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Quantum Computing and the Technical Vitality of Materials Physics

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Quantum Computing and the Technical Vitality of Materials Physics Quantum Computing and the Technical Vitality of Materials Physics David DiVincenzo, IBM SSSC, 4/2008 (list almost unchanged for some years) Physical systems actively considered for quantum computer implementation • Liquid-state NMR • Electrons on liquid He • NMR spin lattices • Small Josephson junctions • Linear ion-trap – “charge” qubits spectroscopy – “flux” qubits • Neutral-atom optical • Spin spectroscopies, lattices impurities in semiconductors • Cavity QED + atoms & fullerines • Linear optics with single • Coupled quantum dots photons – Qubits: • Nitrogen vacancies in spin,charge,excitons diamond – Exchange coupled, cavity coupled Eriksson Group Electron spins in quantum dots Wisconsin TopTop--GatedGated QuantumQuantum DotsDots • Spin up and spin down are qubit 1 and 0. • One electron per dot • Qubit rotations using ESR • Exchange enables swap operations D. Loss and D.P. DiVincenzo, Phys. Rev. A 57, 120 (1998) Details of Marcus-group structures and measurements -- full electric circuit, pulsed operation -- charging honeycomb, termination at empty dots 9 10 11 12 -5 0 5 a) b) dGS/dV6 (a.u.) (2,0) 0.5 mm (2,1) -500 (1,0) (2,2) (1,1) 8 (1,2) (mV) (0,0) 6 (0,1) (0,2) 1 V G P -550 7 S V2 P -20 dB 6 5 4 3 2 -20 dB V6 -450 V (mV) -400 c) d) 2 P V 2 =25 mV (1,1) t=10 ms (1,1)(1,0) (1,2) -520 (mV) 6 DV (0,1) -(0,1)400 V (0,0) (0,2) Cartoon of double quantum dot Electrons coupled, Exchange coupled to thousands of nuclear spins Spin echo experiment Eriksson Group Si/SiGe Heterostructures Wisconsin Schematic Conduction Band Heterostructures grown by Don Savage Eriksson Group Coulomb blockade in Schottky-gated Si/SiGe Wisconsin quantum dots Ohmic Contacts: Au/Sb(1%) Schottky Top Gates: Pd 0.25 0.20 0.15 I (nA) 0.10 0.05 0.00 -0.95 -0.90 -0.85 -0.80 V G (V) Other routes to spin-based solid state qubits P in Si Electrons on He surface Carbon CNTs II-VI semiconductors (nuclear spin story much more varied) “Yale” Josephson junction qubit Nature, 2004 Coherence time again c. 0.5 ls (in Ramsey fringe experiment) But fringe visibility > 90% ! Shwetank Kumar, IBM Excess noise – Where from? Co-Planar Waveguide Geometry Resonator Geometry Nb (200 nm) Si HFSS- FEM Day et. al (Nature, 2003) Simulation • Crystalline substrate – thin native oxide layer • Oxidised metal surface Most of the EM field resides in the CPW slots • TLS in glassy layer @ interface interact with E field cause phase noise! (Gao et. al. 2007, Martinis et. al. 2005) Shwetank Kumar, IBM Excess Phase Noise • Device performance limited by resonator intrinsic noise • Intrinsic noise of the device >> G-R noise Device (Day et. al., Nature 2004) • Noise primarily in phase direction - frequency jitter (Gao et. al., APL 2007) • Floor set by HEMT noise – amplitude direction • Phase noise > Amplitude noise • Rolls of with device bandwidth Z0 Z0 text text Cc I/P O/P mwave mwave L R C é S 21 min + 2 jQ rdx ù f - f r S 21 (dx ) = G ê ú, dx = ë 1 + 2 jQ rdx û f r -1 -1 -1 S 21 min = 1 - Q r / Q c , Q r = Q i + Q c Shwetank Kumar, IBM Q – factor vs Temperature fr = 4.35 GHz, Qc = 496,000 • Loss increases with temperature Physical picture • Temperature increases - population of higher energy state increases • More loss to phonon bath • Power dependence – saturation of TLS effects? High-Fidelity Josephson Qubits UC Santa Barbara John Martinis GS Markus Ansmann Andrew Cleland Matthew Neeley Ken Cooper (JPL) Radek Bialczak Robert McDermott (UW) Erik Lucero Matthias Steffen (IBM) Aaron O’connell PD Eva Weig (LMU) James Wenner Nadav Katz (HU) Daniel Sank Haohua Wang Max Hofheinz UCR A. Korotkov, Qin Zhang (GS), Abraham Kofman (VS) UCI C. Yu, Magdalena Constantin (PD) UG M. Geller, Emily Pritchett (GS), (Andrei Galiautdinov (PD)) NIST D. Pappas, Jeff Kline Physical Decoherence: Where’s the Problem? Capacitors Inductors & Junctions Circuits Energy eV 2D~4Tc resonator D. of States (X-tal) (amorphous) Superconductors: Good circuit design Gap protects from dissipation (mwave engineering.) Many low-E states X-tal or amorphous metal Only see at low T Protected from magnetic defects Qubit Improvements: Understanding Atomic TLS’s P 1 I1> T1 = 40 ns 40% Al 0.5 IL> E ?tRabi[ns] 0 Al2O3 wafer 0 500 1000 Dielectric SiO2 ð SiNx IR> 1 60% T1 = 500 ns Al 0.5 0 Small junction + external Cap. 1 90% T1 = 470 ns TF ~ 300 ns IJ 0.5 0 TLS Defects and Dielectric Loss -3 • a-oxides have large loss, di ~ 10 – BE CAREFULL • Consistent with 30+ years of LT physics • Predicted how to improve phase qubits • Explains spectroscopy data (size and density) • Explains loss of measurement visibility • Explains loss of Rabi amplitude (coherence) • Explains why small junctions statistically avoid TLS • Lower loss dielectrics: xtal’s or a-Si:H Lossy barriers: a-AlN, MgO (D. Pappas, NIST) • Understand magnitude of 1/f charge noise SQ ~ di (Yu and • Understand magnitude of 1/f critical-current noise Constantin) • TLS produces phase noise (C-fluctuations), theory in progress • New resonator data (J. Gao … Caltech/JPL) -5 -6 di ~ 10 – 10 from surface oxide Dielectric Loss in CVD SiO2 Pin Pout C HUGE Dissipation L T = 25 mK 1/Q = -7 10 d -8 Q P 10 in } = lowering e -9 10 ] -10 10 }/Re{ e mW [ -11 10 Im{ out -12 10 P -13 10 -14 10 -15 2 1/2 10 <V > [V] 6.02 6.03 6.04 6.05 6.06 6.07 f [GHz] Theory of Dielectric Loss E Amorphous SiO2 Two-level (TLS) bath: saturates at high power, decreasing loss = 1/Q d high } = power e }/Re{ e von Schickfus and Hunklinger, 1977 Im{ <V2>1/2 [V] Junction Resonances: Dielectric Loss at the Nanoscale ) New theory (Martin et al, Martinis et al): ' 70 mm2 10.5 70 mm2 20 avg. 5 Al samples: 2-level states . 1.5 nm (TLS) e d AlOx 13 mm2 10.0 10 Al S/h theory wave frequency (GHz) 2 2 1/ 2 m d N [1- (S / Smax ) ] N/GHz (0.01 GHz < S = sA 13 mm2 dEdS S 0 d 0.01 0.1 1 S = 2 E e2 / 2C qubit bias (a.u.) splitting size S' (GHz) max 1.5nm 10 Explains sharp cutoff d=0.13 nm (bond size of OH defect!) Smax in good agreement with TLS dipole moment: Charge fluctuators at ~10 GHz explain resonances Wineland group, NIST boulder Errors and error correction in trapped ion systems D. J. Wineland, Dec. ‘07 Fig. 1. (A) The energy level structure of the NV center L. Childress et al., Science 314, 281 -285 (2006) Published by AAAS Fig. 3. (A) Spin-echo revival frequency as a function of magnetic field amplitude L. Childress et al., Science 314, 281 -285 (2006) Published by AAAS .
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