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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31 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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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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14 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 ...
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9 votes
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Is the Solovay-Kitaev theorem relevant for modern hardware?

I think you'll find that most hardware, at the hardware level, gives you arbitrary single qubit rotations. So, in that sense, it is true that Solovay-Kitaev is not directly applicable to current ...
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8 votes

What limits scaling down the size of superconducting qubits?

Getting enough capacitance and maintaining coherence essentially set the size limit. A superconducting qubit, for the purposes of answering this question, can be imagined as an oscillator consisting ...
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8 votes
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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 ...
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8 votes
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Understanding Google's “Quantum supremacy using a programmable superconducting processor” (Part 2): simplifiable and intractable tilings

TL/DR: The two-qubit gates are going by the moniker "Sycamore gates" in the paper, and it appears that they would ideally want to explore more of the $(\phi, \theta)$ phase-space but for ...
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7 votes
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Interaction of an RF pulse with transmon qubit

Kirchhoff to Lagrangian Let's approximate the transmon as a parallel LC resonant circuit. Suppose we connect a voltage source through a coupling capacitor $C_d$ (d for "drive") to a transmon qubit. ...
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7 votes
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Understanding Google's “Quantum supremacy using a programmable superconducting processor” (Part 3): sampling

What does "obtaining samples" mean in this context? The same thing it means in a more classical context. Consider the probability distribution of the possible outcomes of a (possibly biased) coin ...
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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 ...
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6 votes

What are the pros/cons of Trapped Ion Qubits, Superconducting Qubits and Si Spin Qubits?

It think the (very) short answer is that there is not a preferred platform yet. This is why there are very active research communities around each of these technologies. Often if someone says ...
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6 votes
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Why attenuator and not filters for QC driving line

(1) Both filters and attenuators are used Let me just start by saying that non-attenuating filters have not been completely ruled out by people working in the design of cold quantum computers. I will ...
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6 votes

How are the IBM's and Google's Hadamard gates fabricated and operated?

A Hadamard gate isn't usually a physical object that you pass qubits through. In the case of superconducting qubits, the Hadamard gate is performed by bouncing microwaves off of the qubits. It doesn't ...
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6 votes
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What are the pros and cons of the most popular QC architectures?

The major quantum computing providers that comes to me right the way are usually: IBM Quantum (superconducting qubit) Google Quantum (superconducting qubit) Honeywell Quantum (trapped ions) ...
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5 votes

What are the pros/cons of Trapped Ion Qubits, Superconducting Qubits and Si Spin Qubits?

Here's a paper comparing Trapped Ion and Superconducting (the main competitors right now) from the group at UMD which compares their trapped ion system with IBM's transmon (superconducting) system. If ...
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5 votes
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How is a physical qubit measured and how is the result interpreted?

What follows turned out to be a rather technical explanation, so I'll start with the main point: The qubit state can change the resonator's state, and the resonator's state can be easily measured only ...
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5 votes

What is the difference between transmon and Xmon qubits?

In one sense, the Xmon qubit is a transmon qubit, in that they both operate in the $E_J>>E_c$ regime of the CPB Hamiltonian and take advantage of the exponentially suppressed charge noise vs. ...
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5 votes

Superconducting qubit researchers: Do your TLS's move?

The resonance frequencies of TLS fluctuate due to their interaction with neighboring TLS, which occurs through electric dipole interaction or the local mechanical strain in the material. If a TLS at ...
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5 votes
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Are composite gates within superconducting hardware implemented as a single pulse or as a series of pulses?

Typically superconducting qubits can be controlled independently, eg. due to having separate control wires. So you'd have independent pulses in each control wire applying a gate to the respective ...
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4 votes

Rotating about the y- or z-axis of the Bloch sphere

For superconducting qubits, x and y rotations are usually both done with microwave pulses, and as you said the phase of the pulse determines the rotation axis. See mathematical details in this Physics ...
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4 votes
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Understanding Google's “Quantum supremacy using a programmable superconducting processor” (Part 1): choice of gate set

While a follow-up question asks for the motivation behind the two-qubit gates used in Sycamore, this question focuses on the random nature of the single qubit operations used in Sycamore, that is, the ...
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  • 7,042
4 votes

How are the IBM's and Google's Hadamard gates fabricated and operated?

Fundamentally, a device such as an IBM quantum computer interacts according to a Hamiltonian, which might have some time-varying parameters. For example, for a single qubit, it might look like: $$ H=...
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4 votes
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Is every computer in IBM's fleet based on superconductivity?

Yes. IBM uses superconducting Transmon qubit. Here is a quote from IBM's website: At the heart of IBM quantum systems is the transmon qubit. Successive generations of IBM Quantum processors have ...
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3 votes

Why are superconducting qubits the most popular approach to build a quantum computer?

By coincidence, this article just came out on Ars Technica which might answer some of your questions. (This is not an endorsement of everything written in that article. But the author basically asked, ...
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3 votes
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How do frequency-tunable microwave resonators work?

There's a superconducting circuit element called Josephson junction, which is roughly a nonlinear inductor. The inductance of a Josephson junction depends on current via the relation $$L(I) = \frac{...
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