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34 votes
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How does a quantum computer do basic math at the hardware level?

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 ...
Mithrandir24601's user avatar
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33 votes
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Why do optical quantum computers not have to be kept near absolute zero while superconducting quantum computers do?

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 ...
DanielSank's user avatar
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32 votes
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How do I add 1+1 using a quantum computer?

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. ...
Mithrandir24601's user avatar
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32 votes

Which subatomic particle does each company use in quantum computing?

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 ...
JSdJ's user avatar
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30 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 ...
Andrew O's user avatar
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28 votes
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How are quantum gates implemented in reality?

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 (...
Sanchayan Dutta's user avatar
28 votes

How does a quantum computer do basic math at the hardware level?

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 ...
Craig Gidney's user avatar
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22 votes
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Why do the IBM and Google processors both have 53 qubits?

It's just a coincidence. I can speak from personal recollection on the Google side. Google originally intended to use a 72 qubit chip (Bristlecone) where qubits were essentially directly connected to ...
Craig Gidney's user avatar
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20 votes
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What cryogenic systems are suitable for superconducting qubits?

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 (...
DanielSank's user avatar
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19 votes
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What does it mean for two qubits to be entangled?

For a simple example suppose you have two qubits in definite states $|0\rangle$ and $|0\rangle$. The combined state of the system is $|0\rangle\otimes |0\rangle$ or $|00\rangle$ in shorthand. Then if ...
snulty's user avatar
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18 votes
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How many operations can a quantum computer perform per second?

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 ...
Adrien Suau's user avatar
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18 votes

Is there anything practical that can be done with a single qubit?

Some select examples of single qubit technology that has been used for decades: Atomic clocks are single qubit quantum information processors. (In fact, the natural transition frequency between the ...
Chris Ferrie's user avatar
17 votes
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What is the difference between a qubit and classical bit?

A bit is a binary unit of information used in classical computation. It can take two possible values, typically taken to be $0$ or $1$. Bits can be implemented with devices or physical systems that ...
Kiro's user avatar
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16 votes
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What is the physical representation of a qubit?

This section on Wikipedia collects the most important ongoing attempts to physically implement qubits. For physically implementing a quantum computer, many different candidates are being pursued, ...
Sanchayan Dutta's user avatar
16 votes

Why use U2 and U1 gate in IBM quantum computers?

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 ...
Jay Gambetta's user avatar
15 votes
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What countries are leading this "Global Quantum Computing Race"?

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 ...
whurley's user avatar
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15 votes
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What is the difference between transmon and Xmon qubits?

The transmon is a Josephson junction and capacitor in parallel. Originally, transmons were differential circuits, i.e. two transmons on the same chip were not galvanically connected in any way. In ...
DanielSank's user avatar
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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, ...
auden's user avatar
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13 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 ...
James Wootton's user avatar
13 votes
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What real quantum computers are available for students to use?

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 ...
James Wootton's user avatar
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 ...
Doug McClure's user avatar
13 votes

Why do we have to measure the circuit before sending it to the real quantum computer?

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 ...
Martin Vesely's user avatar
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 ...
auden's user avatar
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12 votes
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What is the difference between a qubit and a quantum state?

There are a few things to distinguish here, which are often conflated by experts because we're using these terms quickly and informally to convey intuitions rather than in the way that would be most ...
Niel de Beaudrap's user avatar
12 votes

How many operations can a quantum computer perform per second?

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 ...
James Wootton's user avatar
12 votes
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The process for transferring qubits between locations

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 ...
Dr. Sarah Kaiser's user avatar
11 votes
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Is Google's 72 qubit device better than D-Wave's machines, which feature more than 2000 qubits?

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 ...
Rob's user avatar
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11 votes

Is Google's 72 qubit device better than D-Wave's machines, which feature more than 2000 qubits?

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 ...
auden's user avatar
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