Silicon Quantum Computers -

Silicon Quantum Computers -

Silicon-based Quantum Computation C191 Final Project Presentation Nov 30, 2005 Cheuk Chi Lo Kinyip Phoa Dept. of EECS, UC Berkeley Silicon-based Quantum Computation: Presentation Outline I. II. III. IV.

Introduction Proposals for Silicon Quantum Computers Physical Realization: Technology and Challenges Summary and Conclusions Introduction: Why Silicon? We know silicon from years of building classical computers Donor nuclear spins are wellisolated from environment low error rates and long decoherence time

Integration of quantum computer with conventional electronics Scalability advantages? Introduction: DiVincenzos Criteria 1. Well-defined qubits 2. Ability to initialize the qubits 3.

Long decoherence time 4. Manipulation of qubit states 5. Read-out of qubit states 6. Scalability (~105 qubits) II. Overview of Silicon Quantum Computation Architectures Silicon Quantum Computer Proposals Shallow Donor Qubits

Deep Donor Qubits Electron Shuttling Exchange Coupling Magnetic Dipolar Coupling Silicon-29 Qubits Silicon Shallow Donor Qubits: Qubit Definition and State Manipulation Spin Resonance Control gate BAC BDC

J-Gate (Exchange Coupling) A-Gate (Hyperfine Interaction) barrier Silicon-28 Qubit magnetic dipolar coupling BE Kane, Nature, 393 14 (1998) AJ Skinner et al, PRL, 90 8 (2003) R de Sousa et al, Phys Rev A, 70 052304 (2004)

S-Gates (Electron shuttling) Summary of Silicon Shallow Donor Qubits Qubit: donor nuclear spin or hydrogenic qubit (nucleus + electron spins) Initialization: Recycling of nuclear state read-out + nuclear spin-state flip via interaction with donor electron Decoherence time: e.g. at 1.5K Qubit Manipulation

nucleus spin T1 > 10 hours electron spin T1 > 0.3hours Single Qubit Manipulation: hyperfine interaction + spin resonance Multi-qubit Interaction: Exchange coupling, Magnetic dipolar coupling or Electron shuttling Read-out: Transfer of nucleus spin state to donor electron via hyperfine interaction, then read-out of electron spin state Physical Realization of a Si QC Some common features that must be realized in a shallow donor Si QC are: Array

of single, activated 31P atoms: Single-spin state read-out: Integrated control gates Process Variations Formation of Ordered Donor Arrays Top-down single ion implantation T Schenkel et al, APR, 94(11) 7017 (2003) Bottom up STM based Hydrogen Lithography JL OBrien et al, Smart Mater. Struct., 11 741 (2002) Spin-State Read-out with SETs & Fabrication of Control Gates Read-out: Spin state Charge state (e.g. measurement by SET) Read-out Challenges: i. SETs are susceptible to 1/f and telegraphic

noises (from the random charging and discharging of defect/trap states in the silicon host) ii. alignment and thermal budget of SETs with the donor atom sites also present as a fabrication challenge. (UNSW) Control Gate Challenges: Qubit-qubit spacing requirements for different coupling mechanisms: Exchange Coupling: 10-20nm Magnetic Dipolar Coupling: 30nm Electron Shuttling: >1m State-of the art electron beam lithography: can do ~10nm, but not dense patterns Qubit interaction control gates extremely challenging!

(L Chang, PhD Thesis, EECS) Process Variations Process Variations may arise from: i. substrate temperature gradient, ii. uneven reagent use during fabrication, iii. differences in material thermal expansion iv. strain induced by the patterning of the substrate (leads to uncertainty in ground state donor electron wavefunction, due to incomplete mixing of states) Consequences: i. Need careful tuning and initialization of qubits

ii. Limit of scalability? iii. Introduce strain in silicon intentionally? lifts degeneracy of electronic state less vulnerable to process variations (IBM) Silicon Deep Donors Proposal Excited State Bi Er Bi Optical Excitation

Ground State Bi Bi Er Bi AM Stoneham et al, J. Phys.: Condens. Matter, 15 (2003), L447 Er Bi Initialization, Manipulation and Readout?

Initialization by polarized light or injection of polarized electron Manipulation with microwave pulses like the work by Charnock et. al. on N-V centers in diamond Readout optically both are not very possible under room temperature

detection of photons emitted potentially require detection of single photon Disorderness of donor ion Irreproducibility and difficult to address qubits Decoherence Time and Thermal Ionization Summary of Silicon Deep Donor Qubits

Qubit: deep donor (e.g. Bismuth) nuclear spin, proposed to work at room temperature. Initialization: Optical pumping or injection of polarized electron, questionable in feasibility. Decoherence time: fraction of nanosecond at room temperature Qubit Manipulation: via applying intense microwave pulse, like N-V centers in diamond Read-out: optical readout of photon emitted from transition between two states Silicon-29 Quantum Computer Overview Manipulating qubits by Dysprosium (Dy) magnet

Readout using MRFM CAI Initialize with circularly polarized light NMR-type quantum computer TD Ladd et. al. , PRL, 89(1) 017901, 2002 Decoherence Times Long decoherence time (T1 and T2) Reported T1 as large as 200 hours, measured in dark Experimentally find T2 as long as 25 seconds

T2 is reduced by the presence of 1/f noise due to the traps at lattice defects and impurities Summary of Silicon NMR quantum computer Qubit: Chains of silicon-29 isotope for ensemble measurement Initialization: Optical pumping with circularly polarized light Decoherence time: measured as long as 200 hours in dark at 77K for T1 but only 25 seconds for T2

Qubit Manipulation: combination of static magnetic field and RF magnetic field Read-out: with cantilever, performing MRFM CAI Problem: RF Coil, Dy Magnet & MRFM The deposition method of Dy magnet is not outlined! It wont be trivial to incorporate The cantilever tip for MRFM is not included in the schematic. How to insert it? TD Ladd et. al. , PRL, 89(1) 017901, 2002 Summary and Conclusions

Several proposals for implementing quantum computer in silicon Shallow donor (phosphorus) qubit Deep donor (bismuth) qubit Silicon-29 NMR quantum computer Difficulties faced in each proposals Arguments on the feasibility Most experimental efforts are on shallow donor qubits Convergence with conventional electronics processing requirements: Currently: 90nm technology node (~45nm features) 22nm technology node in 2016! Strained-silicon: hot topic of research in semiconductor industry Narrower transistor performance window with ordered dopants Single-electron transistors and other nanoelectronics (

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