# 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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### Why do we need physical qubit if qubit can be simulated?

I do not see any advantage in constructing very costly and imperfect physical qubit while this qubit can be simulated with using conventional computer memory (noise-free). So what is the purpose when ...
73 views

### Rxx gate as a set of rotations

I'm trying to represent Rxx gate as a set of physical rotations of two qubits in 3D space (or as rotations of Bloch Spheres that is the same). In some simple cases it works well: If q0 is in the ...
1 vote
52 views

### How do I implement $SU(2)$ rotation on the Bloch Sphere unsing qutip?

Using qutip I am trying to implement a qubit rotation according the formula $(25)$ provided in this document "Lecture notes: Qubit representations and ...
78 views

### How do I stepwise rotate a qubit on a Bloch sphere using $SO(3)$ and $SU(2)$ group?

Let us first introduce two fundamental functions for coordinate transformations: ...
21 views

### Experience on analog chaos based qc?

Any expirience on applied analog qc using a chaos based approach. By chaos I mean using analog components in chaos state thus changing between a wide range of states simultanously. The initial work on ...
4k views

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

Sorry if this is a dumb question. With conventional processors we increase cores, clock speed and IPC etc. With quantum computers the race seems to be to have the most qubits, seemingly in conflict ...
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### What is the dipole moment of a typical transmon qubit?

What is the dipole moment of a typical transmon qubit? I know this depends on the specific size, and that that varies from design to design. So let me provide an example situation: So given this size ...
80 views

### What physically limits qubit connectivity in superconducting chips?

All the superconducting based quantum computers I'm familiar with have a maximum of 4 nearest neighbors per qubit. Trapped ion architectures seem to be able to drive entangling gates between all pairs ...
108 views

### What use cases are there for 127 qubit QPUs?

IBM have recently announced their 127 qubit Eagle processor. Other approaches, such as Rydberg arrays, have now 256 qubits, as for example in QuEra's QPU QPU. While these are without a doubt ...
1 vote
37 views

### How does one design an RF pulse for an Z gate in, for example, a trapped ion qubit?

I am trying to use Schrodinger's equation to describe the time evolution of a qubit under a Z gate operation. For that, I need to know what the RF pulse looks like. For concreteness, let's suppose ...
103 views

### Measurement in the Pauli X basis

A qubit in the state $|1\rangle + |0\rangle$, normalized, is time-dependent, right? So it sweeps around the equator of the Bloch sphere at a frequency proportional to the energy gap according to ...
46 views

### Qubit fidelity of DWAVE device

Since DWAVE quantum device is constructed using superconducting flux qubits, each qubit cannot be produced identically so that the fidelity of the qubit must be different. DWAVE only provides the ...
51 views

### What does the $\sigma_z$ operator correspond to in the context of a Transmon qubit?

In the context of the transmon qubit and an LC circuit with a coupling capacitor and driving voltage. We can write the charge operator $Q$ as $-iQ_{ZPF}\sigma_y$ (since we defined $Q$ using the ladder ...
1 vote
55 views

### Calculate Outputs of a Teleportation Circuit

In this question, I assume we use matrix representation to calculate what each output could be? For example, for the middle qubit in state: $$|00\rangle$$ After the first Hadamard and CNOT gate (so ...
35 views

### Meaning of Continuous Charge in the Inductively Shunted Cooper Pair Box [closed]

In the Cooper Pair box, the conjugate momentum $\hat{n}$ of the reduced superconducting phase variable $\hat{\varphi}$ takes only discrete, integer values due to the $2\pi$ periodicity of the phase. ...
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### Is it possible to build a "Quantum Karnaugh Map"?

Is it possible to build something similar to the Karnaugh Map but instead of classical bits input and output with qubits input and output?
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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 ...
66 views

### 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 ...
1k 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 ...
1 vote
26 views

### 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 ...
218 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 ...
1 vote
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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 ...
86 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://...
52 views

### 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? ...
234 views

### 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 - ...
1 vote
47 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 ...
59 views

### 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 ...
1 vote
66 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 ...
96 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 ...
96 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 ...
1 vote
218 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 ...
161 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 ...
34 views

### 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 ...
61 views

### 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. ...
340 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 ...
141 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 ...
440 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 ...
60 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 ...
1k 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 ...
101 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 ...
793 views

### 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 ...
1 vote
192 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 ...
44 views

### 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?
1 vote
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### 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 ...
1 vote
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### 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 . ...
53 views

### 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 ...
1 vote
70 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_{...
1 vote