1. Institute for Photonics and Nanotechnologies
SQUID
Guido Torrioli
Institute for Photonics and Nanotechnologies
IFN – CNR Rome

2. SQUID Transducer Extreme sensitivity, close to the quantum limit
Introduction
Supercontucting QUantum Interference Device SQUID
SQUID
Magnetic
Flux
Voltage
Transducer
Extreme sensitivity, close to the quantum limit
Measures magnetic flux and any other physical quantity
that can be converted into magnetic flux

3. Superconducting loop + Josephson junctions
Introduction
Superconducting loop + Josephson junctions
Josephson junctions
Josephson junction
dc-SQUID
rf-SQUID
rf bias magnetically coupled to the SQUID
dc current bias

4. Superconductivity Flux quantization Josephson effect Basic phenomena
Cooper pairs
Zero resistance
Low Temperature Superconductors (LTSs)
High Temperature Superconductors (HTSs)
Flux quantization
Magnetic Flux threading a superconducting loop is quantized
where
Often referred to as a weak link
Tunnel
Josephson effect
Tunneling of Cooper pairs through
a thin insulating barrier (weak link)
Critical current

5. 1911 Kamerlingh-Onnes discoverd superconductivity
Brief history
1911 Kamerlingh-Onnes discoverd superconductivity
1957 Bardeen–Cooper–Schrieffer theory
1962 Josephson effect
1964 First dc-SQUID (R. Jaklevic, J. J. Lambe, J. Mercereau, A. Silver)
1965 First rf-SQUID (R. Jaklevic, J. J. Lambe, A. Silver, J. E. Zimmerman)

6. rf-SQUID Single Josephson Junction
Radio frequency bias inductively coupled to the SQUID loop
Larger noise compared to the dc-SQUID
Standard device in the 1970s and 1980s

7. rf-SQUID
rf-SQUID as qubit
Chiarello’s talk on superconducting qubits

8. dc-SQUID working principles

9. dc-SQUID working principles
“One Flux quantum is about the
flux of earth’s magnetic field
through a single human
red blood cell” (John Clarke)

10. Thin-film coupling scheme (Ketchen and Jaycox)
Dc-SQUID layout
Thin-film coupling scheme (Ketchen and Jaycox)

11. Coupled energy sensitivity
P. Carelli et al; Appl. Phys. Lett. 72, 115 (1998); doi: 

12. dc SQUID Readout SQUID readout Small change in applied flux dFa
results in
small change in SQUID voltage dV
Very small voltage across the SQUID: Vpp  mV
Transfer coefficient VF = dV/dF depends on SQUID working point
Very small linear flux range: Flin << F0
Main problems:
Linearize transfer function to provide a larger dynamic range
Amplify the weak SQUID voltage without adding noise
Main tasks of a SQUID readout electronics:

13. Readout electronics FLL
Flux Locked Loop
Feedback flux counterbalances applied flux
Output voltage Vf depends linearly on applied flux
Very large dynamic range possible
Transfer function does no longer depend on SQUID working point

14. SQUID readout – Flux Modulation
Flux Modulation Readout
A modulation flux is applied to the SQUID
Use of a cold transformer
The applied flux is sensed by sincrhonously detecting the SQUID voltage
at the modulation frequency (Lock-in)
Main restrictions
Limited FLL bandwidth
Reduced Slew-Rate
Complicated electronics

15. Additional Positive Feedback
SQUID Readout – Direct
Additional Positive Feedback
APF circuit makes the V-F characteristic strongly asymetric
The voltage transfer coefficient is boosted at the working point
D Drung et at, Appl. Phys. Lett (1990).

16. SQUID Readout other configuration
Two stage SQUID
Large voltage transfer coefficient
Problem: multiple working point
SQUID Array
SQUIDs are connected in series
Flux is alpplied to all SQUIDs through input coils
connected in series
Voltage output is added costructively
Large Voltage swing

17. Non-destructive evaluation (NDE),
SQUID Applications
SQUID measures any quantity convertible into magnetic flux
Magnetic field:
Biomagnetism
Non-destructive evaluation (NDE),
Nuclear magnetic resonance (NMR, MRI),
SQUID microscopes,
geophysics
Electric current:
Cryogenic radiation and particle detectors (TES etc),
Cryogenic current comparators (CCCs) for metrological applications
Mechanical displacement:
Gravitational wave detection

18. TES readout
Transition Edge Sensor
TESs are cryogenic energy sensor suitable to detect
radiation from millimeter-wave to γ-ray
NEP can be in the order of W/√Hz
Squid readout is needed in order to reach such a low NEP
Standard single TES readout
TES is voltage biased
Electro Thermal Feedback (ETF) (negative)
SQUID readout (sensitivity, working temperature)

19. SQUID Multiplexing
Applications with large mosaic array of TESs
Problem with heat load and complexity of the cryo-harness
Multiplexing
Signals are combined at low temperature and then separated
at room temperature
Two main multiplexing schemes
Time Division Multiplexing (TDM)
Frequency Division Multiplexing (FDM)

20. Time Division SQUID Multiplexing
TDM Multiplexing
Time Division SQUID Multiplexing
(developed at NIST)
TESs are dc biased
SQUIDs are turned on and off sequentially for each row
Drawback Many SQUIDs

21. Frequency Division SQUID Multiplexing
FDM multiplexing
Frequency Division SQUID Multiplexing
TESs are AC biased (same frequency for every raw)
One SQUID reads one column
Drawback Very demanding performance for FLL SQUID dinamycs

22. Microstrip SQUID Amplifier
Pushing toward Higher Frequencies
SQUID applications with the FLL scheme are operating in the frequency range
from DC to few MHz
Operating the SQUID in “open-loop” mode, would increase the high frequency cut-off
At frequencies above 100–200 MHz, however, parasitic capacitance between the washer
and input coil drastically reduces the gain .
This problem can be overcome by operating the coil and washer as a microstrip resonator
Conventional SQUID Amplifier
Microstrip SQUID Amplifier
Signal connected to both ends of coil
Signal connected to one end of the coil and SQUID washer
The other end of the coil is left open
Michael Mück, Christian Welzel, and John Clarke, Applied Physics Letters 82, 3266 (2003);
doi: /

23. Microstrip SQUID Amplifier
With Microstrip SQUID Amplifier, frequency detection is pushed up to GHz range
This solution has been adopted in the “Axion Dark Matter Experiment (ADMX) “

24. Conclusions Conclusions
SQUID measures any quantity convertible into magnetic flux
At low temperatures, the resolution of SQUID amplifiers is
essentially limited by Heisenberg’s Uncertainty Principle
SQUIDs are remarkably broadband: from DC to 109 Hz
SQUIDs may make contribution to the search for axions,
both in the TES reading and in the direct detection of microwave photons