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 ...
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  • 1,183
32 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 ...
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  • 3,477
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 ...
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  • 4,792
27 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. ...
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  • 3,477
26 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 (...
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24 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 ...
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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 (...
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  • 1,183
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 ...
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  • 4,542
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 ...
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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 ...
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  • 519
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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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 ...
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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

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 ...
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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 ...
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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 ...
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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 ...
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  • 2,229
11 votes

Why is it harder to build quantum computers than classical computers?

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 ...
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10 votes
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How much faster is “D-Wave Two” compared to its predecessor?

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 ...
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10 votes
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What are some popular myths or common misconceptions about quantum computing?

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 ...
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10 votes
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Do real commercial quantum computers exist?

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 ...
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  • 1,132
10 votes

Topology graphs of IBM's quantum computers

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 ...
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  • 2,403
10 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 ...
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9 votes
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How power-efficient are quantum computers?

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 ...
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9 votes

How to build a quantum computer in your house?

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 (...
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  • 4,792
9 votes
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How are IBM's 127 qubits more potent than the 5760 qubits D-Wave - Advantage_system6.1?

They are different kinds of quantum computers: The D-Wave is a quantum Annealer. A rather specific (non-general purpose) device that can solve QUBO-type problems. IBM offers universal gate-based ...
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