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Entanglement
This word was never used before 1994.
Meanwhile physicists understood:
* The structure of atoms, molecules, and nuclei
* Quantum electrodynamics
* Solid state physics
* NMR
* Superconductivity
* Lasers
* Quantum optics
* Quantum Hall Effect
........
Is this really a useful notion?
Can we have and do we need quantum computation?
Michel Dyakonov
Université Montpellier II, France
Outline:
* Overview by an outsider
Brief history and general ideas
Quantum computing with...
Error correction and threshold theorem
* Relaxation (decoherence)
* Fault-tolerant quantum computation
* Challenge
* The lessons from building perpetual motion machines
* Much more powerful in doing what ?
* Conclusions
Quantum computing: a view from the enemy camp
M.I. Dyakonov, 2001, arxiv.org/abs/cond-mat/0110326
Is fault-tolerant quantum computation really possible?
M.I. Dyakonov, 2007, arxiv.org/abs/quant-ph/0610117
A grand challenge for the Millennium ...
Quantum Computation: A Grand Mathematical
Challenge for the Twenty-First Century and the
Millennium
S. J. Lomonaco, Jr., ed. American Mathematical Society, Providence, Rhode Island (2002)
Philosopher's Stone (Lapis Philosophorum): A Grand
Challenge for the Eleventh Century and the Millennium
S. J. Abrucci, Jr., ed. Roman Philosophical Society, Pisa (1002)
Brief history
R. Feynman (1982): Nature isn’t classical, dammit, and if you want to make a simulation
of Nature, you’d better make it quantum mechanical, and by golly it’s
a wonderful problem, because it doesn’t look so easy
D. Deutsch (1985): Universal quantum computer
P. Shor (1994):
Factoring by quantum computing (Shor’s algorithm)
P. Shor (1995), A. Steane (1996): Quantum error correction by encoding
P. Shor (1996) and others:
Fault-tolerant quantum computation (technical instruction)
D. Aharonov and M. Ben-Or (1999): The “threshold theorem”
L. Vandersypen et al (2001):
15 = 3 × 5 by NMR (experimental)
The general idea of quantum computing
Quantum computing is supposed to use some 103 – 106 two-level systems (spins), called “QUBITS ”
Each spin can be in a state a|↑ + b|↓ or a|0 + b|1
The state of the whole computer is described by the grand wave function
Ψ = A0|000...00 + A1|000...01 + A2|000...10 + ... + A 2N–1 |111...11
with 2N complex amplitudes Ak
NB: 21000 ~ 10300 is much greater than the number of protons in the Universe ( ~1080 )
The computing process consists of manipulating ALL of these continuously changing
variables and measuring all qubits at the end
An elementary unitary transformation of amplitudes is called “QUANTUM GATE ”
IF there is no noise (spin relaxation), IF the gates are ideal, and IF measurements are perfect,
THEN it is proven that after applying ~1010 gates, one can use Shor's algorithm to factorize
numbers like ~ 10130 faster than any classical computer
Quantum computing with (randomely picked proposals):
* neutral atoms
* buckyballs
* trapped ions, atoms, and light
* spin qubits interacting
* neutral atoms in an optical lattice
through delocalized excitons
* quantum dots
* non-deterministic gates
* globally controlled interactions
* optically hole burnt materials
* quantum dots in a microcavity
* global one-and two-qubit gates
* endohedral fullerenes
* magnetic atoms in optical
* “always on” Heisenberg interaction
lattices of reduced periodicity
* spatially delocalized qubits
* superconductors
* Hydrogenic spin in Silicon
* collective ensembles of multi-
* atomic Josephson junction arrays
level systems
* rare-earth-ion doped crystals
* hyperfine clock states
* magnetically interacting atoms
* small-amplitude coherent states
* perpetually coupled qubits
* superconducting circuits
* only one mobile quasiparticle
* trapped electron states
* "enhancement" quantum dots
* highly verified cluster states
* polarization encoding
* toric code states
* phase modulated laser pulses
* bosonic atoms
Quantum computing with (continued):
* superconducting charge qubits
* continuous-variable cluster states
* Kerr-nonlinear photonic crystals
* polar molecules
* transverse phonon modes
* slow measurements
* scattering matrices
* qudits
* unlabeled qubits
* shutter logic
* abelian anyons on the honeycomb lattice
* v = 5/2 fractional quantum Hall state
* diatomic bits in optical lattices
* semiconductor nanostructures
* atoms in periodic potentials
* probabilistic ion-photon mapping
* optical coherent states
* one-dimensional optical lattice
* un-tunable couplings
* vibrationally excited molecules
* squeezed coherent states
* spin-1/2 pairs
* ballistic electrons
* mesoscopic superposition states
* matrix product states
* probabilistic quantum gates
* perpetually coupled spins
* cold bosonic atoms
* para-hydrogen
* ideal Clifford gates
* cluster states
* advice
* unknown parameters
* superconducting circuits
* Heisenberg ABAB chain
* harmonic oscillators
* cavity QED systems
* abelian anyons
Topological quantum computing with anyons
(ν = 5/2 fractional quantum Hall plateau)
The observed ν = 5/2 fractional quantum Hall plateau does not fit into the composite
fermion concept.
Some people think that this is a manifestation of anyons. Others think differently.
Nobody knows.
Hence the obvious proposal: to use these hypothetical anyons for quantum computing.
We must move the anyons around one another making complex topological structures,
so that the knot theory may be used.
Schematic presentation of the Hall bar (3×6 mm),
containing 1010 2D electrons
(and presumably an equal number of anyons)
This
See also « Anyon theory of high Tc superconductivity »
Quantum computing with...
Google gives 55 600 hits for « quantum computing with »
and 75 900 hits for « quantum computation with »
Isn't this crazy?...
Is there anything at all that is NOT suitable for quantum computing?
Some possibilities are still not explored:
Quantum computing with:
* muons
(muon qubits for fault-tolerant quantum computing)
* neutrinos (very good candidate, because of long decoherence time)
* quarks
(easily produced at high energy facilities.
Advantage: one can use flavor and SU(3) symmetry)
* rotating black holes
* Shrödinger cats
(can be used as quantum gates for neutrinos)
(entanglement easily observed at room temperature)
15 = 3 × 5 (experimental application of the Shor algorithm with NMR)
L. Vandersypen et al (2001)
NMR spectrum demonstrating factorization of 15 by Shor
Ideally expected
spectrum
→
Experimental
spectrum
→
Simulation of
decoherence
effects
→
Basic principle of error correction
Postulates of Quantum Mechanics
Ψ = A|0 + B|1
Measure the state of a qubit. The result is (0) or (1) with probabilities |A|2 and |B|2
Measurement gives (0) → the wavefunction collapses to the state Ψ = |0
Moreover:
Consider our spin + the rest of the world (ROTW) !
The wavefunction can be written as:
Ψ = A |0 0 + B |1 1 ,
where
0 and 1 are some two states of the ROTW
We now measure the state of our qubit only. Depending on whether the result is (0) or (1),
not only the wavefunction of our qubit will collapse to |0 or |1, but also the wavefunction of the
ROTW will collapse to 0 or
1 respectively!
Ψ = A|0
+ B|1
Error correction by encoding
Logical qubits are encoded by special superpositions of physical qubits
A. Steane (1996)
Ancilla qubits, syndrome extraction operator, measure ancilla, correct error
Fault-tolerant quantum computation – a technical instruction, P. Shor (1996)
In fact, concatenated encoding should be used, in which each qubit on the
right will be in its turn encoded in the same manner, and so on...
Concatenated encoding
The future quantum engineer will have to encode one logical qubit
by maybe 73=343 qubits...
(J. Preskill, 1997)
Very old Russian joke
View of the Supreme Commander work-desk
Pushing the green button
annihilates the enemy ground forces
White button: same for air force
Blue button: same for the navy
It's up to the engineers to work out the
details
The threshold theorem
 Concatenated encoding
 Parallel error correction for different qubits
 Noise model: uncorrelated in space and time errors in individual qubits,
gates, and measurements
Once the error rate per qubit per gate is below a certain value ε,
indefinitely long quantum computation becomes feasible, even if all of
the qubits involved are subject to relaxation processes, and all the
manipulations with qubits are not exact.
D. Aharonov and M. Ben-Or (1999)
Estimate for the threshold: ε ~ 10–6 – 10–4
Beware theorems
Dream...
The mathematical method for
capturing a Lion in the desert
The desert D being a separable topological
space, it contains a countable subset S that is
everywhere dense therein.
(For example, the set of points with rational
coordinates is eligible as S.)
Therefore, letting x  D be the point at which
the Lion is located, we can find a sequence
x n n  S , with limn→ ∞ xn = x
This done, we approach the point x along the
sequence {xn} and capture the Lion
Beware theorems
... and reality
The real thing...
The Lion, by Rembrandt
Relaxation (decoherence)
Mystification of spin relaxation in QC literature:
“The qubit (spin) gets entangled with the environment…”
“The environment is constantly trying to look at the state of a qubit, a process called decoherence”
* * * * * * * * * * *
Meanwhile relaxation of 2-level systems was studied a lot and is quite well understood
Spin relaxation is a result of the action of fluctuating in time magnetic fields
A randomly fluctuating magnetic field is characterized by its correlation time, c,
and by the average angle of spin precession, α, during time c
For the most frequent case when α << 1 , the spin vector experiences a slow angular diffusion
RMS angle after time t >> c is ε = α(t/c)1/2
Then the relaxation time is  = c  α2
Relaxation (decoherence)
This process goes on continuously for all the qubits within the quantum computer
Each qubit loses memory of its initial state during the relaxation time τ
Important point:
The wavefunction of N qubits deteriorates faster than that,
during a characteristic time τ/N
(correlations are more fragile, they disappear faster)
Thus, at the final stage of the Shor algorithm, supposing that you managed to achieve a
good wavefunction (of 73 × 1000 qubits, encoding 1000 logical qubits), the states of all
343000 qubits should be measured during a time considerably shorter than τ/343000
Surprisingly, nobody has ever discussed this problem
(it is for the future quantum engineers to figure this out)
Quantum computing with “decoherence-free subspaces”
IDEA: Suppose that, while individual qubits experience decoherence, there exist
(due to some symmetry) a subset of many-qubit states that do not relax.
We can then encode information within this decoherence-free subspace
and safely perform fault-tolerant quantum computations of arbitrary length!
(A large and respectable branch of mathematics, considering
which is the best way to do this, see for example Theorem 4 below)
Unfortunately, decoherence-free subspaces do not exist,
except in some imaginary world
How to factorize numbers by quantum computing?
To beat my PC in factorizing numbers,
one must have at least 1000 qubits
(without error correction).
With error correction,
the number of qubits should be considerably increased
Fortunately, the number required to factor integers using Shor's algorithm is still only
polynomial, and thought to be between L4 and L6, where L is the number of bits in the
number to be factored.
For a 1000-bit number, this implies a need for 1012 to 1018 qubits.
The fault-tolerant computing manual
(after J. Preskill, 1997)
The fault-tolerant Toffoli gate
(proposed by P. Shor and designed by J. Preskill)
A minor problem with the cat state
The only problem in this scheme is that
the generation of the cat state is not done fault-tolerantly,
and thus one error can cause the entire cat state to be ruined.
The CNOT gates will then propagate this error to many qubits in the block.
The solution to this problem is to verify that the cat state
is indeed a superposition of |00000000... and |11111111.. , before continuing.
D. Aharonov and M. Ben-Or (1999)
Homework
1. Verify your cat + dog state:
2. How big can the cat be?
Ψ=(
+
)/ 2
Quantum versus classical precision
Consider a classical system of 103 – 105 compass needles
• Uncontrolled rotations due to noise
• Manipulations are not exact
• Undesired interactions between our needles
By a) applying external fields to individual needles and
b) introducing controlled interactions betweens pairs,
we wish to impose a prescribed evolution of the whole system
Some trivial remarks:
We fix a coordinate system xyz related to some physical objects,
with the z axis pointing towards the Polar Star
* This direction, as well as the angles between our axes cannot be defined exactly
* The orientation of the needle with respect to our axes cannot be defined exactly
*
cos 45  0,7071067811...
* Two needles NEVER point in exactly the same direction
* etc, etc
Quantum versus classical precision
Apparently, things are not so obvious in the magic world of quantum mechanics!
There is a widespread belief that the |1 and |0 states “in the computational basis”
are something absolute, akin to the on/off states of a classical switch, but with the
advantage that one can have quantum superpositions of these states
on
Ψ = 2-1/2{
off
+
}
In reality, pure |1 and |0 states can never exist!
Similarly, a classical vector can never point in exactly the z direction.
(We simply never know what is the z direction)
Instead of pure |1, we always have |1 + c |0 with some unknown c, where |c| is
hopefully small.
Quantum versus classical precision
The classical statement:
the orientation of any vector is known only within a certain precision
is translated into quantum language as:
there is always an admixture of unwanted states to any desirable state
Thus the (|1 + |0)/21/2 state, and especially the “cat” state
|cat  = (|1111111 + |0000000)/21/2
are abstractions, that can never exist in reality!
Quantum versus classical precision
So, when we say “prepare a (|1 + |0)/21/2 state”, we must realize that
-1/2
what we really can have, is: a|1 + b|0, where |a| and |b| are close to 2
with a precision not better than our ability to measure angles
(usually, much worse)
If we measure the state a|1 + b|0 and get (0), the wavefunction does NOT
collapse exactly to |0. Rather it becomes |0 + c|1, where |c| is small
The precision of qubit states is never better
than the precision of the direction of a classical vector!
Square root of 2 does not exist in Nature !
The measured diagonal of a unit square is equal to 1.4142 ± 0.0007
This remains true even in the Magic World of Quantum Mechanics !
Quotes
(J. Preskill, 1997)
Doubts...
* The existing scheme for fault tolerant quantum computing is an extremely
complicated construction. It is not sufficiently clear how it is going to work.
* It is not obvious that the scheme takes properly account of:
- impossibility of preparing exactly |0 and |1 states, or any desired state
- continuous nature of decoherence of all qubits
- continuous nature of gate errors
- N times faster deterioration of the wavefunction of N qubits,
compared to the decoherence time for an individual qubit
- the true outcome of imperfect measurements
- the finite duration of gates, measurements, and any action
* Besides, in reality there will always be some correlation in noise for different
qubits, systematic errors of gates, undesired interactions between qubits etc.
(something beyond the adopted model).
What will be the result of even small effects of this kind?
My own Theorem
Theorem 1: No continuous variable can be equal exactly to zero
Consequence:
A continuous variable can not have an exact value ( e.g. √2 )
Example: Any action on the wave function of 1000 qubits with 10300 amplitudes is
described by a matrix 10300 × 10300 . You think, that you can apply a matrix,
corresponding to a two-qubit gate:
........................
... 0 0 0 0 0 0 0...
. . . 0 0 1 i -1 0 0 . . .
. . . 0 -i 0 1 0 0 0 . . .
. . . 0 0 -1 0 i 0 0 . . .
. . . 0 i 0 -1 0 0 0 . . .
... 0 0 0 0 0 0 0...
.......................
However, according to Theorem 1 this is impossible. You will never have zeros
everywhere. In simple words, you can never completely isolate all the other qubits from
the action of the applied field
Doubts...
Especially,
undesired interactions between different qubits = death to quantum computing
How small should be such interactions to make large scale quantum computing possible?
- No answer so far …
Maintaining one qubit in its unknown state
Challenge to the QC community:
Provide a detailed sequence of gates and
measurements that allow to maintain the spin
close enough to its initial direction (along x),
once the error per computation step is less
than some ε
I will then check it on my PC
The lessons from building perpetual motion machines of the second kind
Important differencies: we are doing some work on our system,
we do not want to violate the Second Law (not yet).
Similarity: we are attempting to achieve a reversible evolution of
a very large many-body system (on a microscopic level) in the
presence of noise and using noisy devices.
The idea, that quantum mechanics helps us to do it, looks
strange, since all our experience in physics suggests that quantum
effects are more fragile than classical ones.
The basic point of all the projects of perpetual motion machines of the second kind, existing
to date and yet to come, is some ideal element, which is not sensitive to thermal noise.
It is sufficient to have just one such element, often deeply hidden within a very complicated
construction. Finding and identifying such an ideal element may be a daunting task.
This lesson should make us extremely vigilant to the explicit or implicit
presence of ideal elements within the error-correcting theoretical schemes.
The quantum computer is much more powerful than a classical one.
In doing what ?
Answer: in factoring very large numbers
(provided that it will work)
What else? - Oh yes: it will do simulations of quantum systems...
“The transistors in our classical computers are becoming smaller and
smaller, approaching the atomic scale. The functioning of future devices will
be governed by quantum laws. However, quantum behaviour cannot be
efficiently simulated by digital computers. Hence, the enormous power of
quantum computers will help us to design the future quantum technology”
(A love-song for the military sponsors)
“In addition, efficient quantum simulation will make a revolution in physics,
chemistry, and biology”
Plenty time to think about it…
J. Preskill, 1997
A. Steane, 1997
April 2, 2004
Conclusions
 It is absolutely incredible, that
 by applying external fields, which cannot be calibrated perfectly
 by doing imperfect measurements
 and by using converging sequences of “fault-tolerant”, but imperfect, gates
one can continuously protect the grand wavefunction from the random drift of its 10300 amplitudes
and moreover make these amplitudes change in a precise and regular manner needed for quantum
computations on a time scale greatly exceeding the relaxation time
 It seems very likely that the (theoretical) success of error-correcting schemes is based on
the implicit introduction of ideal elements, like exact |1 and |0 states, and other unrealistic
assumptions. In other words, it seems that the math is detached from the physical reality
 The claim that one qubit can be kept intact in the presence of noise, should be supported by
a detailed description of the needed sequence of manipulations and measurements. A
numerical test for a realistic model of imperfections should be done
 Factoring large numbers is not a goal that can inspire generations of physicists and
engineers. Ideas for other applications, like simulating quantum systems, are vague and far
from being obvious
Summary: we cannot have it and apparently we don’t even need it…