Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session T38: Superconducting Amplifiers and DetectorsFocus Recordings Available

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Sponsoring Units: DQI Chair: José Aumentado, National Institute of Standards and Technology Boulder Room: McCormick Place W195 
Thursday, March 17, 2022 11:30AM  11:42AM 
T38.00001: Feedback stabilization of low frequency noise in tunable microwave cavities with a single photon occupancy. Sisira Kanhirathingal, Bhargava Thyagarajan, Benjamin L Brock, Juliang Li, Josh Mutus, Evan Jeffrey, Miles P Blencowe, Alexander J Rimberg We successfully demonstrate suppression of low frequency noise over an approximately 1.4 kHz bandwidth in the resonance frequency fluctuations of a cavityembedded Cooper pair transistor (cCPT) driven at an average photon number n≤10. The gatedependent tunability of the cCPT allows us to implement a feedback technique derived from the wellestablished method of PoundDreverHall locking, thus diminishing the intrinsic charge noise which interferes with its operation as a near quantumlimited electrometer. We believe our technique can be generalized to achieve frequency stabilization in tunable microwave resonators that play a vital role in today's quantum computing architecture, thereby moderating the limitations in detection caused by the intrinsic 1/fnoise on such samples. The work discusses the various aspects relating to the operation of a fully functional feedback loop at the single photon level, such as circuit benchmarking leading to optimized response, biasdependence and nonlinear characteristics determining the bandwidth of the noise reduction, and the interference of quasiparticle poisoning typically observed in similar devices that can affect the feedback parameters. 
Thursday, March 17, 2022 11:42AM  11:54AM 
T38.00002: Nonlinear charge sensing with the cavityembedded Cooper pair transistor Bhargava Thyagarajan, Benjamin L Brock, Juliang Li, Sisira Kanhirathingal, Miles P Blencowe, Alexander J Rimberg The cavityembedded Cooper pair transistor (cCPT) has been shown to be an extremely sensitive linear charge detector when operating at singlephoton levels [1]. We report on the operation of the cCPT as a nonlinear charge sensor. Using the inherent Kerr nonlinearity, we demonstrate a Josephson bifurcation amplifier [2] inspired dispersive charge sensing technique by driving the cCPT at a detuning close to a bifurcation edge. This is implemented using ~100s of photons, still several orders of magnitude lower than what is used for stateoftheart charge detection techniques using radio frequency single electron transistors. We also explore a second nonlinear detection technique in which we parametrically pump the cCPT using a timevarying flux. Flux pumping the cCPT at a detuning on the edge of the parametric oscillation threshold makes the amplitude of oscillations sensitive to the charge environment. 
Thursday, March 17, 2022 11:54AM  12:06PM 
T38.00003: Magnetic fieldresilient quantumlimited parametric amplifier Mingrui Xu Superconducting quantumlimited parametric amplifiers (QPA), ideal for linear detection of weak signals, have become indispensable components for microwave quantum systems. The stateofart QPA's exploit the strong nonlinearity of a Josephson junction. However, the junction based QPA is sensitive to magnetic fields, neccessating the use of a magnetic shield in operation. Therefore they are incompatible with quantum systems that operate with magnetic fields. In this work, we present a quantumlimited amplifier made from NbN thin films. The nonlinearity originates from the kinetic inductance of the NbN nanobridge. We show that when operated in either phasesensitive or phasepreserving mode, the added noise of our amplifier approaches the quantum limit. 
Thursday, March 17, 2022 12:06PM  12:42PM 
T38.00004: HighFidelity Measurement of a Superconducting Qubit Using an OnChip Microwave Photon Counter Invited Speaker: Alexander M Opremcak We describe an approach to the highfidelity measurement of a superconducting qubit using an onchip microwave photon counter (Phys. Rev. X 11, 011027). The protocol relies on the transient response of a dispersively coupled measurement resonator to map the state of the qubit to "bright" and "dark" cavity pointer states that are characterized by a large differential photon occupation. Following this mapping, we photodetect the resonator using the Josephson photomultiplier, which transitions between classically distinguishable flux states when cavity photon occupation exceeds a certain threshold. Our technique provides access to the binary outcome of projective quantum measurement at the millikelvin stage without the need for quantumlimited preamplification and thresholding at room temperature. We achieve raw singleshot measurement fidelity in excess of 98% across multiple samples using this approach in total measurement times under 500 ns. In addition, we show that the backaction and crosstalk associated with our measurement protocol can be mitigated by exploiting the intrinsic damping of the Josephson photomultiplier itself. 
Thursday, March 17, 2022 12:42PM  12:54PM 
T38.00005: Broadband and high compression power SNAIL parametric amplifier: part 1/2 Vidul R Joshi, Gangqiang Liu, Maxime Malnou, Wei Dai, Gene C Hilton, Leila Vale, Michel H Devoret Resonatorbased Josephson parametric amplifiers play a crucial role in superconducting quantum information processing. These amplifiers, such as the Josephson parametric amplifier (JPA) and the SNAIL parametric amplifier (SPA), are widely used due to their ease of operation and near quantumlimited noise performance. However, their limited compression power and bandwidth are not suitable for reading out multiple qubits simultaneously. Although amplifiers which have been optimized for either one of the desired metrics have been amply demonstrated, a single amplifier with significant improvement in both metrics is yet to be made available. In this talk, we present SNAIL parametric amplifiers (SPAs) with lumped element resonators fabricated by niobium trilayer technology. In these devices, compression power is improved by using Josephson junctions with large critical current. Broader bandwidth is obtained by coupling the input signal to the device through a multipole filter. As a result, 10fold improvements in compression power and bandwidth are achieved comparing to standard SPAs fabricated with single layer aluminum technology. In the first part of the talk, we will present the design principles of this amplifier. 
Thursday, March 17, 2022 12:54PM  1:06PM 
T38.00006: Broadband and high compression power SNAIL parametric amplifier: part 2/2 Gangqiang Liu, Vidul R Joshi, Maxime Malnou, Wei Dai, Gene C Hilton, Leila Vale, Michel H Devoret Resonatorbased Josephson parametric amplifiers play a crucial role in superconducting quantum information processing. These amplifiers, such as the Josephson parametric amplifier (JPA) and the SNAIL parametric amplifier (SPA), are widely used due to their ease of operation and near quantumlimited noise performance. However, their limited compression power and bandwidth are not suitable for reading out multiple qubits simultaneously. Although amplifiers which have been optimized for either one of the desired metrics have been amply demonstrated, a single amplifier with significant improvement in both metrics is yet to be made available. In this talk, we present SNAIL parametric amplifiers (SPAs) with lumped element resonators fabricated by niobium trilayer technology. In these devices, compression power is improved by using Josephson junctions with large critical current. Broader bandwidth is obtained by coupling the input signal to the device through a multipole filter. As a result, 10fold improvements in compression power and bandwidth are achieved comparing to standard SPAs fabricated with single layer aluminum technology. 
Thursday, March 17, 2022 1:06PM  1:18PM 
T38.00007: Robustness against pump noise of 3wave mixing Josephson parametric amplifiers Wei Dai, Gangqiang Liu, Vidul R Joshi, Alessandro Miano, Michel H Devoret Josephson Parametric Amplifiers (JPAs) are a key component in superconducting quantum information processing. Traditionally, the cable delivering rf pump to a JPA are attenuated heavily at cryogenic stages for thermalization, as all the input lines inside a dilution refrigerator. It is possible however, to afford less attenuation on the pump line without harming the noise performance of a 3wave mixing JPA, whose pump is spectrally distant from the signal. In this talk, we discuss how does input noise in signal band and pump band affects JPA performance separately. We show experimentally that a resonant 3wave mixing JPA based on SNAILs can provide near quantumlimited signal amplification under pump band noise temperature of more than several hundred Kelvin. 
Thursday, March 17, 2022 1:18PM  1:30PM 
T38.00008: Practical quantum receivers for phaseshift keying at the singlephoton level Jasminder Sidhu, Shuro Izumi, Jonas S NeergaardNielsen, Cosmo Lupo, Ulrik L Andersen Increasing channel capacities in optical communications is a constant and non abating technological demand. While multilevel encoding and phaseshift keying schemes have allowed more information storage in optical fibres, the extent of increase to channel capacities ultimately depends on how well receivers decode the stored information through state discrimination. This is of particular importance for the efficient discrimination of weak coherent states. Quantum enhanced receivers are endowed with resources that can achieve higher sensitivities than conventional technologies. For application in optical communications, they provide improved discriminatory capabilities for multiple nonorthogonal quantum states. In this work, we propose and experimentally demonstrate a new decoding scheme for quadrature phaseshift encoded signals. Our receiver surpasses the standard quantum limit and outperforms all previously known nonadaptive detectors at low input powers. Unlike existing approaches, our receiver only exploits linear optical elements and onoff photodetection. This circumvents the requirement for challenging feedforward operations that limit communication transmission rates and can be readily implemented with current technology. 
Thursday, March 17, 2022 1:30PM  1:42PM 
T38.00009: Statistics of Broadband Microwave Photons Simon Bolduc Beaudoin, Bertrand Reulet, Stephane Virally, Christian Lupien Much effort has been put in the conception and fabrication of large bandwidth amplifier such as travellingwave parametric amplifiers and quasiparticle based SIS junctions. With such device, we can expect new interesting state such as large bandwidth squeezed states to be achievable in the lab, and new measurement tools will be necessary to correctly describe their quantum nature. In this work, we describe how to recover large bandwidth photon statistics from continuous operator measurement in the microwave domain. The proposed solution involves the use of time domain quadrature which are approximated using an on the fly numerical transformation of the 32GSa/s voltage samples. Crucially we show that a large bandwidth photon detector does not correspond, as one might expect, to the sum of monochromatic photo detectors over the same bandwidth. The proposed measure leads to photon statistics no matter if the chosen photon basis is narrow or large in bandwidth. 
Thursday, March 17, 2022 1:42PM  1:54PM 
T38.00010: Microwave Detection with a Superconducting Kinetic Inductance Amplifier Wyatt Vine, Anders Kringhoej, Mykhailo Savytskyi, Daniel Parker, Brett C Johnson, Jeffrey C McCallum, Timothy Duty, Andrea Morello, Jarryd J Pla Parametric amplifiers constructed from nonlinear superconducting microwave resonators have become essential components in circuit quantum electrodynamics experiments. Most parametric amplifiers are driven at powers where they linearly amplify incoming microwave fields, but it is also possible to realize highly nonlinear amplifiers by driving with a strong pump [1]. One class of nonlinear amplifier is based on parametric selfoscillations induced by strong fluxpumping of a Joesphson parametric amplifier and has and has been used to measure superconducting qubits [2,3]. In this work, we present a new nonlinear microwave amplifier based on parametric selfoscillations, whose nonlinearity is engineered instead from kinetic inductance [4]. We characterize the device’s sensitivity using calibrated microwave pulses. We also use it to implement a highgain latchedreadout of spin echo signals from an ensemble of ^{209}Bi that are directly inductively coupled to the device. 
Thursday, March 17, 2022 1:54PM  2:06PM 
T38.00011: Maximized dynamic range over a continuum of operating frequencies with Gradiometric SNAILs – based parametric amplifier Alessandro Miano, Gangqiang Liu, Volodymyr Sivak, Maxime Malnou, Luigi Frunzio, Vidul R Joshi, Wei Dai, Nicholas E Frattini, Gene C Hilton, Leila Vale, Michel H Devoret Superconducting quantumlimited parametric amplifiers are key devices for high fidelity readout of superconducting qubits. Recently, these amplifiers have been implemented with SNAILs as fluxtunable nonlinear elements in order to offer a maximized dynamic range when fluxbiased at a Kerrfree point. However, for a SNAIL – based parametric amplifier, a Kerrfree point corresponds to a single value of operating frequency, limiting the range of available high performances operations. In this work, we demonstrate how to break this constraint with a Gradiometric SNAIL, a doublyflux biased nonlinear element with enhanced tuning of nonlinear processes. When employed to implement parametric amplifiers, Gradiometric SNAILs provide enough tuning capabilities to maximize dynamic range over a continuum of operating resonance frequencies. Experimental results on Gradiometric SNAIL – based parametric amplifiers will be shown and discussed. 
Thursday, March 17, 2022 2:06PM  2:18PM 
T38.00012: Radiofrequency response of a graphene Josephson junction based superconducting oscillator circuit Joydip Sarkar, Subhamoy Ghatak, Pratap C Adak, Biswajit Datta, Supriya Mandal, Lucky N Kapoor, Kishor V Salunkhe, Suman Kundu, Rajamani Vijayaraghavan, Mandar M Deshmukh Recent experimental realizations on SuperconductorNormalSuperconductor (SNS) junctions provide a platform to study gate tunable Josephson junctions (JJ), where one can tune the junction properties like critical current by means of electrostatic gating in a fieldeffect transistor (FET) based device geometry. Because of the nonlinear inductive nature of JJs at finite frequencies, the SNS junctions are potential candidates to be used in gate tunable resonators in circuit QED experiments. Such resonators can have a variety of applications like  superconducting Qubits, lownoise amplifiers, highsensitive radiation detectors, etc. The ability to tune JJ inductance with electrostatic gating gives freedom in frequency biasing at a desirable frequency window. We report graphene JJbased LC oscillators which are promising candidates for lownoise RF signal amplifiers. We implement MoReGrapheneMoRe junctions as the gate tunable JJs. The nature of the JJ nonlinearity is vital for such applications. Hence such measurements can also provide information about the current phase relationship (CPR) and nature of nonlinearity in SNS JJs which is supposed to be different from known sinusoidal CPR in SuperconductorInsulatorSuperconductor (SIS) junctions. 
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