31 votes
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Why do optical quantum computers not have to be kept near absolute zero while superconducting quantum computers do?

I was looking for why optical quantum computers don't need "extremely low temperatures" unlike superconducting quantum computers. Superconducting qubits usually work in the frequency range 4 GHz to ...
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10 votes
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State produced by spontaneous parametric down-conversion (SPDC)

Background First of all, I'll use $\lvert H\rangle$ as a horizontally polarised state and $\lvert V\rangle$ as a vertically polarised state1. There are three modes of light involved in the system: ...
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9 votes

Is it possible to "calculate" the absolute value of a permanent using Boson Sampling?

It appears to be true, up to a point. As I read Scott Aaronson's paper, it says that if you start with 1 photon in each of the first $M$ modes of an interferometer, and find the probability $P_S$ that ...
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9 votes

State produced by spontaneous parametric down-conversion (SPDC)

The existing answer does a good job at describing the state that comes from a SPDC configuration at low conversion efficiency, but it's also worth noting that the single-photon behaviour is not all ...
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8 votes
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How do you apply a CNOT on polarization qubits?

A standard reference for linear optical quantum computing is Kok et al. 2009 (quant-ph/0512071). If one qubit is encoded in the polarization degree of freedom of a single photon, and the second qubit ...
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7 votes

Why do optical quantum computers not have to be kept near absolute zero while superconducting quantum computers do?

Because light, at the right frequencies, interacts weakly with matter. In the quantum regime, this translates to single photons being largely free of the noise and decoherence that is the main ...
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7 votes

Is it possible to "calculate" the absolute value of a permanent using Boson Sampling?

You cannot efficiently recover the absolute values of the amplitudes, but if you allow for arbitrary many samples, then you can estimate them to whatever degree of accuracy you like. More ...
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6 votes

Is "probabilitistic,universal, fault tolerant quantum computation" possible with continuous values?

To start off, I would really suggest you to read this review on "Quantum information with continuous variables(cv)". It covers most of your questions with cv architecture. Since it is a very big ...
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  • 388
6 votes
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What is the status of quantum computing compared with other (photonic) quantum technologies?

According to this UK-oriented report by Gooch and Housego dated May 8, 2018, quantum computing is only one of several main key applications expected to have a market impact: Clock technology/...
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6 votes
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Physical qubit of optical quantum computer

At Xanadu, we're using integrated quantum photonics to build our photonic quantum computing chips. In this case, we have integrated chips containing waveguides --- these are coupled to lasers to ...
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6 votes
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What are the pros and cons of the most popular QC architectures?

The major quantum computing providers that comes to me right the way are usually: IBM Quantum (superconducting qubit) Google Quantum (superconducting qubit) Honeywell Quantum (trapped ions) ...
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6 votes
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How are two photons entangled in the Xanadu computer?

QML researcher at Xanadu here. Our X-series chip produce entangled states by squeezing light and then combining it at beam splitters: those 'cables' are waveguides in a chip, which when they are close ...
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  • 176
5 votes
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Does a photonic quantum computer control a single photon?

There are various ways in which one or multiple photons can be used to encode qubits. Potentially the most widely used encoding (at least when quantum communication is assumed to within the scope of '...
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5 votes
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What is the relation between single photon qubits and squeezed light qubits?

By photon qubits, I'm assuming that you meant single-photon qubit systems. Can one use squeezed light to effect multi-qubit operations on photon qubits, or are these completely independent ...
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4 votes
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What is the correct sign in the unitary evolution operator of a beam splitter?

First, the operators $a$ and $b$ ($a^{\dagger}$ and $b^{\dagger}$) are the annihilation (creation) operators of the two photonic modes in your problem. For an introduction to the subject I recommend ...
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4 votes

Improving probability of spontaneous parametric down conversion

Your question asks two questions that are less-related than you might hope. First, how do we increase the probability of down-conversion occuring? This is fundamentally a question about material ...
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3 votes
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Multi-photon states in photonic quantum computing?

Yes. The kets themselves can have arbitrary labels, and it's just for you to establish the connection between them and the physical scenario. There's no reason why you can't have the physical scenario ...
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3 votes

Does this quantum algorithm to check for a permuting function make sense?

One problem with this approach is that in quantum state space, permuting the inputs looks a lot like permuting most of the inputs but then doing something that isn't permutation with the remainder of ...
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3 votes

Comparing technical limitations of major quantum computing approaches

For a general overview about hardware and the difficulties they present, I recommend section three of Quantum Computing: An Overview Across the System Stack. Another good introduction would be Quantum ...
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3 votes
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How is measurement performed on a stream of polarized photons?

One idea is to do polarimetry. By using a polarizing beam splitter, the polarization qubit can have each of its polarization components directed to a different detector for photon counting (ideally a ...
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2 votes

Improving probability of spontaneous parametric down conversion

Here's some relevant work in Optimizing type-I polarization-entangled photons-Radhika Rangarajan, Michael Goggin, Paul Kwiat. Abstract: Optical quantum information processing needs ultra-bright ...
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2 votes

Why do optical quantum computers not have to be kept near absolute zero while superconducting quantum computers do?

DanielSank is correct, but I think the answer is actually even more subtle. If there was no loss there would also be no way the background radiation leaking into your quantum device. Even if it was ...
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2 votes

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

Strictly, what you have to calculate is that for all $i$ and $j$ $$ \langle 0_L|U_iU_j|1_L\rangle=0 $$ and $$ \langle 0_L|U_iU_j|0_L\rangle=\langle 1_L|U_iU_j|1_L\rangle. $$ (I've ignored the ...
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2 votes
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Hamiltonian for Single-photon, Single-atom QED Cavity

While the Pauli-$Z$ matrix is a 2 x 2 matrix, there are more basis states that need to be considered, namely the vacuum. The atomic basis states are $\left|0\right>$ and $\left|1\right>$, ...
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2 votes
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What are some good resources for learning about photonic qubits?

Here is a handful of Linear Optical Quantum Computation (LOQC) resources I have found useful in the past: "Linear Optical Quantum Computing" (2005) by Kok et. al.: this is probably the best review ...
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2 votes

What are some good resources for learning about photonic qubits?

a good stater i would say is the paper of Knill and Laflamme about LOQC (Linear Optical Quantum computing) from 2001, that says that quantum computing can be achieved with linear optic. Photons are ...
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2 votes

How to solve non-"cross-damping off" Linblad equation in QuTiP?

In QuTiP, one can construct generalized dissipators with the lindblad_dissipator function listed under "Superoperators and Liouvillians" here. You can ...
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1 vote
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QuTIP tensor products

good question - indeed the qutip API could use some improvements when dealing with tensor products. However, here python comes to the rescue with argument unpacking. To create a 4 qubit basis state, ...
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1 vote
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Knill Laflamme conditon

The Hilbert space of the environment part of the state must have at least as many dimensions as the minimum number of Kraus operators $N$ used to describe the local state's evolution: $$|\psi\rangle\...
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1 vote

Can such a transformation be implemented by using just polarizers?

With polarizers you can implement projections. The Pauli Z oerator has an eigenvalue $-1$, so no, you cannot implement $U_a$ with only polarizers.
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