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31 votes

Which subatomic particle does each company use in quantum computing?

Google, IBM and Rigetti use transmon qubits; these are basically fancy LC circuits where a Josephson junction and capacitor connect two superconducting islands. Because of this, they are also often ...
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18 votes

Why do the IBM and Google processors both have 53 qubits?

It's just a coincidence. I can speak from personal recollection on the Google side. Google originally intended to use a 72 qubit chip (Bristlecone) where qubits were essentially directly connected to ...
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  • 21.9k
15 votes
Accepted

What does Google's claim of "Quantum Supremacy" mean for the question of BQP vs BPP vs NP?

Google's paper/results are kind of sideways to questions in computational complexity about the relation between $\mathrm{BPP}$ and $\mathrm{BQP}$ (and even further from questions about whether $\...
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  • 6,787
8 votes
Accepted

Understanding Google's “Quantum supremacy using a programmable superconducting processor” (Part 2): simplifiable and intractable tilings

TL/DR: The two-qubit gates are going by the moniker "Sycamore gates" in the paper, and it appears that they would ideally want to explore more of the $(\phi, \theta)$ phase-space but for ...
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  • 6,787
7 votes
Accepted

Making sense of the Sycamore's computing prowess - power consumption

They say in Section X.H of the supplement that the Summit supercomputer has a power capacity of 14 megawatts. They compare that to their own setup. Their power consumption is mainly their dilution ...
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7 votes
Accepted

Understanding Google's “Quantum supremacy using a programmable superconducting processor” (Part 3): sampling

What does "obtaining samples" mean in this context? The same thing it means in a more classical context. Consider the probability distribution of the possible outcomes of a (possibly biased) coin ...
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  • 18.3k
6 votes
Accepted

Do all physical architectures for quantum computers use the same universal gate sets?

So any universal gate set can replicate any other, since both are universal, but different architectures generally have different physical gates. While Clifford+T is a universal gate set that is very ...
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6 votes
Accepted

Why Google has used $\sqrt{X}$ and $\sqrt{Y}$ instead of $X$ and $Y$ in supremacy experiment?

While Craig Gidney (from Google) is correct in his comment which says that $X$ and $Y$ do not create superpositions on states that are not in superposition, such as $|0\rangle$ and $|1\rangle$; even ...
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  • 11.9k
6 votes

How are the IBM's and Google's Hadamard gates fabricated and operated?

A Hadamard gate isn't usually a physical object that you pass qubits through. In the case of superconducting qubits, the Hadamard gate is performed by bouncing microwaves off of the qubits. It doesn't ...
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  • 21.9k
6 votes
Accepted

What is the basic hardware gate library in the IBM & google?

IBM You can view the basis gates that supporting at the hardware level for IBM's hardware through your dashboard. All the devices with more than 1 qubit have the same set of basis gates $\{CX, ID, RZ, ...
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  • 12.3k
4 votes

Location of "bad" qubits on Sycamore

Although it doesn't explicitly say it in the paper from Google, the diagrams in the paper are missing a qubit along the top edge. Most likely this is the "bad" qubit that wasn't used.
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4 votes
Accepted

Location of "bad" qubits on Sycamore

When I visited the Google Hardware Lab, they were extremely secretive about everything. It is unlikely anyone will be able to answer this question except for the narrow range of Google Hardware Lab ...
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4 votes

Quantum Supremacy: Some questions on cross-entropy benchmarking

That seems to restrict the output probability distributions of all quantum circuits to rather high entropy distributions. The output of a typical randomly chosen quantum circuit is rather high ...
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  • 21.9k
4 votes
Accepted

Understanding Google's “Quantum supremacy using a programmable superconducting processor” (Part 1): choice of gate set

While a follow-up question asks for the motivation behind the two-qubit gates used in Sycamore, this question focuses on the random nature of the single qubit operations used in Sycamore, that is, the ...
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  • 6,787
4 votes

How are the IBM's and Google's Hadamard gates fabricated and operated?

Fundamentally, a device such as an IBM quantum computer interacts according to a Hamiltonian, which might have some time-varying parameters. For example, for a single qubit, it might look like: $$ H=...
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  • 46.4k
4 votes
Accepted

What did exactly Google do in simulating a random quantum circuit on a classical computer in supremacy experiment?

All quantum circuits can be simulated on a classical computer, but not all circuits take the same amount of time to simulate. If information about the circuit is known in advance, certain patterns may ...
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4 votes
Accepted

Where are the physical gates in the Google processor?

In relating quantum computing to classical computing there may be a small conceptual hurdle that needs to be overcome. Although a classical $\mathsf{NAND}$ gate may be implemented in hardware (say ...
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  • 6,787
4 votes
Accepted

Can we conclude that errors on Sycamore are Poisson-distributed Pauli errors?

The model's accuracy is purely empirical observation. The error trend (Fig 4, or 41:50 in the video) demonstrates that the error of the system (cross entropy fidelity with respect to simulated results)...
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  • 4,547
3 votes

How exactly is solving the random circuit sampling problem a computation in the Church-Turing thesis sense?

In the framing of the question (which I believe to be asked in good faith), there seems to be at least two objections. Sampling from a set of strings is not clearly a function, and Sampling is a ...
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  • 6,787
3 votes
Accepted

How exactly is solving the random circuit sampling problem a computation in the Church-Turing thesis sense?

The Church-Turing thesis is not in and of itself a rigorous concept, but rather a judgment on rigorous concepts of computability. As such, it's negotiable. The language in Rosser's 1939 expository ...
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3 votes
Accepted

Quantum Supremacy: Some questions on cross-entropy benchmarking

After some further consideration I think it's quite clear that the only probability mass function evaluated in the computation of $\mathcal{F}_{\text{XEB}}$ is that of the classically computed ideal ...
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3 votes

What does Google's claim of "Quantum Supremacy" mean for the question of BQP vs BPP vs NP?

Paraphrasing some tweets on the matter earlier, the result is rather underwhelming because it plays on a discrepancy between what they mean by quantum supremacy (QS) and what people tend to think QS ...
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3 votes
Accepted

Do quantum supremacy experiments repeatedly apply the same random unitary?

Generally speaking, to prove quantum supremacy, you don't need to sample several times from the same unitary/circuit/output probability distribution. If you extract even a single sample from the ...
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  • 18.3k
3 votes
Accepted

What determines the repetition rate in Google's Weber datasheet?

How is this repetition rate calculated? The repetition rate is how many samples can be collected per second. You compute it by collecting some large number of samples and checking how much time it ...
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  • 21.9k
3 votes
Accepted

Can Google Sycamore's benchmark for quantum supremacy be simulated on Qiskit?

the benchmarking method used in this paper is called cross entropy benchmarking (XEB). An example circuit implementation for a 5 qubit XEB sequence is shown in fig. 3 of the paper. For further info, I ...
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  • 988
2 votes

Quantum Supremacy: Some questions on cross-entropy benchmarking

As an initial matter, I think the Supplementary Information (linked in some other answers on this sight) has a significant amount of discussion on $\mathcal{F}_{XEB}$. However, as I understand it (...
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  • 6,787
2 votes

Understanding Google's “Quantum supremacy using a programmable superconducting processor” (Part 1): choice of gate set

This answer only addresses the part about the necessity of the randomness of the circuit because I am by no means familiar with the physical implementation of the qubits at Google and what kind of ...
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  • 789
2 votes

Do quantum supremacy experiments repeatedly apply the same random unitary?

In the Sycamore paper linked in the comments, in the description of FIG. 4, the authors state: ...For each $n$, each instance is sampled with $N_s$ between 0.5 M and 2.5 M... For $m=20$, obtaining 1M ...
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  • 6,787
2 votes
Accepted

Why does Google's quantum processor outperform IBM's?

Both IBM and Google unveiled 53-qubit processors. At this time, only Google published performance metrics such as 1- and 2-qubit gate errors. Until IBM publishes similar metrics we simply cannot even ...
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2 votes

Why do the IBM and Google processors both have 53 qubits?

My guess is that this is an example of co-opetition, i.e. collaborative competition. Number of qubits is just a single characteristic of a quantum processor, but there are a lot more, like tolerance, ...
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  • 5,453

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