Optical Characterization of the Quantum Capacitance

Optical Characterization of the Quantum Capacitance Detector

Optical Characterization of the Quantum Capacitance Detector (QCD)

J. Bueno*, N. Llombart**, P. K. Day, J. Kawamura, K. Cooper, and P. M. Echternach

Jet Propulsion Laboratory, California Institute of Technology

* present address: Centro de Astrobiología (CSIC-INTA), Madrid (Spain)** present address: School of Optics, Universidad Complutense de Madrid, Madrid (Spain)

Many thanks to: Matt Shaw, Richard Muller, Jonas Zmuidzinas, Per Delsing

This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

Funding for this research was provided by a grant from the National Security Agency.

Juan Bueno


OutlineOutline

1.- Introduction1. Introduction

• Proof-of-Concept of the QCD• Dark NEP

2.- Coupling Radiation to the QCD

• Optical System • Experimental Setup

3.- Quantum Capacitance Detector Characterization

• Signal, noise and NEP• Quantum Capacitance Trace

• Conclusions

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Proof of concept of the QCDProof-of-concept of the QCD

• Quasiparticles injected with a SIS junction

• NEP on the order of 10-18 W/Hz1/2

• Large scalabilityLarge scalability

• Next step: couple light to the detector

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Detector SchemeDetector Scheme

• Radiation is absorbed by the antenna which is coupled to the absorber

• Cooper-pairs are broken and quasiparticles tunnel through the junctions

• The quasiparticle density is proportional to the quasiparticle tunneling rate

(our measurable quantity)

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( q y)


Optical System (I)Optical System (I)29µm

2.1µm1.5µm

0.8µm

1.7µm

0.05µm15.9µm

1.4µm

• Double-dipole antenna

• Frequency = 1 5THz (λ = 200µm)

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• Frequency = 1.5THz (λ = 200µm)


Optical System (II)Optical System (II)

• Z = 32Ω

• Resonance @ 1.5 THz

• 30% bandwidth

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Optical System (III)Optical System (III)

Blackbody source

50mm 1.5THz band pass filter%

Apperture0.08” diameter

10.19mm3 26mm

10% band3THz low pass filter0.03” teflon

3.26mm

13mmTeflon lensR=13.5mm

Silicon lensSilicon lensR=6.8mmSample

13.6mm

25.34mm

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4K Still Temp. MC Temp.


DetectorDetector

• Cooled down with a dilution refrigerator experiments done at 100mKCooled down with a dilution refrigerator, experiments done at 100mK

• Nb λ/2 resonator, Au antenna with Al absorber with Nb plug for quasiparticle trapping

• QCD out of Al/AlOx/Al

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ResonanceResonance

• Resonance frequency = 3.328118 GHz

• Q = 150000• Q = 150000

• Peak depth = 6.5 dB

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Qubit signalQubit signal

• Measured with a Lock-in amplifier technique

• Qubit biased with an AC tone at 25kHz

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Sending light to the QCDSending light to the QCD

• Step the blackbody temperature from 5 to 40K

• Resonance moves towards the right

• Consistent with a drop of• Consistent with a drop of the quantum capacitance

signal

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Measuring the noiseMeasuring the noise

• Noise measured with a spectrum analyzer at the

resonance frequencyresonance frequency

• Phase and amplitude noise measured for each

temperaturetemperature

• Ellipse of noise multiplied by 50 in the figure

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Phase NEPPhase NEP

• Blackbody radiation couples to the detector in

the single mode

• Filter bandwidth = 10%

• Transmission = 60%

• Resonance frequency shifts 400Hz/K

• NEP about 10-17 W/Hz1/2

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Thermal behavior of the resonatorThermal behavior of the resonator

• Two level systems could be the cause of the

frequency shiftq y

• Step the mixing chamber temperature

• In order to get the same frequency shift, the mixing

chamber should be at 300mK (too high!)( g )

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Dark runDark run

• Close all the windows to the experiment

• Step the blackbody temperature

• Resonance does NOT change at all

• Different shape due to magnetic flux trapped

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Quantum Capacitance Trace (I)Quantum Capacitance Trace (I)

• Send a ramp to the gate and look at the signal with

an oscilloscopean oscilloscope

• No clear quantum capacitance peaks visible

• However, the signal drifts when an offset is applied to

the ramp

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Quantum Capacitance Trace (II)Quantum Capacitance Trace (II)

St th t lt d• Step the gate voltage and use a lock-in technique to

look at the signal

N l t• No clear quantum capacitance peaks visible.

• Step the blackbody temperat re Some peakstemperature. Some peaks

grew, some peaks decreased

No clear conclusion can• No clear conclusion can be drawn

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Quantum Capacitance Trace (III)Quantum Capacitance Trace (III)

~320 µm long 1 µm gap1 5 µm strip1.5 µm strip1 µm gap3 micron strip6 micron gap10 micron stripC f 10 micron strip6 micron gapCc = 0.45 fF

Vgate VCPWCc = 0.45 fF

C i l = 4 5pF

• Voltage divider

• VCPW = Vgate Cc / Cspiral

QCDCspiral 4.5pF

• This is possibly the reason why we do not see a clear quantum capacitance trace

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Room for ImprovementRoom for Improvement

• Adding an extra line for gating the qubit and biasing g g q g

it at its degeneracy point

• Designing a new optimized optical systemp y

• Coupling the QCD to a better quality resonator

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C l iConclusions

1.- Light has been coupled to the QCD1. Light has been coupled to the QCD

• Antenna couple radiation has been succesfully detected by the QCD• Response in both the phase and the amplitude

2.- Optical NEP in the order of 10-17 W/Hz1/2 have been obtained for the phase signal

• Comparable to current detectors?

3.- Lots of room for improvement

• Separate gate line for the qubit• Better optical system

• Better quality resonator

Juan Bueno

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Optical Characterization of the Quantum Capacitance