33 votes
Accepted

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 ...
  • 1,193
32 votes
Accepted

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 ...
  • 3,497
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 ...
  • 24.8k
20 votes
Accepted

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 (...
  • 1,193
15 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, ...
15 votes
Accepted

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 ...
  • 1,193
14 votes
Accepted

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, ...
  • 3,349
13 votes

How should different quantum computing devices be compared?

This is a greatly debated topic, and I'm not sure there is an answer to your question at the current time. However, the IEEE (Institute of Electrical and Electronics Engineers) has proposed PAR 7131 - ...
  • 519
11 votes

What is the leading edge technology for creating a quantum computer with the fewest errors?

That is indeed the most important question at the moment! Superconducting qubits currently have the biggest devices. But will they continue to scale? Will short coherence times make it too hard for ...
9 votes
Accepted

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
Accepted

How are gates implemented in a continuous-variable quantum computer?

Taking an $n$-mode simple harmonic oscillator (SHO) in a (Fock) space $\mathcal F = \bigotimes_k\mathcal H_k$, where $\mathcal H_k$ is the Hilbert space of a SHO on mode $k$. This gives the usual ...
  • 3,497
9 votes

Is Quantum Biocomputing ahead of us?

"Is Quantum Biocomputing ahead of us?" There has been some work done on biocomputing, quantum computing, spin chemistry, and magnetochemical reactions. Correlated radical pairs — pairs of ...
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9 votes
Accepted

Are there any estimates on how complexity of quantum engineering scales with size?

This is a question that I have been thinking about for more than 10 years. In 2008 I was a student, and I told my quantum computing professor that I wanted to study the "physical complexity" of ...
  • 12.1k
9 votes

Why IBM devices have the same number of qubit and the same architecture, but the error rates are different?

They have different error rates because they are two different physical devices! This relates to the manufacturing processes of these chips. Every device is unique and will have its own fingerprint ...
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 (...
  • 4,898
8 votes

How should different quantum computing devices be compared?

While number of qubits should be part of such a metric, as you say, it's far from everything. However, comparing two different completely different devices (e.g. superconducting and linear optics) is ...
  • 3,497
8 votes
Accepted

What are the fundamental differences between trapped ion quantum computers and other architectures?

Disclosure: while I am not an experimental physicist, I am part of the NQIT project, which is aiming to develop quantum hardware which is suitable to realise scalable quantum computers. The ...
7 votes
Accepted

Can quantum computers design quantum computers autonomously better than other methods?

Sort of, quite possibly, if by degrees This is a speculative, but plausible, answer First of all, how do qubits interact and states evolve with time? The description of how individual qubits evolve ...
  • 3,497
7 votes

How should different quantum computing devices be compared?

IBM is promoting their quantum volume (see also this) idea to quantify the power of a gate model machine with a single number. Before IBM, there was an attempt from Rigetti to define a total quantum ...
7 votes

How are quantum gates realised, in terms of the dynamic?

Generally speaking, a realization of a quantum gate involves coherent manipulation of a two-level system (but this is nothing new to you, maybe). For example, you can use two long-lived electronic ...
7 votes

Why do optical quantum computers not have to be kept near absolute zero while superconducting quantum computers do?

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 ...
  • 19.6k
7 votes
Accepted

How do quantum computers prevent "quantum noise"?

How do we prevent quantum noise in a quantum computer? Well, technically the answer is (at least for most systems): we use ridiculously low temperatures (much colder than space), we shield everything ...
7 votes

Scalability of ion trap quantum computers

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 ...
  • 3,667
7 votes
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Why D-Wave 2000Q requires such a low pressure?

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 ...
7 votes

Is the "Quantum Volume" a fair metric for future, elaborate, high value quantum computations?

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 ...

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