterization of addressability by simultaneous randomized benchmarking John Smolin The control and handling of errors arising from cross-talk and unwanted interactions in multi-qubit systems is an important issue in quantum information processing architectures. We introduce a benchmarking protocol that provides information about the amount of addressability present in the system and implement it on coupled superconducting qubits. The protocol consists of randomized benchmarking each qubit individually and then simultaneously, and the amount of addressability is related to the difference of the average gate fidelities of those experiments. We present the results on two similar superconducting transmon qubits with different amounts of cross-talk and unwanted interactions, which agree with predictions based on simple models for the amount of residual coupling. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G26.00007: Implementation of a Robust Tomography Toolbox Colm Ryan, Blake Johnson, Marcus Da Silva, Shelby Kimmel, Thomas Ohki Recent advances in coherence times and control techniques have dramatically improved gate fidelities in superconducting qubits. Already, estimates of these small errors are dominated by errors in the state preparation and measurment pulses of quantum process tomography. Randomized benchmarking (RB) provides a way to isolate gate errors, but only for estimating the fidelity of Clifford operations. Here we implement several extensions to RB that provide more detailed information about specific gates while maintaining the key RB advantage of being robust to state and measurement errors. We will show: interleaved benchmarking results to characterize the average fidelity of specific gates; simultaneous benchmarking to characterize addressabilty errors with multiple qubits; and robust tomography results that show a full unital characterization of a trace preserving operation. Taken together these provide a full suite of characterization tools useful to any quantum experimentalist. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G26.00008: Robust Tomography using Randomized Benchmarking Marcus Silva, Shelby Kimmel, Blake Johnson, Colm Ryan, Thomas Ohki Conventional randomized benchmarking (RB) can be used to estimate the fidelity of Clifford operations in a manner that is robust against preparation and measurement errors --- thus allowing for a more accurate and relevant characterization of the average error in Clifford gates compared to standard tomography protocols. Interleaved RB (IRB) extends this result to the extraction of error rates for individual Clifford gates. In this talk we will show how to combine multiple IRB experiments to extract all information about the unital part of {\em any} trace preserving quantum process. Consequently, one can compute the average fidelity to {\em any} unitary, not just the Clifford group, with tighter bounds than IRB. Moreover, the additional information can be used to design improvements in control. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G26.00009: Experimental realization of non-abelian geometric gates with a superconducting three-level system Abdufarrukh Abdumalikov, J. M. Fink, K. Juliusson, M. Pechal, S. Berger, A. Wallraff, S. Filipp Geometric gates hold promise to provide the building blocks for robust quantum computation. In our experiments, we use a superconducting three-level system (transmon) to realize non-adiabatic non-abelian geometric gates. As computational basis we choose the ground and second excited states, while the first excited state acts as an ancilla state. The gates are realized by applying two resonant drives between the transmon levels. During the geometric gate ration of the amplitudes of the two drive tone is kept constant. Different gates are obtained for different ratio of the drive tones. We implement a Hadamard, a $NOT$ and a phase gates with the fidelities of $95\%$, $98\%$, and $97\%$ as determined by full process tomography and maximum likelihood methods. We explicitly show the non-abelian nature of gates by applying two non-commuting gates in alternating order. The demonstrated holonomic gates are not exclusive to superconducting quantum devices, but can also be applied to other three level systems with similar energy level structure. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G26.00010: Implementation of a five-cavity / four-qubit 3D circuit QED system Douglas McClure, Chad Rigetti, Jay Gambetta, Stefano Poletto, Erik Lucero, Mark Ketchen, Matthias Steffen Surface code error correction schemes, which have emerged as a guiding paradigm for the development of small prototype quantum processors, have a natural implementation on a skew square 2D lattice of cavities and qubits. We describe the experimental realization of a modular segment containing a unit cell of this lattice in a device consisting of five 3D waveguide cavities and four superconducting transmon qubits. In this system, we demonstrate high-fidelity one- and two-qubit gates with low crosstalk. Moreover, this device provides an extensible framework for tests of protocols needed for error correction in much larger systems. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G26.00011: Quantum lost property: A possible operational meaning for the Hilbert-Schmidt product Matthew Pusey, Terry Rudolph Minimum error state discrimination between two mixed states $\rho$ and $\sigma$ can be aided by the receipt of ``classical side information'' specifying which states from some convex decompositions of $\rho$ and $\sigma$ apply in each run. I will quantify this phenomena by the average trace distance, and give lower and upper bounds on this quantity as functions of $\rho$ and $\sigma$. The lower bound is simply the trace distance between $\rho$ and $\sigma$, trivially seen to be tight. The upper bound is $\sqrt{1 - {\rm tr}(\rho\sigma)}$, and we conjecture that this is also tight. I will show how to reformulate this conjecture in terms of the existence of a pair of ``unbiased decompositions'', which may be of independent interest. Time permitting, I will outline the evidence for this conjecture. Based on http://arxiv.org/abs/1208.2550 [Preview Abstract] |
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