[itcrowd]

Essentially, they use superconducting electronics to create a quantum circuit. It's not the same logic gates as a traditional computer chip and not based on the same physics. The details are difficult & messy.

There are several reasons it doesn't scale easily to more qubits, but you can imagine that you don't want the chip to be large (must be cooled to 25mK!) but the qubits should be spaced quite far apart so they don't influence each other. Also, it's not a very simple circuit, so the layout of the transmission lines ("wire on a chip") becomes difficult to manage. The last problem also scales badly with the number of qubits (the middle qubit becomes progressively harder to reach).

There is a paragraph in the (leaked) paper that describes their chip:

> In a superconducting circuit, conduction electrons condense into a macroscopic quantum state, such that currents and voltages behave quantum mechanically [2, 30]. Our processor uses transmon qubits [6], which can be thought of as nonlinear superconducting resonators at 5 to 7 GHz. The qubit is encoded as the two lowest quantum eigenstates of the resonant circuit. Each transmon has two controls: a microwave drive to excite the qubit, and a magnetic flux control to tune the frequency. Each qubit is connected to a linear resonator used to read out the qubit state [5].

If you want a physical picture, check out [6]: https://arxiv.org/abs/cond-mat/0703002

edit: source [6] is more appropriate and open to access

[outworlder]

Holy hell.

> > In a superconducting circuit, conduction electrons condense into a macroscopic quantum state, such that currents and voltages behave quantum mechanically [2, 30]. Our processor uses transmon qubits [6], which can be thought of as nonlinear superconducting resonators at 5 to 7 GHz. The qubit is encoded as the two lowest quantum eigenstates of the resonant circuit. Each transmon has two controls: a microwave drive to excite the qubit, and a magnetic flux control to tune the frequency. Each qubit is connected to a linear resonator used to read out the qubit state [5].

I understand most of the words in isolation – and I know that they are valid, even if combining them in a useful manner to understand exactly what they are describing is eluding me.

But if there was ever a paragraph that sounded like pure technobabble, this is it. Replace the technobabble found in the star trek matter transporter with quantum lingo, and it would sound very similar.

[galonk]

"For a number of years now, work has been proceeding in order to bring perfection to the crudely conceived idea of a transmission that would not only supply inverse reactive current for use in unilateral phase detractors, but would also be capable of automatically synchronizing cardinal grammeters..."

[AndrewKemendo]

Honestly, this is how I read every quantum thing. Not because I don't believe it's real, it's just so far outside of my area of education that it might as well be describing the turbo encabulator.

[aphyr]

So I'm not super familiar with quantum computation, but I did do my undergrad research in QM (specifically, how chaotic behavior depends on the scale of nonlinear quantum systems) and I can take some informed guesses about what these words mean. It's actually super cool!

In a superconducting circuit

A circuit is a loop of something. Probably a solid material, like a metal or carbon, though it could be something more exotic. A superconducting circuit means the electrons in that material move without any resistance. This tells us the circuit is probably very cold--superconductors tend to break down at warm temperatures, like the ones in your house.

conduction electrons

Conductors have electrons in them. Some are "stuck" to atoms, others get to move around. Conduction electrons are the ones that move.

condense into a macroscopic quantum state

Macroscopic means "big", and for QM, "big" means, like, more than a handful of atoms or particles. At least as far as QM is concerned, everything--rocks, electrons, photons, people, etc., has a quantum state, but we use the phrase "quantum state" to mean a state that's, like, WEIRDLY QUANTUM. For instance, a pencil sitting on your desk is normal. A pencil that's like, half on your desk and half on mine is "quantum". Condensing means the electrons are going to change from doing normal individual electron things into acting like some sort of Big But Weirdly Quantum system, likely as a group. Like a crowd becoming a flash mob, they might do some sort of synchronized dance, only except the dance involves, say, every dancer doing two or three or ten dance moves at the same time.

such that currents and voltages behave quantum mechanically [2, 30].

Specifically, we're gonna be able to see quantum effects like superposition in Big Things like "current" and "voltage". The circuit might be in a combination of 3 volts and 5 volts at the same time. Also some of those voltages might be partly real and partly imaginary. Long story.

Our processor uses transmon

What the fuck is a transmon? I had to look this one up; it's a way of making these qubits less sensitive to voltage fluctuations.

qubits [6]

Qubits are quantum bits. A bit can be either 0 or 1. A qubit can be 0 or 1 or (and this is the quantum part) any state in between. Let's call the 0 state |0>, and the 1 state |1>. A qubit can be |1>, but it could also be (1/sqrt(2) |0>) + (1/sqrt(2) |1>). We call that a "cat" state, incidentally, because it's "half 1, half 0"--like Schroedinger's Cat, half alive and half dead. Again, the coefficients here are, in general, complex numbers, but we're gonna gloss over that.

which can be thought of as nonlinear

Nonlinear means they don't respond linearly to some input. Ever had someone do a series of small, mildly annoying things, and at some point you snapped and yelled at them? That's called "going nonlinear".

superconducting resonators

Oscillators are things that vibrate, like strings. Resonators have preferred frequencies to vibrate at. I don't exactly know what this means in this context, though. I'm guessing the circuit has some preferred frequencies it really likes to oscillate at.

at 5 to 7 GHz.

Voltages or currents or whatever are gonna go back and forth 5-7 billion times a second. That's about the same frequency as wifi signals, or microwaves.

The qubit is encoded

A qubit is an abstract thing on a whiteboard. There lots of ways we could actually make a thing that looks like a qubit. "Encoded", here, means "turned into an actual machine you can build in a lab".

as the two lowest quantum eigenstates of the resonant circuit.

An eigenstate, loosely speaking, is a state that has nothing in common with any other eigenstate. For instance, if we wanted to measure a particle's position on a line, we could take x=0 as one eigenstate, x=1 as another, x=2.5 as yet another, etc etc. An infinite number of eigenstates. Quantum systems can be in any (well, normalized) sum of eigenstates. My cat loves being inside and outside at the same time, so they're always trying to occupy 0.2|x=0> + 0.6|x=4> + 0.2|x=5>.

An operator is a thing you can do to a quantum state. Think of operators like functions on values, if you're a programmer, or like matrices that can be applied to state vectors, if you know linear algebra. For instance, I might have a measurement operator, which I use to look at my cat. There's also a special operator called the Hamiltonian, which (loosely) tells you what a state will look like after an infinitely small step in time.

Each operators has associated eigenstates, and those eigenstates have a magic property: if you apply that operator to one of its eigenstates, you get back the exact same state, times some complex number, which we call an eigenvalue. This means eigenstates for the Hamiltonian are, in a sense, stable in time. When we talk about the eigenstates of a system, we usually mean the eigenstates of the Hamiltonian. They could also be talking about measurement eigenstates--I'm not sure.

For the Hamiltonian, eigenvalues are, for Really Fucking Cool Reasons, energies. When we talk about "the two lowest quantum eigenstates", we mean the two states with the lowest energy. So maybe the circuit's eigenstates are, I dunno, 5 Ghz, 6 Ghz, 7 Ghz, etc. We'd take 5 and 6 as our |1> and |0> states.

Each transmon has two controls

A control is a thing we can use to change the transmon.

a microwave drive

Something like the microwave in your kitchen, but very small, and probably expensive.

to excite the qubit

This probably means changing the qubit from |0> to |1>. Microwaves carry energy, right? That's how they heat food. If they microwave the circuit at the right frequency, that microwave energy probably helps it jump from a lower frequency/energy to a higher one.

and a magnetic flux control

This feels like something specific to transmons. Flux has to do with the density of stuff moving through a surface. Magnetic flux probably has to do with how strong and close field lines are in some part of the transmon machinery.

to tune the frequency.

How fast the circuit wobbles depends on a magnetic field, I guess?

Each qubit is connected to a linear resonator

Huh. So we've got nonlinear resonators (the qubits) connected to linear resonators (some sort of measurement device?)

used to read out the qubit state

We need a way to actually look at the qubits, and I guess the linear resonator does that. I assume that the linear resonator is isolated from the qubit during computation, and once the computation is over, it gets connected somehow, and vibrates at the same frequency as the qubit. That process probably "spreads out" the quantum state of the system, pushing it REAL CLOSE to an actual eigenstate of the measurement system, which looks like a probabilistic measurement of the actual qubit state.

Like... my cat could be 3/4 inside and 1/4 outside, so long as the room is really dark. If I turn on the light, suddenly my cat is coupled to a MUCH BIGGER system--the room, and that "quantum" state gets diffused into that larger system, in what looks like a measurement like "cat definitely inside". I don't know a simple way to explain decoherence, haha, but if you like math, try Percival's "Quantum State Diffusion".

Hope this helps, and I also hope I got at least some of this right. Maybe someone with a better/more recent command of QM can step in here.

[galaxyLogic]

Very good explanation. Even if it would not be 100% correct, and I can not say yes or no, it gives a good overall introduction to the concepts involved.

[QuinnWilton]

I'm not a quantum physicist either, but I did study quantum mechanics in college for a bit!

> A qubit can be |1>, but it could also be (1/sqrt(2) |0>) + (1/sqrt(2) |1>). We call that a "cat" state, incidentally, because it's "half 1, half 0"--like Schroedinger's Cat, half alive and half dead. Again, the coefficients here are, in general, complex numbers, but we're gonna gloss over that.

In case anyone is interested in not glossing over this part, this [0] lecture by Scott Aaronson is an excellent introduction to the crazy world of complex probability amplitudes. It doesn't assume much more than some basic linear algebra, and does a good job of developing at least a little bit of an intuition for some of the concepts in Aphyr's comment.

[0] https://www.scottaaronson.com/democritus/lec9.html

[viraptor]

At the extreme, it approaches art: https://www.reddit.com/r/VXJunkies/

[tagrun]

We do want the chip to be "large" eventually. The scalability doesn't have anything with those, however. Refrigerators are pretty large and these devices are really really tiny; also qubits shouldn't be spaced "far" apart, this would kill all the (controllable) couplings.

[itcrowd]

I was a little too fast and loose in my previous post. You are correct that we want a large [number of qubits] on a chip eventually. The chips are tiny and the fridges large enough for now (50 qubits). It's not clear to me that they can handle thousands of qubits as imagined. The cryostat will undoubtedly be able to house the chip, but the extra electronics / cables must run in there as well. With 1000 qubits and 2 cables per qubit this will be a major challenge.

You are correct about the coupling of the qubits, I was simplifying too strongly there.

I do stand by my scaling point (center qubits harder to reach) but I'm open for counter arguments.

Thanks.

[tagrun]

There is a scalability problem but due to different reasons. See my other comment in this thread for details.

Classical circuitry is an issue, but not as much as you think. What happened is Martinis' group and others moved forward with a quick & dirty design which worked well for their device but can't be scaled (they basically didn't have the expertise like silicon people had). Nevertheless, it's not a fundamental problem, the circuitry for silicon based spin qubits never had this problem for example and xmons won't either, they just keep reiterating as the number of qubits increase, it's the least of their worries regarding scalability. There are far bigger problems regarding the scalability.

[itcrowd]

I agree on your other points and trust that you are more qualified to judge what the biggest obstacles are.

Do you have a paper I can look into that goes into the chip architectures in more detail (not specifically for this new device)? Otherwise I'll await the science / nature paper of this demonstration.

The classical circuit is mostly outside the cryo I assume since it's GHz's and LNAs are available. Do you know if the microwave readout signal is frequency multiplexed to reduce cables?

[tagrun]

When you say chip architecture, I assume you mean how to assemble together qubits like an integrated circuit. Here's one proposal for silicon based qubits: https://www.nature.com/articles/s41467-017-01905-6

Microwave signals are typically used for control rather than readout. I don't know if this experiment does it or not, and I am not an experimentalist. Multiplexing is more typically required for reducing timing errors of simultaneously driven signals (for better synchronization) and it really depends on the device and the mode of operation, plus whether the experiment they're doing needs it or not. The same experimental group sometimes do it one experiment and not do it in another, despite using the same device.

[itcrowd]

Thanks for the reference and your great insights. On reading the new Google paper more closely, they do FM the readout.