35
votes
Accepted
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 ...
- 1,223
10
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 ...
- 203
10
votes
Accepted
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: ...
- 3,746
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 Aaronson and Arkhipov'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 $...
- 60.5k
8
votes
Accepted
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 is the polarization-encoded photon (at least when quantum communication ...
- 5,639
8
votes
Accepted
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 ...
- 196
8
votes
Accepted
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 ...
glS♦
- 26.3k
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 ...
glS♦
- 26.3k
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 ...
glS♦
- 26.3k
6
votes
Accepted
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)
...
- 14k
6
votes
Accepted
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/...
- 3,877
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 ...
- 388
6
votes
Accepted
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 ...
- 869
5
votes
Accepted
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 ...
- 388
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 ...
- 229
4
votes
Accepted
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 ...
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 ...
- 2,925
3
votes
Accepted
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 ...
- 4,549
3
votes
Accepted
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 ...
- 60.5k
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 ...
- 7,423
3
votes
Modular exponentiation quantum circuit design for Shor's algorithm
There are many quantum implementations of modular exponentiation to choose from depending on whether you want to optimize qubit count vs time and other similar tradeoffs.
However, for the method ...
- 1,186
3
votes
Are there practical uses for boson sampling?
There is a number of techniques that use Gaussian boson sampling for approximate solution of some practical problems:
https://iopscience.iop.org/article/10.1088/2058-9565/ab8504/meta
For example, some ...
- 706
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 ...
- 17.7k
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 ...
- 201
2
votes
Accepted
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 ...
- 1,646
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 ...
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 ...
- 60.5k
2
votes
Accepted
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 $\left|1\right>$, ...
- 3,746
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 ...
- 1,416
2
votes
Modular exponentiation quantum circuit design for Shor's algorithm
The references below explain the construction of modular exponentiation circuit. I also attempt to implement the circuit accordingly in https://github.com/thyung/qiskit_factorization using qiskit and ...
- 21
Only top scored, non community-wiki answers of a minimum length are eligible