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Real Quantum Computers

Real Quantum Computers. Sources Richard Spillman Mike Frank Julian Miller Isaac Chuang, M. Steffen, L.M.K. Vandersypen, G. Breyta, C.S. Yannoni, M. Sherwood. Requirements for quantum computation. 1. Robust representation of quantum information

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Real Quantum Computers

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  1. Real Quantum Computers Sources Richard Spillman Mike Frank Julian Miller Isaac Chuang, M. Steffen, L.M.K. Vandersypen, G. Breyta, C.S. Yannoni, M. Sherwood

  2. Requirements for quantumcomputation • 1. Robust representation of quantum information • 2. Perform universal family of unitary transformations • 3. Prepare a fiducial initial state • 4. Measure the output result

  3. Outline • Hurdles to building quantum computers • Decoherence • Error Correction • Requirements for workable quantum computers • NMR quantum computers • Other quantum computers

  4. Why build a quantum computer? Where is the market? Banks? Military? Security agencies? Physicists? Simulation of quantum systems for drug design?

  5. Why not build a quantum computer?

  6. Implications of building a quantum computer

  7. Why is building a quantum computer so difficult?

  8. Physical implementation • Ion traps, 2 & 3 qubit systems • Nuclear spins in NMR devices, 4 (5?, 6?) qubits • So far: very few qubits, impractical • A lot of current research • Two 9Be+ Ions in an Ion Trap • Wineland’s group, NIST

  9. Main Contenders • 1. NMR (nuclear magnetic resonance), invented in the 1940's and widely used in chemistry and medicine today • 2. Ion traps - single atoms • 3. Optical lattices • 4. Quantum dots • 5. Electrons on liquid helium etc.

  10. Quantum Technology Requirements for Physical Implementation Quantum Technology Requirements[Di Vicenzo ‘01] • 1. A Scalable physical system with well-characterized (well-defined) qubits • 2. An ability to initialize the system to Initializable to a pure basis states such as 00…0 • 3. Relatively long decoherence time, longer than the gate operation times. • 4. “Universal” set of quantum gates • 5. Qubit-specific measurement capability • Ability to faithfully communicate qubits

  11. Di Vincenzo Criteria Additional Di Vincenzo Criteria

  12. Decoherence • Quantum computations rely on being able to operate on a set of qubits in an entangled/superimposed state • Allows computation on all possible inputs to a computation in parallel • Problem: Interaction of qubits with environment affects their state, causing them to not be entangled/superimposed • Can partially address this by designing computer to reduce interaction with environment, but this may make it impractical (for example, running at very low temperatures) • General result: a quantum computation can only proceed for a limited period of timebefore a measurement must be performed • Measurement forces the system into a more-stable classical state • Measurement destroys superposition • System limited by ratio of decoherence time to operation latency

  13. Decoherence

  14. Decoherence

  15. Decoherence

  16. How decoherence happens

  17. Decoherence-related Figure of Merit

  18. Quantum Computer uses a single molecule • Protons and Neutrons have spin. • In a normal atoms these spins cancel out. • In isotopes there are extra neutrons. • These extra neutrons create a net positive or negative spin in an atom.

  19. Spins and Coherence • Most advanced demonstrated technology for quantum computation • Use nuclei with spin ½ as qubits • Spin straight up = |0> • Spin straight down = |1> • Other directions indicate superpositions of |0> and |1> • Long coherence times (seconds) • Electron spins (alternate technology) have coherence times of nanoseconds • In a magnetic field, spin direction precesses about the field’s axis at a rate that is proportional to the field strength

  20. Quantum Computer uses a single molecule • A nearly ideal physical system that can be used as quantum computer is a single molecule, in which nuclear spins of individual atoms represent qubits. • Using NMR techniques, these spins can be manipulated, initialized and measured. • The quantum behavior of the spins can be exploited to perform quantum computation; for example, the carbon and hydrogen nuclei in a chloroform molecule (as shown) represent two qubits. Single molecule or ensamble?

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