Materials Science in Quantum Computing

Materials scientist view of qubit Materials –SiOx sub substrate –Superconductor (Al,Nb) –SiO x dielectric –Al0 x tunnel barrier

The quantum computing challenge superconductors

Superconducting Josephson junction phase qubit principle Tunnel Junction ~1.5 nm Top superconductor Bottom superconductor I  top =  0  bot =  0 e i  Cooper pair wavefunction Josephson relations I depends on  Voltage only when phase is changing

Electrical circuit model of Josephson junction I = CJCJ L J ~1/cos  Potential: , V across junction will slosh around in minimum cos  is x 2 potential => simple harmonic oscillator Need to change potential to get two state system

Quantum behavior - potential that phase qubit lives in Increase bias => cubic potential lifts degeneracy Use the |0> and |1> states for information

Insert qubit pic here Qubit L Stripline (C-SiO 2 ) Josephson Junction (L&C) => Measure “Q” of LC resonators Qubit has SiO 2 Cap in || with J.J. SiO 2 AlO x

Power dependence to parallel plate capacitor resonators  wave resonator L f [GHz] P out [mW] P in lowering Q of the resonator goes down as power decreases! Parallel plate capacitor resonators w/SiO dielectric C L

Room-temperature deposited SiO2 over the capacitor C/2 L Data with and without SiO2 on Cap Dissipation is in SiO2 dielectric of the capacitor! ~P out Interdigitated capacitor resonators

Power dependence of Q LC for parallel plate capacitors HUGE Dissipation Q decreases with at very low power (where we run qubits) N photons Q LC Explains small T 1 ! L C

|E| [V/m] 1/loss tangent Spin (TLS) bath: saturates at high power, decreasing loss high power SiO x (amorphous) Schickfus and Hunklinger, Loss “saturates” from each TLS P abs ~ number saturated ~ d*E Solve Bloch eqn’s for bath  : Theory: Why 1/E Dependence? Uncompensated spins in SiO x E d

~T R SiO2 =2.1k  Temperature Dependence of Q Q also decreases at low temperature!

Problem - amorphous SiO 2 Why short T 1 ’s in phase Josephson qubits? Dissipation: Idea - Nature: At low temperatures (& low powers) environment “freezes out”: dissipation lowers dissipation increases, by 10 – 1000! Change the qubit design:  single crystal sapphire substrates  SiN dielectric & minimize dielectric in design

SiN/sapphire: Significant improvement in T 1, T 2 * 0 Time (  s) 2 0 Time (  s) 1 P(1) 0.4 P(1) T 1 increased to nearly 600 ns T 2 * nearly 300 ns Still need to deal with low fidelity => junctions Do spectroscopy on qubits 1 1 1

Qubit spectroscopy Increase the bias voltage (tilt) Frequency of |0> => |1> transition decreases Resonances Increase bias Resonance density increases with junction size

Microscopic two-state fluctuators in junction Amorphous AlO tunnel barrier Continuum of metastable vacancies Changes on thermal cycling Origin? uncompensated spins in barrier O atoms tunneling between sites

Resonators must be 2 level, coherent with qubit! qubit - |0> or |1> res. - |g> or |e> |0e> |0g> 0 |1e> |1g> On resonance  E=0 Anti-crossing, splitting S |1e> |1g> EE Off resonance

Qubit-resonator coupled interaction Off resonance - |1> decays On resonance, put qubit in |1> Wait some time,  int /2 Qubit goes into |0> =>Wait  int  Qubit goes back to |1> with enhanced amplitude!  States oscillate |1g> |0e> Resonator has longer coherence time than qubit off on

What we need: Crystalline barrier  -Al 2 O 3 ? Interfaces: Smooth Stable No dangling bonds Poly - Al Existing technology: Amorphous tunnel barrier a -AlO x Rough interfaces Unstable at room temp. Dangling bonds No spurious resonators Stable barrier Amorphous Aluminum oxide barrier Spurious resonators in junctions Fluctuations in barrier Silicon amorphous SiO 2 dangling bonds at interface Low loss substrate/dielectric : SiN Re-design tunnel junctions SC bottom electrode Top electrode

Q: Can we prepare crystalline Al 2 O 3 on Al? Binding energy of Al AES peak in oxide Annealing Temp (K) AES Energy of Reacted Al (eV) Al in sapphire Al Metallic aluminum Aluminum Melts Å AlO x on Al (300 K + anneal) 10 Å AlO x on Al (exposed at elevated temp.)  Anneal the natural oxides  Oxidize at elevated temp. A: No

Chose bottom superconducting electrode to stabilize crystalline Al 2 O 3 or MgO tunnel barrier Elements with high melting temperature

Elements with T C > 1K

Elements that lattice match sapphire (Al )

Elements that form weaker bond with oxygen than Al

Elements that are not radioactive

LEED, RHEED, AES Re Sputtering Load Lock STM Ex-situ AFM Al Oxygen O2O2 Al 2 O 3 growth: Al thermal deposition under O 2 exposure on top of base Epi Re. UHV system: P base < 5x Torr

1.5 nm RMS roughness 1-2 atomic layer steps Screw dislocations on mesas Stranski-Krastanov growth –Initial wetting of substrate Formation of 3-d islands –Islands fill in gradually 0.5 x 0.5  m 100 nm Re Base 850 C on sapphire

Epi Re Grow Al 2 O RT ◦ C 4x10 -6 Torr O 2 3m3m Single crystal Al 2 O 3 on Re(0001) Re(0001) Al 2 O 3

Fabricate test junctions with epi-Al 2 O 3 barrier First high quality junctions made with epitaxial barrier!! Re(0001) Al 2 O 3 Al 1/R vs. Area at 300 K V(mV) I-V curve at 20 mK ReRe AlAl

Conclusions Amorphous dielectrics can have HUGE loss due to two level system (spin bath) –Problem with phase qubit: Loss in dielectric – Fix by using SiN dielectric Tunnel junctions have coherent two state systems that are detrimental to the fidelity of the measurements Materials optimization is critical to long coherence times Status –Testing qubits w/epi-barriers –Eliminating/improving dielectrics around qubit 1