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From: Terje Mathisen <terje.mathisen@tmsw.no>
Newsgroups: comp.arch
Subject: Re: Is Intel exceptionally unsuccessful as an architecture designer?
Date: Thu, 19 Sep 2024 12:59:42 +0200
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David Brown wrote:
> On 19/09/2024 09:44, Niklas Holsti wrote:
>> On 2024-09-19 2:47, Lawrence D'Oliveiro wrote:
>>> On Wed, 18 Sep 2024 20:09:53 GMT, Anton Ertl wrote:
>>>
>>>> He mentioned that several physics breakthroughs
>>>> are needed for quantum computing to become useful.
>>>
>>> The biggest one would be getting around the fundamental problem that =
you
>>> can=C3=A2=E2=82=AC=E2=84=A2t get something for nothing.
>>
>>
>> Stupid argument. Look at the effort and tech it takes to make quantum =

>> computers... that is not "nothing".
>>
>>
>>> The promise of an exponential increase in computing power for a linea=
r
>>> increase in the number of processing elements sounds very much like
>>> =C3=A2=E2=82=AC=C5=93something for nothing=C3=A2=E2=82=AC=C2=9D under=
 another name, wouldn=C3=A2=E2=82=AC=E2=84=A2t you say?
>>
>>
>> No, it is exploiting the very non-intuitive nature of quantum=20
>> entanglement to create an exponential number of collective states of a=
=20
>> linear number of elements. Medieval arguments about "nothing" vs=20
>> "something" don't work there.
>>
>=20
> Quantum computing certainly gives you some tricks that are hard to=20
> replicate with classical computers.=C2=A0 (And of course some quantum e=
ffects=20
> are impossible to replicate classically, but those are not actually=20
> computations.)
>=20
> But it is still ultimately limited in many ways.=C2=A0 Landauer's princ=
iple=20
> about the minimal energy costs of calculations applies equally to=20
> quantum calculations.
>=20
> The practical limitations for quantum computers are far more=20
> significant.=C2=A0 Roughly speaking, when you entangle more states at o=
nce,=20
> you need tighter tolerances to maintain coherence, which translates to =

> lower temperatures, higher energy costs, and lower times to do your=20
> calculations.=C2=A0 And to be useful, you need large numbers of qubits,=
 which=20
> again makes maintaining coherence increasingly difficult.
>=20
> I'm sure that there will be breakthroughs that improve some of this, bu=
t=20
> I am not holding my breath - I don't believe quantum computers will eve=
r=20
> be cost-effective for anything but a few very niche problems.=C2=A0 Cur=
rently=20
> they have only beat classical computers in tasks that involve simulatin=
g=20
> some quantum effects.=C2=A0 That's a bit like noticing that soap bubble=
=20
> computers are really good at solving 2D minimal energy surface problems=
=2E
>=20
> Remember, the current record for Shor's algorithm is factorising 21 int=
o=20
> 3 x 7.=C2=A0 Factorising 35 is still beyond current engineering levels.=

>=20

 From my recent reading, it seems like factoring 21 (5 bits) requires at =

least 5+10=3D15 bits all staying entangled, plus a number of additional=20
bits for error correction. I'm guessing you also need some extra=20
bits/redundancy in order to successfully read out the results?

Getting to at the very least 3K entangled bits in order to speed up RSA=20
1024 decryption will certainly be out of the question for the remainder=20
of my professional career, and most probably also the rest of my life.


Terje

--=20
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"