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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 10 GHz. The energy associated with a transition frequency $f_{10}$ in quantum mechanics is $E_{10} = h f_{10}$ where $h$ is Planck's constant. Comparing the ...

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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 there is to the process. Thus, in particular, if your conversion efficiency (or you detection time / efficiency / SNR) is good enough that you can detect (and ...

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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: pump (p), taken to be a coherent light source (a laser); as well as signal and idler (s/i), the two generated photons The Hamiltonian for SPDC is given by $H = \... 7 Because light, at the right frequencies, interacts weakly with matter. In the quantum regime, this translates to single photons being largely free of the noise and decoherence that is the main obstacle with other QC architectures. The surrounding temperature doesn't disturb the quantum state of a photon as much as it does when the quantum information is ... 7 A standard reference for linear optical quantum computing is Kok et al. 2009 (quant-ph/0512071). If one qubit is encoded in the polarization degree of freedom of a single photon, and the second qubit in the path degree of freedom of the same photon, then a CNOT gate is trivially implemented by a polarizing beamsplitter. This is a kind of beamsplitter that ... 7 It appears to be true, up to a point. As I read Scott Aaronson's paper, it says that if you start with 1 photon in each of the first$M$modes of an interferometer, and find the probability$P_S$that a set$s_i$photons is output in each mode$i\in\{1,\ldots, N\}$where$\sum_is_i=M$, is $$P_s=\frac{|\text{Per(A)}|^2}{s_1!s_2!\ldots s_M!}.$$ So, indeed, ... 6 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/timing (e.g. bridging between the optical frequencies typical of atomic clocks and electrical/microwave frequencies typical of timing signals within ... 6 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 specifically, if the input state is a single photon in each of the first$n$modes, and one is willing to draw an arbitrary number of samples from the output, then it is ... 5 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 review, I will try to address your questions with what I can remember from reading that paper and glancing over it again now. For discrete variables(dv), as you ... 5 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 approaches? There are two protocols in quantum communication namely, discrete-varibale (dv) and continuous variable (cv). Squeezed light qubits are a part of cv ... 4 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 properties: the chance per unit length of down-conversion occurring is proportional to$\chi^{(2)}$; if our material of choice doesn't have good phase-matching ... 4 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 generate input resource states, undergo manipulation on the chip, and then are measured via a variety of detectors available in quantum optics. These can include ... 3 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 you've specified and, indeed, people frequently do. 2 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 sources of entangled photons, especially from synchronizable femtosecond lasers and low-cost cw-diode lasers. Decoherence due to timing information and ... 2 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 initially thermally excited, one could actively reset the state of the qubits. Thus, in addition to thermal excitations of microwave qubits, the fundamental ... 2 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$\left|1\right>$, representing the number of excitations in the atom (as it's only a two-level atom, there can't be more than 1 excitation) and this is what$Z$acts on. Similarly, the ... 1 This is very much possible. And is a very general technique of how product systems in composite states are coupled. Here of the form$|n_1\rangle |n_2\rangle$. This kind of general ket is a solution of the Hamiltonian interaction/coupling terms like$V\sim (a_1^\dagger a_2 +h.c)\$ which describe the exchange of one quanta (between the two optical cavities ...

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