Dripto Debroy
  • Member for 3 years, 6 months
  • Last seen more than a month ago
Non-layperson explanation of why a qubit is more useful than a bit?
2 votes

So there are two major advantages qubits have over classical bits: superposition and entanglement. Superposition is the one more often discussed, and is the "0 or 1" phrase you always hear. ...

View answer
How is it not a contradiction that it is possible to build fault tolerant circuits with strictly contractive (e.g.: depolarizing noise) channels?
0 votes

So that's where quantum error correction comes in. By measuring the stabilizers, the code is projected back into a pure state. I'll give an example using a Bell state: Imagine the state $$\frac{1}{\...

View answer
What is the pseudo threshold of a QECC using stabilizer formalism
3 votes

So the general definition of pseudothreshold is when the logical qubit outperforms a physical qubit. If your error model is idling error, for example, you want to find the physical value of T1 and T2 ...

View answer
Where to publish quantum algorithm related paper
2 votes

This is a good list made by Prof. Rod Van Meter. Maybe you could look at the papers you based your work off of and see where they were published? That should give you a sense of which journals are ...

View answer
How the simulator work?
Accepted answer
4 votes

There are three major levels of simulation difficulty (broadly, there are a bunch of others, but these are the main levels.) Clifford simulators can simulate circuits composed of only Clifford ...

View answer
What's the purpose of phases in quantum mechanics?
3 votes

Phase is really one of the things that makes quantum computing what it is! In fact, I think there is a quote by Aaronson like how quantum is "probability theory with negative numbers." The ...

View answer
How to implement a while loop in a quantum computer?
6 votes

I think the closest thing to a "while loop" in a quantum algorithm is something like the Variational Quantum Eigensolver (VQE) or other classical-quantum hybrid algorithms. In these, a ...

View answer
Other than distance, what other metrics can be used to compare quantum error correcting codes?
Accepted answer
1 votes

So the biggest number used when comparing families of QECCs is the threshold, which is the error rate (generally depolarizing noise or XZ noise, depends on the paper) for which increasing the size (...

View answer
Creative way to clone quantum data?
1 votes

So the no cloning theorem doesn't preclude you from creating the state $$\alpha|0\rangle + \beta|1\rangle \rightarrow \alpha|00\rangle + \beta|11\rangle,$$ it just says you cant create $$\alpha|0\...

View answer
Prove that the depolarizing channel is completely positive
Accepted answer
2 votes

Ok, so starting with Trace Preserving, since it's easier: $$Tr(I/2) = 1$$ $$Tr(\rho) = 1$$ $$Tr((1-\lambda)I/2 + \lambda\rho) = (1-\lambda)Tr(I/2) + \lambda Tr(\rho) = 1$$ Now for a map to be ...

View answer
A question about a real programmable quantum computer
Accepted answer
1 votes

The author in that paper allows himself arbitrary angle single and two qubit gates. With this set it is generally pretty easy to exactly match a given unitary, since the two qubit gates can give you ...

View answer
Where can I find tutorials about correcting gate errors in the quantum circuit?
Accepted answer
1 votes

For quantum circuits: So the main answer is that far-term quantum computers will implement Quantum Error Correction, where each logical qubit is composed out of a number of physical qubits and the ...

View answer
How do two qubit states differing by a global phase relate to each other?
Accepted answer
1 votes

1) So $|\psi_1\rangle \neq |\psi_2\rangle$, but it effectively is since they give the exact same distributions for any measurement in any basis. 2) Same discussion as above. 3) True 4) States in ...

View answer
Do all physical architectures for quantum computers use the same universal gate sets?
Accepted answer
6 votes

So any universal gate set can replicate any other, since both are universal, but different architectures generally have different physical gates. While Clifford+T is a universal gate set that is very ...

View answer
Software used for modelling quantum processors
1 votes

Are you looking for something that simulates the actual hardware, or something that runs quantum circuits? If you're looking for circuit simulation with the widest variety of possible operations, the ...

View answer
Smallest Distance-5 Quantum Error Correction Code?
0 votes

Another good place to find codes with your desired parameters is this website: http://www.codetables.de/ The standard format to describe a quantum code is [[n,k,d]], where n is the number of physical ...

View answer
Are all operations simply matrix multiplication?
1 votes

I think all unitary operations can be thought of as matrix multiplication, but something like a measurement is non-unitary, and looks like a partial trace over the density matrix, so there is a bit ...

View answer
The meaning of the city plot in Qiskit
Accepted answer
4 votes

A density matrix $\rho$ on two qubits has 16 complex amplitudes (although not all are free variables due to constraints from normalization and Hermeticity), so the City plot is showing those ...

View answer
What is the notational convention for the single-qubit states $\lvert0\rangle\pm i\lvert1\rangle$?
3 votes

I think that most people would understand what you mean, although maybe mentioning they are the eigenstates of $Y$ wouldn't go amiss depending on your target audience.

View answer
Density matrix after measurement on density matrix
6 votes

So Alice sends Bob a qubit with the density matrix $$\rho = \frac{1}{2}|0\rangle\langle 0| + \frac{1}{2}|1\rangle\langle 1| = \begin{bmatrix} .5 & 0 \\ 0 & .5 \end{bmatrix}$$ as you said. (I'...

View answer
Tools for creating quantum circuit diagrams
Accepted answer
17 votes

Depending on how involved your circuit is you could use Quantikz (written by @DaftWullie I believe) or Q-circuit by Bryan Eastin and Steve Flammia. These are tools to make circuit diagrams in ...

View answer
Symmetry in Conditional Phase Shift Gates and Realizing CNOT through HCZH
5 votes

It's not that both the qubits are independently picking up a phase, it's that the two qubit state itself is picking up that phase. $$\mathbf{CZ}|1\rangle\otimes |1\rangle = |1\rangle\otimes(-1\times |...

View answer
How do you implement the Toffoli gate using only single-qubit and CNOT gates?
Accepted answer
13 votes

is the decomposition (I took this from google images, originally on this website.) In order to understand how to decompose it, we can look at it's base structure. The idea is that we combine gates ...

View answer
Good metaphors for n-level quantum systems
2 votes

A rolling n-sided die would be a good analogy that follows the coin example very closely. Until the die settles you can think of it as having not "collapsed", and die can come in whatever side number ...

View answer
Can we perform quantum mechanical simulations using a quantum computer?
Accepted answer
4 votes

What do you mean by "Quantum Mechanical Simulations" ? One of the primary motivations in the early history of quantum computing was a statement from Richard Feynman that a quantum computer would be ...

View answer
What are the pros/cons of Trapped Ion Qubits, Superconducting Qubits and Si Spin Qubits?
6 votes

Here's a paper comparing Trapped Ion and Superconducting (the main competitors right now) from the group at UMD which compares their trapped ion system with IBM's transmon (superconducting) system. If ...

View answer
Is entanglement an operation or a stored state for qubits?
0 votes

To add on to what @DaftWullie said, in my mind it might make more sense to think of entanglement as a property of a multi-qubit state, instead of as a state itself. There is no special "Entangled" ...

View answer
How many qubits are simulable with a normal computer and freely accessible simulators?
-1 votes

Along with depending on time constraints, as Craig mentioned, you also need to specify how accurate/what gates you want the simulation to have. CHP (CNOT, Phase, Hadamard) simulations can do ...

View answer
Is it possible to purchase quantum computing hardware?
2 votes

The only commercially available quantum machines are from a company called D-Wave however there is debate on exactly how "quantum" their machines are. True quantum computers in the way most of the ...

View answer
What are reliable references on analytical and/or numerical studies of threshold theorems under faulty quantum error correction?
1 votes

This paper by one of my group mates includes faulty prep on ancilla qubits and other errors in an analysis of Bacon Shor. I'm not sure what you mean by faulty error extraction (measurement errors?) ...

View answer