# Tag Info

7

A logical qubit is a very fluid concept. You could use physical qubits as logical qubits. Or, you can encode multiple physical qubits as a single logical qubit. The more physical qubits you use, the better the resistance to noise. So, I would suggest that you question isn't exactly the right one to ask, and a better question is whether something useful can ...

5

What follows turned out to be a rather technical explanation, so I'll start with the main point: The qubit state can change the resonator's state, and the resonator's state can be easily measured only if there is a large different in frequencies between the qubit and the resonator. Let's model a qubit as a two-level system and a resonator as a harmonic ...

4

There is not anything that you cant do with U3 so ideally there is no reason for U1 and U2. Eventually, as the transpilers gets better we may remove them and just have U3 and CNOT. So why did we make U1 and U2? It is because of the hardware. The U1 is done using a frame change (see https://arxiv.org/abs/1612.00858) which means they are done in software (...

4

I am going to try to give guesses that can make sense: More qubits does not mean better machines. They may be less noise-tolerant and with less connectivity between qubits. That is why, when you benchmark them (with or without error-correction), you look first at the simplest implementations of state of art algorithms. Plus, you may change some calibrations ...

4

At Xanadu, we're using integrated quantum photonics to build our photonic quantum computing chips. In this case, we have integrated chips containing waveguides --- these are coupled to lasers to generate input resource states, undergo manipulation on the chip, and then are measured via a variety of detectors available in quantum optics. These can include ...

4

Actually, after having researched the question over the last months, the two answers (one above and one below) are correct, but we can build upon them to get something more up to date. The first answer, however, relies on figures and data which are slightly obsolete, while the source is uncertain (it is impossible to know if the source is McKinsey or The ...

3

So, to begin, I would point out that the 500 micosec T1 time is for a single qubit in isolation, while the GHZ results are on a 20 qubit device. This device has an avg T1 time of around ~75 microsec. The GHZ results were done by Ken Wei from IBM, and will be published shortly. In short, the circuit is a standard GHZ building circuit, with a hadamard ...

3

There's a superconducting circuit element called Josephson junction, which is roughly a nonlinear inductor. The inductance of a Josephson junction depends on current via the relation $$L(I) = \frac{L_0}{\sqrt{1 - (I/I_c)^2}}$$ where $L_0$ is the inductance of the junction with no bias current and $I_c$ is the so-called "critical current" which is the maximum ...

3

When referring to the commercial quantum computers of both parties, it is that both are based on a different quantum principles. The D-Wave machine works via quantum annealing and is suited for optimization problems. The machine by IBM is a gate-based quantum computer, similar to how digital computers work at the elementary level. As the two quantum ...

3

It's unlikely. And even if there is, they've not announced it publicly. Most of the private companies and startups in this area are still in the stealth mode. This is the most complete list of quantum computing startups that I know of. Among the companies listed, Atom Computing may be working on diamond-based quantum computers, but they haven't released much ...

3

By an example with a control qubit in superposition and the target in $|0\rangle$ state: $$\frac{|0\rangle + |1\rangle}{\sqrt{2}} |0\rangle = \frac{|0\rangle|0\rangle + |1\rangle |0\rangle}{\sqrt{2}}$$ Applying a CNOT will have the following result: $$\frac{ CNOT(|0\rangle|0\rangle + |1\rangle |0\rangle)}{\sqrt{2}} = \frac{ CNOT(|0\rangle|0\rangle) + ... 3 It does, unless you have a way to tune your Hamiltonian such that \Delta becomes zero. Since a tunable Hamiltonian is something you usually want in a quantum computer implementation, this should not be a problem. If this term is non-switchable, it just means that the basis in which you are working is continuously rotating, and you have to keep track of ... 3 You can look up work by Gil Kalai, who is a longstanding and outspoken critic of quantum computing (his most recent essay: Kalai, 2019). He often bases his view on assumptions that I entirely disagree with, but its a refreshing reminder that certain ideas are taken for granted in the industry, namely that NISQ computers will yield practical applications. ... 2 They are always rotating in the lab reference frame, but most quantum algorithms take a rotating reference frame to simply things, so that a z rotation only happens when you want it to. The rotating reference frame spins along with the natural spinning rate of each qubit, so in general in can spin at different rates for different qubits. 2 I think the subject matter of supercondcuting qubits is rather broad and diverse, making it challenging to accurately capture it in a 'brief explanation'. With that said, this recent review (Krantz et al., Applied Physics Reviews 6, 021318 (2019)) - "A Quantum Engineer's Guide to Superconducting Qubits" (arXiv:1904.06560) from the MIT group may be a good ... 2 No, as point 4 is not satisfied. The D-Wave machines are quantum annealers and thus not universal. See this question on how to make from the D-Wave machine a universal quantum computer. 2 Hope this late contribution won't be a meaningless contribution, but as mentioned in one of the comments above, by using D-Waves version of NetworkX you can visualize the Pegasus network. I have attached a few images here of the Pegasus 2 (P2) and Pegasus 6 (P6) architectures using the D-Wave NetworkX. The reason that I find Pegasus interesting is that the ... 2 In 1996, David DiVincenzo listed five key criteria to build a quantum computer: A quantum computer must be scalable, It must be possible to initialise the qubits, Good qubits are needed, the quantum state cannot be lost, We need to have a universal set of quantum gates, We need to be able to measure all qubits. Two additional criteria: The ability to ... 2 This is certainly how theorists think of this being done. I don't know if there's an experimental reality to compare this to. Whether they actually decompose it in terms of the eigenvectors, or find some other terms to decompose it as. Just as an example of what I mean, let$$ W=\left(\begin{array}{cccc} 1 & 0 & 0 & 0 \\ 0 & 0 & 1 & ...

2

The Quantum Volume is a benchmark for near term, noisy quantum systems. Indeed, like other random unitary benchmarks, you need to be able to sample the ideal distribution. This distribution comes from classical simulations, so your limited to about the ~40 or so qubit limit. However, the Quantum Volume itself was designed to benchmark not only the quantum ...

2

Quantum volume is a bad metric for this purpose. For example, suppose you have a ten thousand by ten thousand grid of qubits with a gate error rate of 1 in one thousand. The quantum volume of this grid is basically 0, because if you pick two qubits at random they will on average be more than one thousand steps apart. So an error will almost certainly occur ...

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Easiest thing talk about the algorithms for each architecture and the difference between physical and logical qubits. As far as I know we do not know yet how to perform quantum error correction efficiently on an adiabatic machine. Most computations on these devices are just repeated lots and lots of times without much error correction. For the gate model ...

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Parallel computation of the sum of two qubits I wanted to experience parallel computation of the sum of two qubits, a superposition of 0 and "1 with phase -1" added to 1; and I was inspired by Mithrandir24601's answer. The results are below. I hope my answer is within the context of what was asked. It shows how 1 is added 1, and to 0, at the same time, ...

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