Qubit Definition and 82 Threads

QM uses separable Hilbert spaces as model to represent quantum system's states. Take for instance a 1/2-spin particle: its quantum pure state is represented by a ray in the abstract Hilbert space ##\mathcal H_2## of dimension 2. Take an observable represented by an Hermitian unitary operator...

A doubt related to the use of density matrix associated to a density operator ##\rho##. A quantum pure state ##\ket {\psi}## can be written as density operator as ##\ket{\psi} \bra{\psi}##. In this link the density matrix ##\rho## for the pure state ##\ket {\psi} =(\ket{\psi_1} +...

Very simple question: for a two-state quantum system (qubit), is its state space a two-dimensional Hilbert space over the complex field ##\mathbb C## isomorphic to ##\mathbb C^2## or is it ##\mathbb C^2## itself ?

Dear PF Forum, It's been a while since I logged in. And I hope everybody is in good health. Here I have a question concerning quantum computer. First, I'm no unfamiliar with a computer. I'm a computer programmer, but I don't know any physics. How can a quantum computer be a lot faster then an...

I had written a simulator for a quantum computer and wanted to be able to calculate the values of the "purity of reduced states". "Purity of reduced states" is a quantity that IBM provides on their simulator for each qubit. They used to call it "entanglement", and I believe that the two terms...

Hello All, 2nd year undergrad taking my first course in modern physics. We have been given this question in a mock exam and at the bottom is the solution. When looking at a general cubit it seems the argument of both sin and cos functions should be (pi/2) not (pi/3). I have figured out a...

According to this Wikipedia entry a quantum pure qbit state is a ray in the Hilbert space ##\mathbb H_2## of dimension 2. In other words a qbit pure quantum state is a point in the Hilbert projective line. Now my question: is an arbitrary vector in ##\mathbb H_2## actually a "mixed" state for...

[Mentor Note: Two similar thread starts merged] The questions are from MIT OCW. First of all, I cannot understand what is the meaning of the measurement outcome being 0. How can an eigenvalue be 0? I tried doing the problems guessing that by 0 they mean the posterior state will be |0>. The only...

Hi, unfortunately, I am not sure if I have calculated the task a correctly. I calculated the eigenvalues with the usual formula ##\vec{0}=(H-\lambda I) \psi## and got the following results $$\lambda_1=E_1=-\sqrt{B^2+\nabla^2}$$ $$\lambda_2=E_2=\sqrt{B^2+\nabla^2}$$ I'm just not sure about...

When I observe a qubit's state, decoherence happens such that I find the qubit in a particular state. After I cease observing a qubit's state, what physical process causes a fresh superposition of states to develop? Is zero-point energy at least a contributor?

Summary:: How to calculate qubit states with the Schrodinger eq I'm writing something about the relation between quantum computers and the Schrodinger equation. One of the requirements is there has to be an experiment. So I thought I could measure some qubits that have results and then do the...

Hello, I have a question about the measurement of a qubit in the computational basis. I would like to first state what I know so far and then ask my actual question at the end.What I know: Let's say we have a qubit in the general state of ##|\psi\rangle = \alpha|0\rangle + \beta|1\rangle##. Now...

I have just started reading Neilson and chuang's book on quantum computing and two times already have they said that when a qubit is not observed, it can contain infinite information. "How much information is represented by a qubit? Paradoxically, there are an infinite number of points on the...

I know |GHZ>=(1/sqrt(2))[1; 0; 0; 0; 0; 0; 0; 1], and |000>= the tensor product |0> x |0> x |0> = [1; 0; 0; 0; 0; 0; 0; 0]. Can I apply single qubit gates (i.e. 2x2 matrices) and CNOT (a 4x4 matrix) to 8x1 column vectors? If so, does anyone know a good starting point or a hint to get me moving...

I have numerous points of confusion: what does it mean that the matrices are within the exponential? How do I go about doing the matrix multiplication to prove the given form of CZ matches the common form, the 4x4 matrix? Update: using the fact that exp(At)=∑ ((t^n)/n!)*A^n, where A is a...

Am I correct in thinking that the system measures the probability |<f|1>|^2 for some state <f|? Then the probabilities for each of the six states would be: |<0|1>|^2= 0 |<1|1>|^2= 1 |<+x|1>|^2= |(1/√2)|^2 = 1/2 |<-x|1>|^2= |(-1/√2)|^2 = 1/2 |<+y|1>|^2= |(-i/√2)|^2 = 1/2 |<-y|1>|^2= |(i/√2)|^2...

Part a: Gate H X Y Z S T R_x R_y Theta pi pi pi pi pi/2 pi/4 pi/2 pi/2 n_alpha (1/sqrt(2))*(1,0,1) (1,0,0) (0,1,0) (0,0,1) (0,0,1) (0,0,1) (1,0,0) (0,1,0) Using the info from the table and equation 1, I find: U_H=(i/sqrt(2))*[1,1;1,-1] U_X=i*[0,1;1,0] U_Y=i*[0,-i;i,0] U_Z=i*[1,0;0,-1]...

And I was asked to include the deviation and the inherent process of the effective Hamiltonian of the charge qubit and the equation as well. And some of the derivation of the Hamiltonian as well. P.S The effective Hamiltonian formula is from the reference of ...

I have a simple question...the control qubit is A and the the target is B. The cnot is applied on |1A> <0A|⊗|0B0C>. ... How does it work. Thanks in advance.

I been reading up on quantum computers and in particular ones based on quantum harmonic oscillators. In all the articles, I have come across they mention a regular, linear LC oscillator cannot be used for a Qubit because it's energy levels are evenly spaced. Therefore, they use a non-linear...

Hi, I'm trying to understand an outer product |1>_a<1| where |1>_a is the ket for one qubit (a) and <1| is the bra for another qubit. Does this make sense and is it possible to express it in terms of tensor products or pauli matrices?

Consider the first qubit (subsystem A): First, the density operator for the system AB is ## \rho ^{AB} =\frac { \left |00 \right > \left <00 \right |+ \left |01 \right > \left < 01\right |+\left | 11\right > \left < 11\right | } 3 ##. Then, the reduced density operator of subsystem A is ##...

I am confused about the vector notation of quantum states when I have a 2 qubit system. For 1 qubit, I just write l1> = (0 ;1 ) for representing 1, and l0> = (1;0) for representing 0. Dirac notation is straightforward However when it comes to representing two qubits in linear algebra I...

Greg Bernhardt submitted a new blog post A Classical View of the Qubit Continue reading the Original Blog Post.

Hi everyone, I’ve performed microwave measurements on a 3D transmon and want to find the E_J value of the qubit. I’ve tried searching through many papers, particularly Koch et al, about how to do this, but I am stumped. Could someone please help me out? Thanks in advance.

I was wondering how to measure the first or even the second qubit in a quantum computing system after for example a Hadamard Gate is applied to the system of these qubits: A|00>+B|01>+C|10>+D|11>? A mathematical and intuitive explanation would be nice, I am a undergraduate sophomore student...

Hello, The state | W \rangle = \frac { 1 } { \sqrt { 3 } } ( | 001 \rangle + | 010 \rangle + | 100 \rangle ) is entangled. The Schmidt decomposition is : What would the Schmidt decomposition be for | W \rangle ? I am also intersted in writing the reduced density matrix but I need the basis...

I'm reading the following paper. https://arxiv.org/abs/1409.1570 Is there an epistemic model of a qubit in which the number of ontic states is finite? I realize Spekkens toy bit discussed in the paper has only 4 ontic states, but it seems to only model a qubit that was prepared in one of 3...

I found this interesting article on theory work done to create qubit flipflops that can be adjusted via electric fields making them easier to integrate into existing computing systems...

What is exactly Weizsäcker's ur-alternatives theory? How is it related to digital physics theories? Is it related to pancomputationalism? Does it defend that a universe can be described as being fundamentally made of qubits? Would this mean that that universe would be fundamentally made by...

In a 2 level quantum system, should I consider the states |0> and |1|> to be qubits by themselves? Or is only the SUPERPOSITION of these two states, \alpha |0> + \beta |1> considered to be a qubit?

Homework Statement For the state ##(4|00\rangle+3i|11\rangle)\otimes (|0\rangle+i|1\rangle) + (2|01\rangle -i|10\rangle)\otimes(|0\rangle-|1\rangle)## What's the probability of zero being the outcome of measuring the second bit and what is the state of the other two qubits after measurement...

Hello everyone, I don't know how to measure the QBER of BB84 protocol in a realistic experiment.In the most paper show the data which is only the number but do not show the unit of data, what is the unit of these data?

Hi! So I'm studying Gover's Algorithm and I have this doubt: Does 'Phase inversion gate' grows exponentially? I mean, if I want to signal the one combination that is the answer, I must be able to represent all 2^N states, where N is the number of qubits in the system. How do I do this without...

What are the coordinates on the 3D Bloch ball of a qubit's mixed state of the form: ##\rho=p_{00}|0\rangle \langle 0|+p_{01}|0\rangle \langle 1|+p_{10}|1\rangle \langle 0|+p_{11}|1\rangle \langle 1|##

Is it in a Quantum superposition, and then only in a mixed state when finding the reduced state of a qubit from an entanglement of multiple qubits?

Hello everyone! I'm having trouble understanding a specific aspect of qubits, maybe someone among you clever guys can help me. I understand that a qubit is in superposition, we can store information (a quantum property equivalent of true or false) in it. I also understand that reading that...

https://en.wikipedia.org/wiki/Qubit (copy and paste) "In quantum computing, a qubit (/ˈkjuːbɪt/) or quantum bit (sometimes qbit) is a unit of quantum information—the quantum analogue of the classical bit. A qubit is a two-state quantum-mechanical system, such as the polarization of a single...

Hi! I'm going on to the masters year of a theoretical physics course and I need some inspiration for my dissertation. Last year I did a one semester long project on quantum computation. (More specifically I discussed the general idea of a qubit, a simple method of realising a qubit using spin...

Qubit can be ZERO and ONE at the same time. Right? 1 and 0 can represent TWO (10) in a binary system. Right? Therefore one qubit can represent number 2. Right? My question. When this qubit is used to give a result of a calculation (is measured/evaluated somehow at the end of a calculation) it...

Is it possible yet to store a single qubit for an extended period of time? If I only care about storing a BB84 state, is it possible to take (say) 128 of these single qubit storages? They're not entangled and I don't need to send them through any general quantum circuitry.

Can a Markov chain be used to generate a set of qubits given their initial state?

The paper Optimal Cloning of Pure States by R. F. Werner describes a method for approximately expanding an unknown state ##\rho## containing n copies of a qubit, so ##\rho = (\alpha \left| 0 \right\rangle + \beta \left| 1 \right\rangle)^{\otimes n}##, into a larger state with d more qubits...

What does one mean when one says "high-fidelity qubit readouts"?

Hi PF I wonder how qubits interfere in interferometers when they are not in pure states. Let us take qubits with density matrix = Id/2. There lay at the center of the Bloch sphere. Half of them can travel unchanged through the left arm. In the other arm they become ##UP_Z##. Then there is...

Hi everyone. I'm just looking for schematics of cnot quantum gate, but on the Internet it only talks about it from a mathematical point of view. I want to ask you if you have some drawing, or schematics of it, from a "Physical" implementation, in the sense, how are the spins prepared, how we...

Hi :-) for my master thesis I'm working with qubits in the Bloch-sphere representation ##|q\rangle = cos(\frac{\theta}{2})|0\rangle + e^{i\phi}sin(\frac{\theta}{2})|1\rangle##. Side question: why is only the second amplitude complex? But let's move to my main question. I need to know how the...

Homework Statement Suppose that two measurements are made on a qubit in rapid succession. The first is δz and the second is δn. Suppose the first results is always +1. Calculate the probabilities of obtaining the results +/- 1 for the second measurement in terms of the angle θ between z and n...

I am working on a project for our local science fair and decided on researching quantum computers. (This is what my focus will be in college.) Though, I expect to work on this far past the time allotted for that. I would like to build a superconductor based qubit using Josephson Junctions. The...

Hi :-) I'm working on my master thesis in the field of quantum theory; currently I investigante No-Go-Theorems like the No-Cloning, No-Deleting, No-Hiding, No-Communication-Theorems ans so on. There is a fundamental question which is somehow linked to the No-Communication-Theorem. Is it - in...