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For questions related to measurement and its effects as relevant to quantum computation and quantum information

3
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1answer
42 views

How to calculate the off-diagonal elements of a density matrix using the measurement result?

For example, one have measured some states like $|0\rangle$ in the computational basis for many times and got the approximate probability of getting 0 and 1 ($P(0)$ and $P(1)$). Then how does he ...
6
votes
1answer
60 views

Does the need of many quantum algorithms to be repeated several times impair the efficiency gains?

As I understand so far, in some algorithms such as Simon's algorithm, swap-test algorithm or quantum k-means algorithm, we repetitively perform a measurement yielding a classical result. Consequently, ...
-4
votes
2answers
132 views

Scale for Quantum Computing [closed]

Does something like Mohs' scale exist for quantum computing? (eg. classical=0, hybrid=5, pure quantum=10) Mohs' scale: a scale of hardness used in classifying minerals. It runs from 1 to 10 using a ...
6
votes
3answers
2k views

Reverse Quantum Computing: How to unmeasure a qunit

After taking some measure, how can a qunit be "unmeasured"? Is unmeasurement (ie reverse quantum computing) possible?
4
votes
1answer
110 views

What are the standard eigenvalues in $\mathbb{C^2}\otimes\mathbb{C^2}$?

In $\mathbb{C^2}$, we generally take $+1$ and $-1$ as the standard eigenvalues, that's what Pauli-X, Pauli-Z measurements, etc will give us. Is there a similar standard while measuring in the Bell ...
5
votes
2answers
78 views

How to check states?

I don't know much about physics so maybe the question is stupid, but I wonder how engineers detect that the state function did not collapse due to the environment while a calculation is performed? ...
4
votes
1answer
73 views

Measuring ancillas in Shor's algorithm

When considering Shor's algorithm, we use ancilla qubits to effectively obtain the state $$\sum_x \left|x,f(x)\right>$$ for the function $f(x) = a^x \mod N$. As I have learned it, we then measure ...
3
votes
1answer
78 views

Realization of Quantum Shadowgraphy

I recently found out about shadowgraphy and was wondering if a technique like this could be used to: Visually show entanglement Suffice as a measure (e.g. continous partial trace) Most useful ...
11
votes
3answers
1k views

How can I build a circuit to generate an equal superposition of 3 outcomes for 2 qubits?

Given a $2$ qubit-system and thus $4$ possible measurements results in the basis $\{|00\rangle$, $|01\rangle$, $|10\rangle$, $|11\rangle\}$, how can I prepare the state, where: only $3$ of these $4$ ...
7
votes
2answers
232 views

Measuring in standard basis meaning

What does it mean to measure a qubit (or multiple qubits) in standard basis?
6
votes
1answer
58 views

Rank of a measurement

I am thinking about the following question: Assuming that we have some given state $\rho$ and we perform a measurement with $k$ outcomes on this state. Then we can describe the measurement in ...
3
votes
0answers
39 views

Projective measurements: aftermath and restoration

I have some practical difficulties with projective measurements, so I'd welcome inspiration from others. This is beyond the question "Are true Projective Measurements possible experimentally?" in that ...
4
votes
2answers
60 views

'Rectilinear' and 'Diagonal' Basis in BB84 Protocol

A lot of the tutorials on BB84 protocol talks about these two measurement bases, 'Rectilinear' or 'Vertical-Horizontal' and 'Diagonal'. I understand that it is possible to create a physical device ...
8
votes
1answer
77 views

Are true Projective Measurements possible experimentally?

I have heard various talks at my institution from experimentalists (who all happened to be working on superconducting qubits) that the textbook idea of true "Projective" measurement is not what ...
5
votes
1answer
57 views

Accessible information of system vs system, apparatus and environment

Suppose we have a quantum system $Q$ with an initial state $\rho^{(Q)}$. The measurement process will involve two additional quantum systems: an apparatus system $A$ and an environment system $E$. We ...
4
votes
2answers
114 views

What does it mean for a result of a measurement to be known/unknown?

Let me start the question with two examples. First, I am reading Nielsen & Chuang section "8.3.3 Bit flip and phase flip channels". There is a description of a quantum operation $\rho \to \...
13
votes
3answers
494 views

What does “measurement in a certain basis” mean?

In the Wikipedia article about Bell states it is written: Independent measurements made on two qubits that are entangled in Bell states positively correlate perfectly, if each qubit is measured in ...
7
votes
2answers
148 views

Is it true that observing a quantum state will end the superposition of states? How can I not observe?

I have no background in quantum physics, and no understanding of most formulas used in this context. I'm not looking for an in depth answer, i'd just like to vaguely understand the concept. The way i ...
13
votes
2answers
903 views

Do multi-qubit measurements make a difference in quantum circuits?

Consider the unitary circuit model of quantum computation. If we need to generate entanglement between the input qubits with the circuit, it must have multi-qubit gates such as CNOT, as entanglement ...
7
votes
2answers
75 views

Does Barkhausen noise affect the measurement of magnetic particle based qubits?

Q1: I've tried to find out if Barkhausen noise affects the measurement of spin-wave excitations in magnetic particle material based qubits. I prefer implementations such as those described in "...
14
votes
2answers
149 views

Representation of real numbers in quantum computers

In classical binary computers, real numbers are often represented using the IEEE 754 standard. With quantum computers you can of course do this as well - and for measurements this (or a similar ...
12
votes
2answers
187 views

What makes quantum computations different from randomized classical computations?

One of the many thing that confuse me in the field of QC is what makes the measurement of a qubit in a quantum computer any different than just choosing at random (in a classical computer) (that's not ...
15
votes
2answers
162 views

How does measurement of one qubit affect the others?

To represent a quantum computer's state, all the qubits contribute to one state vector (this is one of the major differences between quantum and classical computing as I understand it). My ...
19
votes
6answers
367 views

If all quantum gates must be unitary, what about measurement?

All quantum operations must be unitary to allow reversibility, but what about measurement? Measurement can be represented as a matrix, and that matrix is applied to qubits, so that seems equivalent to ...