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 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: T Q =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

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

Photon Counting in Optical PMTPhotons Signal Source Background Radiation N tot =(n  + n dark )  t  N tot = n background +n source n dark Rate of detector false counts n  =Rate of incoming photons

Photons Direct Detection with Photoconductor Bandpass Filter, B Background Radiation, e.g. CMB, Atmosphere... Signal Source Typical V

Infrared What is the Sub-Millimeter?

How Many Photons in the Sub-mm “Dark?” 3 K blackbody 10 % BW single-mode Photon-counting (background) limit: see e.g. SPECS mission concept, Mather et al., astro-ph/ Future NASA projects need NEP’s < W/rt(Hz) in sub-mm ! NEP ~ h (n  ) 1/2

The SQPC: Single Quasiparticle Photon Counter Antenna-coupled Superconducting Tunnel Junction (STJ) Photoconductor direct detector Each Photon with excites 2 quasiparticles Nb antenna Al absorber (Au) ~ 1 mm STJ detector junction sub-mm photon AuNbAl AlO x Responsivity = 2e/photon = e/  = 5000A/W

Incident photons converted to current Lower I dark => Higher sensitivity What is measured Nb antenna (Au) STJ detector junction sub-mm photon Ultimate Sensitivity V Current readout should not add noise to measurement FET or RF-SET should have noise RF-SET is fast and scalable PhotocurrentDark current

Integration of RF Circuits, SETs, and sub-mm Detectors 16 lithographic tank circuits on one chip one of four e-beam fields, with SETs and SQPC detectors, and bow-tie antenna

Sensitivity and Charge Sharing with Amplifier Q ~ 1000 e - C STJ ~ 250 fF C SET ~ 1/2 fF FET (2SK152; 1.1 nV, 20 pF) RF-SET (30 nV, ½ fF) Either FET or SET can readout Fano limit, But only SET is scalable for > readouts 0.15 e/rt(Hz)1 x e/rt(Hz) Collects all chargeCollects C SET /C STJ ~ 0.2% still ~ 3 times better

Experimental Set-up and Testing Small area junctions fabricated using double angle evaporation 1µm1µm Bow Tie Antenna Detector 140 µm Device mounted in pumped He 3 cryostat (T~250mK)

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. 1 µm junction detector strip SQUID loop quasiparticle trap antenna

Al/AlOx/Al Junctions: ~ 60 x 100 nm X B Detector Junctions form a SQUID Supercurrent Suppression

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: Z en I c sin(  J t) V Z en SQPC RF PowerDC Power * *Holst et al, PRL 1994

Magnetic Field Dependence of Sub-gap Current

BCS Predictions for Dark Current T=1.6 K T=250 mK  { } eV

Thermal Dark Current Measurements BCS Predicts: Tc =1.4 K 50  V Current [pA] Voltage [µV]

Recombination and Tunneling Times absorber lead (large volume) sub-mm x-ray V abs RNRN  tunnel 1000  m  m 3 ½  2  s 50 k  2  s V abs  thermal  recomb ~ 100  0.26 K  tunnel ~ V abs R N  tunnel <<  recomb so quantum efficiency is high at low power: False count rate = I dark /e = 3 MHz for ½ pA

Saturation: Recombination vs. Tunneling Current Power (P) I dark (or photon rate, N  ) Noise N  ~ I d /e  rec ~  tunn N sat ~ (  th /  tun ) I d /e P sat ~ 0.02 pW; scales as 1/R N Absorber gap reduced by excess q.p.’s I ~ P NEP ~ P 1/2 NEP ~ P 1/4 I ~ P 1/2

Demonstration of an RF-SET Transimpedance Amplifier Trim gate Input gate 0.5 fF

Electrical Circuit Model and Noise Shot Noise Johnson Noise Amplifier Noise V RbRb enen SQPC

Problem: Need to couple known amount of sub-mm radiation to detector Solution: Use blackbody radiation from a heat source in the cryostat Future Work: Detecting Photons

Cryogenic Blackbody as Sub-mm Photon Source 1 cm V Hopping conduction thermistor Micro-machined Si for low thermal conduction

Coming Soon: Photoresponse Measurement T= 1-10K T= 250 mK Quartz Window Si Chip with SQPC

Advantages of SQPC Fundamental limit on noise = shot noise of dark current Low dark currents imply NEP’s < W / rt.Hz High quantum efficiency – absorber matched to antenna High speed – limited by tunneling time ~  sec Can read out with FET, but SET might resolve single  ’s Small size and power (few  m 2 and pW/channel) Scalable for arrays w/ integrated readout

Summary When hf>kT bb, 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