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

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

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

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

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)

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

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

dc-SQUID working principles

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)

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

Coupled energy sensitivity P. Carelli et al; Appl. Phys. Lett. 72, 115 (1998); doi: http://dx.doi.org/10.1063/1.121444

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  10...50 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:

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

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

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. 57 406-408 (1990).

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

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

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 10-19 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)

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)

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

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

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: 10.1063/1.1572970

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) “

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