Quantized Majorana conductance

[shpalman]

rps20221215_212002-01.jpeg (175.5 KiB) Viewed 27624 times

Spin qubits in germanium work though.

[shpalman]

NISQ Is Dead, a Dying Dead End, With No Prospects for a Brighter Future or Practical Quantum Computing

You just need "near-perfect qubits" instead. Well ok then why didn't you say.

[shpalman]

In addition to not existing as quasiparticles in solid state systems, Majorana fermions also do not exist as fundamental particles.

"The fact that we didn't detect anything demonstrates the feasibility of spending a whole bunch more money to not detect anything on a much bigger scale" said a particle physicist, again.

[shpalman]

Long Microsoft paper about not finding Majoranas

[IvanV]

In addition to not existing as quasiparticles in solid state systems, Majorana fermions also do not exist as fundamental particles.

"The fact that we didn't detect anything demonstrates the feasibility of spending a whole bunch more money to not detect anything on a much bigger scale" said a particle physicist, again.

Only just got around to looking at this. At some point it says, "While neutrinos have already been found to have a mass..."

I think to say that a neutrino has a mass, you have to demonstrate a non-zero lower bound for it. That does not seem to have been achieved.

It seems the justification for the above statement is neutrino oscillations. By some well-established theory, that can only happen if they have a mass. But whilst our well-established are excellent in their core zones, they are rather less reliable outside their known reliable areas of application, to the extent of appearing to be completely inconsistent with major facts. So I'm not convinced by this demonstration by a theory that has not been proven to be correct in relation to neutrinos.

[shpalman]

In addition to not existing as quasiparticles in solid state systems, Majorana fermions also do not exist as fundamental particles.

"The fact that we didn't detect anything demonstrates the feasibility of spending a whole bunch more money to not detect anything on a much bigger scale" said a particle physicist, again.

Only just got around to looking at this. At some point it says, "While neutrinos have already been found to have a mass..."

I think to say that a neutrino has a mass, you have to demonstrate a non-zero lower bound for it. That does not seem to have been achieved.

It seems the justification for the above statement is neutrino oscillations. By some well-established theory, that can only happen if they have a mass. But whilst our well-established are excellent in their core zones, they are rather less reliable outside their known reliable areas of application, to the extent of appearing to be completely inconsistent with major facts. So I'm not convinced by this demonstration by a theory that has not been proven to be correct in relation to neutrinos.

The neutrino has to have mass otherwise it would travel at the speed of light, and at the speed of light time would not evolve for it in our frame. So to be able to oscillate, time must be passing in its frame. Special relativity is simple and fundamental enough that for it to be wrong would be a major upset.

[IvanV]

The neutrino has to have mass otherwise it would travel at the speed of light, and at the speed of light time would not evolve for it in our frame. So to be able to oscillate, time must be passing in its frame. Special relativity is simple and fundamental enough that for it to be wrong would be a major upset.

Yet several measurements of neutrino speed have been unable to distinguish it from the speed of light.

To me, the neutrino mass is in the same box as supersymmetric particles.

[dyqik]

The neutrino has to have mass otherwise it would travel at the speed of light, and at the speed of light time would not evolve for it in our frame. So to be able to oscillate, time must be passing in its frame. Special relativity is simple and fundamental enough that for it to be wrong would be a major upset.

Yet several measurements of neutrino speed have been unable to distinguish it from the speed of light.

To me, the neutrino mass is in the same box as supersymmetric particles.

Then you're an idiot who doesn't know what you're talking about.

Neutrinos have mass. We know that because they oscillate, which they could not do if they traveled at the speed of light.

If they don't have mass, then special relativity, quantum mechanics and general relativity are wrong.

I'm building a $660 million experiment to determine that mass.

[IvanV]

Then you're an idiot who doesn't know what you're talking about.

Neutrinos have mass. We know that because they oscillate, which they could not do if they traveled at the speed of light.

If they don't have mass, then special relativity, quantum mechanics and general relativity are wrong.

I'm building a $660 million experiment to determine that mass.

And I hope that you show they do have a mass, because we learn something from positive findings, whereas negative findings leave us still unsure.

A few years ago, some very clever people who seemingly utterly believed in super-symmetry used similar language against the doubters, and stated seemingly for certain that the LHC would vindicate them. Clearly super-symmetry isn't disproved, but there is a much narrower window of possibility for it to be true, and such people are less combative about it these days. Likewise, the failure of your experiment wouldn't disprove neutrino mass, we still wouldn't know either way, but it would be looking less likely. And it might be more difficult to get funding for an even more expensive experiment that might put an even lower upper limit on it.

I think we already know that some of these famous theories have to be wrong, but in the positive way that Newton's Laws are wrong - they have a range of application, and you need to know something else at the edges where they start to contradict each other, or where we have physical knowledge that is hard to make sense of. That kind of "wrong theory" is much more useful and insightful than those that are not even wrong.

[dyqik]

Again, you are clearly showing that you don't know what you are talking about.

Supersymmetry has never had a direct evidence base. Meanwhile, special relativity and electroweak theory are proven by huge amounts of evidence.

[Gfamily]

An interesting suggestion that there will be a joint announcement next week from
- a team attempting to detect GW waves through the measurement of deviations to the timing Pulsar signals
- a team using the Ice Cube Neutrino Observatory in Antarctica.

This may allow us to determine the speed at which neutrinos travel relative to the speed of light.
So far, we know (in part following the neutrino burst detected a few hours before* the gamma ray burst for the SN1987A supernova) that if there's a difference between the speed of neutrinos and light, it's only a few parts per billion, It'll be interesting to see if the announcement helps to narrow it down.

* neutrinos arrive first as they are created as the core collapses, and escape almost immediately, whereas the gamma rays emitted have to battle through the outer layers of the collapsing star before they are emitted.

[IvanV]

I did a bit more reading and see I was ignorant, my apologies.

I thought this was going to be an unbounded game where we could just keep pushing it down a few more orders of magnitude, and never come to an end, but I see that is wrong. I also found it a bit hard to believe in a lepton with a positive mass at least 6 orders of magnitude less than an electron.

But now I see there are good reasons to believe the heaviest neutrino should have a mass around about 6 orders of magnitude less than electron. And if it is is excluded from having a mass in that range, then that shows there's something wrong with the theory.

I read the theory of neutrino oscillation depends upon differences in the square of the masses of neutrino flavours. We can estimate the difference of the sum of squares from the the oscillation characteristics. So that implies the heaviest flavour of neutrino should have a mass of at least 0.05eV/c². But clearly the other flavours could weigh rather less.

Meanwhile, considerations from cosmology are suggesting an upper limit of 0.11eV/c² - possibly that's an average of the flavours.

So there is a fairly narrow window to aim for, at least in terms of the mass of the heaviest neutrino. And you'll know something is wrong if they are strongly excluded from that window.

But maybe you don't have experimental methods of making measurements on all 3 neutrino flavours, so you can't necessarily determine the mass or speed of the heaviest type?

[bjn]

AIUI neutrinos are in a superposition of three flavours and depending on when you observe one you get to see one of those three. But each of the three flavours have a different mass. What happens to conservation of mass? If you see it as an tau neutrino then later on as an electron neutrino, what happens to the missing mass? Same when it goes the other way, where does it get the mass from? Or does quantum mechanics manage to ignore that because quantum magic gets to over rule classical nonsense and involves maths way beyond what I’m now capable of?

[dyqik]

AIUI neutrinos are in a superposition of three flavours and depending on when you observe one you get to see one of those three. But each of the three flavours have a different mass. What happens to conservation of mass? If you see it as an tau neutrino then later on as an electron neutrino, what happens to the missing mass? Same when it goes the other way, where does it get the mass from? Or does quantum mechanics manage to ignore that because quantum magic gets to over rule classical nonsense and involves maths way beyond what I’m now capable of?

Mass can be exchanged for energy.

[bjn]

AIUI neutrinos are in a superposition of three flavours and depending on when you observe one you get to see one of those three. But each of the three flavours have a different mass. What happens to conservation of mass? If you see it as an tau neutrino then later on as an electron neutrino, what happens to the missing mass? Same when it goes the other way, where does it get the mass from? Or does quantum mechanics manage to ignore that because quantum magic gets to over rule classical nonsense and involves maths way beyond what I’m now capable of?

Mass can be exchanged for energy.

I know that, so do they radiate/absorb energy or speed up/slow down when they flip between flavours? Or something else?

[dyqik]

AIUI neutrinos are in a superposition of three flavours and depending on when you observe one you get to see one of those three. But each of the three flavours have a different mass. What happens to conservation of mass? If you see it as an tau neutrino then later on as an electron neutrino, what happens to the missing mass? Same when it goes the other way, where does it get the mass from? Or does quantum mechanics manage to ignore that because quantum magic gets to over rule classical nonsense and involves maths way beyond what I’m now capable of?

Mass can be exchanged for energy.

I know that, so do they radiate/absorb energy or speed up/slow down when they flip between flavours? Or something else?

They don't flip between flavors as they travel - they travel as a superposition of the flavors, which has a well defined mass/energy (and thus momentum). But the mass eigenstates aren't the same as the flavor eigenstates, so when a weak interaction with a neutrino happens, the mass eigenstate is reduced to a flavor eigenstate, with different probabilities for each flavor.

The probability of a particular flavor eigenstate being the outcome of an interaction fluctuates as the phases of the flavor elements of the wavefunction of a mass eigenstate change with respect to one another.

[IvanV]

AIUI neutrinos are in a superposition of three flavours and depending on when you observe one you get to see one of those three.

I read that a solar neutrino oscillates on average about once per 1250m. So unless you are detecting neutrinos from rather close to their source, it would seem "when" ceases to be a practical issue, as after a fairly short distance they will become fully randomised. Though maybe at higher energies the oscillation occurs over longer distances, due to greater time dilation, (he hand-waves in his ignorance). The 730km from Geneva to Gran Sasso seems to be enough for the muon-neutrinos emitted at CERN to oscillate to tau-neutrinos in detectable quantities.

The detection of tau-neutrinos in small numbers at Gran Sasso was news, so detecting these other flavours is possible but apparently not easy.

But we have just learned with thanks to dyqik that mass eigenstates are not aligned with flavour eigenstates. So that is implying we can straightforwardly detect the heavier/slower neutrinos in an experiment, because a fraction of the heavier mass eigenstates will interact as each of the flavours?

So is the sensitivity of this new experiment sufficient to close the window on neutrino mass?

[shpalman]

There's a Pinned thread from CMTC about topological quantum computing, which is of course moot as long as the quasi particles they need to be able to do it don't exist.

Meanwhile there's work to induce superconductivity in semiconductors by putting them very close to metallic superconductors.

[shpalman]

I'll bump this because there are posts relevant to the Q day thing, amongst the neutrinos.

[shpalman]

Long Microsoft paper about not finding Majoranas

A NEW Kind of Quantum Computer - The Map of Topological Quantum Computing is a somewhat uncritical video on this, from @domainofscience.

I note that Microsoft Quantum wanted to comment on a paper* which explained that you could think you had the signature of Majoranas from some relatively trivial effects, but their comment was nonsense.

* - Trivial Andreev Band Mimicking Topological Bulk Gap Reopening in the Nonlocal Conductance of Long Rashba Nanowires https://arxiv.org/abs/2210.03507

tl;dr just because Microsoft think their devices pass the "topological gap protocol" doesn't mean there are really Majorana zero modes in there.

https://twitter.com/PhysicsHenry/status ... 2013060357

To summarise: Microsoft Quantum put a comment on our paper claiming that our mechanism would not cause false positives in their topological gap protocol.

1. Our mechanism can quite easily cause TGP false positives.

2. MSFT's comment was based on a demonstrably false figure.

I also note that the International Conference on Reproducibility in Condensed Matter Physics just happened. Calling it "reproducibility" is a bit of a euphemism... nobody could reproduce Jan Hendrik Schön's results because they were fabricated.

[shpalman]

Documentary: Reproducibility in Condensed Matter Physics - 35 minute youtube video on Sergey Frolov's channel.

Meanwhile, Microsoft still haven't demonstrated Majorana qubits but you wouldn't get that idea from their press release about their Nature paper.

Dulwich Quantum Computing on X has had at it, obviously.

The peer review comments are available in this pdf.

After a thorough analysis of the data and comparison with simulations, the main conclusion is that the results are consistent
with a measurement of the fermion parity encoded in a pair of Majorana zero modes in a long topological nanowire.

In my opinion, these experiments are very interesting and certainly relevant for the condensed matter community working on
topological superconductors and Majorana states. What I do NOT like is the way the article is written which, sometimes
subtly and sometimes more crudely, uses a language and wording that at all times leads the reader to think that we are
dealing with a measurement that demonstrates parity in a topological qubit based on Majorana states. The examples are
many and here I highlight only a few:

Page 1: lines 52 and 53 "albeit in a regime that does not allow qubit readout". Here the authors seem to imply that in this
manuscript a qubit (and its parity readout) is demonstrated which is not true.
Page 1: “In this paper we demonstrate an interferometric measurement of the parity of a near-zero-energy state in a 1D
Nanowire, thereby validating a necessary ingredient of topological quantum computation” How is this demonstrated? How
do the authors know that this is a 1D system? Etc.
Page 1: lines 62-68 "By itself, this measurement does not unequivocally distinguish between MZMs in the topological phase
and fine-tuned low-energy Andreev bound states in the trivial phase, but it does require the low-energy state to be supported
at both ends of the wire and very weakly coupled to other low- energy states". This is an interesting sentence, of course I
fully agree with it given its importance, but then the authors cite a series of papers related to Andreev qubit physics [49–53]
but not THE relevant references concerning the trivial Andreev versus Majorana controversy. These papers , which are cited
later in the paper, Refs [98-103] should be cited and highlighted in this important introductory paragraph. Moreover, the
Nature Review Physics discussing in great detail such controversy, Nature Reviews Physics 2, 575 (2020), should be cited.
-Page 1 section2:
Already the title of the section is misleading “Topological qubit…” There is no definitive proof of topology or qubit…
-Line 76: “in this work we introduce a topological qubit design…” the same as above.
-Line 80-82: “The first component is a nanowire, sections of which can be tuned into a 1DTS state, leading to topological
degeneracy of the many-body ground state…”
Page 2:
-Line 104, line 111: “To form a qubit…” “ a full qubit device therefore consists of…” again the same wording which implies
that the authors demonstrate a qubit.
-Line 118: “ we focus on the left topological section…”

I could continue since the text is plagued with such misleading and ambiguous wording where theoretical prediction, device
design and actual proof in experiment/data is mixed in a rather careless manner….

The same referee, on reading the revised version, then commented:

I stand by my previous report and, in fact, I must now be harsher since after one round of evaluation the authors have had the
opportunity to rewrite the article carefully and avoiding the continuous mixing of objective facts with interpretation
(sometimes bordering on a strong bias towards a priori conclusions on the part of the authors) but have largely ignored my
suggestion (and also that of other referees).

The tl;dr is that there's no qubits and no Majoranas in this paper.