Episode #125 of the Stack Overflow podcast is here. We talk Tilde Club and mechanical keyboards. Listen now
26

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 classical algorithms but forgot to mention that the problem can be solved in polynomial time using minimum weight perfect matching. Denchev showing $10^8$ ...


14

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, but superconducting quantum computers do. So, that answers your second question. To answer your first question, superconducting quantum computers (for example) ...


9

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 qubits that lasted for over 39 minutes; these, however, only had an 81% fidelity rate. (This is for qubits used in computation, not memory storage. For memory ...


8

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 demonstrate a violation of Bell’s inequality. My arguments for this are: It uses a physical degree of freedom (photon polarization) which has since been ...


8

$\require{\mhchem}$ There are almost too many ion species to list that have been used in ion trap based quantum computing or related experiments. The usual choice is one that is, when singly ionized, hydrogen-like which has convenient consequences for their laser spectroscopy: Then a strong, typically $20$ MHz wide transition lies in the UV or blue end of ...


7

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 sum: yes, the arrangement of the ions is one key limitation to scalability, but no, configurations are not currently limited to a single line of atoms. On that ...


7

Ion trap quantum computers hold ions in empty space using electric not magnetic fields. That is impossible using static fields (Earnshaw's theorem) so an alternating field is used. The effect is that charged particles such as ions seek a field minimum; this type of ion trap is also called a quadrupole trap because the simplest (lowest order) field having a ...


6

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 with the D-Wave One, an adiabatic ('analog') computer, is to the "south-pointing chariot" or the "Antikythera mechanism". A lengthy explanation is offered in ...


6

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 experimental realization of an algorithm was indeed Jones' and Mosca's implementation of the Deutsch algorithm in 1998 using an NMR setup. Of course previous ...


6

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. However, this isn’t a limitation in terms of the ease of communication because what you’re probably thinking about is when a linear system has nearest neighbour ...


5

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 Hamiltonian to be implemented, the system to be cooled to sufficiently near the ground state, the anyons to be manipulated, etc. So there's a lot to be done, and I ...


5

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 for integers with more than two prime-factors to be composed. See this paper from D-Wave, Prime factorization using quantum annealing and computational ...


5

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 review, I will try to address your questions with what I can remember from reading that paper and glancing over it again now. For discrete variables(dv), as you ...


4

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 properties: the chance per unit length of down-conversion occurring is proportional to $\chi^{(2)}$; if our material of choice doesn't have good phase-matching ...


4

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 the purity, of the full output state. If the oracle corresponds to a unitary evolution, the purity is 1. The less coherent the smaller the purity. Incidentally, ...


4

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 on the chemistry/material science that are working to clean up the surfaces as much as possible. I had a colleague, Carlo Bradac who worked with our chemistry ...


4

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 made out of. You might typically assume that they're all the same technology (e.g. solid state, photonic,...) but there is no need to do so. There are very good ...


3

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 Stack Exchange post: How do we perform transverse measurements in a two level system? Rotations about the z axis are quite different; they are done by ...


3

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: $$ H=\sum_{n=0}^{N}\omega_{n}^L|n\rangle\langle n|_L+\omega_{n}^R|n\rangle\langle n|_R $$ Written like this, the two spins are not interacting, so we won't get a two-qubit gatewithout ...


3

Superconducting qubits There are several possible approaches. In the most popular scheme the qubits are always coupled, but off resonant and thus the energy conservation prevents the exchange of excitation. With external magnetic flux the qubits can be tuned temporarily into a resonance. The qubits will pick up a phase, which will depend on the state of ...


3

This sort of architecture has certainly been studied more, often under the banner of "Global Control", significantly reducing some of the requirements (in particular, only requiring an ABABAB... repeating structure instead of ABCABC...). I am not aware of any of these ideas having been implemented. I assume this is partly because there are large overheads in ...


3

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 field. The goal being to make them always better of course with better connectivity for instance (a possible factor to take into account).


3

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 fidelities and decoherece times for IBM and Rigetti chips (unfortunately they don't give any results on ion traps and photonics, since these machines are not ...


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 ...


2

Let's assume that your black box processes classical inputs (i.e. a bit string) to classical outputs in a deterministic way, i.e. it defines a function $f:x\mapsto y$. If you can only prepare and measure separable states in that basis, all you can determine is what that function $f$ is. Assuming that all the outputs are different, it could have been ...


2

I'm not exactly sure what you mean by quantumness of your black box. So maybe there are some more sophisticated approaches (similar to the other answer you could use an entanglement witness to show that your black box is not entanglement breaking). However, in general you could perform quantum process tomography (see e.g. arXiv:quant-ph/9611013).


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Here's some relevant work in Optimizing type-I polarization-entangled photons-Radhika Rangarajan, Michael Goggin, Paul Kwiat. Abstract: Optical quantum information processing needs ultra-bright sources of entangled photons, especially from synchronizable femtosecond lasers and low-cost cw-diode lasers. Decoherence due to timing information and ...


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