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Questions tagged [quantum-optics]

A branch of atomic, molecular, and optical physics dealing with how individual quanta of light, known as photons, interact with atoms and molecules. It includes the study of the particle-like properties of photons.

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Multimode unitary channel in terms of action on characteristic function

Consider a set of $M$ signal modes described by the creation operators $\mathbf a^\dagger = (a_1^\dagger,...,a_M^\dagger)$, and let $\Phi_U$ be the channel defined by the conjugation $\Phi_U(\cdot)=U(\...
Phil K.'s user avatar
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Two-photon N00N state through Mach-Zehnder interferometer

I am interested in modelling a two-photon N00N state sent through a Mach-Zehnder interferometer, which consists of a beam-splitter (50:50), a phase shift operator on the first mode, a phase shift ...
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Partial decoherence of a general one-photon state

Let $\rho_1$ be the pure one-photon state described by the ket $$|\psi_1\rangle = \int dk\ A(k)a^\dagger(k)|0\rangle$$ for a complex amplitude function $A(x)$ and an empty ket $|0\rangle$. This state ...
Bentanglement's user avatar
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Are there resources to learn about quantum optics that focus on math and/or are addressed to those familiar with the qubit formalism?

Most of my knowledge in the field of Quantum Information comes from the lectures I've had as an MSc Student and from Nielsen and Chuang's textbook. Both of these focused on the "qubit" ...
Tristan Nemoz's user avatar
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Composing beam splitters

Let $a, b$ and $c$ be independent modes in a system $S$ and in environments $E_1$, $E_2$ respectively. Suppose $a$ goes through a beam-splitter characterized by a parameter $\theta$ coupling it to ...
Quantastic's user avatar
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How to use parameters in QuTiP?

I am working with QuTiP to do some calculations based on my proposed Hamiltonian. The Hamiltonian of my system has some coupling constants that show the strength of coupling between matter and light(...
Navid Elyasi's user avatar
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How to compute a partial trace of the form ${\rm Tr}_S [s^\dagger b- s b^\dagger, \vert \psi \rangle \langle \psi \vert \otimes \rho_B]$?

Let us consider a entangled state for the signal-idler system in its Schmidt decomposition form \begin{align} \vert \psi \rangle_{SI} = \sum_{\alpha} \sqrt{p_{\alpha}} \vert w_{\alpha} \rangle_S \vert ...
Michael.Andy's user avatar
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Backwards evolution arising from changing evolution picture

This may not be a question strictly belonging to the realm of quantum computing, but it does arise from problems in QI and optics, so I choose to ask it here. Suppose $A$ is a Hermitian operator ...
user15135's user avatar
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What are the antisymmetric terms in $\sigma_{mn}$ in the expression for the Fisher information?

Given a parameter-dependent density operator $\hat\rho^\lambda$ and its spectral decomposition $\{\rho_m^\lambda, |\psi_n^\lambda\rangle\}$, Eq. $(17)$ from this review shows that one can compute its ...
Bentanglement's user avatar
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Role of Intensity Modulator's Extinction Ratio on transmitter side in COW/DPS QKD using Weak Coherent Pulses

What is the role of having a high extinction ratio (30-40 dB) for the intensity modulator on the transmitter side in COW/DPS QKD using Weak Coherent Pulses, when ultimately the pulses have to be ...
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How to check the jmat operators on Qutip?

I'm trying to reproduce this Qobject form Qutip https://qutip.org/docs/latest/guide/guide-tensor.html#a-two-level-system-coupled-to-a-cavity-the-jaynes-cummings-model but, I'm using the Jmat ...
PCat27's user avatar
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1 answer
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Nielsen and Chuang: Solving equation of motion for amplitude damping

I would like to know how to obtain a solution to the equation of motion given in Section 8.4.1 Master equations of Nielsen and Chuang, 10th edition. The equation of motion that allows getting the ...
MonteNero's user avatar
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QuTiP, Continuous variable systems: Calculating the expectation value of an operator from the covariance matrix

Let's say I have the covariance matrix for a 4-mode system, $\sigma$, which is an $8 \times 8 $ matrix (the first moments are zero). I want to calculate the expectation value for an operator, for ...
Bard's user avatar
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CNOT gate path interference

I have a question on paper https://doi.org/10.1038/nature02054. In this paper, the CNOT gate is realized using a beam displacer for higher stability. The figure below shows a 1:1 mapping from (a) to (...
유승연's user avatar
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QuTiP lecture: Single-Atom-Lasing

I have tried to run one of the examples in QuTip lectures: single atom lasing. Although I have used the prepared code represented in reference, I have received error and I have not been able to ...
mehrosadat ebrahimi's user avatar
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Diagonal elements of the transition dipole moment

I'm currently doing some work in Quantum Optics and now have a question about the transition dipole moment. We were told in class that the off-diagonal elements of this matrix, namely $\langle n|d|m\...
SphericalApproximator's user avatar
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Does the entanglement have the conventional meaning in optical interferometers?

Let's consider a two-mode quantum state described in Fock state. A N00N state can be written as $|\psi _{{\text{NOON}}}\rangle ={\frac {|N\rangle _{a}|0\rangle _{b}+e^{{iN\theta }}|{0}\rangle _{a}|{N}...
narip's user avatar
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How to interpret photon number 'gates' in quantum circuit?

Peng et. al. Describe a circuit for entanglement cloning (i.e. entanglement broadcasting) which on its own looks fairly straightforward: Confusingly however, the gates $U_1$ and $U_2$ are not logical ...
Visipi's user avatar
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What are the measurement operators $F_k$ corresponding to a homodyne measurement?

By definition, a measurement is characterized by a set of positive-semidefinite matrices $\{F_k\}$ satisfying the completeness relation $\sum_k F_k = \textbf{I}$. I am interested in knowing how does ...
User101's user avatar
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How to write the covariance matrix of a quantum gaussian state as a function of photon numbers?

Assume having a one-mode quantum Gaussian state with quadrature observable vector $\hat r = [\hat q , \hat p ] $ and covariance matrix $\sigma$. According to definition [1]: \begin{equation} \sigma = \...
hafezmg48's user avatar
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1 answer
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How are two photons entangled in the Xanadu computer?

I was recently reading about how Xanadu made a scalable, room-temperature-operating quantum computer using photons as qubits (see this link). It said that in the logic gates, it entangles two photons ...
Blue Herring's user avatar
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How to connect the idea of inner product, outer product, matrix element in 3 level composite system?

I was solving steady state solution given by Lindblad equation, and I got stuck on the calculation part like what will be the value of $\langle e, 0|b, 0\rangle$? And how should I rewrite $|d, 0\...
Katem Mitkong's user avatar
2 votes
1 answer
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Comparing technical limitations of major quantum computing approaches

As far as I know, major quantum computing approaches include superconducting qubits, trapped ions, nuclear magnetic resonance qubits, and free space quantum optics. I guess two main technical ...
LearnerAL's user avatar
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1 answer
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How is this expression for a GHZ state obtained in the nature paper by Pan et al. (2000)?

Can someone tell me how the authors of the paper "Experimental test of quantum nonlocality" (Nature link to abstract) have rewritten their equation 1 in terms of equation 2 and 3?
Haxhi Pantina's user avatar
4 votes
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What is an electro-optic switch?

I found an implementation of a CNOT in photonic QC and I just couldn't find any info on a component named "electro-optic switch" What does it do exactly? How does it work? The ...
Mimo Pakou's user avatar
6 votes
1 answer
743 views

Why is a state Gaussian if and only if its covariance matrix satisfies $\boldsymbol\sigma+i\boldsymbol\Omega\ge0$?

Let $\rho$ be a Gaussian state, described by the $2N\times 2N$ covariance matrix $\newcommand{\bs}[1]{{\boldsymbol{#1}}}\bs\sigma$. Denote with $\bs\Omega$ the $N$-mode symplectic form associated with ...
glS's user avatar
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In (O'Brien et al. 2004), why is the target qubit not affected when the control is $\vert 0\rangle$?

I'm currently reading this paper https://arxiv.org/abs/quant-ph/0403062 but I fail to understand how the input of the control qubit affects the output of the target qubit. I do understand that when ...
Haxhi Pantina's user avatar
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1 answer
171 views

Why do we use 'modes' in quantum optics?

In this material, it gives the definition of modes: The modes are basically defined by the properties of coherence and orthogonality: modes are orthogonal solutions of the wave equation. Since the ...
narip's user avatar
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How to build a single-photon emitter at home

Hey any of you people know if it's possible to make a DIY single-photon emitter? I've been searching for an answer for quite some time now and all I've come across that's helpful is this: https://www....
LeDuck's user avatar
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3 votes
1 answer
118 views

How is measurement performed on a stream of polarized photons?

Is it possible to use a simple laser to measure qubits in optical quantum computers, or is a single-photon emitter absolutely necessary? If you can do it with a simple laser, how would you go about ...
Mimo Pakou's user avatar
6 votes
1 answer
2k views

What are spin-coherent states?

Trying to understand the paper; https://arxiv.org/pdf/1702.02577.pdf and ran into "spin-coherent" states. I wonder those are.
John Parker's user avatar
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After a photon passes through the phase shifter, does it gain the geometric phase or dynamical phase?

In Quantum Mechanics, the phase can be divided into geometric phase and dynamical phase. In mach-zehnder interferometer, after the photon passes through the phase shifter, what kind of phase does it ...
narip's user avatar
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4 votes
3 answers
117 views

Why does joint ground state not change under action of beam splitting unitary operator?

How can one show that $\hat{U}|00\rangle=|00\rangle$ where $\hat{U}=e^{-igt(\hat{a}^\dagger_2\hat{a}_1+\hat{a}^\dagger_1\hat{a}_2)}$ and $|00\rangle$ is the unique joint zero eigenstate of the ...
heromano's user avatar
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Sidebands in spin-oscillator coupled devices

I am researching spin-oscillator hybrid devices (particularly those coupled to NV centers), and I have come across this paper investigating the strain field of a cantilever and its effect on the NV ...
bbbb's user avatar
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Solution for the "Input and Output theory" for circuit/cavity-QED Quantum Architecture [closed]

This question consists of the basic calculations which are often referred to as "trivial" in most references in circuit-QED. However, are not so trivial for anyone who starts fresh into the ...
Siddhant Singh's user avatar
1 vote
0 answers
115 views

How is the third-order autocorrelation measured in quantum optics?

The third order autocorrelation is defined as $$\bar g^{(3)} = \frac{\langle \hat a^\dagger \hat a^\dagger\hat a^\dagger \hat a\hat a\hat a\rangle}{\bar n^3}.$$ How is this quantity measured in ...
quest's user avatar
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3 votes
1 answer
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Relation between Wigner quasi-probability distribution and statistical second-moment

Is there any relation between the Wigner quasi-probability distribution function $W$ and the statistical second-moment (also known as covariance matrix) of a density matrix of a continuous variable ...
Kianoosh.kargar's user avatar
1 vote
1 answer
64 views

How do we restrict to a limited number of dimensions, say 3 for qutrits, while using OAM states of light?

When a azimuthal phase $\mathrm{e}^{il\phi}$ is applied to gaussian beams having plane wavefront, they develop a corkscrew sort of structure and therefore possess an orbital angular momentum in ...
Devjyoti Tripathy's user avatar
6 votes
1 answer
219 views

Shor's Code: Understanding how it satisfies Knill Laflamme Theorem

I'm new to Quantum Error Correction, and I have a question on Shor's Code. If we have a protected subspace, $V \subset \mathbf{C}^2\otimes \cdots \otimes \mathbf{C}^2$ $V=\operatorname{span}\{|0_{...
Anon's user avatar
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