Quantum ComputerBuilding Blocks

Paola CappellaroQuantum Engineering Group - MIT

P. Cappellaro —

The approach to QIP

• Challenges in quantum information– Engineer a scalable quantum system– Control a large quantum system…– …before decoherence ruins it

• Bottom-up approach1. Use small quantum building blocks2. Perfect their control3. Combine them in a modular way

P. Cappellaro —

Distributed quantum computing• Modular, hybrid architecture for quantum computing

1. quantum registers for simple algorithms and local memory2. quantum actuators to interface the registers with the

classical controllers 3. quantum wires to connect the registers

P. Cappellaro —

ELECTRON-NUCLEAR SPINQUANTUM REGISTERS

P. Cappellaro —

• Electronic spin surrounded by nuclear spins• Hyperfine interaction creates a local field Bhyp

– Close-by nuclei quantized in Bhyp dephasing

– Farther away nuclei create a fluctuating field: electron spin decoherence

Bhyp

Quantum registers

Closest nuclei have distinct frequency qubits

P. Cappellaro —

• Isolated electronic spin– From 2 unpaired electrons Spin 1

• Optically active– Single electronic spin observed by fluorescence

Nitrogen-Vacancy (NV) centers

Confocal Microscope

P. Cappellaro —

NV centers: optical control

• Spin-selective fluorescence– Spin state read-out

Fluorescence

P. Cappellaro —

NV centers: optical control

• Spin-selective fluorescence– Spin state read-out

Fluorescence

P. Cappellaro —

NV centers: optical control

• Spin-selective fluorescence– Spin state read-out

200 ns

Fluorescence

P. Cappellaro —

NV centers: optical control

• Spin-selective fluorescence– Initialization to ground

state

Effective T ~ mK

at room temperature!

3A2

1A1

3E

P. Cappellaro —

• Optical initialization & readout• Ground state– Control via ESR– Effective qubit

• Complex spin environment– Nuclear spins: N,13C– Epr impurities (nitrogens)

• Close-by nuclear spins quantum register

NV centers: spin control

P. Cappellaro —

ACTUATOR CONTROL

P. Cappellaro —

• Quantum register– Electronic qubit for initialization and communication – Nuclear qubits for memory and error correction

• Only need 2 types of logic gates:

1) Single qubit gate 2) Controlled gate on electronic spin on nuclear spins

• All other gates can be built from these two

Gates for quantum registers

P. Cappellaro —

• Use electron as an actuator– Inducing nuclear rotation about 2 axis

– Can obtain universalcontrol

– Need strong anisotropy– In ms=1, faster

rotation than rf pulse– Good isolation of the registers

Electronic spin-only Control

ms=0 ms=1

Electron pulse

P. Cappellaro —

Advanced techniques• Addressing individual nuclei with rf field– Limits on nuclear Rabi frequency long pulse

times– Off-resonance modulation, pulse errors, spin

couplings, …• Tools– Composite pulses– Numerically optimized pulses

r.f. p

ower

t

AWG

MW Signal source

IQ

Pulse Blaster

50Ω

Power detector

Oscilloscope

Spectrum analyzer

Directional Coupler

SwitchIQ Mixer

Pre-AmpAmplifier Circulator Load

50ΩMixer

RF Control Line

Sample

rf-Switch rf-Amplifier rf -Circulator Load

Pre-Amp

Filter

Filter

Microwave Control Setup

P. Cappellaro —

Clarice Aiello

Masashi Hirose

Ashok Ajoy Honam Yum

Alex Cooper

Gurneet Kaur

Thanks!

MartinGoycoolea

Jonathan Schneider

Gary Wolcowitz

P. Cappellaro —

Funding

AFOSR YIPNSF CUAxQIT (Keck Foundation)

Publications F. Ticozzi, R. Lucchese, P. Cappellaro, L. Viola, "Hamiltonian Control of Quantum Dynamical Semigroups: Stabilization and Convergence Speed" To appear in IEEE Transaction on Automatic Control, arXiv:1101.2452