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1500 questions
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3 answers
How to create the state $\vert 0 \rangle+i \vert 1 \rangle$ using elementary gates?
I am trying to write $|0\rangle+i|1\rangle$ in terms of elementary gates like H, CNOT, Pauli Y, using the IBM QE circuit composer.
I was thinking some kind of combination of H and Y since $Y|0\rangle=i|1\rangle$, so it is close but not…
Natasha
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What does $M_m |\psi_i\rangle$ mean in the equation $p(m|i)=\langle\psi_i|M_m^\dagger M_m|\psi_i\rangle$?
I have trouble understanding two equations in the Nielsen & Chuang textbook. Suppose we perform a measurement described by the operator $M_m$. If the initial state is $|\psi_i\rangle$, then the probability of getting result m…
ZR-
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Topology graphs of IBM's quantum computers
What is the meaning of the topology graphs of IBM's quantum computers? How should it be interpreted?
For example, how should I understand the following graph?
AL_P
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6
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How to decompose/unroll a Qiskit Arbitrary initialization
I'm aware that to randomly initialize a quantum circuit with Qiskit, we can simply write qc.initialize(rand_state, q[0]...). However, when drawing the circuit, I see that the "initialize" function is treated as one big gate that takes in an input…
Rehaan Ahmad
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6
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3 answers
Do there exist problems known to be computationally intractable for quantum computer, but tractable for classical computer?
Or alternatively phrased, is it believed that the complexity class P is a complete subset of BQP?
Consider the following diagram à la MIT OpenCourseWare, which seems to explicitly state as much.
Eoin Dowling
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6
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Why QAOA with $p \rightarrow \infty $ gives the optimal solution?
In the QAOA paper, it is shown that the optimal value of the p-ansatz $M_p$ converges to $\max_z C(z)$ as $p \rightarrow \infty$ on page 10. The proof is to relate to QAOA by considering the time-dependent Hamiltonian $H(t) = (1 − t/T)B + (t/T)C$,…
John Wong
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6
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1 answer
Are applications with only polynomial speedup worth chasing after? (since error correction adds a heavy overhead)
A number of ML algorithms have demonstrated to have polynomial speed-up:
But this (I'm assuming) is without error correcting qubits. How practical are algorithms that only exhibit polynomial speed-up when error correction is taken into account? For…
Steven Sagona
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6
votes
2 answers
Simple Hadamard Circuit gives incorrect results?
I am running the simplest Hadamard circuit possible - one qubit, initialised to 0, apply H gate, measure in Z basis. When I run the circuit on the simulator I obtain the expected result (50% in 0, 50% in 1) but the "measurement probability" and…
Catherine
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1 answer
Do the latest D-Wave computers use Helium compounds for cooling?
Do the latest D-Wave machines use compounds of $\require{\mhchem}\ce{^{3}He}$ and $\ce{^{4}He}$ for cooling? If not, how does D-Wave approach cooling its plates low enough to achieve superconductivity? What compounds does DWave use for the plates in…
Dylan Dodds
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6
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1 answer
My IBM quantum account has been disabled
As shown in the picture above, my IBMQ account is unavailable, and I found that my friend's account is also unavailable, but I have not received any email or message notification. what happened?
wei zi
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6
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1 answer
Role of convexity in proof of Monogamy of Bell correlations
In this paper, the authors give a proof of the monogamy principle in quantum physics. I'm having trouble understanding the convexity argument in the proof of Lemma 2 (penultimate paragraph, page 2). I suggest the reader read the problem first…
Tuneer
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1 answer
What is the difference between quantum control and quantum optimal control?
In the context of quantum control theory, it is common to see references to both quantum control and quantum optimal control (e.g. 0910.2350 or the guide on qutip quantrum control functions).
Sometimes it seems like the two terms are used…
glS
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6
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Trying to understand this measurement of a simple quantum circuit
It's a newbie question, I know. But I was just wondering if someone could help me understand why this simple circuit results with the measurement shown. I've just didn't get the last step. It's not supposed that the last Hadamard gate would cancel…
Maka
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6
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2 answers
In quantum computation, if the fidelity is high enough to promise over 50% success rate, is quantum error-correction still needed?
Let's say we have many qubits and gates. The errors happen randomly, for example with a probability of 0.1% at each place(at this place, no quantum error correction is implemented). If the probability of no error occurs in the whole computation…
Han Bao
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6
votes
1 answer
Could a new benchmark of quantum processors Q-Score by Atos be more useful than quantum volume?
A few days ago, Atos company published new benchmark for quantum computers. The benchmark is called Q-Score and it is defined as follows:
To provide a frame of reference for comparing performance scores and maintain uniformity, Q-score relies on a…
Martin Vesely
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