Questions tagged [physical-qubit]

For questions related to physical realizations of quantum bits i.e. quantum systems living in a 2-dimensional Hilbert space.

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What exactly does it mean to embed classical data into a quantum state?

As the title states. I am a Machine Learning Engineer with a background in physics & engineering (post-secondary degrees). I am reading the Tensorflow Quantum paper. They say the following within ...
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What are some good resources for learning about photonic qubits?

I am interested in resources that would allow me to learn about the physics behind optical quantum computation, and specifically, the physical properties of photonic qubits. I have some good resources ...
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Readout using an NMR spectrum

I'm trying to understand how to read results from an NMR quantum computer. According to Nielsen and Chuang: The principal output is the free induction decay signal $$V(t) = V_0\text{Tr}\left[e^{-...
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How to deal with the relative phase from the evaluation of the not interacting qubit?

Every qubit is a realization of some two-level quantum system with $\left| 0 \right\rangle$ and $\left| 1 \right\rangle$ states. These states have their energies $E_0$ and $E_1$ respectively. By ...
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How do I explain qubits to my cousin who is 8 year old?

How do I explain qubits to my cousin who is 8 years old?
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Is there anything practical that can be done with a single qubit?

Is there anything practical that can be done with a single qubit? And by "practical," I mean a problem that can be solved or information that can be stored. I realize that one practical thing that ...
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Building a DIY quantum computer - how to reduce a noise?

I refer to A DIY Guide to Building a Quantum Computer and Researchers advance noise cancelling for quantum computers. With my friends, I would like to build our simplified quantum computer. We are ...
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Can a classical limit of qubit emerge on a classical computer? [closed]

Hello, I am uncertain; however, today at a toy museum, I might have captured a qubit on a classical computing structure, “ a broken 90-marble abacus,” as shown in the attached photos above. I did ...
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Is quantum superposition state a truth or an assumption?

Please, be patient with my question I already read that there is a heuristic that makes superposition a fact of reality. In addition, this superposition, when observed, it has a state of 0 or 1. ...
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Why are superconducting qubits the most popular approach to build a quantum computer?

There are many different physical implementations of qubits: NMR, spin qubit, N color center in diamond, superconducting qubit, trapped ions, photonics quantum computing, topological qubit, etc. ...
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Atom magnetic moment caused by orbiting electron

In Nielsen and Chuang Quantum Computation and Quantum Information book section 1.5.1 describing the Stern-Garlach experiment, it says: "Hydrogen atoms contain a proton and an orbiting electron. You ...
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What is the problem in demonstrating quantum supremacy?

I am new to the quantum world and it's computing. But it accidentally hit in my mind that DWave built a quantum computer with 2000 qubits which can be use to simulate the whole observable universe or ...
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What is the difference between superposition of $|0\rangle$ and superposition of $|1\rangle$ at physical level?

If we apply Hadamard gate 2 times on a qubit $|0\rangle$ then we will get $|0\rangle$. So there must be something else which is unique for both $|1\rangle$ and $|0\rangle$ which is constant even in ...
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Why do the IBM and Google processors both have 53 qubits?

As I understand from this IBM post both the IBM and Google teams have independently built 53-qubit processors. What is the significance of the number 53? It is purely coincidental, or is there a ...
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What's the difference between observing in a given direction and operating in that same direction?

So starting with an up particle: $$ \lvert \uparrow \rangle = \begin{bmatrix} 1 \\ 0 \\ \end{bmatrix} $$ My understanding is that you can measure $\lvert \uparrow \rangle$ in $X$ and have ...
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What are the physical reasons behind noise/errors in quantum computers?

We know that the current quantum computers are noisy/error-prone. They are susceptible to gate errors (single qubit and multi-qubit), decoherence (relaxation and dephasing), readout error, etc. I want ...
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Why don't qubits continuously rotate in the $z$ direction due to free time evolution?

If we have a physical qubit with energy eigenstates $|0\rangle$ and $|1\rangle$ with energy separation $\Delta E$ its Hamiltonian in the absence of any interaction is $$H=\hbar\frac{\Delta E}{2}\...
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What is the difference between a cooper pair box and a transmon qubit?

These two kinds of qubits are both classified under "charge qubits", looking for them, I always find the same circuit representation. Occasionally, I find one where there's a SQUID instead of the ...
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What design considerations set the frequency bounds for superconducting qubits?

Superconducting qubits generally have frequencies within the range of 4 - 8 GHz. What design considerations give the upper and lower bounds for what is a feasible design. I.e, why can't they be higher ...
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Excitation of a superconducting circuit

What do we mean by exciting a superconducting circuit--which represent a qubit, how they are get excited inside? how the two states of the qubit are represented in this circuit?
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Measuring a quantum wire at t > 0

I saw on the site Brilliant.org a question witch is: "Let's say I'm given a quantum wire, I measure it and find it charged "on". If I come back some time later, what state should I measure?" And the ...
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Charge Qubit vs Transmon

I want to know the big difference, in terms of fabrication (circuit) between a charge Qubit and a Transmon, how is a Transmon less affected by noise ? Many thanks
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I thought we had already passed this error and qubit threshold?

As outlined in Quantum Computing Progress and Prospects published this year, on page 169-171 there is the diagram showing that we are at the 10's of qubits mark. Looking into the diagram further there ...
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States of a qubit in a DC-SQUID

Does anybody of you know what are the two states $|0\rangle$ and $|1\rangle$ of a qubit in a DC-SQUID (2 Josephson junctions in a loop)?
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Physical realization of qubits with superconducting Josephson junctions

Does anybody know how to explain briefly how to realize qubits (initialization, states transition, measurements,..) with superconducting Josephson junctions, say DC-SQUIDS?
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How much power do quantum computers use based on the number of qubits being allocated?

How much power do quantum computers use based on the number of qubits being allocated? If it's not based on qubits, What determines how much power quantum computers use? Note: A similar question was ...
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Can entangled qubits be disentangled without using code or software?

Is there a way to revert two entangled qubits to a separate state without using code...like restarting a quantum computer? Note: By "restarting a quantum computer", I do not mean restarting a virtual ...
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Converting a qubit's state to a binary value in Q#

In Q#, How do I store a qubits state in a binary-based disk / hard drive for use by regular digital programs? Is this even possible?
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What “states” can a qubit have if it doesn't have values?

I was told that qubits don't have a value, they have a "state". What does that mean? What are the different "states" that a qubit can have (like bits can be either 1 or 0)?
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How to interpret $-\rvert1\rangle \otimes \rvert1\rangle = -\rvert11\rangle$?

I'm having trouble accepting, intuitively, that $-\rvert1\rangle \otimes \rvert1\rangle = -\rvert11\rangle = \rvert1\rangle \otimes -\rvert1\rangle$. It's my understanding that $ -\rvert1\rangle$ ...
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How do physical implementations of Z gate selectively affect $\lvert1\rangle $ basis vector?

The Pauli Z gate inverts the phase of $\lvert1\rangle $ while leaving $\lvert0\rangle$ unaffected. When I think about how $\lvert1\rangle $ and $\lvert0\rangle$ are physically realized, however, as ...
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Physical qubit of optical quantum computer

I was researching how optical quantum computers work and discovered a KLM protocol that allows for building quantum gates using linear optics elements like mirrors, beam splitters, and phase shifters. ...
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How is a physical qubit measured and how is the result interpreted?

To my understanding, most of the qubits we use today are just Josephson junctions coupled to a resonator that is triggering the qubits to go to different states by using microwave pulses. The same ...
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What is the charging energy in relation to a superconducting transmon?

The transmon achieves its reduced sensitivity to charge noise by significantly increasing the ratio of the Josephson energy to the charging energy. What is this charging energy?
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How many physical qubits are used when I run a program on a IBM Q computer?

How do I find out how many physical-qubits I'm using when I run my program in an IBM Q backend quantum computing system?
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What noise channel that is not coming from the control can lead to XY qubit rotations?

In a semiclassical treatment, the noise Hamiltonian of a quantum system can be treated as a summation over its noise channels in the following way: $$\hat{H}_{\text{noise}}=\sum_{k}\beta_{k}\left(t\...
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What does an observable in a different basis mean physically?

So when you have an observable which is the measurement operator acting on the state you get a different result than in a different basis. What does it mean in physical terms? Does that mean writing ...
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Can one make NV centred type qubits from analogus materials in the periodic table?

Can one replace Nitrogen with Phosphorus and Carbon with Silicon and so on? What would be the conditions that would preclude such a realisation? Are there any implemenations so far?
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Interaction of an RF pulse with transmon qubit

When an RF pulse is interacting on resonance with a transmon qubit, it leads to rotation of the qubit around an axis in the XY plane (in a reference frame rotating in the transition frequency of the ...
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Rabi oscillations with different energy differences

Please check this Rabi oscillations image1: Now, let's say I want to create a superposition of E1 and E2, so I started an EM field tuned to a frequency related to E2-E1. But, let's say there is ...
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Hamiltonian of the valence electron of Yb+ ion

Let's say I have chosen the valence electron of Yb+ as my qubit. I want to consider it's hyperfine structure as the two-level energy states. Are my following assumptions correct? If the magnetic ...
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Non-layperson explanation of why a qubit is more useful than a bit?

I have a computer science and mathematics degree and am trying to wrap my head around quantum computing and it just doesn't seem to make sense from the very beginning. I think the problem is the ...
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Polarization and qubit information

It is my understanding that light, and its polarization, is used to transfer information in quantum computers, but how can the information encoded in say, an electron also be stored in light? I ...
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Is there any single-logical-qubit physical device out there as of end 2018?

By my first impression, there are many-qubits computers out there and more to come, as to follow the press. Now a closer look reveals that it's all about designing and building physical qubits. ...
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What limits scaling down the size of superconducting qubits?

There are multiple ways of building a qubit: Superconducting (transmons), NV-centers/spin-qubits, topological qubits, etc. The superconducting qubits are the most well-known qubits and are also the ...
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How do we physically initialize qubits in a Quantum register?

In quantum algorithms we need to initialize the qubits at the start of our algorithm in some quantum register. Suppose that if we are working with a four qubit quantum register we can initialize the ...
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Are qubits the only elements required to build a quantum computer?

The power of a quantum computer is often described in terms of how many qubits it has. Does that mean that quantum computers can be run using only qubits, and nothing more? Wouldn't we require some ...
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What is the significance of recent demonstration of a passive photon–atom qubit swap operation?

In reference to this recent nature article: https://www.nature.com/articles/s41567-018-0241-6 Specifically, does this warrant a new type of gate?
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Difference between coherence transfer, polarization transfer and population transfer?

I asked a question on Physics Stack Exchange but no one answered the question and I didn't get enough views on it. I am asking it on QCSE because the question is related to experimental quantum ...
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Transmons and cooper pair box qubits: two islands or a single island and a reservoir

In scientific literature, one typically describes Cooper pair boxes as a small superconducting island coupled to a superconducting reservoir (say, a large ground plane of superconducting metal, or a ...