34

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 ...


27

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 ...


24

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 ...


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 ...


19

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 =...


16

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

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 ...


15

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 ...


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 (...


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) ...


14

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

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 ...


11

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 ...


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

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

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. ...


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

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 ...


10

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 (...


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 ...


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

The DWave machine relies heavily on single-flux-quantum digital control for setting up qubit and coupler operating points, and for carrying out the annealing protocol. Any stray magnetic flux, if present while the chip is cooled through its superconducting transition, will be trapped inside the circuit and can cause it to fail. You can calculate how much ...


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 ...


8

Pressure implies the presence of stray atoms flying around messing things up. The use of a vacuum is required to prevent this, as one of the ways of keeping the device well isolated from unwanted effects. I think that they are just intending the "10 billion times lower than atmospheric pressure" statement to demonstrate how good their vacuum is.


8

An undergrad can create an account with the IBM quantum experience. Users have some points that they can use to run simulations of their design on a real quantum computer. You can use five qubits. I am unaware of any way for someone to use Google's quantum computer unless you get a job with them. Rigetti has a device, but what people use online is a ...


8

Yes, the physical implementation is the constraint. If you look at the image of the processor you'll notice the connections between qubits. This gives you an idea of how you can perform two qubit gates between particular qubits. Here's the documentation on the Tenerife backend. In the section titled Two Qubit gates at the bottom you can read the details. ...


8

As a start, you might want to look at https://arxiv.org/abs/1605.03590, which lays out conservative (i.e., high) qubit and gate requirements for a meaningful quantum chemistry calculation under some pretty reasonable assumptions. The estimates there are on the order of $10^{15}$ total logical gates (not gate depth) over roughly 100 logical qubits, which ...


7

As far as I know the closest answer to your question for applications is given in the recent (still unpublished) work presented at the March meeting by Bibek Pokharel, where he compares graph 3-coloring instances on D-Wave Two, D-Wave 2X and D-Wave 2000Q, all other things staying reasonably equal. The short answer is that all the performance increase is ...


7

Because light, at the right frequencies, interacts weakly with matter. In the quantum regime, this translates to single photons being largely free of the noise and decoherence that is the main obstacle with other QC architectures. The surrounding temperature doesn't disturb the quantum state of a photon as much as it does when the quantum information is ...


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