7 votes
Accepted

What is the clear definition of coherent versus incoherent errors?

Coherent vs incoherent: as a rule of thumb, coherent is unitary, incoherent is stochastic. The distinction is not entirely clear when your channel is a mixture of these. Thus, it makes sense to ...
Balint Pato's user avatar
7 votes

Does post selection of a qubit introduce non-linearity?

Linearity before normalization Yes this map is linear (up to normalization) if we don't care if the output of the map is normalized (i.e., the output is a vector but not a valid quantum state). To see ...
Rammus's user avatar
  • 5,503
5 votes
Accepted

Does post selection of a qubit introduce non-linearity?

This transformation is almost linear, but normalization makes it non-linear in general. The state update rule following a generalized measurement on some state $\rho$ is $$\rho\to \frac{A_i\rho A_i^\...
Quantum Mechanic's user avatar
4 votes
Accepted

What do the numbers mean in the elements used in ion traps?

The numbers you are seeing are labeling the isotope, not the charge. For example, they are specifying the isotope of calcium that has 43 nucleons. Since calcium by definition has 20 protons, this ...
Quantum Mechanic's user avatar
3 votes
Accepted

How to create a electron in a superpositon?

But I am not clear how to keep the electron in a particular superposition of our wish. (for e.g.: $1/\sqrt 2 |0\rangle + 1/\sqrt 2 |1\rangle$). Consider a Stern-Gerlach apparatus with it's field in ...
hft's user avatar
  • 868
3 votes

Qudits in NISQ Devices: Benefits Beyond Dimensional Advantages?

Preamble Versions of this question arise over and over again, probably because most of us are accustomed to thinking that "more is better" rather than "less is more", for example ...
user1271772's user avatar
  • 13.5k
3 votes
Accepted

Quantum and DNA computing

The NMR quantum computing implementation you mentioned used cytosine as a 2-qubit platform because cytosine has two adjacent, carbon-bound hydrogen atoms with slightly different hyperfine couplings ...
Chris E's user avatar
  • 1,398
2 votes

What use cases are there for 127 qubit QPUs?

This paper may be of interest in regards to this question; we were able to execute QAOA circuits using all 127 qubits of ibm_washington (and the computation was not complete noise, due to the short ...
Elijah Pelofske's user avatar
2 votes

How many physical qubits are needed to encode a logical qubit on ion trap, superconducting, neutral atom, photonic QC?

The answer depends on several factors: The desired error rate of the logical qubit. The physical qubit fidelities in the given platform (e.g. two-qubit gate fidelity, idling errors, measurement ...
QuantumEngineer's user avatar
2 votes

Measuring Ising anyons: What is a fusion measurement?

A bit late, but some comments for the sake of posterity: Yes, the claim is that in certain systems, a pair of anyons can be brought close together -- then measuring (for instance) the energy of the ...
Sachin Valera's user avatar
2 votes
Accepted

Physical interpretation of a 2-photon-qubit system occupying the antisymmetric Bell state

The confusion here arises because of not being careful about what the labels inside the kets mean. When we talk about the Bell states (or about the phenomena of entanglement), we consider two ...
Abdullah Khalid's user avatar
2 votes

Is there any recent review on the state of the art of NMR quantum computing?

Not as far as I know. NMR quantum computing isn't a particularly active area of research these days for a few reasons including: You're not working with a single qubit, but rather an ensemble of ...
Chris E's user avatar
  • 1,398
2 votes

What is complete structures of the photonic quantum computers?

Chapter 7 of Nielsen and Chaung goes through (some of) the physical realizations of quantum computers. However, it's important to note the sheer complexity involved in building a quantum computer from ...
banercat's user avatar
  • 753
2 votes

Example of two-level systems which has very long lifetime "in nature" but as soon as we manipulate them their lifetime is very small

A notable counter example to this idea is dynamical decoupling / spin echo. Qubits often have longer lifetimes when you're constantly flipping them, instead of letting them sit still, because the ...
Craig Gidney's user avatar
  • 35.3k
2 votes
Accepted

What is the purpose of Doppler cooling in neutral atom devices?

We generally think about temperature as how hot or cold something is, but in physics it's really a measure of the average speed of the atoms that make up an object, with hotter things have atoms ...
Chris E's user avatar
  • 1,398
2 votes
Accepted

What are the limitations of Harvard's and Google experiment on quantum error-correction

Yes. Sounds right. Assembling stabilizers by destructively measuring the state is typically less noisy. They're certainly working on extending to multiple rounds of syndrome extraction. Probably first ...
squiggles's user avatar
  • 698
1 vote

Phase accumulation for multiple Rabi driving pulses

Good question! Your first equation mixes a couple of abstractions that we typically sweep under the rug, so let's start by taking a step back. When we apply a control pulse to a qubit, it's usually ...
Chris E's user avatar
  • 1,398
1 vote

What is the clear definition of coherent versus incoherent errors?

The need for these terms might be helpful to understand the intuition. Coherent errors are statistical characterizations while still respecting unitary evolution. Incoherent errors are the traditional ...
Sam's user avatar
  • 187
1 vote
Accepted

Number of qubits in quantum circuits in ibm qiskit

TL;DR: With QuantumCircuit.num_qubits: >>> print(qc.num_qubits) 12 A list of them, via ...
luciano's user avatar
  • 5,743
1 vote

Why are we only interested in the linear combination $|\phi\rangle = a |0\rangle + b |1\rangle$?

A qubit is defined as living in a 2-dimensional Hilbert space. This means one cannot define more than 2 linearly independent basis states. Since $|0\rangle$ and $|1\rangle$ are linearly independent (...
Quantum Mechanic's user avatar
1 vote

Physical interpretation of a 2-photon-qubit system occupying the antisymmetric Bell state

You're exactly right that indistinguishable bosons are represented by symmetric states. However, when we think about a photons in QIS, we often think about them interacting with beamsplitters. When a ...
Chris E's user avatar
  • 1,398
1 vote
Accepted

How to measure an unknown state produced by a source of qubits?

I thought I'd elaborate a little on @Chris E's answer with some details about what kind of precision we can expect in our approximation as we increase the number of measurements we make. I'll be ...
Franklin Pezzuti Dyer's user avatar
1 vote

Randomness and precision of Quantum Number Random Generators

My naive take is that the state of any classical generator may be gigantic but enumerable. Hence the generated sequences should repeat eventually. Perhaps that's overly naive.
rhundt's user avatar
  • 923
1 vote

How does a quantum computer do basic math at the hardware level?

It's worth pointing out this review article that attempts to compare the many approaches to quantum adders, based on several different metrics for efficiency that one may care about: https://www....
Max's user avatar
  • 171
1 vote

How are elementary quantum gates realised?

I'll posit that, much as classical NAND and NOR gates can be generated with NMOS and PMOS transistors arranged in series and/or in parallel, quantum gates such as CCNOT (Toffoli) and CSWAP (Fredkin) ...
Mark Spinelli's user avatar

Only top scored, non community-wiki answers of a minimum length are eligible