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 DiVincenzo s 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 O Brien et al Smart Mater Struct 11 741 2002 Spin State Read out with SET s Fabrication of Control Gates Read out Spin state Charge state e g measurement by SET Read out Challenges i SET s 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 SET s 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 1 m 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 won t 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 http www ITRS net Thank You Thank You