Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

Questions tagged [qubit]

For questions related to quantum bits, quantum systems living in a 2-dimensional Hilbert space. Consider also using the 'qubit-state' tag when relevant

6
votes
3answers
207 views

Why is the state of multiple qubits given by their tensor product?

How did we derive that the state we get by $n$ qubits is their tensor product? You can use $n=2$ in the explanation for simplicity.
3
votes
2answers
47 views

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 ...
3
votes
1answer
726 views

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. ...
7
votes
0answers
69 views

Is it better to use fewer gates or fewer working qubits? [closed]

I have a script that takes a while to simulate. I can modify it in such a way where I can use fewer qubits at a time, but it will require more iterations of manipulation. I believe this will cut down ...
7
votes
2answers
116 views

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 ...
6
votes
1answer
37 views

Mapping Algebraic Normal Form of Exclusive Sum of Products to Toffoli Network

How does one map an ANF to a toffoli network? Is there a straight-forward procedure for doing this? For example, given the ANF for the Sum function of an adder: $$S = A \oplus B \oplus C \oplus ABC$$ ...
7
votes
2answers
379 views

Transformation of a Bell state

I am relatively new and interested in quantum computing. Specifically, I am interested in transforming an equation that I found on Wikipedia. But I did not quite understand the transformation. $ \...
4
votes
1answer
82 views

BB84 attack with entangled qubits example

BB84 attack with entangled qubits example Hi, I'am interested in an attack for BB84 protocol with entangled quibits. Lets say Alice sends a qubit $x$ in state $\left|1\right>_x$ to Bob and Eve ...
2
votes
2answers
158 views

Many-Worlds Interpretation and GHZ States

I'm working through a problem set, and I've come across the following problem: In this problem, you'll explore something that we said in class about the Many-Worlds Interpretation of quantum ...
3
votes
1answer
55 views

Protocol for entaglement swapping

Suppose there are 3 parties, of which 2 pairs share an EPR pair and can communicate classically. What is a protocol that results in the third pair sharing an EPR pair? That is the problem I'm given, ...
2
votes
1answer
47 views

Let $\psi$ be a two-qubit state, $U \in \mathbb{C}^{2\times2}$ be a single-qubit unitary. Measure second qubit after applying U to the first qubit?

How can one show that measuring the second qubit of $\psi$ is the same as measuring the second qubit after applying $U \otimes I$ to $\psi$? I know that $\psi = \Sigma_{ij} a_{ij}|i⟩|j⟩$, $(U \otimes ...
5
votes
3answers
131 views

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 ...
7
votes
1answer
60 views

Can a stored programming model be applied to a Quantum Computer?

A stored programming computer model is that where a central memory is used to store both instructions and data that they operate on. Basically all the classical computers of today that follow the von ...
5
votes
3answers
150 views

What does it mean by qubit can't be copied

What does it mean by ''qubit can't be copied'' ? In a note, it is saying that: Copying a qubit means $$U|\psi\rangle_A|0\rangle_B=|\psi\rangle_A|\psi\rangle_B$$ i.e; applying a unitary ...
3
votes
3answers
110 views

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 ...
2
votes
2answers
74 views

Tensor product between operators

If the state of one qubit can be described by a ray in $\mathbb{C}^2$, then the combined state of an $n$-qubit system can be described by a ray in $(\mathbb{C}^2)^{\otimes n}=\mathbb{C}^{2 n}$. ...
6
votes
1answer
107 views

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 ...
5
votes
4answers
426 views

Why do Bloch sphere wavefunctions have half angles?

Suppose we have a single qubit with state $| \psi \rangle = \alpha | 0 \rangle + \beta | 1 \rangle$. We know that $|\alpha|^2 + |\beta|^2 = 1$, so we can write $| \alpha | = \cos(\theta)$, $| \beta | ...
5
votes
1answer
69 views

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 ...
7
votes
3answers
170 views

What does it mean for a quantum computer to have $X$ qubits?

I want to preface with a disclaimer that I am a physicist with minimal knowledge of computer hardware. I have a solid understanding of quantum information from a theoretical standpoint but zero ...
2
votes
4answers
162 views

Are classical bits quantum?

A bit is a binary digit, typically 0 or 1. Until a value is assigned (or a measurement is made) a bit is in a superposition of the entangled binary pair, is it not?
6
votes
2answers
107 views

The process for transferring Qubits between locations

I understand that right now Qubits are physical entities in a Quantum Computer and I am playing around on the IBM Quantum Computer as well as the Q# language and dipping my toes into the Quantum world ...
6
votes
2answers
159 views

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 ...
4
votes
2answers
150 views

How many qubits are simulable with a normal computer and freely accessible simulators?

I want to simulate an arbitrary isolated quantum circuit acting on $n$ qubits (i.e. a pure state of $n$ qubits). As I know RAM is the bottleneck for quantum simulators, you can consider a "normal" ...
6
votes
1answer
162 views

Are qubits preferred over qumode, and if so, why?

Qubit and qumode are different forms of quantum computation. But most existing quantum computers/chips seems to be of discrete variables. I heard that a group chose qubit for a quantum optical ...
4
votes
1answer
125 views

Projection operator on Time evolution Operator

From a 9×9 Hamiltonian lying 9D space, I choose a certain subspace of 4D for designing a two qubit gate. Now the original unitary time evolution operator also lies in 9D space and it's a 9×9 size ...
4
votes
1answer
121 views

What are the standard eigenvalues in $\mathbb{C^2}\otimes\mathbb{C^2}$?

In $\mathbb{C^2}$, we generally take $+1$ and $-1$ as the standard eigenvalues, that's what Pauli-X, Pauli-Z measurements, etc will give us. Is there a similar standard while measuring in the Bell ...
5
votes
3answers
266 views

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

I had asked this question earlier in the comment section of the post: What is a qubit? but none of the answers there seem to address it at a satisfactory level. The question basically is: How is a ...
4
votes
1answer
43 views

Generalization of n-th level entangled system

I have seen qubits, qutrits & entangled bits (e-bits) a decent amount. I have also seen qunits/qudits for n-th level qubits. What I am trying to wrap my head around is the differences between n-th ...
7
votes
1answer
160 views

Quantum algorithm for linear systems of equations (HHL09): Step 1 - Number of qubits needed

This is a continuation of Quantum algorithm for linear systems of equations (HHL09): Step 1 - Confusion regarding the usage of phase estimation algorithm Questions (contd.): Part 2: I'm not exactly ...
8
votes
2answers
382 views

Quantum algorithm for linear systems of equations (HHL09): Step 1 - Confusion regarding the usage of phase estimation algorithm

I have been trying to get my head around the famous(?) paper Quantum algorithm for linear systems of equations (Harrow, Hassidim & Lloyd, 2009) (more popularly known as the HHL09 algorithm paper) ...
10
votes
6answers
793 views

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 ...
7
votes
1answer
197 views

Changing indices of qubits in QISKit

I was wondering if something like this is possible in QISKit: let's say we have two registers containing target and ancilla qubits: $a_0$ ------------------- $a_1$-------------------- $\vdots$ $...
6
votes
1answer
203 views

How to find a density matrix of a qubit?

If we are given a state of a qubit, how do we construct its density matrix?
7
votes
2answers
396 views

Measuring in standard basis meaning

What does it mean to measure a qubit (or multiple qubits) in standard basis?
11
votes
1answer
763 views

How to interpret a quantum circuit as a matrix?

If a circuit takes more than one qubit as its input and has quantum gates which take different numbers of qubits as their input, how would we interpret this circuit as a matrix? Here is a toy example:...
5
votes
1answer
107 views

What is the $\left| 22\right>$ state?

I came across with a problem that involves $2$ quantum trits in state $\left| 22 \right>.$ What is it's tensor product interpretation and a matrix interpretation?
4
votes
2answers
331 views

How to check if 2 qubits are entangled? [duplicate]

I know that 2 qubits are entangled if it is impossible to represent their joint state as a tensor product. But when we are given a joint state, how can we tell if it is possible to represent it as a ...
6
votes
1answer
232 views

How to apply single and two qubit gates to 2 qubits multiple times?

Let's say we have the following quantum circuit: Let's say we input the state $|00\rangle$ . Both of the $H$ gates produce the output $1/\sqrt{2}$, but which one of the following $2$ vectors is the ...
7
votes
2answers
963 views

How to input 2 qubits in 2 Hadamard gates?

Let's say we have a circuit with $2$ Hadamard gates: Let's take the $|00\rangle$ state as input. The vector representation of $|00\rangle$ state is $[1 \ 0 \ 0 \ 0]$, but this is the representation ...
6
votes
1answer
260 views

How to calculate the state given by two qubits?

Let's say two qubits are both in $|+\rangle$ state. We need to find $a_1$, $a_2$, $a_3$, and $a_4$ in $|\phi\rangle = a_1|00\rangle + a_2|01\rangle + a_3|10\rangle + a_4|11\rangle$, how do we find ...
6
votes
1answer
384 views

Resources for quantum algorithm basics

I have just started to learn about quantum computing, and I know a little bit about qubits. What is a resource where I can learn a basic quantum algorithm and the concepts behind how it works?
6
votes
1answer
166 views

No-cloning theorem and distinguishing between two non-orthogonal quantum states

I'm currently reading Quantum Computation and Quantum Information and I'm not sure if I correctly understand this exercise (on page 57) : Exercise 1.2: Explain how a device which, upon input of one ...
5
votes
4answers
240 views

How to check if 2 quantum bits are orthogonal?

How would you check if 2 qubits are orthogonal with respect to each other? I need to know this to solve this problem: You are given $2$ quantum bits:$$ \begin{align} |u_1\rangle &= \cos\left(\...
7
votes
2answers
194 views

What's the difference between a set of qubits and a capacitor with a subdivided plate?

This is probably just a misunderstanding on my part, but everything I've seen on what quantum computers do thus far seems to suggest that the actual process of reading the entangled qubits would be ...
7
votes
1answer
77 views

How did the authors manage to simulate and get the error estimate for a neural network with greater than 7840 qubits?

In the paper A quantum-implementable neural network model (Chen, Wang & Charbon, 2017), on page 18 they mention that "There are 784 qurons in the input layer, where each quron is comprised of ten ...
17
votes
3answers
927 views

Is entanglement transitive?

Is entanglement transitive, in a mathematical sense? More concretely, my question is this: Consider 3 qubits $q_1, q_2$ and $q_3$. Assume that $q_1$ and $q_2$ are entangled, and that $q_2$ and $q_3$...
4
votes
2answers
104 views

'Rectilinear' and 'Diagonal' Basis in BB84 Protocol

A lot of the tutorials on BB84 protocol talks about these two measurement bases, 'Rectilinear' or 'Vertical-Horizontal' and 'Diagonal'. I understand that it is possible to create a physical device ...
4
votes
1answer
41 views

Do avoided crossings / CTs /ZEFOZs optimize quantum fidelity in practice?

CTs / ZEFOZs: Energy level structures that include avoided crossings at accessible energies tend to be resilient to noise and therefore present high coherence times, at least in the case of spin ...
7
votes
1answer
118 views

Does superdense coding allow to double the information capacity of a set of qubits?

I read the basic introductory information about qubits on Wikipedia: There are two possible outcomes for the measurement of a qubit—usually 0 and 1, like a bit. The difference is that whereas the ...