An Update on Google's Quantum Computing Initiative

An Update on Google’s Quantum Computing Initiative

January 31, 2019 [email protected]

Our Brains are Wired for Newtonian Physics

Brains that recognize and anticipate behaviors of Heat, Light, Momentum, Gravity, etc. have an Evolutionary Advantage.

Quantum phenomena contradict our intuition.

Quantum Phenomena Contradict Intuition

Interference, “Erasure”, etc.

0

(₁)

Quantum Theory Explains Cleanly…

...but the Math looks Strange

1

i

1
√2
1
√2

1 i i 1

How can a Particle be On Two Paths at the Same Time?

1
1

i

√2

P_u

1
√2

1 i i 1

(_P)

r

Superposed States, Superposed Information

l0〉

( |0〉+|1〉 )²
= |00〉+|01〉+|10〉+|11〉
|0〉 + |1〉

l1〉

01

Macroscopic QM Enables New Technology

Control of single quantum systems, to quantum computers

1 nm

1 μm

1 mm

H atom wavefunctions:

Problem: Light is 1000x larger

Large “atom” has room for complex control

Xmon: Direct coupling + Tunable Transmons

● Direct qubit-qubit capacitive coupling ● Turn interaction on and off with frequency

g

control

“OFF”

“ON”

f₁₀f₂₁

Δ

η

Coupling g

SQUID

Qubit

Coupling rate Ω_zz≈ 4ηg²/ Δ²

Qubit A

Qubit B

Logic Built from Universal Gates

Classical circuit:

Quantum circuit:

1 qubit rotation 2 qubit CNOT No copy
1 bit NOT 2 bit AND Wiring fan-out
1 Input Gates
2 Input Gates

(space+time )

time

Execution of a Quantum Simulation

arXiv:1512.06860

Quantum Simulation Results, H₂Molecule

Control

Qubits

arXiv:1512.06860

Office of the CTO

Space-Time Volume of a Quantum Gate Computation

2 qubit gate fidelity = 99.5%

Uncorrected Gate “Circuits” Limited by Fidelity of Operations and Decoherence Times

Fidelity is the Third Dimension

Gate Depth

Goals for Near-Term Scaling

10^-2

Error correction threshold

∗

“Quantum Supremacy”

10^-310^-4

Useful error corrected QC

Near-term applications

Classically simulatable

✔

✘

?

10⁰

10¹

10²

10³

10⁴

10⁵

10⁶

10⁷

10⁸

Number of Qubits

Which computer is better at landing samples the “Bright Spots” ?

Quantum “Supremacy”

Do what classical CPUs Cannot do:

2ⁿ

index k

0

Ideal distribution

Multiple errors

>50 Qubits, >40 Steps

2ⁿ

ordered index k

0

Toward Universal Fault-Tolerant QC

● Qubit error rates ~10^-2-10^-3per operation

● Universal QC requires ~10^-10● Error correction:
○ Low error logical qubit made with many physical qubits
● Surface code error correction:
○ 2D array of qubits (n.n. coupling) ○ Modest error rates (1% threshold,
0.1% target)
○ Useful at 10⁶physical qubits

9 Qubit: Good performance, Limited Scaling

9 qubit device has good performance
● Err_CZdown to 0.6% ● Err_SQ< 0.1% ● Err_RO= 1% Limited to 1D connectivity (planar geometry)

Scale-up strategy: move qubits, control to different planes

Bump-Bond Architecture

● Bond together two separate chips
○ Qubits → “Chip”

“Chip” bumps

○ Control → “Carrier”

“Carrier”

● Superconducting interconnect ● Use lossless vacuum as dielectric

In TiN Al

30 μm

Qubits (chip)

Control (substrate)

Scaling to 2D

Design must be “tileable” (control fits in qubit footprint)
● Readout resonator ● XY coupler ● SQUID coupler

Need to shield qubits from interior wire routing
● Small coupling to 50Ω line will decohere qubit

“Foxtail” 22 Qubit Device

“Chip”
● Qubits

“Carrier”
Readout XY control Z control
●●●

2D Unit Cell

● Diagonal for surface code: all “measure” qubits on same line
● Condense footprint across 2 chips ● Introduce shielded wiring between qubits ● Tile unit cell for 2D array Unit cell: Condensed, diagonal linear chain

Readout line

RO XYZ
RO XYZ
RO XYZ

RO

XYZ
RO XYZ

rotate

XYZ

Unit cell designed for surface code

“Bristlecone” Architecture

Tile
12 unit cells of 6 qubits = 72 qubits

Tile for a 2D grid of n.n. coupled qubits
Bonus: Looks like a pine cone!

Photo: Erik Lucero

“72 qubits cold in fridge”