Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A39: Superconducting Circuits: Measurement IFocus

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Sponsoring Units: DQI Chair: Andrew Jordan, University of Rochester Room: LACC 501B 
Monday, March 5, 2018 8:00AM  8:36AM 
A39.00001: Fast, highfidelity, QND measurements of superconducting qubits using a transverse interaction? Invited Speaker: Kurt Jacobs The standard procedure for measuring superconducting qubits is to couple them to a cavity mode via a tranverse interaction, and to detune the cavity from the qubit to obtain an effective "dispersive" interaction in which the qubit shifts the frequency of the cavity. This allows the state of the qubit to be read out by measuring the phase shift of a signal that is passed through the cavity. This technique is often referred to as providing a quantum nondemolition (QND) measurement, since the effective dispersive interaction has the QND form, but in fact it will only approximate a QND measurement to any reasonable degree when used with a detuning so large as to significantly reduce the effective interaction strength and thus the measurement speed. As a result, measurements that use this technique so as to achieve stateoftheart speeds (e.g., below 100 ns) are not operated in the QND regime. Here we show that there is signifcantly more to understand about the transverse interaction than the received wisdom that we have just described. In particualr we show that by using a circuit configuration in which the qubit/cavity interaction strength can be controlled in a timedependent fashion, and by carefully choosing the timeenvelope of this strength, it is possible to exploit a recurrance phenomena in the transferverse interaction to acheive fast QND measurements. This technique appears to be quite feasible even though it requires fairly precise control. We will discuss a circuit design that could be used to implement it as well as some further properties of the transverse interaction. 
Monday, March 5, 2018 8:36AM  8:48AM 
A39.00002: High Fidelity Measurement of Single and Multiqubit Superconducting Quantum Circuits Suman Kundu, Sumeru Hazra, Tanay Roy, Madhavi Chand, K Salunkhe, Meghan Patankar, Rajamani Vijayaraghavan Fast, high fidelity measurements of multiqubit devices are crucial for quantum error correction and fault tolerant quantum computation. In superconducting qubits, dispersive circuitQED readout using a Josephson parametric amplifier (JPA) has been the common approach to obtain highfidelity QND measurement. A standard way to improve speed and/or measurement fidelity is to use higher measurement power. However, that often leads to unwanted transitions both within and outside of the computational subspace, resulting in reduced fidelity. Moreover, this problem can get exacerbated when performing joint readout of multiple qubits. Using the 3D circuitQED architecture, we explore the optimization of various parameters like measurement power, qubitcavity coupling, qubitcavity detuning to enhance measurement fidelity in both single and multiqubit circuits. We will also discuss the optimization of the JPA so that it can handle the larger measurement signals needed for high fidelity. 
Monday, March 5, 2018 8:48AM  9:00AM 
A39.00003: High fidelity dispersive qubit readout with twomode squeezed light Gangqiang Liu, Xi Cao, TzuChiao Chien, Pinlei Lv, Olivia Lanes, Michael Hatridge High fidelity qubit measurement is essential for scalable, faulttolerant quantum computing. In superconducting circuits, qubit readout with fidelity above 99% has been achieved by using a quantumlimited parametric amplifier such as the Josephson Parametric Converter (JPC) as the first stage amplifier. However, the SignaltoNoise Ratio (SNR) of such readout is fundamentally limited by quantum fluctuations in the coherent readout pulse. Alternatively, readout with squeezed light can be used to reduce fluctuation along certain quadratures and thus improve the SNR. In this talk, we investigate a readout scheme with twomode squeezed light both produced and amplified by JPCs in a simple interferometer unbalanced by a transmon qubit/cavity. This configuration has been predicted to improve the SNR compared to readout with both coherent states and singlemode squeezed light [1]. This readout method, which sends twomode squeezed light along two paths simultaneously is also of interest for remote entanglement of qubits. We will present preliminary data and discuss optimization in the presence of JPC imperfections and photon loss. 
Monday, March 5, 2018 9:00AM  9:12AM 
A39.00004: To catch and reverse a quantum jump midflight Zlatko Minev, Shantanu Mundhada, Shyam Shankar, Philip Reinhold, Ricardo GutiérrezJáuregui, Robert Schoelkopf, Mazyar Mirrahimi, Howard Carmichael, Michel Devoret A quantum system driven by a weak deterministic force while under strong continuous observation exhibits quantum jumps between its energy levels. Employing a threelevel superconducting artificial atom of the Vtype involved in the original observation of quantum jumps, we show that quantum jumps can be caught and even reversed midflight. The three required levels are: G (for Ground), B (for Bright), and D (for Dark). The D level is engineered to be decoupled from both any dissipative environment and any measurement apparatus. Quantum jumps between G and D are monitored indirectly by the combination of a Rabi drive between the G and B levels, together with the monitoring of the occupation of B, itself tracked by a dispersivelycoupled readout cavity. Using digital lowlatency feedback electronics, we demonstrate the catch of a quantum jump midflight, i.e. a coherent superposition of D (corresponding to having jumped) and G (corresponding to having not jumped). The fidelity of the midflight state is above 70%, in agreement with quantum trajectory theory simulations. Our monitoring scheme can be useful for other quantum information tasks, such as the continuous monitoring of error syndromes. 
Monday, March 5, 2018 9:12AM  9:24AM 
A39.00005: Qubit State Measurement using the Josephson Photomultiplier Alexander Opremcak, Ivan Pechenezhskiy, Caleb Howington, Bradley Christensen, Konstantin Nesterov, Maxim Vavilov, Frank Wilhelm, Britton Plourde, Robert McDermott We describe results of highfidelity qubit state measurement based on microwave photon counting. The protocol involves mapping the qubit state onto highcontrast microwave cavity pointer states in a linear readout resonator followed by subsequent photodetection using the Josephson Photomultiplier (JPM). Our technique provides access to the classical binary outcome of qubit state measurement at the millikelvin stage of a dilution refrigerator and does not use any nonreciprocal components between the qubit and JPM. We exploit the intrinsic damping of the JPM to “clean up” following photodetection so that the measurement is highly QND. This approach is wellsuited for integration with proximal cryogenic digital logic. 
Monday, March 5, 2018 9:24AM  9:36AM 
A39.00006: NonDispersive Limit of the Qubit Readout with a Microwave Photon Counter. Konstantin Nesterov, Alexander Opremcak, Ivan Pechenezhskiy, Robert McDermott, Maxim Vavilov The Josephson photomultiplier (JPM) provides a path to fast, high fidelity qubit readout [Govia et al, Phys. Rev. A 90, 062307 (2014)]. The measurement protocol requires the creation of cavity pointer states with high contrast in photon occupation. For such large pointer states the dispersive approximation to the JaynesCummings Hamiltonian breaks down. In this regime, the nondispersive terms reduce the pointerstates contrast and lead to backaction of the measurement on the qubit state, reducing the readout accuracy and affecting its QND nature. We discuss the optimal conditions for JPMbased qubit readout in terms of the microwave drive parameters, the cavity photon decay time, and the detuning between the cavity and qubit. We also show that strong cavity drive facilitates reset of the readout system. 
Monday, March 5, 2018 9:36AM  9:48AM 
A39.00007: SingleShot Quantum NonDemolition Detection of Itinerant Microwave Photons JeanClaude Besse, Simone Gasparinetti, Michele Collodo, Theo Walter, Philipp Kurpiers, Christopher Eichler, Andreas Wallraff We realize a quantum nondemolition detector for propagating single photons in the microwave domain. By using a cavityassisted conditional phase flip gate between a photon and a superconducting artificial atom, the information of the presence of an itinerant photon is mapped into a definite state of the qubit [1]. We test the detector with a single photon source and perform singleshot readout of the state of the atom, whose first to second excited state transition is tuned to be resonant with the photon source. We demonstrate the quantum nondemolition nature of the detector by characterizing the field of the reflected photon, which is never absorbed. The internal detection fidelity is limited by the coherence properties of the qubit, but is independent of the temporal shape of the photon as long as its frequency bandwidth is smaller than the coupling of the atom to the assisting cavity. 
Monday, March 5, 2018 9:48AM  10:00AM 
A39.00008: Quantum nondemolition detection of an itinerant microwave photon Shingo Kono, Kazuki Koshino, Yutaka Tabuchi, Atsushi Noguchi, Yasunobu Nakamura Microwave quantum optics in superconducting circuits enables us to investigate unprecedented regimes of quantum optics. The strong nonlinearity brought by Josephson junctions together with the strong coupling of the qubits with microwave modes reveals rich physics not seen in the optical domain before. However, singlephoton detection in the microwave domain is still a challenging task because of the photon energy four to five orders of magnitude smaller than in optics. 
Monday, March 5, 2018 10:00AM  10:12AM 
A39.00009: Parity measurements using parametrically driven resonators: Part I Shruti Puri, Baptiste Royer, Steven Girvin, Alexandre Blais Multiqubit parity measurements are indispensable for quantum error correction. A possible way to realize these is by homodyne measurement of the field of a cavity which is dispersively coupled to multiple qubits. However such an interaction can lead to dephasing within the parity subspace and results in only halfparity measurement. With the aim of overcoming this shortcoming, in this talk we will introduce the parametrically driven nonlinear resonator as an alternate approach to measure multiqubit parity. With analytical results, we will show that this approach leads to significant reduction of paritysubspace dephasing making fullparity measurements possible. 
Monday, March 5, 2018 10:12AM  10:24AM 
A39.00010: Parity measurement using parametrically driven resonators  part 2 Baptiste Royer, Shruti Puri, Steven Girvin, Alexandre Blais Nondestructive measurement of multiqubit parity stabilizers is essential in quantum error correction. For superconducting qubits, one way to perform such parity measurements is to use an ancilla qubit and multiple twoqubit gates. However, this increases the required number of qubits for a given code and the need for twoqubit gates makes this measurement slow. 
Monday, March 5, 2018 10:24AM  10:36AM 
A39.00011: Implementation of Continuous Parity Measurements and Error Correction William Livingston, Machiel Blok, Emmanuel Flurin, Juan Atalaya, Justin Dressel, Andrew Jordan, Alexander Korotkov, Irfan Siddiqi Continuous monitoring of quantum systems has been successfully used to examine state collapse and quantum jumps in single qubits. Continuous parity measurements allow for the observation of collapse dynamics in multiqubit systems, and naturally motivate a strategy for performing quantum error correction. One can observe the parity of two dispersive superconducting transmons without the need for ancilla qubits by strongly coupling the former to a joint readout resonator. With three qubits pairwise coupled to two resonators, we can measure two generators of the conventional threequbit bitflip code simultaneously. Using highspeed field programmable gate array electronics to continuously monitor these parities, we observe multipartite collapse and can apply correction pulses when an error is detected. 
Monday, March 5, 2018 10:36AM  10:48AM 
A39.00012: Parametrically induced readout of a superconducting qubit Steven Touzard, Angela Kou, Alexander Grimm, Kevin Chou, Katrina Sliwa, Uri Vool, Shyam Shankar, Luigi Frunzio, Michel Devoret One of the basic operations of quantum information is to nondestructively identify the state of a quantum system with high fidelity and within a time that is short compared to its natural decay. In circuit quantum electrodynamics, this goal is achieved by the dispersive readout scheme, in which the frequency of an ancillary cavity is pulled by an amount depending on the qubit state. Despite its successes, this scheme suffers from the Purcell limit set on the decay time, and from the dephasing induced by spurious photons in the readout cavity. We address these shortcomings with a parametrically driven 3rd order interaction between the qubit excitation number and the cavity coherent state displacement. This interaction can also be described as a socalled “longitudinal” coupling between the qubit z operator and the readout cavity field amplitude, and it differs markedly from the usual "transversal" JaynesCummings coupling between the qubit x operator and the field amplitude. This interaction can be tuned insitu, and can be turned off altogether. The presentation will report experimental progress towards the realization of this scheme. 
Monday, March 5, 2018 10:48AM  11:00AM 
A39.00013: Fast and Efficient AllMicrowave Reset of a TransmonQutrit Coupled to a Large Bandwidth Resonator Paul Magnard, Philipp Kurpiers, Theo Walter, Marek Pechal, Baptiste Royer, JeanClaude Besse, Simone Gasparinetti, Alexandre Blais, Andreas Wallraff Fast and efficient reset of qubits is a key operation in many quantum algorithms, and particularly in error correction codes. We experimentally demonstrate a reset scheme of a qutrit coupled to a low Q resonator. The reset protocol uses the microwave induced interaction between the f,0〉 and g,1〉 states of the system, with g〉 and f〉 denoting the ground and second excited state of the qutrit, and 0〉 and 1〉 the photon Fock states of the resonator. We characterize the reset process and demonstrate reinitialization of the transmonresonator system to its ground state with 99.8% fidelity in less than 500 ns. Our protocol is of practical interest as it has no requirements on the chip design, in addition to those for fast and efficient singleshot readout of the transmon [1], and does not require any feedback for initialization. 
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