Most Popular
1500 questions
7
votes
3 answers
What is the difference between $\vert 0 \rangle + \vert 1 \rangle$ and $\vert 0 \rangle \langle 0 \vert + \vert 1 \rangle \langle 1 \vert$?
In a discussion with Jay Gambetta on the QISKit Slack channel, Jay told me that "T2 is the time that $\vert 0 \rangle + \vert 1 \rangle$ goes to $\vert 0 \rangle \langle 0 \vert + \vert 1 \rangle \langle 1 \vert$".
My question is: what is the…
Adrien Suau
- 4,927
- 20
- 58
7
votes
2 answers
How do you represent the output of a quantum gate in terms of its basis vectors?
I'm stuck while trying to understand the Hadamard Gate in a more linear algebra understanding. (I understand the algebraic way). This is because I want to program a simulation of a quantum computer. To apply a gate you multiply each ket by the…
Vitulus
- 173
- 6
7
votes
3 answers
Is entanglement an operation or a stored state for qubits?
I am going through this video of Quantum Computing for Computer Scientists. I am not able to understand the entanglement state of qubits.
Is entanglement just an operation, or is it a state which can be stored. If it's a state which is stored then…
rusty
- 171
- 2
7
votes
1 answer
Solving linear systems represented by NxN matrices with N not power of 2
As far as I have seen, when it comes to solving linear systems of equations it is assumed to have a matrix with a number of rows and columns equal to a power of two, but what if it is not the case?
If for instance I have the equation $Ax=b$ where A…
FSic
- 859
- 5
- 18
7
votes
2 answers
What are the pros/cons of Trapped Ion Qubits, Superconducting Qubits and Si Spin Qubits?
Among available qubit technologies for universal quantum computing these 3 come up as promising. NV centers and Majorana qubits also underway but relatively new.
I find superconducting qubits and Trapped Ion qubits very hard to scale. Also…
mando
- 355
- 3
- 11
7
votes
1 answer
How to verify whether a state is a stabilizer state?
Given an arbitrary $n$-qudit state vector $|\psi\rangle =\sum_i c_i| i \rangle \in \mathbb{C}_d^n$ for some orthonormal basis $\{|i\rangle\}$, what is the most efficient way one can:
Verify whether the state is a stabilizer state (i.e. can be…
SLesslyTall
- 1,626
- 8
- 26
7
votes
3 answers
Is there something like GitHub for quantum programming?
I'm just learning about quantum computer but some of it has been available for people to research & practice so I'd like to study it myself. The only kind of quantum computing I found so far is IBM cloud service and Q# quantum simulator but the…
Dan Minh Toan
- 145
- 3
7
votes
2 answers
Does "quantum registers with $n$ qubits are able to hold $2^n$ values and therefore scale exponentially" actually hold that straightforwardly?
A lot of people claim that quantum provides exponential speedup whereas classical computers scale linearly. I have seen examples (such as Shor's algorithm and Simon's) that I believe, but the layman's explanation appears to boil down to "quantum…
Thomas Hubregtsen
- 604
- 1
- 4
- 10
7
votes
1 answer
Are qubits preferred over qumode, and if so, why?
Qubit and qumode are different forms of quantum computation. But most existing quantum computers/chips seems to be of discrete variables. I heard that a group chose qubit for a quantum optical frequency comb experiment relating to quantum computing…
raycosine
- 860
- 8
- 12
7
votes
1 answer
Query regarding BQP belonging to PP
I found the following proof of BQP belonging to PP (the original document is here). There is a part of the proof that I have trouble understanding. First, the structure is given below.
We try to simulate a polynomial-time generated quantum circuit
…
BlackHat18
- 1,313
- 8
- 16
7
votes
2 answers
What are reliable references on analytical and/or numerical studies of threshold theorems under faulty quantum error correction?
By "faulty", I mean that you can have errors on the ancilla qubits, you can have faulty syndrome extraction, etc.
Sherif F.
- 171
- 3
7
votes
1 answer
What is the purpose of the TDG gate in QISKit?
The QISKIT documentation doesn't explain what a TDG gate does and I can't find it anywhere else online.
Theodore Lewitt
- 71
- 2
7
votes
2 answers
Arbitrary powers of NOT and SWAP
The square-root of not and square-root of swap gates are often singled out for discussion of gates displaying important properties relating to quantum computers.
How do I define arbitrary (non-integer) powers of the square-root of NOT or…
DaftWullie
- 57,689
- 3
- 46
- 124
7
votes
2 answers
Are classical shadows useful?
according to the paper https://doi.org/10.1038/s42254-022-00535-2 , the advantage of classical shadows is doing measurements first and asking questions later.
But in real experiments, who would do measurement first and ask questions later, being…
yuanyi_thu
- 71
- 1
7
votes
1 answer
Which Clifford groups are 2-designs?
Let $ X $ be the $ q \times q $ shift matrix sending $ |y \rangle \mapsto |y+1 \rangle $ where the ket index $ y=0,\dots, q-1 $ is taken mod $ q $. Let $ Z $ be the diagonal $ q \times q $ clock matrix sending $ |y \rangle \mapsto (e^{2 \pi i…
Ian Gershon Teixeira
- 3,722
- 3
- 21