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Single Photon Counting Detectors for Submillimeter Astrophysics: Concept and Electrical Characterization. John Teufel Department of Physics Yale University. Yale: Minghao Shen Andrew Szymkowiak Konrad Lehnert Daniel Prober Rob Schoelkopf. NASA/GSFC Thomas Stevenson Carl Stahle

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slide1
Single Photon Counting Detectors for Submillimeter Astrophysics:Concept and Electrical Characterization

John Teufel

Department of Physics

Yale University

Yale:

Minghao Shen

Andrew Szymkowiak

Konrad Lehnert

Daniel Prober

Rob Schoelkopf

NASA/GSFC

Thomas Stevenson

Carl Stahle

Ed Wollack

Harvey Moseley

Funding from NASA Explorer Tech., JPL, GSFC

overview
Overview

Types of detectors

Noise and sensitivity in detectors

What is the Submillimeter?

The “SQPC” – a high-sensitivity sub-mm detector

Dark currents and predicted sensitivities of SQPC

Time scales and saturation effects

Future Work

types of detectors
Coherent

Measures Amplitude & Phase

For Narrow-band Signals

Sensitivity given in Noise Temperature [K]

Adds a 1/2 photon of noise per mode

Minimum Noise Temperature: TQ=hf/2k

Example: a mixer

Incoherent

Measures only Amplitude

For Broad-band Signals

Sensitivity given by NEP [W/rt(Hz)]

No fundamental noise limit on detector

Ideally limited only by photon statistics of signal or background

Example: a photomultiplier

Types of Detectors
when to use an incoherent detector

Raleigh-Jeans

Wien

When to Use an Incoherent Detector
  • Average occupancy
  • per mode
  • In the Wien limit:
  • 1/2 photon per mode of
  • noise is unacceptable!

bb

photon counting in optical

n=Rate of

incoming photons

Ntot =

Photon Counting in Optical

Background Radiation

Signal Source

Photons

PMT

nbackground+

nsource

ndark

Rate of detector false counts

Ntot=(n + ndark)• t

direct detection with photoconductor
Direct Detection with Photoconductor

+

Signal Source

Photons

-

V

+

Background Radiation, e.g. CMB, Atmosphere...

Bandpass Filter, B

-

Typical

how many photons in the sub mm dark
How Many Photons in the Sub-mm “Dark?”

3 K blackbody

10 % BW

single-mode

Photon-counting (background) limit:

NEP ~ h(n )1/2

Future NASA projects need NEP’s < 10-19 W/rt(Hz) in sub-mm !

see e.g. SPECS mission concept, Mather et al., astro-ph/9812454

the sqpc single quasiparticle photon counter
Antenna-coupled Superconducting Tunnel Junction (STJ)

Photoconductor direct detector

Each Photon with excites 2 quasiparticles

Nb

Al

Al

Au

AlOx

The SQPC: Single Quasiparticle Photon Counter

Nb antenna

Al absorber

(Au)

m

~

1

STJ detector

junction

sub-mm

photon

Responsivity = 2e/photon = e/ = 5000A/W

what is measured
What is measured
  • Incident photons converted to current

Lower Idark=> Higher sensitivity

Nb antenna

Photocurrent

Dark current

(Au)

sub-mm

Current readout should not add noise to measurement

FET or RF-SET should have noise

RF-SET is fast and scalable

photon

STJ detector

junction

V

Ultimate Sensitivity

integration of rf circuits sets and sub mm detectors
Integration of RF Circuits, SETs, and sub-mm Detectors

one of four e-beam fields, with SETs and SQPC detectors, and bow-tie antenna

16 lithographic tank circuits on one chip

sensitivity and charge sharing with amplifier
Sensitivity and Charge Sharing with Amplifier

Q ~ 1000 e-

CSET ~ 1/2 fF

CSTJ ~ 250 fF

RF-SET(30 nV, ½ fF)

FET(2SK152; 1.1 nV, 20 pF)

0.15 e/rt(Hz)

1 x 10-4 e/rt(Hz)

Collects all charge

Collects CSET/CSTJ ~ 0.2%

still ~ 3 times better

Either FET or SET can readout STJ @ Fano limit,

But only SET is scalable for > 50-100 readouts

experimental set up and testing

Bow Tie Antenna

Detector

140 µm

1 µm

Experimental Set-up and Testing
  • Small area junctions fabricated using double angle evaporation

Device mounted in pumped He3 cryostat (T~250mK)

slide14

Fig. 2. (a) SQPC detector strip and tunnel junctions are located between two halves of a niobium bow-tie antenna for coupling to submillimeter radiation. A gold quasiparticle trap is included here in the wiring to just one of two dual detector SQUIDs. (b) Close-up view of detector strip and tunnel junctions made by double-angle deposition of aluminum through a resist mask patterned by electron beam lithography. Pairs of junctions form dc SQUIDs, and critical currents can be suppressed with an appropriately tuned external magnetic field.

quasiparticle trap

SQUID

loop

1 µm

junction

antenna

antenna

detector strip

supercurrent suppression

X

B

Supercurrent Suppression

Detector Junctions form a SQUID

Al/AlOx/Al Junctions: ~ 60 x 100 nm

supercurrent contributions to dark current
Supercurrent

Cooper pair tunneling affects the subgap current both at zero and finite voltages

DC Josephson effect:

AC Josephson effect:

V

Supercurrent Contributions to Dark Current

DC Power

RF Power

Zen

Zen

Ic sin(J t)

SQPC

*

*Holst et al, PRL 1994

thermal dark current measurements
Thermal Dark Current Measurements

BCS Predicts:

Tc =1.4 K

I @ 50 mV

Current [pA]

Voltage [µV]

recombination and tunneling times

x-ray

Vabs

1000 mm3

0.01 mm3

½ W

50 kW

RN

ttunnel

2 ms

2 ms

sub-mm

Recombination and Tunneling Times

Vabs

ttunnel ~ VabsRN

lead

(large

volume)

g

thermal trecomb ~ 100 ms

@ 0.26 K

absorber

at low power:

ttunnel << trecomb

so quantum efficiency

is high

False count rate = Idark/e = 3 MHz for ½ pA

saturation recombination vs tunneling
Saturation: Recombination vs. Tunneling

Current

Noise

I ~ P1/2

Absorber gap reduced by excess q.p.’s

trec ~ ttunn

I ~ P

NEP ~ P1/4

Idark

NEP ~ P1/2

Power (P)

(or photon rate, Ng)

Ng~ Id/e

Nsat ~ (tth/ttun) Id/e

Psat~ 0.02 pW; scales as 1/RN

electrical circuit model and noise

Rb

V

en

SQPC

Shot Noise

Johnson Noise

Amplifier Noise

Electrical Circuit Model and Noise
slide24

Future Work: Detecting Photons

Problem: Need to couple known amount of sub-mm radiation to detector

Solution: Use blackbody radiation from a heat source in the cryostat

cryogenic blackbody as sub mm photon source

V

1 cm

Cryogenic Blackbody as Sub-mm Photon Source

Hopping conduction thermistor

Micro-machined Si for low thermal conduction

coming soon photoresponse measurement

T= 1-10K

Coming Soon: Photoresponse Measurement

Si Chip with SQPC

Quartz Window

T= 250 mK

advantages of sqpc
Advantages of SQPC

Fundamental limit on noise = shot noise of dark current

Low dark currents imply NEP’s < 10-19 W / rt.Hz

High quantum efficiency – absorber matched to antenna

High speed – limited by tunneling time ~ msec

Can read out with FET, but SET might resolve single g’s

Small size and power (few mm2 and pW/channel)

Scalable for arrays w/ integrated readout

summary
Summary

When hf>kTbb, a photon counter is preferred

In the sub-mm, no such detector exists

The SQPC would be a sub-mm detector with unprecedented sensitivity

Contributions to detector noise have been measured and are well-understood

Photocurrent measurements in near future

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