Questions tagged [quantum-computer]

The quantum computing tag is relevant for computing that uses quantum states such as superposition and/or entanglement to locate low energy states as solutions to complex problems (rather than laboriously enumerating and checking solutions as would be done with non-quantum traditional computing).

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Has anyone come across the relationship between Higher order confinement wave-functions and corresponding Zeeman Interaction?

I'm studying a hole quantum dot in Germanium. The problem is described in the Luttinger Spin-3/2 basis and takes Fock-Darwin states as solutions in the Quantum Dot plane(xy plane). Furthermore, I ...
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Why/how does Microwave pulse cause electron spin to rotate in the Bloch sphere?

I recently started studying pulse techniques for electrons in the context of quantum computing/sensing/optics/etc. For spins in solid state (like color centers in diamond or SiC, or metal centers in ...
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Is Hamiltonian evolution unique given a fixed input and output state?

Consider a quantum system that is prepared at some initial state $| \text{in}\rangle$ and we are able to measure the output state $| \text{out}\rangle$. Furthermore assume this evolution of the state ...
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Displacement gate on an arbitrary quantum state

I am reading this reference. In this article, the author define the Heisenberg-Weyl operators (Eqs. [148] in the reference): $$X(s)=e^{-is\hat{p}/2} \tag{1},$$ $$Z(t)=e^{it\hat{q}/2} \tag{2},$$ where $...
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The number of qubits required in Shor's algorithm

People say that the number of qubits required in Shor's algorithm for factorizing $N$ should be $2\log N$ for control register and $\log N$ for target register. I want to know the reason why these ...
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Can we make a microstrip analog of a 3D microwave E plane Tee

I need to make the 2d analog of a microwave E plane tee.The microwave Eplane tee has 3 ports and it divides a wave input at port 3 into two 180 degree out of phase waves at the 2 output ports. I want ...
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Johnson noises in the Josephson junction based devices

I have difficulty understanding that superconductors being non-dissipative; have resistive noises. Even if we consider the case of charge noise and minimal thermal noises (if present) how can we ...
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How can time crystals be useful in qRAM design?

A time crystal is a phase of a matter which is ordered in time, similar to classical crystals which are ordered spatially. In other words, the structure of a time crystal is ever-changing but with ...
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Any Books about Superconducting Quantum Computing?

I'm an undergraduate majored in physics and I'm quite interested in superconducting quantum computing. Are there any books or maybe reviews about this field (suitable for beginners)?
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Are electrons in a semiconductor quantum dot is a real electron or just a quasiparticle with a well-defined spin?

It seems there is a consensus that in the gate-defined quantum dot exists isolated electron with well-defined spin. And I understand that by tuning the voltages applied to the 2DEG at quantum well, ...
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Which other physical quantities, other than the polarization of classical light be used as a qubit?

This article mentions that polarization of classical light can be used as a qubit. link Are there other physical quantities in the classical world which can be act as qubit.
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Why does the no-cloning theorem imply the monogamy of entanglement?

I have seen papers where they mentioned that the monogamy of entanglement can be seen as a consequence of the no-cloning theorem. Could somebody sketch this out? N.B.: I have seen the argument that ...
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What entanglement measures are known to satisfy entanglement monogamy?

As the title says, what entanglement measures are known to satisfy the entanglement monogamy principle? Im specially interested in those entanglement measures that apply to mixed states.
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Quantum computer: What are the carriers of information and what is the spatial arrangement of the individual information carriers?

I assume that the carriers of the information bits are electrons with their spin or magnetic dipoles. Are there other bit carriers for quantum computers? And, how are these bits arranged spatially in ...
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Can you explain a quantum computer to someone who knows physics a little over high school level? [closed]

How do quantum computers work ? Please provide an explanation that is suitable for a person who has studied chemistry and had some courses of physics at university level. But still is not a genius at ...
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Why are quantum computers faster at breaking RSA-2048 encryption than classical supercomputers are?

One night whilst browsing the web I came across a video on Youtube called "The Hype Over Quantum Computers, Explained" by CNBC. Naturally, I became curious as to why quantum computers are so ...
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Braiding anyons in one dimension

In the Rev. Mod. Phys. 80, 1083 (2008) Non-Abelian Anyons and Topological Quantum Computation, they make an aside in Section II.1.a that as an aside, we mention that in 1 + 1D, quantum statistics is ...
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Quantum volume required to break a 2048 bit number used in RSA encryption

I am curious as to whether there is a specific amount of quantum volume that will allow a quantum computer to break a 2048 bit number used in RSA encryption, and if so, what that number is. (within a ...
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Optimal control for single qubit gate

Suppose we want to implement a single qubit gate $U$ and are only able to apply rotations around axes in the $xy$ plane of the Bloch Sphere, i.e. we can only apply Hamiltonians of the form $$H(t) = \...
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What optical element can you use to implement a square root of NOT gate

How do you implement a square root of NOT gate in an optical quantum computing circuit? What optical element or combination of elements would you use - especially, if you want to manipulate ...
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Initial condition of the data in the quantum computation?

So here's something that's been bothering me. Given the time evolution of the wavefunction can only be unitary or discontinuous as a process of the measurement. So let the observables for our ...
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Shor's algorithm entanglement verification

I would like to ask whether the entanglement verification is necessary in Shor's algorithm In the paper, Nature Photon 6, 773–776 (2012), they mentioned that they tried to factorize 21 to avoid the ...
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How do I prove the identity for ${\rm tr}_p [e^{-iS\Delta t}(\rho\otimes\sigma)e^{iS\Delta t}]$ in Seth Lloyd's 2014 Quantum PCA Paper?

Equation (1) in Seth Lloyd's paper on Quantum PCA says: $\text{tr}_{p}\text{e}^{-iS\Delta t} \rho \otimes \sigma \text{e}^{iS\Delta t} = \cos^2(\Delta t)\sigma + \sin^2(\Delta t) \rho - i \sin(\Delta ...
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Could trapped ion quantum computers be used as (useful) atomic clocks?

Building a trapped ion quantum computer requires optimizing for different things than a cold atomic clock system. However, a lot of money is currently being dumped into rapidly developing this quantum ...
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Proof: A channel is entanglement breaking if it separates maximally entangled states

I'm working my way through Mark M. Wilde's Quantum Information Theory (2nd edition) and am stuck on a proof regarding entanglement breaking channels (Exercise 4.6.2) Prove that a channel $\mathcal{N}...
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In the context quantum computation, what is the evidence that querying classical oracles in superposition makes sense?

In quantum computation it is often assumed that if $f$ denotes some (classical) Boolean circuit $\{0,1\}^n \rightarrow \{0,1\}$, then a quantum circuit can have oracle access to $f$, that is the ...
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Is there a comparison table for quantum algorithms like VQE, QPE, QAOA, and so on?

I have one question: Recently, I studied about several algorithms like VQE, QPE, QAOA and so on. I would like to make some comparison tables about those algorithms, their strengths and weaknesses. If ...
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Why are electrons usually used to define qubit in silicon-based quantum dot, but not holes?

Usually, electrons spin are used to define qubit in sillicon. In contrast, holes are usually used in description of germanium qubit. Why is that?
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In spin qubit of quantum dots, why $|S>$ and $|T0>$ state have energy difference?

As we know, the Zeeman splitting by magnetic field causes the energy difference between |↑> and |↓>, therefore states like |T+>=|↑↑> have a different energy from others. But why states ...
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Is $\{Z, CZ\}$ an universal quantum gate set for diagonal gates with eigenvalues +1 and -1?

Consider diagonal quantum gates with eigenvalues $\pm 1$, i.e. all diagonal elements are either $+1$ or $-1$. Can these gates always be decomposed into a finite number of Z and controlled-Z gates? My ...
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Simplification in operator-sum representation (Nielsen and Chuang)

First remark: there is a similar question: Operator-sum representation of a quantum operation with different input and output spaces which helped more generally, but I am still stuck on a part unasked ...
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Proof of Uhlmann's theorem (explicit decomposition )

I am having difficulty with Wiki's proof of the Uhlmann's theorem (https://en.wikipedia.org/wiki/Fidelity_of_quantum_states#Proof_with_explicit_decompositions) The purifications of the states is given ...
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Why does entanglement negativity not satisfy the triangle inequality in the usual sense?

I am a bit puzzled, I’ve read in some places, like the original paper by Vidal, that $$ \mathcal{N}(\sum_n a_n \rho_n)\leq \sum_n a_n \mathcal{N}(\rho_n) $$ whenever $a_n \geq 0$ and $\sum_n a_n =1$. ...
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How does this quantum circuit work? [closed]

I am getting a bit confused about how to trace through a quantum circuit. So, for example, if you have the circuit below: I know that the first gate would split an arbitrary state alpha|0> + beta|...
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Set of unitary operators such that $U|0\rangle = |\psi\rangle$

In quantum computing, one of the central areas of study is to determine efficient quantum circuits - described by unitaries - to prepare a state $|\psi\rangle$ from the initial computational basis ...
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How is many world interpretation of quantum mechanics compatible with no cloning theorem?

In many worlds interpretation of the quantum mechanics all possible outcomes of a measurement are realized, however, in different universes. Everytime a measurement occurs we register one outcome and ...
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Leakage of superconducting qubit: why it occurs with short driving pulses and not as well with long ones

My question in very short: Short pulses on superconducting qubit usually induce leakage. But a long pulse can be seen as a sequence of short pulses (imagine all of the short pulse as square pulses: ...
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Final Hamiltonian for Adiabatic Grover

X-Posted on Quantum-Computing Stack Exchange In quantum computation, there is a famous algorithm to search a marked item in an unstructured database called Grover's algorithm. It achieves a quadratic ...
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How do $|1⟩$ and $-|1⟩$ represent the same state?

I am a newbie to quantum mechanics (I come from more of an engineering/CS background), and today my quantum computing professor stated that $\def\ket#1{\left|#1\right\rangle}\ket{1}$ and $-\ket{1}$ ...
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Can I put 2 Majoranas particles (instead of just 1) on each vertex of any lattice (rectangular, hexagonal, triangular, etc ...)?

I am trying to study correlations between Majorana particles on the 1D and 2D lattice. Most parts of the literature considers just 1 Majorana in each site of the lattice.
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Is there an algorithm for taking generators of a stabilizer code and finding an encoder?

Given a stabilizer $[[n, k, \_]]$ code $\mathcal{S}$, its normalizer $\langle g_1, \dots, g_{n-k}\rangle$, and encoded Pauli operators $\bar{Z}_1, \dots, \bar{Z}_k, \bar{X}_1, \dots, \bar{X}_k$ I know ...
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Can the Hamiltonian be upscaled to speed up quantum gates?

In quantum computing, gates are performed unitarily, i.e. $|\psi\rangle \mapsto U|\psi\rangle$, driven by some Hamiltonian, i.e. $U = \exp(-iHt)$. Consider $H$ as time independent, if it is ...
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CNOT over a $|+\rangle$ qubit

I want to use the CNOT gate over a qubit $|0+\rangle$, but the definition says that CNOT flips the second qubit if a $1$ is found in the first one (i.e. 0 to 1, and 1 to 0). However, what is is to ...
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Can classical spin-glass problems be solved exactly in a more efficient way than bringing them to the quantum level?

It is known that classical spin-glass problems are NP-hard, just like their quantum counterparts. Does this truly mean that finding their Ground-State is equally hard in both cases? The most ...
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Does uncopying an information requires a copy of the same program in this scheme? If so, how?

In the paper Information is Physical by Rolf Landauer (reference), it is claimed that Uncopying is not equivalent to erasure and, just like copying, can be done with a dissipation per step ...
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What gate combinations create entangled two-qubit states?

I know that in order to make a two qubit entangled state, this quantum circuit is used: But I was wondering if there are any other gate combinations which also create entangled two quit states. If ...
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Is quantum computation possible in a superdeterministic universe?

I've seen many renowned scientists arguing that the universe is superdeterministic, but superdeterminism is a "hidden variable" theory. Is this compatible with the concept of quantum ...
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Quantum computing resource [duplicate]

I am a beginner in quantum computing, and I do not have any background in Physics. Can anyone refer me a good material to study the subject? By assuming quantum mechanics like black box is also making ...
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How to write the general state of a corrupted $n$-qubit codeword in quantum error correction?

I am reading the chapter on quantum error correction in Quantum Computer Science by David Mermim. Which says, for a single qubit in a random state $|\Phi\rangle$ of the superposition of $|0\rangle$ ...
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Why does changing $X\to iX$ in the definition of the CNOT make an important difference?

How to find a ket $|\psi \rangle$ that illustrates how changing X to iX in the definition of the CNOT gate makes an important difference because of what happens when CNOT is applied to $|\psi \rangle$?...

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