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Quantum Computing from theory to experiments
Artur Ekert
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Every 18 months microprocessors double in speed
Motivation faster smaller shrinking computer 1m 1nm Every 18 months microprocessors double in speed FASTER = SMALLER
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Towards the quantum limit
Quantum technology Limits or Opportunities?
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What is so special about quanta?
50% 1 50% 1
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They do weird things 1 1 1
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They defy logic 1 1 1 1 1 NOT
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Logic or Physics? Why shall I accept this
Niels Bohr & Albert Einstein Why shall I accept this logically impossible operation Because its physical representation does exist in Nature! It can be performed! Alan Turing
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This is for real! …with neutrons… With photons…
Light enters from the left hitting a cube beamsplitter, splitting the beam in two arms and recombining on a second cube beamsplitter. The mirror of the top arm was moved by a piezo-electric transducer. Interference signal was recorded as a function of the voltage on the piezo. A second beam splitter sent part of the light to a photodiode detector (top-aluminum box). The other part of the beam was sent and to a series of neutral density filters placed before a photomultiplier (in black). Below we see inteference fringes seen both with the photodiode voltage and in the photon counts recorded by the photomultiplier. In Spring 2001 Lauren Heilig constructed a Mach-Zehnder interferometer as part of her Phys410 research project: © Lauren Hellig With photons… © NIST Boulder
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…and with internal states of atoms!
© ENS Paris
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Experiment © ENS Paris
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With pairs of electrons in superconductors…
Ramsey interferometry on the internal states of QUANTRONIUM © CEA Saclay
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…and with ions 0.2 mm © NIST Boulder Beryllium ions
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From logic gates to computers
I can build any computer as a network composed of logic gates. Can you?
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Theoretical physicist perspective
Sure, we can ! H H U H H U U H U Quantum logic gates in a network = Quantum Computer
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Deterministic classical computation
Intermediate configurations Initial configuration (input) Final configuration (output)
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Probabilistic computation
Input Possible outputs
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Quantum computation sensitive to decoherence
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Building quantum computers
In fact, there are many ways of implementing quantum interference… Testing H
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Any unitary operation can be constructed as a quantum network !
H H U H H U U U H
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Power of quantum physics
The quantum taketh away… …and the quantum giveth back! Quantum factoring Quantum search Finding hidden subgroups Quantum simulations… Quantum cryptography © DRA Malvern (1990)
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Impact on Logic Traditional approach: proof = physical record
Is A true or not ? Yes, A is true! Testing different possibilities in quantum superpositions Proof = physical process
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Quantum computing with trapped ions
qubits = 2 internal state / ion 10 mm individual manipulation with laser pulses interaction via collective phonon modes "phonon data bus" 30 m
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Ion collective motion here: classical motion of the ion chain
phonon data bus: quantized motion of the ion chain with one or no phonon
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One, two, many… Quantum Charge-Coupled Device (QCCD)
Efficient coherent transport of a qubit between two traps demonstrated. Decoherence free subspaces in action… © NIST Boulder
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Two traps on a chip © NIST Boulder ions in trap #4 RF electrodes
control electrodes central slot side slots 4 rf electrode 2 alumina wafers trap axis wafer spacing 4 rf electrode control electrodes 2 bare alumina gold coating © NIST Boulder
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Optical lattices An array of potential wells
created by a pattern of crossed laser beams © NIST Potential depends on internal states of atoms-qubits conditional dynamics for quantum gates © University of New Mexico
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Source of power & source of problems
sensitive to decoherence
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Stabilising quantum computation
Projections on symmetric subspaces (Deutsch 93) Decoherence free subspaces (Palma et al, 95) Quantum error correcting codes (Shor, et al 95,…) Geometric/holonomic computation (Jones et al, Zanardi et al 99) Anyons etc…
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…and many other good ideas
Entangled rubidium vapour cells in Århus Cavity QED at CalTech
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?? Timelines 55 years 47 years Classical computers 1947 2002
single transistor, 10 kHz 55,000,000 transistors, 2.8 GHz Ion trap quantum computer NIST, 1995 Quantium® ? 2050 NIST, 2002 47 years ?? single qubit, 20 kHz 4 qubits, 30 kHz
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So what have we learned ?
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