[Cryptography] Some quantum computers might need more power than supercomputers

[Henry Baker]

-----Original Message-----
From: Peter Gutmann <pgut001 at cs.auckland.ac.nz>
Sent: Jan 10, 2026 11:50 AM
To: Jon Callas <jon at callas.org>, cryptography at metzdowd.com <cryptography at metzdowd.com>
Subject: Re: [Cryptography] Some quantum computers might need more power than supercomputers

Jon Callas writes:

>Some quantum computers might need more power than supercomputers

The German government (via the BSI) wrote a report on this and estimated that
it would take 100 days and €4M in electricity to recover a single 2048-bit key
on a quantum computer that doesn’t exist. That's one single key. There are 7
trillion keys negotiated each year just for TLS web connections.

Oh, and that's for integer factorisation, not (EC)DLP, so not actually useful
for attacking the shopping list of common crypto protocols I mentioned
earlier.

Peter.

---
Lemme see...

Google tells me that quantum computers operate at 0.1 - 1 degree Kelvin,
so the Carnot efficiency of a 1 degree Kelvin QC would be 1/300, while the
Carnot efficiency of a 0.1 degree Kelvin QC would be 1/3000, meaning
that rejecting 1 Joule of energy from a 0.1 degree QC would require at a
*minimum* 3000 Joules of energy, about 1 watt-hour.

The "quantum" part of a QC dissipates *zero* energy, as quantum evolution
is reversible/conservative.

The "measurement" part of QC does dissipate energy -- the minimum is
about the same as what it would take to clear the (classical) bit to zero.

Q: Why do QC's need to operate near 0K ?

A: Because there is no known "shield" for quantum interactions (unlike
electrical interactions, which can (and are) shielded by *conductors*),
the vicinity of the QC must be cleared of as much thermal noise as
possible, implying temperatures near 0K.

Here's the Debby downer on quantum shielding:

https://en.wikipedia.org/wiki/Aharonov%E2%80%93Bohm_effect

(Remember the 3rd Law of Thermodynamics? "You can't get out of
the game" aka "Nothing is 0K".)

Bottom line:

I'm not holding my breath waiting for QC, although it should be
obvious that an excellent place for current QC's to operate would
be in outer space, somehow shielded from the Sun.