Quantum Computer Building Blocks

1. Quantum ComputerBuilding Blocks Paola CappellaroQuantum Engineering Group - MIT

2. 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 • Use small quantum building blocks • Perfect their control • Combine them in a modular way

3. Distributed quantum computing • Modular, hybrid architecture for quantum computing • quantum registers for simple algorithms and local memory • quantum actuators to interface the registers with the classical controllers • quantum wires to connect the registers

4. Electron-nuclear spinquantum registers

5. Quantum registers • 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 Closest nuclei have distinct frequency qubits

6. Isolated electronic spin From 2 unpaired electrons  Spin 1 Optically active Single electronic spin observed by fluorescence Nitrogen-Vacancy (NV) centers

7. Confocal Microscope

8. NV centers: optical control • Spin-selective fluorescence • Spin state read-out Fluorescence

9. NV centers: optical control • Spin-selective fluorescence • Spin state read-out Fluorescence

10. NV centers: optical control • Spin-selective fluorescence • Spin state read-out Fluorescence 200 ns

11. NV centers: optical control • Spin-selective fluorescence • Initialization to ground state Effective T ~ mK at room temperature! 3E 1A1 3A2

12. 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

13. Actuator Control

14. Gates for quantum registers • 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

15. Electronic spin-only Control • 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 Electron  pulse ms=0 ms=1

16. 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

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

18. Alex Cooper Gary Wolcowitz Clarice Aiello Masashi Hirose Thanks! Honam Yum Ashok Ajoy GurneetKaur Jonathan Schneider Martin Goycoolea

19. 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:1101.2452