Transcript Slides - Agenda INFN

SUPERCONDUCTING NANOWIRES
DETECTING SINGLE PHOTONS FOR
INTEGRATED QUANTUM PHOTONICS
Roberto Leoni
IFN-CNR, Istituto di Fotonica e Nanotecnologie, Via Cineto
Romano 42, 00156 Roma, Italy
FRONTIER DETECTORS FOR
FRONTIER PHYSICS
13th Pisa Meeting on Advanced
Detectors 24-30 May 2015La Biodola, Isola d'Elba (Italy)
Coauthors:
- Alessandro Gaggero (IFN-CNR)
- Francesco Mattioli (IFN-CNR)
- Andrea Fiore (Eindhoven University
of Technology, The Netherlands)
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WHY SUPERCONDUCTING
DETECTORS?
Superconducting detectors are the natural
choice for ultrasensitive optical detection
.
• Small energy gap compared to
semiconductors,
• Cryogenic temperatures allow low
noise, reduction in blackbody radiation
and phonon noise.
With a smaller  a larger
number of quasiparticles are
created
quasiparticles
  meV
Cooper pair qs
 2e
Single Photon Detectors based on
Superconducting Nanowires
Their acronym is SSPDs or SNSPDs
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SSPDS AND INGAAS APD SPECS AT
TELECOM WAVELENGTH (1550NM)
http://www.nict.go.jp/en/press/2013/11/05-1.html
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Superconductors that have demonstrated capability of singlephoton detection are compounds or alloys (classified as Type II
superconductors):
NbN (niobium nitride)
NbTiN (niobium titanium nitride)
WSi (tungsten silicide)
MoSi (molybdenum silicide)
…..
Coherence length
Type II superconductors have short ξ and
~ Cooper pair size
large  (magnetic penetration depth)
They can be grown as ultra thin films
(thickness ~ ξ ~ 4-5 nm)
o  vF / 
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DETECTOR CONCEPT
Goltsman APL (2001)
T<<TC
Vout
JB ~ J C
T ≥ TC
Bias-T
Vout
+
RB
IB ≤ IC
VB
Rn
Rout
+
t=4-5 nm
RN
-
NbN
meander
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SSPDS DESIGNED FOR FREE SPACE COUPLING
Nanowire are made of a Type-II
ultrathin (4-5nm) films.
Not many chances for photons to
be absorbed
SiO2
Si
SSPD on top of a /4 optical cavity
(SiO2) to improve photon absorption
efficiency 
quarter-wavelength optical stack to enhance 
Marsili Nature
Photonics (2013)
http://www.nict.go.jp/en/press/20
13/11/05-1.html
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QUANTUM EFFICIENCY
A Figure of Merit: Detection
Quantum Efficiency DQE
DQE = (Nc − DCR ) / Nph
• Nc = count/sec
• DCR = dark count rate
• Nph = photons/sec
Dark Count rate DCR
DCR is due to spontaneous
transient resistive states
observed even in the
absence of incident photons.
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Z. Zhou PhD thesis 2014
ORIGIN OF DARK COUNTS
In a 2-D Superconductor (thickness ~ξ,
width >>ξ, like the nanowire) vortices
are present even at Ha=0 in form of
vortices- antivortices pairs (VAP) to
meet the condition net B=0.
Semenov et al Physica C
Supercond (2008)
Yamashita APL (2011)
U
− VAP
The bias current IB exerts a Lorentz force
𝑃VAP = ΓVAP 𝑒 kBT
F that can overcome the binding of the
VAP. (UVAP) and break them into single
vortices that move in opposite directions.
Crossing of the unbound vortices create decoherence in the film that can
trigger the superconducting to normal transition (false pulse => DCR)
COOLING THE DETECTORS
SNSPDs are operated in a
Gifford-McMahon (GM)
closed-cycle refrigerator
Noisy environment due
to the presence of the
refrigerator compressor?
CRYOPHOBIA?
extreme fear of cold?
Operating at telecom
wavelength 1550nm, low
loss optical fibers allow
remote photon detection
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EFFICIENTLY FIBER COUPLED DETECTORS
semiconducting substrates
Miller et al. Opt. Express (2011)
Alignment issue:
The diameter of the detector
is 10 um approx. the same
size of the core of single
mode fiber.
IFN implementation
IFN
Labs: NIST, JPL
Companies: Single Quantum,
Quantum Opus
SYSTEM QUANTUM EFFICIENCY
System Quantum Efficiency
•
•
SQE = ε DQE= εαη
T= 0.12 K
λ= 1550 nm
ε optical coupling efficiency
accounts for losses in the optical
fiber system between the (roomtemperature) light input point and
the (cold) detector
SQE= 93%
achieved with :
• High ε using the selfaligned mounting
scheme based on Si
micromachining
• High η using WSi films
Marsili et al. Nature Photonics (2013)
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QUANTUM PHOTONIC INTEGRATED CIRCUITS
To combine active (single photon sources / detectors) and
passive (grating couplers, waveguides, beam splitters, MMI…)
elements to achieve important functionalities of Quantum
Information Processing
QD
Photonics
circuits
SNSPDs
Sources
(Quantum dots,
split ring
resonators)
DESIGN FOR TRAVELLING WAVE COUPLING
GaAs case
[V/m
]
Sprenger et al. APL (2011)
absorption coefficient
κabs(TE)= 452 cm-1
absorption probability α
NbN on top of
GaAs ridge WG
𝜶 = 𝟏 − 𝒆−ℵ𝒂𝒃𝒔 𝑳 = 90%
with L= 90µm
selected as influential paper from Appl Phys Lett
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Roberto Leoni
KEY ENABLING TECHNOLOGIES FOR THE
FABRICATION OF INTEGRATED DETECTORS
dc-magnetron
sputtering
(NbN)
Electron beam
lithography
(HSQ, PMMA,
SU 8)
Electron-beam lithography
(100kV, FEG)
Reactive ion
etching
(NbN, Si3N4 ,
Si)
fabrication facility:
250 m2 Cleanroom
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Roberto Leoni
Ti/Au Electric contacts
1st step. Definition of TiAu
contact pads on top of a
ultrathin (4-5 nm) sputtered
NbN
100 nm
2nd step. Definition and etching
nanowires
3rd step. Definition and etching of
waveguides aligned (better than
~100nm) to nanowires
4th step. Definition and etching of the
vias openings through HSQ to
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contact pads
TRAVELLING WAVE COUPLING
silicon-on-insulator case
WG 500 nm
Grating coupler
300nm ribs
10um
NbN
t=5nm
10um
Coll CNR and UBRI
MMI MULTIMODE INTERFERENCE COUPLER
The combination of
splitting and detection is
at the very heart of
linear-optics quantum
computing
MMI coupler allows
redistributing light from the
N inputs into the M outputs
1.85 m
6 m
Coll UBRI
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MMI AND WSPD OPTICAL CHARACTERIZATION
Gaggero, to be published (2015)
- SQEs of detectors D1
and D2, feeding light
alternatively to the two
inputs, overlap.
- the input light equally
splits between the two
output channels.
λ = 1510, P=85 pW
CONCLUSIONS
- SSPDs in a stand alone multichannel system can
be efficiently fiber coupled to room temperature
experiments
- SSPDs integrated in waveguides represent an
important step towards the realization of fullyfunctional quantum photonic integrated circuits
Laser ranging (LIDAR)
Deep space communications
Single photon sources characterization
Quantum key distribution (QKD)
Linear optics quanum computing (LOQC)
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