Quantum capacitance and charge sensing of a superconducting double dot

Lambert, Nicholas J.; Esmail, A. A.; Edwards, Megan; Pollock, Felix A.; Lovett, Brendon W.; Ferguson, A. J.

doi:10.1063/1.4962811

Cited by 7 publications

(3 citation statements)

References 29 publications

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“…This is particularly crucial for measurementbased quantum computation, such as proposed for MZMbased architectures 4,5,8 . So far, however, the frequency shift of dispersive gate sensors has been fairly small, on the order of a degree 3,6,[9][10][11] ; correspondingly, the required readout times to resolve a difference in tunnel coupling has been in the range of milliseconds [12][13][14] . It is thus of great interest to find avenues toward increasing the attainable SNR, and achieve readout on the submicrosecond scale, as available for other solid-state qubit platforms 15 .…”

Section: Introductionmentioning

confidence: 99%

Rapid Detection of Coherent Tunneling in an InAs Nanowire Quantum Dot through Dispersive Gate Sensing

et al. 2019

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Dispersive sensing is a powerful technique that enables scalable and high-fidelity readout of solidstate quantum bits. In particular, gate-based dispersive sensing has been proposed as the readout mechanism for future topological qubits, which can be measured by single electrons tunneling through zero-energy modes. The development of such a readout requires resolving the coherent charge tunneling amplitude from a quantum dot in a Majorana-zero-mode host system faithfully on short time scales. Here, we demonstrate rapid single-shot detection of a coherent single-electron tunneling amplitude between InAs nanowire quantum dots. We have realized a sensitive dispersive detection circuit by connecting a sub-GHz, lumped element microwave resonator to a high-lever arm gate on one of dots. The resulting large dot-resonator coupling leads to an observed dispersive shift that is of the order of the resonator linewidth at charge degeneracy. This shift enables us to differentiate between Coulomb blockade and resonance -corresponding to the scenarios expected for qubit state readout -with a signal to noise ratio exceeding 2 for an integration time of 1 µs. Our result paves the way for single shot measurements of fermion parity on microsecond timescales in topological qubits.

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Section: Introductionmentioning

confidence: 99%

Rapid Detection of Coherent Tunneling in an InAs Nanowire Quantum Dot through Dispersive Gate Sensing

et al. 2019

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“…11,12 Integrating readout circuitry into an existing electrostatic gate or ohmic contact is useful for reducing device footprint and lead count. 2,8,[14][15][16][17][18][19][20][22][23][24][25] In this case, dispersive readout is performed by monitoring state-dependent shifts in the resonance frequency f R = (LC tot ) −1/2 of an LC circuit connected to a gate, where f R is detuned from the qubit transition frequency. The total capacitance, C tot , comprises geometric capacitance, C g (including parasitic contributions), quantum capacitance, C Q , and tunnel capacitance, C T .…”

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confidence: 99%

Dispersive sensing in hybrid InAs/Al nanowires

Sabonis

O’Farrell

Razmadze

et al. 2019

Applied Physics Letters

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Dispersive charge sensing is realized in hybrid semiconductor-superconductor nanowires in gate-defined single-and double-island device geometries. Signal-to-noise ratios (SNRs) were measured both in the frequency and time domain. Frequency-domain measurements were carried out as a function of frequency and power and yield a charge sensitivity of 1 · 10 −3 e/ √ Hz for an ∼11 MHz measurement bandwidth. Time-domain measurements yield SNR > 1 for 20 µs integration time. At zero magnetic field, photon-assisted tunneling was detected dispersively in a double-island geometry, indicating coherent hybridization of the two superconducting islands. At an axial magnetic field of 0.6 T, subgap states are detected dispersively, demonstrating the suitability of the method for sensing in the topological regime.Readout of quantum systems on timescales short compared to coherence or relaxation times is typically performed by one of a few schemes: (i) the device is incorporated into a resonant circuit, allowing state-dependent changes in the damping or shift of the resonance to be measured, 1,2 (ii) the quantum state is converted to charge, which is then detected by a nearby electrometer, [3][4][5][6] or (iii) a state-dependent capacitive coupling to the system results in a frequency shift in the coupled resonant circuit that depends on the quantum state, 7,8 the latter referred to as dispersive readout. In the context of topological qubits, several proposals for non-locally encoding fermion parity in Majorana zero modes have been made 9-13 . Some proposals use parity-to-charge conversion for readout 10 , while others use state-dependent hybridization of the Majorana mode with an ancillary system, leading to a dispersive readout signal. 11,12 Integrating readout circuitry into an existing electrostatic gate or ohmic contact is useful for reducing device footprint and lead count. 2,8,[14][15][16][17][18][19][20][22][23][24][25] In this case, dispersive readout is performed by monitoring state-dependent shifts in the resonance frequency f R = (LC tot ) −1/2 of an LC circuit connected to a gate, where f R is detuned from the qubit transition frequency. The total capacitance, C tot , comprises geometric capacitance, C g (including parasitic contributions), quantum capacitance, C Q , and tunnel capacitance, C T . 7,26 When the quantum system consists of a Coulomb island tunnel coupled to a reservoir, C Q arises from continuous charge transitions, and is proportional to the curvature of energy with respect to the confining gate voltage. 27 The maximum magnitude of C Q occurs at gate voltages corresponding to charge degeneracy, with opposite signs for ground and first excited states. C T is significant when the energy relaxation rate exceeds f R . The dependence of f R on C Q provides the quantum state selectivity of the dispersive shift. Monitoring phase or magnitude of the signal reflected from the resonant circuit thus allows readout of the quantum state of the system.Recent work on gate-based dispersive sensing has addressed semicondu...

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“…We have recently found the superconducting double dot (SDD) a useful platform to understand and exploit quasiparticles [10][11][12] . The superconducting double dot comprises two superconducting islands, with a charging energy comparable to the superconducting gap, coupled to each other by a Josephson junction.…”

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confidence: 99%

Cooper pair tunnelling and quasiparticle poisoning in a galvanically isolated superconducting double dot

Esmail

Ferguson

Lambert

2017

Applied Physics Letters

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We increase the isolation of a superconducting double dot from its environment by galvanically isolating it from any electrodes. We probe it using high frequency reflectometry techniques, find 2e-periodic behaviour, and characterise the energy structure of its charge states. By modelling the response of the device, we determine the quasiparticle poisoning rate and conclude that quasiparticle exchange between the dot and the leads is an important relaxation mechanism.The presence of excess quasiparticle excitations has a deleterous effect on many superconducting technologies, including resonators 1,2 , single photon detectors 3 , on-chip electronic refrigerators 4 , and superconducting qubits 5-7 . In qubits this is known as quasiparticle poisoning and is often the limiting factor for coherence times. Typically, there is a significant quasiparticle population even at dilution fridge temperature due to incomplete shielding of the qubit from non-equilibrium radiation 8,9 . Understanding and supression of this is therefore important for successful superconducting technologies.We have recently found the superconducting double dot (SDD) a useful platform to understand and exploit quasiparticles 10-12 . The superconducting double dot comprises two superconducting islands, with a charging energy comparable to the superconducting gap, coupled to each other by a Josephson junction. In our previous work, the islands have been connected via tunnel junctions to normal-metal leads, but here we study a galvanically isolated double dot (GIDD). This approach removes one of the key relaxation pathways for the SDD of quasiparticle exchange with the leads 10 , but isolated semiconductor qubits have been shown to have reduced electron temperatures 13 . It also prevents quasiparticle poisoning via tunnelling from the leads. Previous studies on a similar system measured via a charge sensor 14,15 found strictly e-periodic behaviour, implying complete poisoning of the device. This was ascribed to back action from the charge sensor. Here we revisit the system, using radio-frequency reflectometry and microwave spectroscopy to assess the poisoning rate.In Fig. 1(a) we show an SEM of the GIDD, made via double angle shadow mask evaporation 16 , with 20 nm thick aluminium forming each island. The SQUID-like geometry allows for the tuning of the Josephson energy of the junction between the two islands with a perpendicular magnetic field. Electrostatic gates allow control of the chemical potentials of the islands via V 1 and V 2 and the introduction of microwave frequency excitations (V MW ) and a probe tone for reflectometry (V RF ). The device is embedded in a tank circuit ( Fig. 1(b)) comprising a lumped element inductor (L = 560 nH) and its parasitic capacitance (C p = 0.33 pF), and cooled to a base temperature of 35 mK. The inductor has a resonant frequency of f 0 = 370.25 MHz, and, by homodyne detec-(a) (b) 1 um (c) 30 20 10 0 -10 -20 -30 20 10 0 -10 -20 -30 30 0.08 0.06 0.04 0.02 0.00 V RF Bias tee V 1 L C p C g1 V MW C m2 C m1 C g2 V 2 V...

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Quantum capacitance and charge sensing of a superconducting double dot

Cited by 7 publications

References 29 publications

Rapid Detection of Coherent Tunneling in an InAs Nanowire Quantum Dot through Dispersive Gate Sensing

Rapid Detection of Coherent Tunneling in an InAs Nanowire Quantum Dot through Dispersive Gate Sensing

Dispersive sensing in hybrid InAs/Al nanowires

Cooper pair tunnelling and quasiparticle poisoning in a galvanically isolated superconducting double dot

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