Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B39: Superconducting circuits: Measurement II 
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Sponsoring Units: DQI Chair: Kater Murch, Univ of California  Berkeley Room: LACC 501B 
Monday, March 5, 2018 11:15AM  11:27AM 
B39.00001: Decomposing positive operator valued measurements on qubits into destructive weak measurements YiHsiang Chen, Todd Brun Many quantum measurements are destructive. In an example of photodetection, when the detector "clicks" a photon is absorbed. In monitoring the emission of photons from a source, one decomposes the strong measurement into a series of weak measurements, which describes the evolution of the state during the process. Motivated by this destructive photodetection, we study a model of destructive weak measurements using qubits. A system qubit is coupled to a series of ancilla qubits, each prepared in the 0> state. The coupling performs a weak SWAP between the system and the ancilla, which is followed by a strong measurement of the ancilla. The weak measurement is destructive in the sense that the 0> state “leaks" into the system. Surprisingly, this destructive model can achieve any POVM on the system. Showing this requires a chain of arguments. First, any POVM with linearly dependent POVM elements is equivalent to choosing randomly among a set of POVMs with linearly independent elements. Each such POVM, in turn, is equivalent to a POVM whose elements are all proportional to projectors, followed by a probabilistic output step. Finally, any POVM whose elements are proportional to a linearly independent set of projectors can be achieved by a sequence of destructive weak measurements. 
Monday, March 5, 2018 11:27AM  11:39AM 
B39.00002: Ondemand microwave generator of shaped single photons Pol FornDiaz, Chris Warren, Adrian Parra, Chung Wai Sandbo Chang, A. Vadiraj, Laura García Álvarez, Enrique Solano, Christopher Wilson We present results of a microwave singlephoton generator where the emission rate of a superconducting qubit is controlled in real time and photon wavepackets are emitted with a shaped profile. Controlling the radiative properties of quantum emitters has important applications in quantum information science. Arbitrarily shaped photon wavepackets could have an important role in, e.g., quantum networks as well as connecting spatiallyisolated sectors of future quantum processors. The modulation is achieved by controlling the position of the nodes of vacuum modes in a semiinfinite transmission line shunted by a dcSQUID. Besides its practical application as a photon source, this work enables fundamental studies of artificial atoms interacting with a quantum vacuum, leading to controlled dissipative processes and their effect in quantum information applications. 
Monday, March 5, 2018 11:39AM  11:51AM 
B39.00003: General method for extracting the quantum efficiency of dispersive qubit readout in circuit QED Cornelis Christiaan Bultink, Brian Tarasinski, Niels Haandbæk, Stefano Poletto, Nadia Haider, David Michalak, Alessandro Bruno, Leonardo DiCarlo We present and demonstrate a general 3step method for extracting the quantum efficiency of dispersive qubit readout in circuit QED. We use active depletion of postmeasurement photons and optimal integration weight functions on two quadratures to maximize the signaltonoise ratio of nonsteadystate homodyne measurement. We derive analytically that the method robustly extracts the quantum efficiency for arbitrary readout conditions in the linear regime. To demonstrate generality, we extract the quantum efficiency as a function of the readout drive frequency and for different drive envelopes. We use the proven method to optimally bias a Josephon travellingwave parametric amplifier and to quantify the different noise contributions in the readout amplification chain. This method allows insitu characterization and optimization of the quantum efficiency in extensible multiqubit architectures. 
Monday, March 5, 2018 11:51AM  12:03PM 
B39.00004: Fluorescence Readout of a Superconducting Qubit Using a Fluxonium Artificial Atom YenHsiang Lin, Nathanael Cottet, Long Nguyen, Raymond Mencia, Nicholas Grabon, Aaron Somoroff, Vladimir Manucharyan We report the fluorescence "shelving" readout scheme of atomic physics applied to a multilevel fluxonium artificial atom. Here fluxonium circuit is directly coupled to a 3D waveguide which is in turn smoothly connected to a 50Ohm transmission line, i.e. no highQ cavity mode is involved. The qubit is encoded into a lowfrequency transition 01 of fluxonium which is protected from Purcell effect by the cutoff of the waveguide. The readout transition involves higher excited states of fluxonium and is designed to be in the passband of the waveguide. The state of the qubit can be identified by measuring the reflection coefficient at the frequency of the readout transition. In current experiments, T1 > 38μs and T2 > 8μs, while the linewidth of the readout transition is > 6MHz. We discuss the requirements to achieve a QND measurement in such a system and origins of decoherence. Direct connection of a longlived qubit to propagating photons can be useful for realizing quantum networks and quantum internet. Our device can also serve as a tool to characterize and optimize thermalization of the measurement lines in superconducting qubit experiments. 
Monday, March 5, 2018 12:03PM  12:15PM 
B39.00005: Qubit readout using a transmon with a Vshaped energy diagram Remy Dassonneville, Luca Planat, Javier Puertas, Farshad Foroughi, cecile naud, Wiebke Guichard, Nicolas Roch, Olivier Buisson The most widely used scheme to perform qubit readout in cQED relies on the dispersive coupling between a qubit and a harmonic oscillator. However, despite important progresses, implementing a fast and high fidelity readout remains nowadays a major challenge. Indeed, inferring a qubit state is limited by the tradeoff between speed and accuracy due to Purcell effect and transitions induced by readout photons in the resonator. To overcome this, we introduce a new device: a transmon with a Vshaped energy diagram, embedded in 3D architecture. It is made of two transmons coupled via a large inductance [1]. The resulting circuit presents two qubits longitudinally coupled – called qubit and ancilla. Using symmetry rules [2], the ancilla can be strongly coupled to the cavity while the qubit remains uncoupled. However due to their strong longitudinal coupling, the qubit state can still be inferred through the ancilla state. 
Monday, March 5, 2018 12:15PM  12:27PM 
B39.00006: Readout Induced Transitions in a Driven Superconducting Transmon Qubit Zijun Chen, Mostafa Khezri, Ben Chiaro, Andrew Dunsworth, Brooks Foxen, Charles Neill, James Wenner, Alexander Korotkov, John Martinis Dispersive readout via a coupled resonator has become a staple component of high fidelity superconducting qubit systems. Ideally, reading out a qubit should not disturb its state, but previous work (Sank et al, PRL 117) has found that dispersive readout can cause undesired transitions in transmon qubits. In this talk, I will show that additional transitions are possible when the qubit is also driven, and that these transitions can be detected using qubit spectroscopy at high resonator photon numbers. These transitions can be predicted by examining the joint qubitresonator states in the JaynesCummings ladder. I will also show that this phenomenon has important consequences for readout in multiqubit devices. 
Monday, March 5, 2018 12:27PM  12:39PM 
B39.00007: Tracking calibration drifts in continuous quantum measurement Shiva Lotfallahzadeh Barzili, Justin Dressel We investigate how to correct the calibration drifts of a qubit undergoing continuous measurement. Currently, qubit measurement calibrations only remain valid for short measurement durations before naturally drifting due to uncontrolled environmental factors. In order to continuously measure a qubit for a longer duration and correctly interpret its evolution, it is necessary to dynamically update the calibrations while the measurements occur. We explore the use of machine learning models such as recurrent neural networks for predicting the behavior of such calibration drifts over a longer duration. This effort is a step towards autocalibration for the continuous measurement process. 
Monday, March 5, 2018 12:39PM  12:51PM 
B39.00008: Chaos in ContinuouslyMonitored Qubits: An ExtremalProbability Path Approach Philippe Lewalle, John Steinmetz, Andrew Jordan

Monday, March 5, 2018 12:51PM  1:03PM 
B39.00009: Ultrasensitive microwave nanobolometer Dibyendu Hazra, Joonas Govenius, Russell Lake, Roope Kokkoniemi, Visa Vesterinen, Marton Gunyho, Kuan Tan, Jan Goetz, Matti Partanen, Slavomir Simbierowicz, Leif Grönberg, antti Laitinen, Pertti Hakonen, Juha Hassel, olli Pentti Saira, Jukka Pekola, Janne Lehtinen, Mika Prunnila, Mikko Möttönen We have fabricated microwave nanobolometers using normalmetal nanowires. A part of the wire acts as an absorber of the microwave radiation whereas the remaining parts act as a sensitive thermometer with proximityinduced superconductivity. We use electrothermal feedback enhance the coupling of the temperature to the electrical degrees of freedom in the bolometer allowing us to measure a change in the radiofrequency signal caused by the absorption of microwave radiation. At the regime of large positive feedback, two metastable electron temperature states arise enabling threshold detection of microwave pulses down to 200 8.4GHz photons, i.e., 1 zeptojule of energy. The measured noiseequivalent power of our nanobolometer is 2*10^{20} W/Hz^{1/2} at 1ms timeconstant. Finally, I present results on the devices where the normalmetal is replaced by graphene for enhanced performance. 
Monday, March 5, 2018 1:03PM  1:15PM 
B39.00010: Rapid HighFidelity Multiplexed Readout of Superconducting Qubits Christian Kraglund Andersen, Johannes Heinsoo, Ants Remm, Sebastian Krinner, Yves Salathe, Theodore Walter, Simone Gasparinetti, JeanClaude Besse, Anton Potocnik, Christopher Eichler, Andreas Wallraff The duration and fidelity of qubit readout is a critical factor for applications in quantum information processing as it limits the fidelity of algorithms, which reuse qubits after measurement or apply feedback based on the measurement result. In this talk we discuss the results of a fast multiplexed readout experiment performed on five qubits with readout resonators populated for less than 200 ns. We find that the probability of assigning the individual qubit to be in the state we intended to prepare, is above 95% for all five measured qubits. In addition, the data shows that individual Purcell filters used in this experiment lead to reduced measurement induced dephasing of the qubits we did not intend to measure. 
Monday, March 5, 2018 1:15PM  1:27PM 
B39.00011: Cryogenic Digital Readout of Superconducting Qubits Caleb Howington, Alexander Opremcak, Ivan Pechenezhskiy, Maxim Vavilov, Robert McDermott, Britton Plourde We present schemes for interfacing the result of superconducting qubit measurements with SingleFluxQuantum (SFQ) logic, a superconducting cryogenic digital logic family. Using a Josephson Photomultiplier (JPM), we are able to read out a superconducting qubit coupled to a cavity by mapping qubit states onto cavity photon occupation, then mapping the cavity photon occupation onto different classical circulating current states of the JPM. This scheme provides fast, high fidelity readout at the same temperature stage as the qubit device without the need for cryogenic isolators or amplifiers. The results of this readout can then be encoded into SFQ logic by coupling the JPM to an underdamped Josephson Transmission Line (JTL) circuit. Simulations of this protocol, as well as recent JPM measurement results are discussed. 
Monday, March 5, 2018 1:27PM  1:39PM 
B39.00012: Highfidelity readout of bosonic qubits via a dispersively coupled ancilla Connor Hann, Sal Elder, Christopher Wang, Kevin Chou, Robert Schoelkopf, Liang Jiang Highfidelity qubit measurements play a crucial role in quantum computation, communication, and metrology. In recent experiments, readout fidelity has been improved by performing repeated quantum nondemolition (QND) readouts of a qubit’s state through an ancilla. For a qubit encoded in a twolevel system, however, the fidelity of such schemes is limited by the fact that a single error can destroy the information in the qubit. In bosonic systems, this fundamental limit could be overcome by utilizing higher levels such that a single error still leaves states distinguishable. In this talk, we present a robust readout scheme, applicable to bosonic systems with dispersively coupled ancilla, that leverages both repeated QND readouts and higherlevel encodings to asymptotically suppress the effects of qubit/cavity relaxation and individual measurement infidelity. We characterize the fidelity of the scheme and show how it may be calculated in terms of known experimental parameters. 
Monday, March 5, 2018 1:39PM  1:51PM 
B39.00013: Robust measurement of a qubit encoded in the bosonic mode of a superconducting cavity Salvatore Elder, Christopher Wang, Connor Hann, Kevin Chou, Christopher Axline, Luke Burkhart, Luigi Frunzio, Liang Jiang, Robert Schoelkopf Reliable singleshot readout is a necessary DiVincenzo criterion for quantum computation. The readout fidelity of any twolevel system is bounded by the rate of a single error. In contrast, if qubit states are encoded using higher occupation levels of a bosonic mode, a single error does not completely destroy the information: the basis states are still distinguishable. We present experimental results for a highfidelity readout scheme in which information is encoded in higher photon numbers of a superconducting cavity. By performing repeated QND measurements utilizing an ancillary transmon, we can asymptotically reduce the effects of both photon loss and infidelity of individual measurements. We demonstrate the capability of highfidelity state readout in our system and discuss limits on the achievable fidelity. 
Monday, March 5, 2018 1:51PM  2:03PM 
B39.00014: Using Software Defined Radios for Qubit Control and Readout Rodney Sinclair, Mark Suska, Lauren Capelluto, Michael Lenihan, Damon Russell Software Defined Radios (SDR) generating VHF, UHF, and microwave signals can be used to calibrate and operate superconducting qubits with some caveats. The premise of SDR is that a highpower computer, such as an Intelbased server, computes signals to be transmitted by the SDR and processes signals received from the SDR. This usage model treats the SDR as a nonintelligent peripheral that is only capable of forwarding raw signals from a host PC to a signal generator and from a downmixer/digitizer to a host PC. This usage model requires highbandwidth Ethernet activity, particularly when operating multiple qubits, and limits the repetition rate of qubit operations and measurements. Incorporating custom HDL into the SDR, making it more like a piece of custom measurement equipment, increases the achievable repetition rate and substantially increases communications and system reliability. This paper describes practical details of using SDRs to operate a quantum computer. This paper describes Rigetti Computing's approach and experiences using National Instruments Universal Software Radio Peripherals (USRP) to reduce the cost of implementing multiqubit control systems. 
Monday, March 5, 2018 2:03PM  2:15PM 
B39.00015: Development of microwave single photon detectors in lumped element and traveling wave architectures John Mark Kreikebaum, Kevin O'Brien, Arne Grimsmo, Baptiste Royer, Alexandre Blais, Irfan Siddiqi Detection of microwave photons is an important capability for superconducting quantum computing and microwave quantum optics. Promising single photon detectors have been demonstrated at microwave frequencies, but a single detector with high quantum efficiency, nondestructive detection, a low dark count rate, and broad detection bandwidth has thus far not been demonstrated. An enticing prospect is using the travelingwave architecture, which has been used for broadband nearquantumlimited Josephson parametric amplifiers, for detection of discrete signals. Here we report progress towards experimental demonstration of single photon detection in lumpedelement and traveling wave devices. 
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