Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session L55: Quantum Dot/ Microwave Photon EntanglementFocus

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Sponsoring Units: DQI Chair: Mark Gyure, HRL Laboratories, LLC Room: LACC 515A 
Wednesday, March 7, 2018 11:15AM  11:51AM 
L55.00001: Quantumlimited measurement of spin qubits via curvature coupling to a cavity (and more) Invited Speaker: Charles Tahan There are still new physics to discover in circuitQEDlike systems. We investigate coupling an encoded spin qubit to a superconductor resonator via qubit energy level curvature versus gate voltage [1]. This approach enables quantum nondemolition readout with strength of tens to hundred MHz all while the qubit stays at its full sweetspot to charge noise, with zero dipole moment. A "dispersivelike" spin readout approach similar to circuitQED but avoiding the Purcell effect is proposed. With the addition of gate voltage modulation, selective longitudinal readout and nqubit entanglementbymeasurement are possible. We consider the origin of the transverse, longitudinal, and curvature couplings in "lumpedelement" spin qubit system to a superconducting cavity and make connections with superconducting qubits. 
Wednesday, March 7, 2018 11:51AM  12:03PM 
L55.00002: Transverse vs. longitudinal vs. curvature couplings of spin qubits to superconducting cavity Rusko Ruskov, Charles Tahan We consider in detail the origin of the transverse, longitudinal, and curvature couplings of a spin qubit to a superconducting cavity. On the example of single, double, and triple quantum dot spin qubits we use a general framework to show the interrelations between these couplings, and discuss parameter regions, e.g. sweet spots, when transverse (JaynesCummings) coupling can be eliminated, while curvature coupling can be maximized to reach tens of MHz. With the perspective of using curvature couplings for qubit readout or entangling gates, we estimated the dephasing mechanisms, and also discussed possible higherorder effects for these coupling scenarios. 
Wednesday, March 7, 2018 12:03PM  12:15PM 
L55.00003: Coherent spinqubit photon coupling: Part 1 Udson Mendes, Andreas Landig, Jonne Koski, Pasquale Scarlino, Christian Reichl, Werner Wegscheider, Andreas Wallraff, Klaus Ensslin, Thomas Ihn, Alexandre Blais Coherently coupling a spinqubit to a single microwave photon is one of the main challenges to overcome towards the development of quantum information processors based on spinqubits. The coherent coupling of photons to spin is, however, weak and alternative strategies involving charge can be used. One such strategy is to rely on the exchange interaction, which naturally couples spin to orbital degrees of freedom. In this talk, we investigate theoretically and experimentally a spinqubit, formed by three electrons confined in a triple quantum dot, coupled to a superconducting resonator [1]. We first show the spin nature of the qubit and its coupling to photons in the resonator. Second, we characterize the spinqubit decoherence by analyzing the effects of charge noise due to voltage fluctuations. Finally, we show how the spinqubit can be coupled longitudinally to the microwave resonator. 
Wednesday, March 7, 2018 12:15PM  12:27PM 
L55.00004: Coherent spinqubit photon coupling: Part 2 Andreas Landig, Jonne Koski, Pasquale Scarlino, Udson Mendes, Alexandre Blais, Christian Reichl, Werner Wegscheider, Andreas Wallraff, Klaus Ensslin, Thomas Ihn Coherent coupling of longdistance spins is a crucial step towards quantum information processing with spinqubits. We use a circuit quantum electrodynamics architecture to demonstrate strong coupling between single microwave photons in a high impedance NbTiN cavity and a threeelectron spinqubit in a GaAs triple quantum dot. We resolve the vacuum Rabi mode splitting with a coupling strength of g/2π≈31MHz and a qubit decoherence rate of γ_{2}/2π≈20MHz. The spinqubit is formed by exchange interaction, which couples the spin with the orbital degrees of freedom. We can directly access the amount of spincharge coupling strength electrostatically. This allows us to tune both the qubitphoton coupling strength as well as the qubit decoherence. From dispersive twotone spectroscopy measurements we extract a minimum qubit decoherence rate of γ_{2}/2π≈10MHz for a coupling strength of g/2π≈23MHz. We also observe an ac Stark shift of the qubit frequency, which allows us to calibrate the number of photons in the resonator and gives direct access to the qubitphoton coupling strength. 
Wednesday, March 7, 2018 12:27PM  1:03PM 
L55.00005: Strong coupling of a microwave photon to spin and charge qubits in GaAs quantum dots Invited Speaker: Klaus Ensslin We demonstrate the strong coupling limit of cavity QED with individual charges and spins in GaAs quantum dots by using the enhancement of the electric component of the vacuum fluctuations of a resonator with impedance beyond the typical 50 Ohm of standard coplanar waveguide technology. In a first experiment [1], we have realized a frequency  tunable microwave resonator with high impedance implemented by using the large inductance of a SQUID array combined with a small stray capacitance. Its inductance, and thus also its impedance and resonance frequency, is tunable by applying a small magnetic field using a mmsized coil mounted on the sample holder. In the resonant regime, we resolve the vacuum Rabi mode splitting of 238 MHz at a resonator linewidth 12 MHz and a charge qubit decoherence rate of 40 MHz extracted independently from microwave spectroscopy in the dispersive regime. In a second experiment [2], we couple a spin qubit in a GaAs triple quantum dot to a high impedance NbTiN superconducting resonator which can sustain finite magnetic fields. We resolve the vacuum Rabi mode splitting with a coupling strength of 31 MHz and a spinqubit decoherence rate of 20 MHz. We can tune coupling and decoherence electrostatically and obtain a minimal decoherence rate of 8.6 MHz. We directly measure the dependence of coupling strength on the tunable electric dipole moment of the qubit using the ac Stark effect. 
Wednesday, March 7, 2018 1:03PM  1:15PM 
L55.00006: Coupling Two Spin Qubits with a HighImpedance Resonator Shannon Harvey, Charlotte Bøttcher, Lucas Orona, Stephen Bartlett, Andrew Doherty, Amir Yacoby Fast, highfidelity single and twoqubit gates are essential to building a viable quantum information processor, but achieving both in the same system has proved challenging for spin qubits. In this talk, I will propose an approach to perform a twoqubit controlled phase gate between two singlettriplet qubits using an electromagnetic resonator. We use the longitudinal coupling of the qubits to the resonator to devise a gate relying on a single tone applied to the qubits near the resonator's frequency that is independent of the qubits' splitting. By using high impedance resonators, we predict an increase in twoqubit gate speed of up to two orders of magnitude. Simulations show average gate fidelities of over 96% using currently achievable experimental parameters and over 99.5% using stateoftheart resonator technology. By optimizing the gate fidelity in terms of parameters tuneable insitu, we find that it takes a simple powerlaw form in terms of the resonator's impedance and quality and the qubits' noise bath. I will also discuss our experimental progress towards implementing this gate. 
Wednesday, March 7, 2018 1:15PM  1:27PM 
L55.00007: Superconducting Kinetic Inductance FieldFrequency Sensors: HighSensitivity Magnetic Field Sensing in Moderate Background Fields Abraham Asfaw, Stephen Lyon We present a lumpedelement superconducting resonator magnetometer fabricated from a thin film of NbTiN. By taking advantage of the kinetic inductance nonlinearity of the superconductor, we achieve frequency shifts of order 1 MHz for 1 mG field changes in a perpendicular magnetic field of 600 G. We show that field modulation up to 10 kHz can be sensed at a temperature of 2K. Our device operates in background fields as large as 2000 G, and can be designed to operate at even higher fields. This feature makes it attractive for applications requiring high sensitivity magnetometry at moderate to high magnetic fields, where superconducting quantum interference devices are typically unable to operate. One such application is xband pulsed electron spin resonance, which typically requires large magnetic fields and becomes particularly susceptible to magnetic field fluctuations of the order of 10 ppb when measuring spin systems with long coherence times. While traditional fieldfrequency locking techniques based on NMR have been used to compensate for slow magnetic field drifts, our resonator devices can provide the required field sensitivity for higher noise frequencies. 
Wednesday, March 7, 2018 1:27PM  1:39PM 
L55.00008: Magnetic field resilient superconducting coplanar waveguide resonators for hybrid circuit QED experiments James Kroll, Francesco Borsoi, Kian van der Enden, Willemijn Uilhoorn, Damaz de Jong, Alessandro Bruno, Maja Cassidy, Leo Kouwenhoven Superconducting coplanar waveguide (CPW) resonators have proven to be powerful tools in astronomy, microwave microscopy and quantum information processing. They are crucial components in superconducting circuits, and numerous proposals aim to utilise them in scalable solidstate quantum computers, topological computing schemes and hybrid qubit candidates. Many of these proposals require application of magnetic fields strong enough to introduce Abrikosov vortices or destroy the superconductivity entirely, significantly reducing the internal quality factors (Q_{i}) and causing frequency fluctuations that would render the resonator useless. Here we report the design and fabrication of NbTiN superconducting CPW resonators that retain stable resonance frequencies and single photon Q_{i}s ∼ 100,000 in parallel fields up to B_{} = 5.5 T and perpendicular fields of B_{⊥} = 20 mT. We demonstrate this field resilience results from two factors: expulsion of vortices by careful reduction of the superconducting film thickness and control of vortex dynamics via patterning with etched vortex pinning sites. Finally, we apply these techniques to perform fast charge readout of a hybrid InSb nanowire double quantum dot device at 1 T. 
Wednesday, March 7, 2018 1:39PM  2:15PM 
L55.00009: Strong SpinPhoton Coupling in Silicon Invited Speaker: Nodar Samkharadze Long coherence times of single spins in Silicon quantum dots make these systems highly attractive for quantum computation. A crucial missing ingredient for realization of large networks of quantum dot based spin qubits has been long range coherent interconnects between them. 
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