Superconducting Photodetectors David Schuster Assistant Professor University of

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 Martinis, NIST Photon number resolving S. W. Nam, NIST Low noise LLE review vol 101 High throughput > 1 Gbps S. W. Nam, NIST

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 -60 K • Large spectral range THz - Xray • Rely heavily on microfabrication

Cascade of broken Cooper pairs 10 0 Photon hn e-e interaction • Photon breaks a cooper pair 10 -1 e. V phonons • Thermalizes making hn/D qp’s • # gain but no E gain yet e-e interaction 2 D 10 -3 Quasi particles k b. T • 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) Nb. N 4 nm thick <100 nm 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 Nb. N 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/Hz 1/2 NEP) Multiplexable (cameras) • Cryogenic operation still a limitation but getting better

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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 2 g = vacuum Rabi freq. k = cavity decay rate g = “transverse” decay rate Strong coupling: 2 g > k, g m out c 5. 2 ~ l L = Transmission line “cavity” 10 mm 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 Di. Carlo, 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) Ultracold atoms See SYHQ 2! 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 < 8 K He Levitates 8 nm above surface (in vacuum) Clean 2 DEG : Mobility = 1010 cm 2/Vs + Bare electron: meff = 1. 005 me, g = 2 <1 ppm 3 He nuclear spins e = 1. 057 a 0 = 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 mm)

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 • Electron motion couples to cavity field Cavity-electron coupling • Can achieve strong coupling limit of cavity QED • Couple to other qubits through cavity bus Predicted decay rate <10 k. Hz 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 k. Hz/m. A

Motional Decoherence Mechanisms • Relaxation through bias electrodes • Dephasing from level fluctuations • Emission of (two) ripplons • Emission of phonons dephasing relaxation Ø 10 us motional decoherence time … 10, 000 x longer than Ga. As Ø Spin coherence time predicted > 100 s

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 IV V

Detecting trapped electrons on helium Electrons No electrons

Making an eonhe transistor (eon. FET) DVgate Modulate density without losing electrons Measure density ~109 e/cm 2 (~few e/um 2)

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 m. K without much heating (~100 m. K) • Can hold them for hours Next up: Trapping single electrons Recruiting! Check out: schusterlab. uchicago. edu for more info

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Experimental Setup Pulse-tube cooled dilution refrigerator Hermetic sample holder top bottom • Indium sealing & stainless capillary • No superfluid leaks down to 10 m. K

Additional slides follow