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 ...


6

There is not a complete story from "run quantum computation" to "make a billion dollars via slightly better batteries". The vague idea is that a new tool capable of giving new insights into the behavior of materials will lead to important discoveries. It's unrealistic to expect a complete quantum-compute-to-engineering-improvement story since the most ...


6

We start with the following qbit state: $|\psi\rangle = |1\rangle = \begin{bmatrix} 0 \\ 1 \end{bmatrix}$ Then, we apply the Hadamard gate to that qbit: $H|\psi\rangle = \begin{bmatrix} \frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} \\ \frac{1}{\sqrt{2}} & -\frac{1}{\sqrt{2}} \end{bmatrix}\begin{bmatrix} 0 \\ 1 \end{bmatrix} = \begin{bmatrix}\frac{1}{\...


5

Some near-term quantum algorithms rely on getting lucky with the measurements, and in fact these algorithms will not scale efficiently to large sizes. But most quantum algorithms don't have this problem; it is required that the amount luck needed [i.e. retries] scales only polynomially with the problem size. For example, Shor's algorithm fails if the ...


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 ...


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

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

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 ...


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

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

Yes, this has been thought about. For example, the plan for scaling up ion trap computers involves having multiple "modules", each with a few dozen qubits. When qubits in separate modules need to interact, they are moved to the same module using quantum teleportation or some other quantum channel. Each "module" is like a little quantum computer in a cluster, ...


3

None. The quantum race is lead by those entities capable of building the most powerful quantum computer and it are enterprises like IBM, Google, Intel, Microsoft, D-Wave that are currently building the most powerful quantum computers. So it are enterprises that are leading this race and not countries.


2

When we take about hardware specs for classical computers, we are getting some information about the kind of things we can do with the device. For a circuit based quantum computer, the relevant number is how many fault-tolerant qubits we have. We can then computer this to the required qubit number for given instances of our favourite algorithm and see what ...


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

Before answering your question, let's go back to classical computing. The classical computer has a processor, which implements functions on bits, and it has varying levels of memory which trade speed of access against volatility and capacity. So, there are hard drives with large, non-volatile, but comparatively slow storage, and RAM, which is volatile, ...


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

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 ...


1

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 ...


1

1 - Not completed in the sense computations suffer from noise. Doing fault-tolerant quantum computations would be a major advancement. 2 - Unless you are wealthy, no. They are hosted by big groups generally. For how many, it is non-exhaustive but you can refer to this link 3 - So far it is just a chip, with big cooling system which takes some storage place....


1

Currently, as far as I know, all gate-model quantum architectures start computation in a ground state, normally denoted as the zero state $|0\rangle$. In theory, any desired input state can be created from this by use of a correct unitary transformation $\rm U$, since by definition the Hilbert space of quantum states is invariant under unitary ...


1

Qubits are indeed the main elements. But in the future hopefully, we will have complementary devices like a qRAM and this will be helpful for many quantum algorithms like the HHL to input in a quanntum form classical data. For a more visual presentation of the qRAM, you can check these slides. The architecture proposed at that time was the bucket-brigade ...


1

One advantage of the transmon design is the additional loop you gain from what you called two-island-design. The yellow flux bias line changes the Josephson energy, thus the resonance of the qubit. You can imagine this as changing the (Josephson) inductance of the SQUID loop being a non-linear LC-resonator. This helps for example in two-qubit gate ...


1

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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