Quantum Computing and the Technical Vitality of Materials Physics

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1 Quantum Computing and the Technical Vitality of Materials Physics David DiVincenzo, IBM SSSC, 4/2008

2 (list almost unchanged for some years) Physical systems actively considered for quantum computer implementation Liquid-state NMR NMR spin lattices Linear ion-trap spectroscopy Neutral-atom optical lattices Cavity QED + atoms Linear optics with single photons Nitrogen vacancies in diamond Electrons on liquid He Small Josephson junctions charge qubits flux qubits Spin spectroscopies, impurities in semiconductors & fullerines Coupled quantum dots Qubits: spin,charge,excitons Exchange coupled, cavity coupled

3 Electron spins in quantum dots Eriksson Group Wisconsin Top-Gated Quantum Dots 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)

a) Details of Marcus-group structures and

operation -- charging honeycomb, termination at

5 mm 1 b) -500 V 6 (mv) -5 0 5 (2,0) (1,0) (0,0) dg

) (2,1) (2,2) (1,1) (1,2) (0,1) (0,2) c) V P 6 7-20

4 a) Details of Marcus-group structures and measurements -- full electric circuit, pulsed operation -- charging honeycomb, termination at empty dots mm 1 b) -500 V 6 (mv) (2,0) (1,0) (0,0) dg S /dv 6 (a.u.) (2,1) (2,2) (1,1) (1,2) (0,1) (0,2) c) V P db (1,1) (1,0) G S (1,1) (1,2) -20 db V P d) V 2 (mv) P -400 V 2 =25 mv t=10 ms (0,1) (0,0) -400 (0,1) (0,2) V 6 (mv) DV

5 Cartoon of double quantum dot Electrons coupled, Exchange coupled to thousands of nuclear spins

6 Spin echo experiment

7 Si/SiGe Heterostructures Eriksson Group Wisconsin Schematic Conduction Band Heterostructures grown by Don Savage

8 Coulomb blockade in Schottky-gated Si/SiGe quantum dots Eriksson Group Wisconsin Ohmic Contacts: Au/Sb(1%) Schottky Top Gates: Pd 0.25 I (na) V G (V)

9 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)

10 Yale Josephson junction qubit Nature, 2004 Coherence time again c. 0.5 λs (in Ramsey fringe experiment) But fringe visibility > 90%!

EM field resides in the CPW slots Day et.

11 Shwetank Kumar, IBM Excess noise Where from? Co-Planar Waveguide Geometry Resonator Geometry Nb (200 nm) Si HFSS- FEM Simulation Most of the EM field resides in the CPW slots Day et. al (Nature, 2003) Crystalline substrate thin native oxide layer Oxidised metal surface TLS in glassy interface interact with E field cause phase noise! (Gao et. al. 2007, Martinis et. al. 2005)

12 Excess Phase Noise Shwetank Kumar, IBM 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 text Z 0 Z 0 text Cc I/P µwave L R C O/P µwave S S 21 ( δx ) = G 21 min = 1 Q S r 21 min 1 + / Q c + 2 jq rδx, δx = 2 jq rδx, Q 1 r = Q 1 i + Q 1 c f f r f r

13 Q factor vs Temperature Shwetank Kumar, IBM f r = 4.35 GHz, Q c = 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 PD John Martinis Andrew Cleland Ken Cooper (JPL) Robert McDermott (UW) Matthias Steffen (IBM) Eva Weig (LMU) Nadav Katz (HU) Haohua Wang Max Hofheinz

14 High-Fidelity Josephson Qubits UC Santa Barbara PD John Martinis Andrew Cleland Ken Cooper (JPL) Robert McDermott (UW) Matthias Steffen (IBM) Eva Weig (LMU) Nadav Katz (HU) Haohua Wang Max Hofheinz GS Markus Ansmann Matthew Neeley Radek Bialczak Erik Lucero Aaron O connell James Wenner Daniel Sank 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

15 Physical Decoherence: Where s the Problem? Capacitors Inductors & Junctions Circuits Energy ev resonator 2 4T c D. of States Superconductors: Gap protects from dissipation X-tal or amorphous metal Protected from magnetic defects Good circuit design (µwave engineering.) (X-tal) (amorphous) Many low-e states Only see at low T

16 Qubit Improvements: Understanding Atomic TLS s 1 P I1> 40% T 1 = 40 ns Al ?t Rabi [ns] % T 1 = 500 ns Al 2 O 3 wafer SiO 2 SiN x Dielectric Al IL> IR> E % T 1 = 470 ns T F ~ 300 ns Small junction + external Cap. I J 0

17 TLS Defects and Dielectric Loss a-oxides have large loss, δ i ~ 10-3 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 S Q ~ δ i (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) δ i ~ from surface oxide

Dielectric Loss in CVD SiO 2 P in C P out HUGE Dissipation L P out [mw] 10-7 10-8 10-9 10-10 10-11 10-12 Q T =

18 Dielectric Loss in CVD SiO 2 P in C P out HUGE Dissipation L P out [mw] Q T = 25 mk P in lowering Im{ε}/Re{ε} = δ = 1/Q f [GHz] <V 2 > 1/2 [V]

19 Theory of Dielectric Loss E Amorphous SiO 2 Two-level (TLS) bath: saturates at high power, decreasing loss von Schickfus and Hunklinger, 1977 high power Im{ε}/Re{ε} = δ = 1/Q <V 2 > 1/2 [V]

20 Junction Resonances: Dielectric Loss at the Nanoscale µwave frequency (GHz) µm 2 S/h 13 µm 2 qubit bias (a.u.) N/GHz (0.01 GHz < S < S') µm 2 avg. 5 samples: 13 µm splitting size S' (GHz) theory New theory (Martin et al, Martinis et al): 2-level states (TLS) e. d Al AlO x Al 1.5 nm 2 2 1/ d N [ 1 ( S / Smax ) ] = σa deds S d S = 2 max 2 E10e / 2C 1.5nm Explains sharp cutoff d=0.13 nm (bond size of OH defect!) 2 S max in good agreement with TLS dipole moment: Charge fluctuators at ~10 GHz explain resonances

21 Wineland group, NIST boulder

22 Errors and error correction in trapped ion systems D. J. Wineland, Dec. 07

23 Fig. 1. (A) The energy level structure of the NV center L. Childress et al., Science 314, (2006) Published by AAAS

24 Fig. 3. (A) Spin-echo revival frequency as a function of magnetic field amplitude L. Childress et al., Science 314, (2006) Published by AAAS

Josephson phase qubit circuit for the evaluation of advanced tunnel barrier materials *

Qubit circuit for advanced materials evaluation 1 Josephson phase qubit circuit for the evaluation of advanced tunnel barrier materials * Jeffrey S Kline, 1 Haohua Wang, 2 Seongshik Oh, 1 John M Martinis