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Quantum Information and
Computation for Dummies
Peter Samuelsson, Mathematical Physics
Outline
What is quantum
information?1
What is a quantum
computer?2
Why build a quantum
computer?3
How to build a
quantum computer?4
Outline
What is quantum
information?1
What is a quantum
computer?2
Why build a quantum
computer?3
How to build a
quantum computer?4
Basic unit of information
Classical bit
Quantum bit - qubit
“Quantum information is physical information that is held in the
state of a quantum system” - wikipedia
Definition
Two-state quantum system
V 1
0
Two-state classical system
More qubit ”hardware” examplesE
lectr
ons
spin
Photo
ns
polarization position
atomic level
n
n+1
Bloch sphere representation
Vector on the unit sphere
State parametrization
More qubits
- classical 00,01,10,11
2
N
- ”analog information”
Quantum vs classical
Teleportation
No cloning theoremCopy information
Quantum teleportation
Entanglement
Qubit A and B
Resource for quantum information processing
Hotly debated
non-local properties
Bob
Alic
e
Alice measures Bob must measureNo information
is transferred
Teleportation
Photon experiment
source of
entangled pairs
Alice
initial state
Bob
final state
classical communication
two-qubit
measurement
single qubit
rotation
Superdense Coding
Cryptography
- sending 2 classical bits in 1 qubit
1
Alice Bob
2 00, 01,
10, 11
300
01
10
11
Share en-
tangled pair
A rotates
- codes
B measures
- decodes
- quantum key distribution
Eve
classical
quantum
What is a quantum
computer?
Outline
What is quantum
information?1
2
Why build a quantum
computer?3
How to build a
quantum computer?4
“A quantum computer make use of quantum-mechanical phenomena, such as
superposition and entanglement, to perform operations on data.” - wikipedia
Definition
Fundamentals
Quantum computers can be programmed to carry out
the same operations as classical computers – Deutch
Any operation on a reversible Turing machine can
be simulated quantum mechanically – Benioff
A reversible Turing machine can perform the same
operations as a standard Turing machine – Bennett
1
2
3
4
Any algorithm can be
implemented on a (classical)
Turing machine – Church,Turing
One bit gates
Classical one-bit gate
= NOT aa
a c
0 1
1 0
NOT c
Single qubit gates
X
Rq
H
gateunitary transformation
operator
X
Quantum mechanics
Two qubit gates
a
bANDc a bAND
0 0 0
0 1 0
1 0 0
1 1 1
a b c
=
Classical two-bit gate
…. also AND, OR, XOR, NAND,
Two qubit gate
Controlled NOT
::
Circuits
Universal computation
Entangler
H
… and multi qubit circuits
With (for example)
Rp/4H
single qubit gates two qubit gate
any multi qubit circuit (quantum computer) can be constructed.
Quantum parallelism
f
Superposition of computations
f f f
f
Classical circuit
Quantum circuit
All function
evaluations in a
single run
Why build a quantum
computer?
Outline
What is quantum
information?1
What is a quantum
computer?2
3
How to build a
quantum computer?4
Motivation
1
Computationally hard problems
2
Quantum simulations
3Energy saving
Quantum computers can solve
certain problems much faster than
any classical computer
Grover Shor
Quantum computers can simulate
many-particle systems much faster
than any classical computer
Feynman
Quantum computers are reversible
and can consume much less energy
than (standard) classical computers
Landauer
Shor’s algorithm
Prime factorization problem: Given an integer N, find its prime factors
15 = 3 x 5 9999999942014077477 = 3162277633 x 3162277669
Time to solve:classical
quantum
Public key cryptography
transfers
identity
…
internet security
Product of two unknown primes
Exponential speed-up
Grovers algorithm
Unsorted data base search: Find a given element in a data base of size N
Time to solve:
classical
quantum
Quadratic speed-up
Quantum simulations
Classical computers require exponentially long time to
simulate many-particle quantum systems
Feynmans observation
Quantum computers
require polynomial timeExponential speed-up
Many-body fermion system
Lloyd
Initial state preparation
Time evolution
Charge density
Correlation functions
….
Simulate
Bosonic systems, chemical reactions, …
Outline
What is quantum
information?1
What is a quantum
computer?2
Why build a quantum
computer?3
How to build a
quantum computer?4
Di Vincenzo criteria
Requirement on physical system (hardware)
Scalable with well characterized qubits1
2
3
4
5
Initial state preparation
A universal set of quantum gates
Long (relevant) decoherence times
Reading out the result
Quantum error
correction
Linear quantum optics Trapped ions
Nuclear magnetic resonance Superconducting circuits
Electrons in solid state
Spins in quantum dots Dopants in silicon
Nitrogen vacancies in diamond Topological states in solids
Status and outlook
Achieved to date
Controlling & coupling individual qubits
Needed to beat classical computers
Full quantum computer : 104 - 105 qubits
Limited tasks: 102 - 103 qubits
Running algorithms with <10 qubits
Error correction with <10 qubits
Coherent transfer of individual qubits
…
Still plenty of work to do….
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