Most Popular
1500 questions
6
votes
3 answers
How do we physically initialize qubits in a Quantum register?
In quantum algorithms we need to initialize the qubits at the start of our algorithm in some quantum register. Suppose that if we are working with a four qubit quantum register we can initialize the register into values such as $|0000\rangle$ or…
K Sarkar
- 171
- 3
6
votes
2 answers
Randomness from deterministic machine
It is known that in a classical computer we can't generate a purely random number by deterministic process. I am taking a course in Quantum Computing, recently I learnt that using state we can generate a deterministic process which will produce…
tarit goswami
- 315
- 3
- 10
6
votes
1 answer
How scalable are quantum computers when measurement operations are considered?
From a high-level point of view, given a quantum program, typically the last few operations are measurements.
In most cases, in order to extract a useful answer, it is necessary to run multiple times to reach a point where it is possible to…
Juan Leni
- 165
- 8
6
votes
2 answers
Symmetry in Conditional Phase Shift Gates and Realizing CNOT through HCZH
Why are conditional phase shift gates, such as CZ, symmetrical? Why do both the control and target qubit pick up a phase?
Furthermore, assuming that they are symmetrical, when using a CNOT gate as an H gate on the target qubit, a CZ gate, and…
Eric Li
- 133
- 6
6
votes
1 answer
Classical XOR gate in Quantum Circuit
Can we use classical XOR gate in a quantum circuit? Or are there any alternatives for XOR gate?
Govind
- 123
- 1
- 5
6
votes
0 answers
Photon Entanglement using SPDC
Are there any good references where from I can learn about photon entanglement using spontaneous parametric down-conversion (SPDC)?
Govind
- 123
- 1
- 5
6
votes
1 answer
Trace of projected Gaussian operator product: puzzling difference between projection onto filled or empty fermionic mode
This question was motivated by a Mathoverflow posting.
I am comparing two traces of fermionic creation/annihilation operators $a_n,a_n^\dagger$ ($n=1,2,\ldots N$):
$$T_{\rm empty}= \operatorname{tr}\bigl(a_1 a_1^\dagger e^{a^\dagger X_1 a}a_1…
Carlo Beenakker
- 151
- 8
6
votes
1 answer
Given an entangled state on two systems, when can a subsystem of one decouple from the other?
Suppose you have the following quantum state:
$$\frac{1}{\sqrt{2^n}}\sum_{i}|\bar{i}\rangle_A|i\rangle_S$$
where $\{|i\rangle_A\}$ and $\{|i\rangle_S\}$ are orthonormal bases for an $n$-qubit ancilla and $n$-qubit quantum system respectively. The…
ssd42
- 61
- 2
6
votes
2 answers
Why aren’t electrons in our atoms suffering from decoherence?
I know about how the current era of quantum computing is trying to find ways in order to improve the coherence times of the quantum states and so on and that decoherence and noise are the greatest obstacles, some needing extremely isolated…
yousef elbrolosy
- 337
- 1
- 9
6
votes
2 answers
How do we derive the density operator of a subsystem?
The density operator can be used to represent uncertainty of quantum state from some perspective, aka a subsystem of the full quantum system. For example, given a Bell state:
$|\psi\rangle = \frac{|00\rangle + |11\rangle}{\sqrt{2}}$
where Alice has…
ahelwer
- 4,288
- 2
- 15
- 36
6
votes
0 answers
Can a half-classical, half-quantum rat settle on finding the cheese faster than a fully classical or fully quantum one?
TL/DR: With $t$ being the classical mixing time of an connected, non-bipartite, unweighted, undirected graph on $n$ vertices, and $\pi_n=\frac{1}{n}$ being the stationary probability at vertex $v_n$, what strategy, if any, is there to interpolating…
Mark Spinelli
- 15,378
- 3
- 26
- 83
6
votes
1 answer
Inclusion of the normalizer in the centralizer
From wiki, a centralizer $C(S)$ and a normalizer $N(S)$ of stabilizer group $S$ are defined as (1), (2):
$$C(S) = \{g \in G \,|\, gs = sg \, \text{for all}\, s \in S\} \tag{1}$$
$$N(S) = \{g \in G \,|\, gSg^{\dagger} = S\} \tag{2}$$
where G is Pauli…
taketoshi kinoshita
- 1,249
- 1
- 2
- 11
6
votes
1 answer
How is measurement modelled when using the density operator?
I've just learned about the density operator, and it seems like a fantastic way to represent the branching nature of measurement as simple algebraic manipulation. Unfortunately, I can't quite figure out how to do that.
Consider a simple example: the…
ahelwer
- 4,288
- 2
- 15
- 36
6
votes
1 answer
To what degree could additional ancilla qubits affect the complexity of a quantum algorithm?
It is a known trick in quantum computing to use additional ancilla qubits and uncomputation to construct efficient quantum circuits.
I wonder, are there some rigorous results that estimate how big this effect could be?
For example, is it possible to…
Danylo Y
- 8,076
- 13
- 24
6
votes
1 answer
Transmons and cooper pair box qubits: two islands or a single island and a reservoir
In scientific literature, one typically describes Cooper pair boxes as a small superconducting island coupled to a superconducting reservoir (say, a large ground plane of superconducting metal, or a large piece in any case) via a Josephson junction.…
user129412
- 170
- 5