# Presentation: 205929

## GHz-Rate Superconducting Photon Counting Detector

**Verevkin, R. Sobolewski**
*Department of Electrical and Computing Engineering*
*and Laboratory of Laser Energetics*
*University of Rochester, Rochester NY 14627*
**G. N. Gol’tsman**
** ***Moscow State Pedagogical University*

***NIST***
***Single Photon Workshop***
***April 1, 2003***

**GHz-Rate Superconducting Photon Counting Detector**

## Acknowledgements

**Dr. A.Semenov (DLR, Berlin, Germany)**
**	****Dr. C. Williams (Corning, Corning NY)**
**J. Zhang, X. Zheng, Y. Xu,A. Pearlman, UR**
**O.Okunev, G. Chulkova, A. Lipatov, A. Korneev, N.Kaurova, V. Drakinsky, K. Smirnov, B. Voronov (MSPU, Moscow)**
**-****	****Dr. D. Nicholson, AFRL/IFGC, Rome, NY**
**	****Dr. K. Wilsher, Dr. W. Lo, NPTest, San Jose, CA**
**-****	****Support: NSF, AFOSR, ONR,  NPTest Inc.**

**Acknowledgements**

## Outline

**Introduction and Motivation:**
**NbN Superconducting Single-Photon Detector:**
**	****	****- how does it work?**
**	****	****- performance and experimental results**
**Applications:**
**	****	****- quantum cryptography and communications**
** ****	****	****- noninvasive testing of VLSI chips**
**Conclusions**

## Why superconducting detectors are promising?

- One optical photon creates only one
- 	electron-hole pair
-       (typical bandgap 1-2 eV)
- At low temperatures,
- 	relaxation and response times are long in APDs

-           Superconductors
- One optical photon creates ~100–1000 excited quasiparticles
- 	(superconducting gap
- 	  ~ 2 meV for NbN)
- b)   Relaxation is ultrafast
- 	even at low temperatures

**Low temperature environment reduces background noise**
**and thermal fluctuations responsible for dark counts.**

**Why superconducting detectors are promising?**

## Slide 5

## Hotspot formation in a thin superconducting metallic film occurs upon the absorption of a photon

**-****	****Initial hotspot forms due to electron thermalization and the generation of ****	****a large number of excited quasiparticles (quasiparticle avalanche process).**
**-****	****The hotspot grows due to the quasiparticle diffusion and electro-phonon ****	****secondary excitation.**
**-****	****The hotspot collapses due to the quasiparticle recombination and ****	****	****out-diffusion.**

**Notes:**
The thermalization process results in a local increase of electron temperature above Tc --a hotspot—a local nonsuperconducting region. 
After the initial thermalization, the resistive hotspot size grows as hot electrons diffuse out of its hotspot region. 

## Slide 7

## Slide 8

## Slide 9

## Meander-structured thin-film NbN SSPDs

**10x10 ********m****2**** meander-type 10-nm-thick SSPD structures**

**Due to the device geometry, some radiation flux in not absorbed.**
**Intrinsic Quantum Efficiency (QE) is larger than experimental ****	****Detection Efficiency (DE)**

**early devices**

**later devices**

## Inter-digitated structure of thin-film NbN SSPD

**3.5-nm-thick device with 0.5 fill factor**

***T*****c**** of 3.5-nm-thick NbN**
**SSPD is >10 K**

## SSPD experimental setup

## SSPD response versus the incident photon flux

**Illumination source:**
**100-fs pulses, 810-nm wavelength, 82-MHz repetition rate**

**In the single-photon**
**counting regime,**
**the number of photoresponse pulses is proportional to the incident photon flux.**

**A**

**B**

**C**

**D**

**E**

**F**

**~20000 photons/pulse**
**~2400 photons/pulse**
**~2400 photons/pulse**
**~80 photons/pulse**
**~1 photon/pulse**
**~1 photon/pulse**

## Photoresponse statistics of a 3.5-nm-thick NbN SSPD illuminated by photons with810-nm and 1540-nm wavelengths

## Hotspot size is a function of incident photon energy (wavelength)

## Detection efficiency is an exponential function of the SSPD bias current

- 2

**10-nm-thick**
**device**

## Spectral dependence of the detection efficiency (DE) for 3.5-nm and 10-nm SSPDs

**Intrinsic QE**
**is approx. 30 larger than experimental DE**

**SSPDs are effective**
**from UV to IR**

**10 x 10 ********m****2**** devices**

**For visible-light photons QE reaches 100%**

## Dark counts and NEP for 3.5-nm-thick SSPD

## SSPD jitter measurements

- The system jitter is 35 ps with Tsunami Ti:Sapphire laser, which includes contributions from the laser the amplifier.
- Signal FWHM < 150 ps, negative voltage due to quasiparticle
- recombination is observed.

## 1-GHz-rate photoresponse trainof a 3.5-nm-thick NbN SSPD

**1-GHz-rate photoresponse train****of a 3.5-nm-thick NbN SSPD**

- (Pritel Inc. fiber laser)

## SSPD Counting speed is above 2GHz

- Jitter is <18 ps at 775 nm
- and 1550 nm fiber Pritel
- Mode-locked laser

## Quantum Cryptography (QC) based on single-photon communication assures unconditional security

**Unconditionally secret, quantum key distribution is possible in actual physical environments due to Heisenberg Indeterminacy Principle: **
***	******It is impossible to measure the state of a quantum bit without altering it.***
**Ultrafast Alice (Transceiver) already exists**
**Ultrafast Bob (Receiver): we propose to implement a novel SSPD counter.**

**Alice**
**(Transceiver)**

**Bob **
**(Receiver)**

- [from Simon Benjamin, Science 290, 2273 (2000)]

## Fiber optics versus optical free spacequantum cryptography channels

**-****	****The best optical fibers have losses ~ 0.2 dB/km and introduce uncontrolled polarization changes.**
**-****	****Quantum amplifiers/repeaters are questionable.**
**	****	****Fiber-based QC channels are applicable for local ****	****	****networks (below 100 km).**
**	****	****Global communications and, especially, satellite links ****	****require optical**** ****free-space**** ****QC.**

## Ultrafast Alice

## Ultrafast Bob

**Ultrafast Bob**

## Comparison of our SSPD performance with the best commercially available single-photon detectors at ~1.3 m wavelength 

*******Gated regime with 0.1% per gate after-count probability.**
******** Calculated with 10****-4**** per gate probability.**
*********Data for a high-speed version; standard devices exhibit 1 ********10****5**** s****-1****.**

## PICA® System with NbN SSPD  at Schlumberger ATT (now NPTest Inc.)

## IR radiation image of normally working CMOS

**Weak IR pulsed radiation emitted by switching CMOS inverter. **
**When V****out**** is switching from 1 to 0, nMOS emits photons. When V****out**** is switching from 0 to 1, pMOS emits photons.**

## Single-photon emission from CMOS transistors

**0.35-********m linewidth, 3.3-V bias CMOS circuit running at 100 MHz**

**Mepsicron II**
**detector**

**NbN SSPD**
**detector**

**0.13-********m linewidth, 1.3-V bias CMOS circuit running at 100 MHz**

**Notes:**
Histogram of counts versus time for 0.35um device (Lazarus). Device is stimulated by repetitive signal and data is acquired over many periods of the signal. Acquisition time: minutes. Data taken on different devices in chip so timing does not match.

Pink trace: Peaks only when CMOS inverter switches from high to low since this is only time that significant current flows in nMOS transistor.

Orange trace: Defective, leaky pFET in inverter. Sharp peak = nFET emission when output switching from 1 to 0. Broad ‘peak’ is emission from leaky pFET.  

Statistical analysis allows center of emission peak and width of peak to be determined within some error. How large the error is depends on jitter of the measurement (Transit Time Spread, TTS) and also noise (dark counts).

How efficiently photons are detected is determined in part by detector sensitivity (Quantum Efficiency, QE).

Speed of the acquisition depends on TTS, QE, and Dark Counts.

## Single-photon emission from bothnMOS and pMOS transistors

**0.13-********m linewidth, 1.3-V bias CMOS circuit running at 100 MHz**

**nMOS**

**pMOS**

**Notes:**
Histogram of counts versus time for 0.35um device (Lazarus). Device is stimulated by repetitive signal and data is acquired over many periods of the signal. Acquisition time: minutes. Data taken on different devices in chip so timing does not match.

Pink trace: Peaks only when CMOS inverter switches from high to low since this is only time that significant current flows in nMOS transistor.

Orange trace: Defective, leaky pFET in inverter. Sharp peak = nFET emission when output switching from 1 to 0. Broad ‘peak’ is emission from leaky pFET.  

Statistical analysis allows center of emission peak and width of peak to be determined within some error. How large the error is depends on jitter of the measurement (Transit Time Spread, TTS) and also noise (dark counts).

How efficiently photons are detected is determined in part by detector sensitivity (Quantum Efficiency, QE).

Speed of the acquisition depends on TTS, QE, and Dark Counts.

## Conclusions

**-****	****NbN single-photon devices are sensitive to radiation from UV to mid-IR. In terms of speed and sensitivity they outperform any semiconductor photon counters.**
**Our best devices reach  DE = 10% at 1.3 ********m and**
**	****DE > 15% at 0.4 ********m - 0.8 ********m, leading to intrinsic QE close to 100% for visible light photons.**
**NEP is below 2*10****-18**** W/Hz****1/2****.**
**SSPD photoresponse time is ~150 ps and jitter 18-35 ps. Measured counting speed is up to 2 GHz.**
**-****	****Implemented in IDS****®**** PICA system for VLSI circuit testing.**
**-****	****Devices-of-choice for fast quantum communications.**