1. Quantum Computing from theory to experiments
Artur Ekert

2. Every 18 months microprocessors double in speed
Motivation
faster smaller shrinking computer 1m
1nm
Every 18 months microprocessors double in speed
FASTER = SMALLER

3. Towards the quantum limit
Quantum technology
Limits or Opportunities?

4. What is so special about quanta?
50% 1
50% 1

5. They do weird things 1 1 1

6. They defy logic 1 1 1 1 1
NOT

7. 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

8. 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

9. …and with internal states of atoms!
© ENS Paris

10. Experiment
© ENS Paris

11. With pairs of electrons in superconductors…
Ramsey interferometry on
the internal states of
QUANTRONIUM
© CEA Saclay

12. …and with ions
0.2 mm
© NIST Boulder
Beryllium ions

13. From logic gates to computers
I can build
any computer as a network
composed of logic gates. Can you?

14. Theoretical physicist perspective
Sure, we can ! H H U H H U U H U
Quantum logic gates in a network =
Quantum Computer

15. Deterministic classical computation
Intermediate configurations
Initial configuration
(input)
Final configuration
(output)

16. Probabilistic computation
Input
Possible outputs

17. Quantum computation
sensitive to decoherence

18. Building quantum computers
In fact, there are many ways of implementing quantum interference…
Testing H

19. Any unitary operation can be constructed as a quantum network !H H U H H U U U H

20. 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)

21. 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

22. 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

23. Ion collective motion here: classical motion of the ion chain
phonon data bus: quantized motion of the ion chain with one or no phonon

24. 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

25. 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

26. 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

27. Source of power & source of problems
sensitive to decoherence

28. 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…

29. …and many other good ideas
Entangled rubidium
vapour cells in Århus
Cavity QED at CalTech

30. ?? 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

31. So what have we learned ?