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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  • 1,163
18 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 ...
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18 votes

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 ...
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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 ...
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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, ...
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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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  • 1,163
14 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 ...
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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 ...
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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 ...
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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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10 votes
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State produced by spontaneous parametric down-conversion (SPDC)

Background First of all, I'll use $\lvert H\rangle$ as a horizontally polarised state and $\lvert V\rangle$ as a vertically polarised state1. There are three modes of light involved in the system: ...
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10 votes

What is a qubit?

This is a good question and in my view gets at the heart of a qubit. Like the comment by @Blue, it's not that it can be an equal superposition as this is the same as a classical probability ...
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9 votes

State produced by spontaneous parametric down-conversion (SPDC)

The existing answer does a good job at describing the state that comes from a SPDC configuration at low conversion efficiency, but it's also worth noting that the single-photon behaviour is not all ...
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9 votes

Why do quantum computers have more qubits than classical computers have bits?

"Why do quantum computers have more qubits than classical computers have bits?" They do not. I'm typing on a classical laptop with 500GB (8 x 500 billion bits). Quantum computers do not ...
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8 votes
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How do you apply a CNOT on polarization qubits?

A standard reference for linear optical quantum computing is Kok et al. 2009 (quant-ph/0512071). If one qubit is encoded in the polarization degree of freedom of a single photon, and the second qubit ...
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  • 18.7k
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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How can one imagine entanglement in a non-mathematical way?

What is entanglement? Briefly, entanglement is a type of dependence between subsystems of a composite system. Quantum mechanics can be viewed as a variant or extension of probability theory. In this ...
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7 votes
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Resources for quantum algorithm basics

Most textbooks and lecture courses start with solving the Deutsch problem using quantum computing. Parts 1 to 4 of John Watrous's lecture notes will describe the concepts, starting from basics. By the ...
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7 votes

What is a qubit?

I'll probably be expanding this more (!) and adding pictures and links as I have time, but here's my first shot at this. Mostly math-free explanation A special coin Let's begin by thinking about ...
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  • 3,331
7 votes

Is there any single-logical-qubit physical device out there as of end 2018?

A logical qubit is a very fluid concept. You could use physical qubits as logical qubits. Or, you can encode multiple physical qubits as a single logical qubit. The more physical qubits you use, the ...
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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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What exactly does it mean to embed classical data into a quantum state?

First it is instructive to ask oneself: "how does classical data get into my computer?" In a classical computer, your data is always stored in bits. Because calculations in base 2 are not very ...
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7 votes
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Qubit Connectivity of IBM Quantum Computer

You can pass the parameter coupling_map to execute or transpile. This allows simulating a ...
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  • 3,801
7 votes
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Does a CNOT quantum gate violate no cloning theorem?

Cloning means you have a unitary transforming $U$ that takes $|\psi\rangle|0\rangle $ to the $|\psi\rangle|\psi \rangle $ for all quantum states $|\psi\rangle$. That is, $$ U\big(|\psi\rangle|0\rangle ...
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  • 12.5k
7 votes

Why do quantum computers have more qubits than classical computers have bits?

It is generaly hard to compare classical and quantum computers, as both are based on different calculations paradigmas. Besides number of qubits, it is also important to track inter-qubits ...
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6 votes

How is a single qubit fundamentally different from a classical coin spinning in the air?

How is a single qubit in a Bell state $\frac{1}{\sqrt{2}}(|0\rangle+|1\rangle)$ any different from a classical coin spinning in the air (on being tossed)? For both of them, the probability of ...
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  • 47.1k
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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