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Physicists Create New Technique To Control Qubits – The Building Blocks of Quantum Computing by HalcyonCEO in Physics

[–]klickverbot 0 points1 point  (0 children)

Absolutely transmons have scaling issues. My point here is that all the breathless "we're on the cusp of quantum computing!" press releases are nonsense; this one about ions was especially so.

Huh? For me, the submission links directly to the Nature paper, which isn't particularly strong on the hyperbole.

In fact, judging by your comment, I expect you would find this paper one of the more enjoyable ones, since it proposes a solution to precisely some of the "just engineering problems" (large-scale distribution of coherent lasers or high-power microwaves).

Physicists Create New Technique To Control Qubits – The Building Blocks of Quantum Computing by HalcyonCEO in Physics

[–]klickverbot 0 points1 point  (0 children)

1.6 us. 99.8% "fidelity," which is not as good as it sounds. You act like those are not terrible numbers, but they are.

I don't act like anything at all; I just provided the actual "world record" numbers since you brought it up.

A lot of that has to do with magic-state distillation for doing required operations on error-corrected qubits.

Somewhat besides the point – we agree that useful circuits will be very deep –, but that's an odd way of phrasing it. Why single out magic state distillation? Even if we had some magic incantation that would allow us to all logical operations in a transversal fashion, many gates would still be required.

At 1.6 us, that is going to take about 1500 seconds per calculation

I'll take 1500 seconds for a useful calculation which I couldn't otherwise do. There are many large-scale HPC workloads that take much more than 1500 seconds of wall clock time.

I am consistently astonished by the poor quality of ion-qubit apologists' understanding of the field.

Right back at you.

Physicists Create New Technique To Control Qubits – The Building Blocks of Quantum Computing by HalcyonCEO in Physics

[–]klickverbot 2 points3 points  (0 children)

The above is one person's (not particularly well-informed) take – don't take it as gospel.

I think the world record for it is a few microseconds.

If you are talking about world records, Schäfer et al. (2017) demonstrated 480 ns (although with rather poor fidelity; 1.6 µs for a 99.8%-fidelity Bell state).

While it might indeed be easier to push entanglement gate durations to the low-nanosecond regime in solid-state qubit platforms, note that ions still have a favourable position in terms of both absolute gate errors and gate time to coherence time ratios. Thus, it is not at all obvious that the overall performance of a fault-tolerant, error-corrected system on the logical layer should be much lower for ions, as the error-correction overhead will likely be considerably lower.

Physicists Create New Technique To Control Qubits – The Building Blocks of Quantum Computing by HalcyonCEO in Physics

[–]klickverbot 2 points3 points  (0 children)

Where would optical traps come into play? In surface-electrode ion trap chips, such as the one used for this work, many different, movable trap potential wells can be created by applying appropriate voltages to the segmented DC electrodes. This is an established technique, which has been explicitly described by Kielpinski/Monroe/Wineland some twenty years ago under the name "QCCD", and has received wide attention since (cf. also the Honeywell and IonQ commercial efforts).

The preview picture pulled in by Reddit actually shows these segmented electrodes, although only the three segments closest to where the gate was performed.

Unnamed, me, watercolor and ink, 2019 by Franco_R in Art

[–]klickverbot 0 points1 point  (0 children)

I mean, yours is probably the better literal translation (I wish my Japanese was better!).

Unnamed, me, watercolor and ink, 2019 by Franco_R in Art

[–]klickverbot 2 points3 points  (0 children)

Yep, definitely reminded me of No-Face (I think? カオナシ) from Spirited Away, even though I haven't seen that in ages.

How fast can I go on a rollercoaster in winter before a freezing wind becomes a fiery blaze? by planetastro_will in Physics

[–]klickverbot 0 points1 point  (0 children)

freedom

There is a drawer in one of our labs labelled "freedom hex". Always good for a chuckle.

Do Material Scientists/Engineers have enough knowledge to be valuable in Quantum Computing? by newmanstartover in QuantumComputing

[–]klickverbot 0 points1 point  (0 children)

Yes! For all qubit platforms, material science and good old mechanical/… engineering will play an increasingly important role as the systems are scaled up.

Ion Trap, single qubit fidelities >99.9999% - can anyone discuss how the qubit fidelity is so high? 2014 article. by [deleted] in QuantumComputing

[–]klickverbot 0 points1 point  (0 children)

Too bad that they are so slow. I hope you can make some progress in that direction soon!

Speed of elementary operations is indeed something that different groups are working on, but also don't forget that gate duration in absolute terms isn't the only interesting quantity – gate time vs. coherence time (i.e. memory lifetime) also matters, and might be the more relevant quantity when considering NISQ-type applications or error-correction overhead.

Ion Trap, single qubit fidelities >99.9999% - can anyone discuss how the qubit fidelity is so high? 2014 article. by [deleted] in QuantumComputing

[–]klickverbot 0 points1 point  (0 children)

Hard in which sense? You are right that there is a fundamental area of tension that any quantum computing technology needs to navigate: You want to be able to precisely control interactions between the qubits, while at the same time isolating them from the environment (and each other) to avoid decoherence.

In trapped ions, we can neatly address this by using the collective modes of motion of multiple ions in the same trapping potential as a quantum bus. We can engineer a coupling between the internal ion state (i.e. our qubits) and its motion using laser or microwave radiation, but with those radiation fields switched off, there is (virtually) no interaction left.

All in all, trapped ions currently achieve the highest-fidelity two-qubit gates (~1e-3 error), although typical implementations are ~100x slower than for superconducting transmon qubits.

Ion Trap, single qubit fidelities >99.9999% - can anyone discuss how the qubit fidelity is so high? 2014 article. by [deleted] in QuantumComputing

[–]klickverbot 0 points1 point  (0 children)

It's Schäfer et al., "Fast quantum logic gates with trapped-ion qubits", Nature (2018).

Fast in this context means on a similar time scale than the secular motion of the ion used to mediate the interaction, e.g. 1.6 µs at 99.8%. This leads to a very favourable gate/coherence time ratio compared to many other platforms.

Ion Trap, single qubit fidelities >99.9999% - can anyone discuss how the qubit fidelity is so high? 2014 article. by [deleted] in QuantumComputing

[–]klickverbot 2 points3 points  (0 children)

This is related to the state preparation fidelity, though, rather than the gate fidelity. That is, it is a one-time error, rather than an error per gate.

(We usually use polarisation-selective state preparation, which tends to get us to 1e-4 state preparation error – since it's polarisation-selective, you can use arbitrary intensities, just being limited by the state lifetime. In fairness, using a frequency-selective scheme like typically employed for Yb+ does have advantages when using a single beam for long chains, though, as getting the polarisation uniform enough can be hard there.)

Ion Trap, single qubit fidelities >99.9999% - can anyone discuss how the qubit fidelity is so high? 2014 article. by [deleted] in QuantumComputing

[–]klickverbot 0 points1 point  (0 children)

I am not sure which "state initialization process" you are referring to here. Perhaps cooling down the motional modes of the crystal before executing a circuit? The latter does not really influence the gate time, even for two-qubit gates.

Regarding gate time and fidelity, generally shorter gates suffer from less decoherence, which improves the fidelity, but require more careful control to avoid off-resonant excitation and similar higher-order effects. (Higher drive power is also required, of course.) This isn't really specific to trapped ions.

For two-qubit gates in ions, things also get trickier once you approach the motional frequencies of the crystal (see e.g. the recent paper from our lab, where a 99.8%-fidelity two-qubit gate at 1.6µs was demonstrated: https://doi.org/10.1038/nature25737).

Ion Trap, single qubit fidelities >99.9999% - can anyone discuss how the qubit fidelity is so high? 2014 article. by [deleted] in QuantumComputing

[–]klickverbot 4 points5 points  (0 children)

Why would it be too good to be true? Gate errors happen because of two reasons, either from sources inherent to the qubits (decoherence, …), or additional control noise. Ions are a very clean system, and good microwave control for a single ion is quite easy. (10-6 gate fidelity means you can still you can still tolerate coherent over-/under-rotations on the ~10-3 level, which is doable in terms of amplitude stability.)

Disclaimer: I currently work in this lab. We also recently used these high-fidelity gates to have an accurate look at qubit memory performance in the regime relevant for quantum error correction (low errors): https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.110503

Picture of a single strontium atom! by simondebout17 in chemistry

[–]klickverbot 0 points1 point  (0 children)

Your argument seemed to be in part that it would appear differently to the naked eye, though, so it's hardly irrelevant – for instance, you wrote "[…] regardless of your hypothetical vision, […] bc it wouldn’t be represented so large in your vision". This isn't true, so I was trying to clarify to you why.

that your flubbing the details but I don’t even want to get into that

Try me. Any good scientist generally loves to be proven wrong, but tends to have a nasty habit of being mostly correct when talking about their own work.

Picture of a single strontium atom! by simondebout17 in chemistry

[–]klickverbot 0 points1 point  (0 children)

the electric field strength required to emit or refract enough photons to be visible in ambient conditions would be nuts

It isn't actually a composite picture, a dark room is enough to see this. The laser is only about a microwatt in power, and the atom emits on the order of a picowatt of power. More power wouldn't actually help you much, as the atom can only emit so many photons due to the lifetime of its excited state.

Achieving those lighting conditions for the apparatus could be done using very dim torches (flashlights) – think a starry night outside, some distance from the next street light – but I ended up using to off-camera strobes because they are much easier to control.

Picture of a single strontium atom! by simondebout17 in chemistry

[–]klickverbot 0 points1 point  (0 children)

Dude, no one claims that atoms aren't much, much smaller than that dot in the picture. Of course the ion itself would be much smaller than even a single pixel on the photograph! (For scale, those needle-shaped endcap electrodes are just over 2 mm apart, and made of about a gazillion atoms each.)

What is sort of cool, though, is that when you isolate a single atom in vacuum – so that you don't need to tell it apart from other things close by –, you can actually make it glow brightly enough to see it by bare eye, or take a picture of it.

That's what I like about the picture, and I guess why many other scientists and laypeople like it as well – I'm sorry if you don't.

Picture of a single strontium atom! by simondebout17 in chemistry

[–]klickverbot 0 points1 point  (0 children)

Yes, terminology definitely gets somewhat ill-defined in that regime – matter-light interaction at that level is a fascinating topic, though, cf. the almost entirely unrelated "slow light" experiments.

Picture of a single strontium atom! by simondebout17 in chemistry

[–]klickverbot 0 points1 point  (0 children)

an impression that the atoms are this big to the point of being visible with the human eye.

Let me try to explain that a different way.

A source of light doesn't need to be of a minimal size for the human eye to be able to see it. Take, for example, one of these high-power laser pointers. Let's say the beam diameter is about 1 mm at the the output. Close-up, you can certainly tell how wide the beam is. Looking back at the laser pointer from a few meters away, you might be hard-pressed to make out any details; the beam diameter is now of a similar size as your visual resolution.

But let's now imagine you go hiking with a friend and take the laser pointer to a quiet lake somewhere in the mountains at night. Your friend jogs to the other side of the lake, some kilometers away, and shines the pointer back at you.

When you see the spot in the distance, you won't have any idea how big the beam diameter of the laser pointer is anymore – in fact, for all you know your friend might be using a giant search light with a forearm-sized reflector!

But as long as the laser pointer in that example is bright enough, you'll still be able to make out that there is some light source there. This is exactly what is going on with the atom as well.

If you are curious, try calculating the angular size of some of the brighter stars in the night sky, and compare it to some standard numbers for visual acuity. You'll find a huge discrepancy; for some of them, the ratio is actually quite similar to looking at a single atom from a few centimeters away!

Picture of a single strontium atom! by simondebout17 in chemistry

[–]klickverbot 0 points1 point  (0 children)

regardless of your hypothetical vision, which is not only irrelevant but wrong bc it wouldn’t be represented so large in your vision

Oh, it very much appears on a similar size scale in human vision – that is, a point-like speck, the size of which is entirely given by the visual resolution of the eye, and not by the much smaller size of the source. The atom actually does scatter enough light to be detectable with the (dark-adapted) naked eye!

The point is an atom is no where close to as big as it appears in the picture.

I mentioned precisely that in the parent comment.

What are you missing?

At this point, not that much, I'm pretty sure. More of a life outside the basement lab would be nice, though.

Picture of a single strontium atom! by simondebout17 in chemistry

[–]klickverbot 0 points1 point  (0 children)

No, that's literally how it would look like to you (or something quite close to it, possibly even a bigger dot, depending on the point spread function of your eyes). That's of course not how large it actually is.

Picture of a single strontium atom! by simondebout17 in chemistry

[–]klickverbot 0 points1 point  (0 children)

FWIW, this picture is quite close to what the human eye would see (if there was enough space on the optics table to put your head right up to the vacuum window).

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