# Tag Info

## Hot answers tagged physical-realization

37

We know exactly, in theory, how to construct a quantum computer. But that is intrinsically more difficult than to construct a classical computer. In a classical computer, you do not have to use a single particle to encode bits. Instead, you might say that anything less than a billion electrons is a 0 and anything more than that is a 1, and aim for, say, two ...

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Firstly, a classical computer does basic maths at the hardware level in the arithmetic and logic unit (ALU). The logic gates take low and high input voltages and uses CMOS to implement logic gates allowing for individual gates to be performed and built up to perform larger, more complicated operations. In this sense, typing on a keyboard is sending ...

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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 referred to as superconducting qubits. The qubit states are the various charge levels that can exist on the circuit; since the lowest two levels are separated in ...

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I was looking for why optical quantum computers don't need "extremely low temperatures" unlike superconducting quantum computers. Superconducting qubits usually work in the frequency range 4 GHz to 10 GHz. The energy associated with a transition frequency $f_{10}$ in quantum mechanics is $E_{10} = h f_{10}$ where $h$ is Planck's constant. Comparing the ...

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As per the linked question, the simplest solution is just to get the classical processor to perform such operations if possible. Of course, that may not be possible, so we want to create an adder. There are two types of single bit adder - the half-adder and the full adder. The half-adder takes the inputs $A$ and $B$ and outputs the 'sum' (XOR operation) $S =... 25 One can replicate any quantum gate or at least get arbitrarily close using sufficient number of CNOT, H, X, Z and$\pi/8$rotation gates. That is because they form a universal set of quantum gates (refer to: M. Nielsen and I. Chuang, Quantum Computation and Quantum Information, Cambridge University Press, 2016, page 189). Be careful here. Clearly, we cannot ... 22 Here is my process for doing arithmetic on a quantum computer. Step 1: Find a classical circuit that does the thing you're interested in. In this example, a full adder. Step 2: Convert each classical gate into a reversible gate. Have your output bits present from the start, and initialize them with CNOTs, CCNOTs, etc. Step 3: Use temporary outputs. If ... 19 Is a dilution refrigerator the only way to cool superconducting qubits down to 10 millikelvin? There's another type of refrigerator that can get to 10 mK: the adiabatic demagnetization refrigerator (ADR).$^{[a]}$why is dilution refrigeration the primary method? To understand that, let's talk about one of the main limitations of the ADR. How an ADR ... 18 Giving an estimate for a generic quantum chip is impossible as there is no standard implementation for the moment. Nevertheless, it is possible to estimate this number for specific quantum chip, with the information provided online. I found information on the IBM Q chips, so here is the answer for the IBM Q 5 Tenerife chip. In the link you will find ... 15 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) ... 15 There are several countries that are actively participating in the "Quantum Race", most of which are making significant investments. The estimated annual spending on non-classified quantum-technology research in 2015 broke down like this: United States (360 €m) China (220 €m) Germany (120 €m) Britain (105 €m) Canada (100 €m) Australia (75 €m) Switzerland (... 15 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 (... 13 The first cloud device was made available back in 2013. It is a photonic chip at the University of Bristol. Though it is an example of something we could build quantum computers from, it is quite different from the usual 'circuit model' architecture. Then 2016 brought some devices from IBM. There are 5 qubit processors anyone can use with the Quantum ... 12 You are totally right in your assumption about transporting qubits from Alice to Bob implies something physical. Usually problems/situations that have this setup of a transmission between two parties are called quantum communications. These problems/situations sometimes disambiguate by calling their qubits "flying qubits" which are almost always photons. ... 11 There are two points I'd make here. D-Wave's computer and Google's computer are fundamentally different. D-Wave's computer is a quantum annealer. Imagine a landscape with some grassy hills. If you put a ball at the top of the hill, it will roll to a local minima, or even the minimum - in this case, a valley. Similarly, a quantum annealer has the qubits as ... 11 Short explanation: D-Wave implements quantum annealing, while Google has digitized adiabatic quantum computation. Lengthy Explanation: D-Wave advertises their line of quantum computers as having thousands of qubits, though these systems are designed specifically for quadratic unconstrained binary optimization. More information about D-Wave's manufacturing ... 11 What you call a black box is simply isolating the quantum system that stores (or represents) your qubits from the environment. This can be done in several ways depending on your physical realization. For example, in an ion trap based quantum computer, one uses states of a single ion to represent a qubit, and isolates that from the environment by levitating ... 11 There's many reasons, both in theory and implementation, that make quantum computers much harder to build. The simplest might be this: while it is easy to build machines that exhibit classical behaviour, demonstrations of quantum behaviour require really cold and really precisely controlled machines. The thermodynamic conditions of the quantum regime are ... 11 There is an important difference between physical operations and logical operations. Physical operations that will be slightly imperfect, performed on qubits that are also imperfect. The rate at which these can be performed depends on what physical system is being used to realize the qubits. For example, superconducting qubits can perform two qubit gates (... 10 As Troyer and Lidar saw no speed increase with the D-Wave 1 compared to classical computers, the D-Wave 2 benchmark figure reported in 2013 of 3600 times as fast as CPLEX (the best algorithm on a conventional machine) suggests the D-Wave 2 is 3600 times as fast as the D-Wave 1. However: the results are in a pretty restricted set of parameters, so this may ... 10 1. Quantum computers are powerful because they act in many universes at once This is an oversimplification based on the MWI at best. I don't think it has any pedagogical value. It needs to stop being repeated. Every journalist I talk to asks whether it is a good thing to write. I always say no. 2. Quantum computers/physics is weird and random Anyone not ... 10 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 ... 10 That depends on your definitions of "commercial" and of "quantum computer". The company D-Wave Systems has been offering what they call quantum computers commercially since 2011. Many things seem to point towards those being adiabatic quantum computers (though people disagree on this). That doesn't quite fit the kind of quantum computers that are becoming ... 10 The graph shows you how the physical qubits are connected together on the real device you will be using. For example, on the graph you put, qubit 0 has a physical connection to qubit 1 and qubit 14 on the quantum device but is not connected to qubit 12. This graph is really important when Qiskit tries to map a circuit to the quantum device because it shows ... 10 Note that you do not measure anything before sending to QC. What you actually send to QC is a sequence of instructions telling QC what to do. The measurement is the last instruction necessary for obtaining results as pointed out in the other answer. 9$\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 ... 9 As usual, it is too soon to make comparisons like this. The power consumption of a device will depend strongly on the architecture it uses, for one. However, in principle, there is no reason to suspect that quantum computers would consume more energy than classical devices performing the same operations. Indeed, one would expect the opposite, the ... 9 If your intent is to understand Gil Kalai's arguments, I recommend the following blog post of his: My Argument Against Quantum Computers: An Interview with Katia Moskvitch on Quanta Magazine (and the links therein). For good measure, I'd also throw in Perpetual Motion of The 21st Century? (especially the comments). You can also see the highlights in My ... 9 A serious answer: you pretty much can't. It's not that you in particular can't, it's that no one can. Huge companies pour in huge amounts of money to try and make a proof-of-concept quantum computer (there is actually no 'proper' quantum computer yet). A slightly less serious answer: some odd$10-100\$ Million would get you started I would say. It all depends ...

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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 ...

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