Quantum Computer Building Blocks Paola Cappellaro Quantum 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 approach 1. Use small quantum building blocks 2. Perfect their control 3. 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 memory 2.quantum actuators to interface the registers with the classical controllers 3.quantum wires to connect the registers

P. Cappellaro — E LECTRON - NUCLEAR SPIN QUANTUM REGISTERS

P. Cappellaro — Electronic spin surrounded by nuclear spins Hyperfine interaction creates a local field B hyp – Close-by nuclei quantized in B hyp  dephasing – Farther away nuclei create a fluctuating field:  electron spin decoherence B hyp Quantum registers Closest nuclei have distinct frequency  qubits

P. Cappellaro — Isolated electronic spin – From 2 unpaired electrons  Spin 1 Optically active – Single – 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! 3A23A2 1A11A1 3E3E

P. Cappellaro — Optical initialization & readout Ground state – Control via ESR – Effective qubit Complex spin environment – Nuclear spins: N, 13 C – Epr impurities (nitrogens) Close-by nuclear spins  quantum register NV centers: spin control

P. Cappellaro — A CTUATOR C ONTROL

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 universal control – Need strong anisotropy – In m s =1, faster rotation than rf pulse – Good isolation of the registers Electronic spin-only Control m s =0 m s =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. power t

AWG MW Signal source IQ Pulse Blaster 50Ω Power detector Oscilloscope Spectrum analyzer Directional Coupler Switch IQ Mixer Pre-Amp AmplifierCirculatorLoad 50Ω Mixer RF Control Line Sample rf-Switchrf-Amplifier rf -Circulator Load Pre-Amp Filter Microwave Control Setup

P. Cappellaro — Clarice Aiello Masashi Hirose Ashok Ajoy Honam Yum Alex Cooper Gurneet Kaur Thanks! Martin Goycoolea Jonathan Schneider Gary Wolcowitz

P. Cappellaro — Funding AFOSR YIP NSF CUA xQIT (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: