28 votes
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Is there proof that the D-wave (one) is a quantum computer and is effective?

There is still a search for problems where the D-Wave shows improvement over classical algorithms. One might recall media splashes where the D-Wave solved some instances $10^8$ times faster than a ...
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  • 1,679
14 votes
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Why must quantum computers be kept near absolute zero?

Well, first, not all systems must be kept near absolute zero. It depends on the realization of your quantum computer. For example, optical quantum computers do not need to be kept near absolute zero, ...
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  • 3,341
13 votes
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Changing one qubit causes changes in another independent qubit on ibmqx2 in IBM Q Experience

Thanks for pointing this out! It turns out that this device was mis-calibrated in a way that was leading to that behavior. We just fixed the calibrations, so the problem should be gone now. I ...
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12 votes
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Who built the first quantum computer using at least two qubits?

What is a qubit? And what is a quantum computer? Any claim about about which is first will depend on our definitions. One suggestion might be the 1981 experiment by Aspect, Grangier and Roger to ...
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12 votes
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What is the longest time a qubit has survived with 0.9999 fidelity?

Well, for the longest coherence time ever, I'm finding this Science from 2013 entitled Room-Temperature Quantum Bit Storage Exceeding 39 Minutes Using Ionized Donors in Silicon-28, which indicates ...
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  • 3,341
8 votes

What is the status of confirming the existence of anyons?

It depends what you mean by the 'existence' of anyons. One way is to engineer a Hamiltonian which leads to quasiparticles (or other defects) that have anyonic statistics. This will require the ...
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8 votes

Is there proof that the D-wave (one) is a quantum computer and is effective?

Is there proof that the D-wave (one) is a quantum computer and is effective? D-Wave Video - Offers an explanation of: "How do we know ...": https://youtu.be/kq9VqR0ZGNc One analogy you might make ...
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  • 2,229
8 votes

Who built the first quantum computer using at least two qubits?

It's difficult to define the point where an experimental setup is a quantum computer. But the crucial feature of a quantum computer is that it's able to perform a quantum computation. The first ...
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  • 2,363
7 votes

Scalability of ion trap quantum computers

You may want to check out this Schaetz et al, Reports on Progress in Physics of 2012 "Experimental quantum simulations of many-body physics with trapped ions" (alternate link in semanticscholar). In ...
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  • 3,647
6 votes

Scalability of ion trap quantum computers

While I’m not an experimentalist, and have not studied these systems in any great depth, my (crude) understanding is the following: In ion traps you (more or less) have to trap the ions in lines. ...
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6 votes
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Quantum simulation of environment-assisted quantum walks in photosynthetic energy transfer

One major idea there seems to be that the "environment" (quantum decoherence) assists or optimizes the transport of a signal The idea that photosynthetic systems are doing a Grover search or ...
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6 votes

Is "probabilitistic,universal, fault tolerant quantum computation" possible with continuous values?

To start off, I would really suggest you to read this review on "Quantum information with continuous variables(cv)". It covers most of your questions with cv architecture. Since it is a very big ...
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  • 388
6 votes

Changing one qubit causes changes in another independent qubit on ibmqx2 in IBM Q Experience

This was a readout crosstalk error that has now been resolved.
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  • 2,159
5 votes

Passive improving of nanodiamond surfaces for NV centers?

I have worked with NVs in nanodiamonds a little bit, and you are totally right, surface characteristics have a huge influence on how far we can push them. There are definitely multiple groups working ...
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5 votes
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Which is the highest number factorized by QC in a non-specific experiment?

The answer is $N = 200\,099$. Shor's algorithm is not the only way to factorize integers. In fact, it is also possible to factorize integers with an optimization approach. This approach even allows ...
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  • 1,422
5 votes

What is the status of confirming the existence of anyons?

A more definite claim of detection of abelian anyons appeared in 2020: H. Bartolomei et al.: "Fractional statistics in anyon collisions", Science 368, 173-177 (2020) (arXiv:2006.13157) J. ...
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5 votes

State of the art gate speeds and decoherence times

I guess your best shot would be to look for experimental comparisons like this one on Arxiv. But I am not aware of a tracking. I do not think we can consider having a "state of the art" in this ...
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  • 4,574
4 votes

What is the longest time a qubit has survived with 0.9999 fidelity?

Answer: Fidelity of 0.9999 at 1.08 seconds in 2013: http://science.sciencemag.org/content/342/6160/830.full?ijkey=uhZaDNPnwgTdA More details: The $T_2$ was 180 minutes, or 3 hours. What about the ...
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4 votes

Can one interrogate black boxes for quantum coherence?

Why not input one half of a maximally entangled state as the input to the black box (so that half has the same dimension as the input dimension)? Then you could test your favourite measure, such as ...
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4 votes

Improving probability of spontaneous parametric down conversion

Your question asks two questions that are less-related than you might hope. First, how do we increase the probability of down-conversion occuring? This is fundamentally a question about material ...
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4 votes

Rotating about the y- or z-axis of the Bloch sphere

For superconducting qubits, x and y rotations are usually both done with microwave pulses, and as you said the phase of the pulse determines the rotation axis. See mathematical details in this Physics ...
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  • 1,183
4 votes
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Breakthroughs in quantum computing using non-standard quanta

The only two quasi-particle quanta for which I know there to be active research in quantum computing are phonons and anyons. Phonons: That state-of-the-art is given my answer here: Phononic Quantum ...
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4 votes

State of the art gate speeds and decoherence times

You could also look at the following webpage: https://quantumcomputingreport.com/scorecards/qubit-quality/ where they provide recent (I'm not sure how often they update this scores) values for gate ...
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4 votes
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What is the significance of recent demonstration of a passive photon–atom qubit swap operation?

This does not warrant a new type of gate. When we write down quantum circuits, each 'wire' corresponds to a single qubit. However, we do not (usually) specify what technology any of these qubits is ...
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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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4 votes
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Simple program produces consistently skewed results in IBM Q Experience

It seems like the skew is indeed high on qubit 0. I ran a single Hadamard followed by measure on this qubit, and see about 13% skew. The other qubits on this device seem fine (less than 2% skew). ...
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  • 1,562
3 votes

Conditional Phase Gate Superconducting Qubits

Each of the two spins, $q\in\{L,R\}$, has a bunch of energy levels $\{|n\rangle_q\}$, each at energy $\omega_{n}^q$. In other words, the basic Hamiltonian of the spins is: $$ H=\sum_{n=0}^{N}\omega_{n}...
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