Re: Latest from ICANN: Quantum Computers + N85 Peering Forum

[Masataka Ohta]

As I wrote:


Nanog News wrote:


Latest from ICANN: Quantum Computers are "Interesting"…
But Don't Lose your Head



Uselessness of quantum logic gate style quantum computers
will be discussed in a separate mail.


Quantum logic gate style quantum computers use qubits,
which have two orthogonal quantum state |0> and |1>
(which may be a horizontally and vertically polarized
photon, correspondingly, which is familiar for us,
communication engineers) used as bit values of 0 and
1. As they are orthogonal, we can consider |0>+|1>,
(which may be a diagonally polarized photon, easy
to understand classically). But such addition is
improperly called quantum superposition.

Two qubit states are represented as |00>, |11>
and |00>+|11>.

Then, if we can construct a linear circuit to
compute f, a 3 input and 3 output bits function,
f(a, b, c)=(d, e, f) as f(|abc>)=|def>. Then,
we can compute f(|000>)+f(|001>>+...+f(|111>) as
f(|000>+|001>+...+|111>), that is evaluating
f not 8 times but only once, which is quantum
parallelism. It can be extended for N qubit
cases for 2**N parallelism by 2**N terms.

However, though an N qubit state, like an N bit string,
can naturally distinguish 2**N cases, 2**N term
quantum superpositioned states require distinguishing
2**(2**N) cases, which is obviously impossible because
of noise/error (theory of Shannon).

So, QEC (Quantum Error Correction) was invented,
which was expected to reduce noise/error.

QEC certainly work for single term states. Small
non linear error on single term states is no different
from linear error.

The problem, however, is that, QEC is non-linear,
which means not applicable to 2**N term
superpositioned states.

But, it is implicitly and wrongly assumed that
all the 2**N terms will suffer from identical
error, in which case, QEC become linear.

That is single, so called, bit flip error on
the first qubit of:

|000>+|111>

occurs as:

|100>+|111>

or:

|000>+|011>

not (unless strong correlation exists, which
is an improper assumption):

|100>+|011>

which is the reason why quantum logic
gate quantum computer with practical
number of qubits is impossible.

A complication is that it is possible
to construct complex QEC scheme applicable
for 2 term states. So called Shor code
is such QEC. But Shor code itself use
states with more than 2 terms. Anyway,
once QEC scheme is fixed, it is not
applicable to 2**N term states for
large N.

It should also be noted that though
non-linear error on two or more term states
imply errors caused by interactions of multiple
terms, they are ignored.

Masataka Ohta

Re: Latest from ICANN: Quantum Computers + N85 Peering Forum

[Masataka Ohta]

Nanog News wrote:


Latest from ICANN: Quantum Computers are "Interesting"…
But Don't Lose your Head


But, quantum computers are mocked up by theoretical
physicists, IT amateurs who don't know basics of
computational and/or information theory at all, and,
as such, just do not work better than classical ones.

As for adiabatic quantum computers (those of D-wave),
they can be configured to form various quantum circuits.
Thus, a quantum circuit, the minimum energy state of which
is a solution to some optimization problem, may be
constructed. In addition, according to quantum mechanical
adiabatic theorem, the state can be reached within
polynomial time (w.r.t. problem size), *IF* (very big
if) energy difference between the lowest and the
second lowest energy states is not very small.

However, it is obvious that errors to construct the
circuit will badly affect the result. IIRC, D-wave
quantum computer has 8bit ADC to control coupling
strength between qubits, which is poorly accurate.

Worse, there are known classical, not quantum,
approximation algorithms to compute the lowest
energy state with certain accuracy (w.r.t. energy)
within polynomial time.

As such, "if energy difference between the
lowest and the second lowest energy states
is not very small", the classical algorithms
can find the correct answer and quantum
computing is no better than the classical
algorithms.

It should be noted that, with really hard problems,
the energy differences are very small (become
exponentially smaller as problem size increases),
which can not be solved by classical approximation
nor quantum algorithms in polynomial time.

Though it is possible to construct quantum circuits
to enhance energy differences a lot, such circuits,
like high Q resonators, require extreme accuracy,
which is impossible to construct for large problem
size, which means such attempts do not contribute
scalability.

Uselessness of quantum logic gate style quantum computers
will be discussed in a separate mail.

Masataka Ohta

Latest from ICANN: Quantum Computers + N85 Peering Forum

[Nanog News]

Latest from ICANN: Quantum Computers are "Interesting"… But Don't Lose your
Head

*An Interview with ICANN, Paul Hoffman*
In a recent publication, written by ICANN (Internet Corporation for
Assigned Names and Numbers), chief technology officer Paul Hoffman
discussed the hot topic of Quantum Computing and the DNS.

NANOG had the opportunity to talk to Hoffman to better understand the
current status + future of quantum computing.
Learn what questions our community should be asking + and how much
discussion we should be giving this topic at this time.


*READ NOW
*
Peering Forum: Now Accepting Applications

Peering Coordination Forums are an incredible way to network with others in
the industry!

The 90-minute session will occur during the hybrid NANOG 85 conference,
which will kick off in Montreal on Jun. 6. Please note that participants
must be physically present to attend. NANOG 85 Peering Forum applications
will remain open until we have received twenty applications or the deadline
date of May 31.


*SUBMIT NOW  *

[NANOG-announce] Latest from ICANN: Quantum Computers + N85 Peering Forum

[Nanog News]

Latest from ICANN: Quantum Computers are "Interesting"… But Don't Lose your
Head

*An Interview with ICANN, Paul Hoffman*
In a recent publication, written by ICANN (Internet Corporation for
Assigned Names and Numbers), chief technology officer Paul Hoffman
discussed the hot topic of Quantum Computing and the DNS.

NANOG had the opportunity to talk to Hoffman to better understand the
current status + future of quantum computing.
Learn what questions our community should be asking + and how much
discussion we should be giving this topic at this time.


*READ NOW
*
Peering Forum: Now Accepting Applications

Peering Coordination Forums are an incredible way to network with others in
the industry!

The 90-minute session will occur during the hybrid NANOG 85 conference,
which will kick off in Montreal on Jun. 6. Please note that participants
must be physically present to attend. NANOG 85 Peering Forum applications
will remain open until we have received twenty applications or the deadline
date of May 31.


*SUBMIT NOW  *
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