Speaker: Sridhara Dasu, UW-Madison department of Physics
Date: 7/16/2012
Location: UW-Madison Department of Physics
The LHC probe is studying conditions of the "Electro-weak Era" a nanosecond after the Big Bang.
Collisions:
Classical picture: Momentum and Kinetic Energy are conserved.
Increase Speeds: Kinetic energy not necessarily conserved, but total energy is.
Increase speeds near the speed of light (c): E=M (take c=1); energy can be converted into mass. In motion, |P|=E^2-(p_x)^2-(p_y)^2-(p_z)^2=m^2 (invariant mass). Units are eV (electron-Volts). Currently, LHC can accelerate particles to 4 trillion eV.
Relativistic collisions: New particles produced, often much more massive than initial colliding particles.
Fields create quanta: Matter/anti-matter manifest out of vacuum. Particles exchange quannta: photons, gluons, W, Z.
Quantum electrodynamics works well for massless quanta, but W and Z are not massless. A Higgs field is needed to provide mass and complete the Standard Model of particle physics.
Higgs Field as medium: increase interaction with higgs field, increase mass.
Requires that it is self-interactive: Higgs Boson, has mass, but we cannot determine what mass would be.
7/4/12: CMS/ATLAS experiments obverseve new particle with mass approximately 125-126 GeV.
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-LHC: 2 rings driving protons in opposing directions.
-CMS: Compact Muon Solenoid. Giga-pixel Camera.
Observes photons, electrons, muons, charged hadrons (composed of quarks: pions, K, protons), neutural hadrons (neutrons, K_L). Leave electronic signals in silicon diodes in detector used to track particle.
Muons go straight through; charged hadron deposits all energy in calorimeter; photons don't ionize gaseous medium, ineract in inner part of calorimeter.
-Measuring Heavier Particles
decay near collision
collision point found with tracks
low-momentum: "underlying event"
electron/positron momentum measured in tracker; energy measure in calorimeter.
Colloquium: "Quantum computer: Dream and Realization"
Speaker: Rainer Blatt, Institute for Experimental Physics, University of Innsbruck
Date: March 19, 2010 (Watched via DVD recording July 2011)
Location: UW-Madison Department of Physics
-Moore's law: Every 18 months, computer power doubles. Faster=smaller. Can we go all the way down to a single atom?
-How many atoms per bit? Extrapolating: 2015 to 2020, if we were to follow Moore's law, we would have to do it with a single atom. Limit reached, quantum mechanics required.
-Applications in Physics and Mathematics
Factorization of large numbers can be achieved much faster on a quantum computer than with a classical computer. Classical computer: exponential time; quantum computer: polynomial time.
Fast database search: search data base with N entries. Classical computer: O(N); quantum computer: O(N^1/2)
Simulation of Schrodinger Equations
Spectroscopy: quantum computer as atomic state synthesizer.
Quantum physics with "information guided eye".
Requirements for quantum computation (Divincenzo criteria)
Scalable physical system, well characterized qubits.
Ability to intialize the state of the qubits
Long relevant coherence times, much longer than gate operation time.
Universal set of quantum gates
Measurement capability specific to implementation
Ability to interconvert stationary and flying qubits (photon, for example)
Ability to faithfully transmit flying qubits between specified locations.
Quantum bits and Quantum registers
Classical big: physical object in state 0 or 1.
Register: Bit rows 011
Qubit: Superposition of two orthogonal quantum states |psi>=c0|0>+c1|1>
Quantum register: L 2-level atoms, 2^L quantum states that correspond to number 0,...,2^L-1.
Most general state of register is the superposition
Universal Quantum Gates
Operations with single qubit: 1 bit rotations.
Operations with two qubits (like if/then decisions)
CNOT-gate operation (controlled-NOT)
analogous to XOR.
|control bit>|target bit>
|0>|0> --> |0>|0>
|0>|1> --> |0>|1>
|1>|0> --> |1>|1>
|1>|1> --> |1>|0>
Together, these two gate operations are a universal set for quantum computation.
How Quantom computing works: Start out with superposition; quantum processor can be broken down into sequence of single-qubit and two qubit operations, order found by compiler/programmer; project register to eigenbases. Output is done by reading measurements.
Realization concepts:
Ion Traps
Neutral Atoms in traps (optical traps, optical lattices, microtraps)
Neutral atoms and cavity QED
NMR (in liquids)
Superconducting qubits (charge-, flux qubits)
Solid state concepts (spin systems, quantum dots, etc.)
Optical qubits and LOQC (Linear optics quantum computations)
Electrons on L-He surfaces
Spectral hole burning
Quantum dot structures: Two atoms. Interaction given by dipole-dipole interaction. Problems: coherence time, very small distances, single spin detection not easy.
Coherent manipulation of single spins in semiconductors: Self-assembled quantum dots, or nitrogen vacancy colour centres that have two level systems that can be used. Encode quantum information in nuclear spin, but hard to place to them deliberately.
NMR in solids: Suppose you had a single phosphorous atom. Mediate nuclear spin coupling between two sites by switching electron clouds, controlled by j-gate (junction gate) by changing coupling constant and address individual sides.
10^18 moleculte; each has nuclear spins. 2 qubit gates done by field interactions. Signals and noise problems. However, quite a few algorithms have been shown, but problem remains: scalability. Add more spins, people think it is impossible to go above 12 or 14 qubits at this time, not feasible (unless you do optical pumping or produce pure states)
Ultracold atoms and optical lattices attractive because of techniques available now. Individual atoms in lattice, and manipulate them individually. Instead of using sequential gate operations, entangle large area of ions. Make measurements, depending on outcome, go to other qubits, and what remains is result of computation.
Microtrap--Rb lab--two atoms sitting side by side with cnot operations using the fact that single atom in Rydberg state prevents other atom from going into excited state.
Figured out in 2001: using photons. How to make photons interact? Take linear optical network and take measurements and take conditional operations on other qubits. Interferometer with conditional operations.
Trapped ion quantum computing
Many gate proposals available. Geometric phase gates, quantum bus, etc.
Storing and keeping quantum info requires long-lived atomic states:
L ions in linear trap. Encode qubits in narrow optical transition or groundstate Zeeman coherences. State vector of quantum computer comprised of internal degrees of freedom and direct product with center of mass motion. Assume atoms are still. Point laser to controlling ion and pulse to entangle with motion. target ion: rotate system if and only if motion in the bus.
Spectroscopy with quantized florescence (quantum jumps). Suppose single atom with three level system. Once in a while, electron shelved to state, so doesn't scatter light. High detection efficiency, know immediately what state is.
Lasers point to atom. Atom needs to be rewritten in terms of manifold of two atom systems. Light scatters and climbs down to ground state: three transitions, carrier transition; higher order transitions do not contribute because atom is so cold.
Coherent state manipulation. Two level system connected with carrier transition. Arbitrary angles theta, phi. Take coherence state manipulation of carrier. Stop after a few microseconds and have superposition. Sideband transitions entangle motion and internal excitation.
Toffoli gate; controlled-controlled NOT gate. Use 2-phonon excitation.
Molmer-Sorensen gate-operation: bichromatic laser excitation close to upper and lower sidebands induces collective state changes (spin tips). Avoids Stark shifts.
Measuring entanglement: measure the parity pi=sum from j=0 to 2 of (-1)^j P_j.
P_0=P_DD, P_1=P_SD,DS, P2=P_SS. Pi oscillates with 2phi; spectroscopic tool to measure entanglement.
Deterministic Bell states using the Molmer-Sorensen gate.
Scaling the ion trap quantum computer
more ions, larger traps, phonons carry quantum information. Slow for many ions (few 10 ions may be possible)
move ions, carry quantum information around. requires small, integrated trap structures; miniaturized optics and electronics.
Ion chip trap quantum microprocessors a worldwide effort. NIST in USA.
Scaling the ion trap quantum computer.
cavity QED: atom-photon interface, use photons for networking
trap arrays, using single ion as moving head
ion-solid state qubits (charge qubits)
Future goals and developments
more qubits
better fidelities
faster gate operations
faster detection
development of 2d trap arrays, onboard addressing, electronics
entangling of larger systems
implementation of error correction, keep qubit "alive"
Quantum computer can be applied to algorithms, doesn't pertain to quantum cryptography. This has to do with using random quantum numbers to do information exchange. Use qubits as storage devices for quantum repeaters, so when you want to do quantum cryptography over longer distances, then you would need to have repeater stations.
