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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How do we know that a superconducting circuit is a quantum system?

In introductions to superconducting qubits, we often take the classical LC circuit and then quantize the operators. Why are we allowed to do this? I do not see this derivation anywhere -- what ...
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How are gates implemented in a transmon qubit?

A transmon qubit is fundamentally in LC circuit. How are gates implemented in a transmon qubit? How do we know what voltage corresponds to the $\sigma_x$ gate for example?
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In a transmon qubit, what corresponds to the ground and excited states?

In a transmon qubit, we have ground and energy eigenstates but what is actually happening in the circuit here? How do the current and voltage look? It looks like its made up of things you would find ...
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911 views

Why can't we simulate a Qubit using classical computer?

I am completely a noob in terms of quantum computing, have watched several videos to understand what Quantum computers are trying to achieve. I am a programmer of classical computers. We have a ...
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What are the natural, synthetic, identical, heterogeneous qubits? Can anyone explain with examples?

I am reading a paper from 2016, "Quantum computing with atomic qubits and Rydberg interactions: Progress and challenges", In section 2.1, a debate on whether identical or heterogeneous ...
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Is there an intuitive way to conceptualize the ground and excited states of a transmon qubit?

I understand how a transmon qubit is analogous to an LC circuit, but has an anharmonic energy spectrum due to the nonlinearity of the Josephson junction. I also understand how to write out the ...
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1answer
166 views

Does a photonic quantum computer control a single photon?

Does a photonic quantum computer control a single photon and use it to represent a single qubit? I think ion trapped quantum computers use a single ion to represent a qubit. I would like to know how a ...
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What is the role of Josephson junctions and biases in the D-Wave qubit structure?

On the D-Wave 2000Q there are up to 2048 qubits and 5600 coupler. A commonly used picture of one of such qubits is attached below. Each qubit is realized by a loop of niobium controlled by Josephson ...
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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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1answer
57 views

Why in quatum coin toss, when starting from state $|1\rangle$, we get $|-\rangle$?

I was reading qiskit's text book, there I found that for a Double quantum coin toss, we have negative probability amplitude for $|1\rangle$ state when we starts from $|1\rangle$ state. Link : https://...
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How to design a RF-SQUID qubit by Qiskit Metal?

I am now learning how to use Qiskit Metal to design superconducting circuits. I found that there are only several transmon qubits in the library. Can I design an RF-SQUID qubit by using Qiskit metal? ...
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What is a qubit?

What is a "qubit"? Google tells me that it's another term for a "quantum bit". What is a "quantum bit" physically? How is it "quantum"? What purpose does it serve in quantum computing? Note: I'd ...
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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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What are the pros and cons of the most popular QC architectures?

I notice numerous companies and research groups around the world trying to build QC's with wildly different approaches. Intuitively, this suggests to me that QC is still very early in its maturation - ...
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1answer
37 views

How to re-ordering physical qubit location into desired one?

Suppose original quantum circuit have 4 qubits and some quantum operation. Also suppose the result of original quantum circuit state is '1100'. Then apply qubit mapping then mapping state is like ...
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60 views

Why does $(XZ\otimes I)|\Phi^+\rangle$ equal the Bell state $|\Psi^-\rangle$?

I'm slightly confused by the solution provided below by a suggested solution online to convert |$\phi^+$⟩ to |$\psi^-$⟩. I tried doing the operation XZ but I got $\frac{1}{\sqrt2}$(|10⟩-|01⟩) instead ...
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83 views

How do the extra energy levels of a transmon qubit affect computation/fidelity?

I was reading about transmon qubits, and I know that they are not true two-level systems. Are there any math/papers which talk about how those extra energy levels affect the computation? I'm assuming ...
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1answer
64 views

What's free evolution for a period T?

I am currently studying a model of a quantum (atomic) clock. And in this paper, I came across the term "Free evolution for a period T": Free evolution for a period T where a phase ...
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129 views

Does a quantum computer exist today? [closed]

I agree that this may not be an easy question to answer. But lately, I increasingly come across the fact that many news materials and research papers in the field of quantum technologies say that at ...
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Qubit Connectivity of IBM Quantum Computer

When simulating a quantum circuit using the qiskit qasm_simulator, the qubits that you assign to the quantum circuit may be ...
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1answer
45 views

What does a gate electrode do in the context of a quantum dot?

I am trying to understand the creation of qubits through the use of quantum dots, but I have always had some trouble understanding the function of the gate electrode. I know it can control the ...
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1answer
95 views

Does a CNOT quantum gate violate no cloning theorem? [duplicate]

I am a curious quantum computing learner. :) Once observe the CNOT gate: as you can see there it converts a |+> to |-> in the top or to say in another way it clones the |-> state. So does ...
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Clean way of approximating an oscillator by a qubit

Let me consider a (quantum) Harmonic oscillator of (bare) Hamiltonian: $$H_0=\frac{p^2}{2m}+\frac{1}{2} m \omega^2 x^2$$ (I can add to it an anharmonicity $V=\alpha x^4$ if necessary for the purpose ...
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1answer
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Is it possible to perform quantum computation between different Hilbert spaces?

Let us consider a protocol between Alice and Bob. Alice works in a $2^n$-dimensional Hilbert space $\mathcal{H}_A$, using $n$ qubits. Bob works in a $(1+2^n)$-dimensional Hilbert space using qdits. ...
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1answer
130 views

How significant is achieving electronic transport in a single qubit at room temperature?

This is a follow-up to a previous question I asked: How promising is the possibility of carbon-based qubits to make a qubit that’s stable at room temperature? I am trying to assess the significance of ...
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1answer
145 views

What is the difference between SWAP & Bridge gates?

While I'm studying about Qubit allocation (mapping), I wonder what the differences between SWAP and Bridge gates are. I know that a SWAP gate and Bridge gate can be represented with 3 CNOT gates and 4 ...
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363 views

What's the default value for a quantum register?

I've just started to learn Quantum computing with Qiskit: I'm reading the Qiskit documentation and the book "Quantum Computing in Practice with Qiskit® and IBM Quantum Experience®". In this ...
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991 views

How can one imagine entanglement in a non-mathematical way?

I understand the theoretical concept of an entanglement and what it means for our computation to have two entangled qubits, but what does it really mean when two qubits are entangled? How are they ...
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1answer
110 views

What is the meaning of measuring a Bell state with Pauli operators?

There will be a certain value of getting the probability when measuring any Bell's state with Pauli operators such as observable X, Y, or Z. What is the meaning behind all this measurement? the result ...
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2answers
91 views

How instantaneous is state preparation in a quantum register, if all possible superpositions are to be initialized equally?

Before the start of a quantum algorithm qubits need to be initialized into a quantum register. How fast can a quantum register of length $n$ be initialized in a way that all possible superpositions of ...
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1answer
133 views

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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2answers
422 views

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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1answer
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What are some resources on qubits based on quantum dots?

I would like to learn a bit on the physics and details of manufacture of a qubits based on quantum dots. Can anyone recommend some papers?
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What are the pros/cons of Trapped Ion Qubits, Superconducting Qubits and Si Spin Qubits?

Among available qubit technologies for universal quantum computing these 3 come up as promising. NV centers and Majorana qubits also underway but relatively new. I find superconducting qubits and ...
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86 views

How do quantum gates work on qubits in the physical world?

How do quantum gates work on qubits in the physical world? What are different ways through which qubits can be manipulated in the physical world? For example, by analogy a classical NOT gate uses ...
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1answer
69 views

In $|x_1,...,x_n\rangle$, can the individual constituents be called qubits?

A qubit is a quantum system in which the Boolean states 0 and 1 are rep- resented by a prescribed pair of normalised and mutually orthogonal quantum states labeled as ${|0⟩, |1⟩}$ According to [1]. ...
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61 views

Why do we use two optical modes per qubit in the KLM scheme?

The original KLM paper says: The basic states of a bosonic qubit encoded in modes $l_1$ and $l_2$ are $|0\rangle\mapsto|0\rangle_{l_1}|1\rangle_{l_2}$ and $|1\rangle\mapsto|1\rangle_{l_1}|0\rangle_{...
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What determines the topology/connectedness of quantum computers?

Some quantum computers hold their qubits in a 2D array. I've even seen a 1D topology I think. On IBM Q, you can see which qubits talk to which qubit. I would like to know what determines/limits the ...
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How do you CNOT between qubits faraway from each other?

I'm just curious. Say I have 3 qubits on a straight line 0-1-2. How do you guys (on the IBMQ or Rigetti team for example, or anyone who actually works on quantum computer) entangle 0-2 if I, say, am ...
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How promising is the possibility of carbon-based qubits to make a qubit that’s stable at room temperature?

Here is the first article I could find on this idea in 2016: https://arxiv.org/abs/1611.07690 And here is a patent in 2017 for a quantum electronic device developed with one of the authors of the ...
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59 views

What are some general concepts of how a classical computer would interface with a quantum computer?

Very general question so I'm not looking for an exact answer. I just want a basic description of certain ways it can be done and then if possible the names of those ways, so that I can look them up ...
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Question about DIY Quantum Computer Prototype

I recently came across these 2 videos on Coursera which show how to build a simple quantum computer that can implement the simplest case of the Deutsch-Jozsa algorithm (which uses only 2 qubits). ...
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Can we have multiple-state unit for quantum computer?

All quantum computers now are working with qubit which is zero or one. Can we have quantum computer with multiple-state unit? Such as trit (trinary, 3 states), digit (10 states) instead of qubit (2 ...
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Calculating the state of the qubit when $\alpha$ and $\beta$ are given [closed]

I am new to quantum computing. Can someone help me in calculating state of the qubit when $\alpha = \frac{3}{5}e^{i\pi/7}$ and $\beta = -\frac{4i}{5}$. Thanks
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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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108 views

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 qubit phase is build in real hardware?

How qubit phase is build in real hardware? On IBM Quantum, D-Wave, etc?
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173 views

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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84 views

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?