Superconducting Photodetectors
David Schuster
Assistant Professor
University of Chicago
Figures from:
Yale:
Schoelkopf Group
Prober Lab
NIST:
S.W. Nam
J.M. Martinis
Manipulating microwaves one photon at a time
?
Outline
• Applications of superconducting photodetectors
• Overview of superconducting photodetectors
• Kinetic Inductance Detectors
• Nanowire Superconducting Single Photon Detectors
• Practical considerations
Applications for superconducting detectors
• Astronomy
– Low dark noise
– High absorption efficiency
– Multi-pixel
• X-ray analysis
– Good energy resolution
• Quantum Computing / Quantum Key Distribution
– Low dark noise
– Fast response/recovery time
– Broadband
SC detectors have great performance!
High resolution
Low noise
LLE review vol 101
Martinis, NIST
High throughput > 1Gbps
Photon number resolving
Histogram of photon number for a pulsed laser
50
Signal from a TES for 0, 1, 2, 3, 4 photons
1000
Output signal [a.u.]
40
35
30
100
co u n ts
Counts [thousands]
45
25
-5
0
5
10
time [s]
20
15
20
S iA P D
SSPD
10
15
10
1
5
0
0
1
2
3
4
5
6
Photon number
S.W. Nam, NIST
7
8
0
2
4
6
8
tim e (n s)
S.W. Nam, NIST
10
Most SC detectors work like calorimeters
Energy
deposition
Thermometer
Rn
Absorber, C
R
Weak thermal link, g
Thermal sink
T
• Many types of detectors: Transition Edge/Tunnel Junction/KID/nanowire
• Operating temperatures range from ~ 0.1-60K
• Large spectral range THz - Xray
• Rely heavily on microfabrication
Cascade of broken Cooper pairs
100
Photon h n
e-e interaction
• Photon breaks a cooper pair
10-1
eV
phonons
• Thermalizes making hn/D qp’s
• # gain but no E gain yet
e-e interaction
2D
10-3
Quasi particles
k bT
• E resolution / photon # counting
determined by shot noise
• Gain comes from change R or L
Cooper pairs
Quasiparticles change surface impedance
Shunted normal resistance
Kinetic inductance
LK
R
Broadband
R
Resonant
Rn
T
Day, et. Al. Nature (2003)
Multiplexing Kinetic Inductance Detectors
Nanowire Superconducting Single Photon Detector (SSPD)
NbN
4nm thick
<100nm wide
Annunziata JAP 2010
• Current Biased
• Very fast ( 10’s of ps)
• Usually cooled by phonons
Other innovations…
High Tc
Williams IEEE ASC Proc. 2010
Multiwire detectors
Lincoln labs
But is it practical?
Already in use for some applications:
• X-ray analysis
• Ground based telescopes
Major limitations:
• Cryogenic operation
• Not enough pixels
Way forward:
• Closed-cycle Cryo systems
• Multiplexed detection, SC cameras
• Even better performance
NIST NbN detector
Summary
• Lots of SC detector technologies
• Kinetic Inductance Detectors, Nanowire Single Photon
Detectors
• Transition Edge Sensors/Bolometers/Tunnel Junction
• Many applications
• Astronomy
• Analysis
• Quantum computing / cryptography
• Excellent Performance
•
•
•
•
Wide spectral coverage (Terahertz – X-ray)
Fast (10 ps)
Sensitive (10-21 W/Hz1/2 NEP)
Multiplexable (cameras)
• Cryogenic operation still a limitation but getting better
Additional slides follow
Outline
• Types of superconducting photodetectors
• Speed limitations of SC detectors
• Super-sensitive level meter and preliminary measurements of
electrons on helium
Cavity QED with circuits and floating electrons
2g = vacuum Rabi freq.
k = cavity decay rate
g = “transverse” decay rate
Strong coupling: 2g > k, g
out
Transmission line “cavity”
10 m
10 GHz in
Trapped electron
Theory: Blais, Huang, et al., Phys. Rev. A 69, 062320 (2004)
What to do with hybrid systems and cavity QED?
Quantum Optics
Measure individual photon # states
Produce single photon states
Tomography of arbitrary quantum states
DIS*, Houck*, et. al., Nature, (2007)
Fundamental Quantum physics
Measurement of field quantization
Tests of quantum gravity, etc.
Bishop, Chow, et. al.,
Nature Physics, (2009)
Quantum Computing
Two qubit gates
Quantum algorithms
Process tomography
DiCarlo, Chow, et. al.,
Nature, (2009)
Hybrid quantum systems
Nanomechanics
Solid-state spins
Y. Kubo, F. Ong, P. Bertet et. al. PRL (2010)
DIS, A. Sears, E. Ginossar, et. al. PRL (2010)
Teufel, et al., Nature (2011)
See SYHQ 2!
Ultracold atoms
Verdu, Zoubi, et. al. PRL (2009)
Hunger, Camerer, Hänsch, et. al.
PRL (2010)
Electrons on helium
See SYHQ 3-5!
Polar Molecular Ions
DIS, Bishop, et. al.
PRA (2011)
DIS, Fragner, et. al.
PRL (2010)
Seeing a puddle of electrons on helium
Low energy electrons get
stuck on the surface
Force from positive
electrode causes a
dimple
M.W. Cole. Rev. Mod. Phys. 46, 3 1974
An electron on helium?
See Jackson 4.4
Electron bound at < 8K
He
Levitates 8nm above
surface (in vacuum)
R
e
V   En   2
4 e 0 nz
1
+
2
Clean 2DEG :
Mobility = 1010 cm2/Vs
Bare electron:
R / h  157G H z meff = 1.005 me, g = 2
<1 ppm 3He nuclear spins
e = 1.057
a0 = 7.6 nm
QC Proposal w/ vertical states: Dykman, Science 1999
An electron in an anharmonic potential
•
DC electrodes to define trap for lateral motion
•
Nearly harmonic motion with transitions at a few GHz
•
Anharmonicity from small size of trap (w ~ d ~ 1m)
CCD’s for electrons on helium
• Massive CCD of electrons on helium
• Control many electrons with
just a control inputs
Courtesy Lyon group
• Needed: to load/detect exactly 1 electron/pixel
• Needed: way to entangle pairs of pixels together
Detection of single electrons on helium
Electrons transferred 1 at
a time from a resevoir
into a 10 micron size trap
Charge is quantized but no
detection of coherent motion
or spin
Rousseau, et. al. PRB 79 045406 (2009)
An electron in a cavity
E0 
V0
w
• Electron motion couples to cavity field
Cavity-electron coupling
• Can achieve strong coupling limit
of cavity QED
• Couple to other qubits through cavity bus
g ~ ex 0
V0
~ h  25M H z
w
Predicted decay rate
<10 kHz
Schuster, Dykman, et. al. Phys. Rev. Lett. 105, 040503 (2010)
Accessing spin: Artificial spin-orbit coupling
• Electricaly tunable spin-motion coupling!
• With no flux focusing and current geometry: 100 kHz/mA
Motional Decoherence Mechanisms
• Relaxation through bias electrodes
• Dephasing from level fluctuations
• Emission of (two) ripplons
• Emission of phonons
dephasing
relaxation
 10 us motional decoherence time … 10,000x longer than GaAs
 Spin coherence time predicted > 100s
Anatomy of an “eon” trap
Cavity level meter
Drive plate
Guard ring
Gate plate
Sense plate
Superconducting Cavities as liquid He-Meters
Experiment
Q~105
I
II
III
IV
V
Detecting trapped electrons on helium
Electrons
No electrons
Making an eonhe transistor (eonFET)
DVgate
Modulate density without losing electrons
Measure density ~109 e/cm2 (~few e/um2)
Conclusions
Electrons on Helium:
•Rich physics - single electron dynamics, motional
and spin coherence, superfluid excitations, etc.
• Strong coupling limit easily reached
• Good coherence times for motion and spin
We see electrons on helium!!
• Can trap at 10 mK without much heating (~100mK)
• Can hold them for hours
Next up: Trapping single electrons
Recruiting!
Check out: schusterlab.uchicago.edu for more info
Additional slides follow
Experimental Setup
Pulse-tube cooled dilution refrigerator
Hermetic sample holder
top
bottom
• Indium sealing & stainless capillary
• No superfluid leaks down to 10mK
Additional slides follow