Questions tagged [quantum-computing]

Quantum computation and computational issues related to quantum mechanics

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6
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1answer
741 views

Analytic solutions in semidefinite programming (SDP)

From my experience in the application of semidefinite programming (SDP) to quantum information, I have learnt that the solution to an SDP can sometimes be expressed as an analytic formula. For example,...
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2answers
105 views

What is the best lower bound for the fault-tolerance threshold in quantum computing?

It is well established that there exists a noise threshold for quantum computation, such that below this threshold, the computation can be encoded in such a way that it yields the correct result with ...
4
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1answer
347 views

Quantum cellular automata

This questions is cross-posted from MathOverflow A little background: As far as I know there is no standard definition of a quantum cellular automaton in the literature. Different authors use ...
10
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2answers
364 views

Does there exist a quantum algorithm ala Deutsch's algorithm that computs AND instead of XOR?

Deutsch's algorithm is a well known quantum computing $f(0) + f(1)\mod{2}$ with only one one evaluation of $f$. If we replace $+$ with $\cdot$ the problem seems to become rather different. My ...
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3answers
1k views

Rigorous security proof for Wiesner's quantum money?

In his celebrated paper "Conjugate Coding" (written around 1970), Stephen Wiesner proposed a scheme for quantum money that is unconditionally impossible to counterfeit, assuming that the issuing bank ...
27
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5answers
872 views

Quantum proofs of classical theorems

I'm interested in examples of problems where a theorem which seemingly has nothing to do with quantum mechanics/information (e.g. states something about purely classical objects) can nevertheless be ...
27
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4answers
1k views

Quantum approximation algorithms

It is generally considered unlikely that quantum computers will be able to solve NP-complete problems efficiently. In the classical case one approach to tackle such problems is to use approximation ...
9
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0answers
112 views

Non-tomographical certification of projectors, using product states?

I'm interested in operational ways of demonstrating (with high probability of confidence, in an error-free setting) that a POVM operator on n-qubit states is a projector. Specifically, I'm interested ...
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0answers
193 views

Are the minimal quantum and classical span programs the same?

A span program is a linear-algebraic way of specifying a boolean function introduced here which has found recent application in quantum query complexity. A span program for a function $f: \{0,1\}^n \...
14
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1answer
677 views

What is known about multi-prover interactive proofs with short messages?

Beigi, Shor and Watrous have a very nice paper on the power of quantum interactive proofs with short messages. They consider three variants of 'short messages', and the specific one I care about is ...
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1answer
349 views

Quantum complexity class vs classical complexity class [closed]

What is the relation between BQP complexity class and P and NP?
7
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3answers
3k views

Ed. Witten's new paper and the simulation of a quantum field theory

Context: Ed. Witten recently wrote a potentially revolutionary paper where he showed that under certain conditions, a Chern-Simons path integral in three dimensions is equivalent to an N = 4 path ...
12
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1answer
317 views

Transitioning from quantum to classical random walks on the line

Quick version Are there models of decoherence for the quantum walk on the line such that we can tune the walk to spread as $\Theta(t^k)$ for any $1/2 \leq k \leq 1$? Motivation Classical random ...
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3answers
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Physical realization of nonlinear operators for quantum computers.

I have read in a paper where nonlinear operators for quantum computers implies the solving of problems in #P time. See http://arxiv.org/pdf/quant-ph/9801041 . What would be the simplest realization of ...
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Computation beyond unitary matrices

Just out of curiosity, if the classical computation is about permutation matrices and quantum computing is about unitary matrices (of which the permutation matrices are a subgroup), then will there be ...
17
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1answer
458 views

Using the extra power of the negative adversary method

The negative adversary method ($ADV^\pm$) is an SDP that characterizes quantum query complexity. It is a generalization of the widely used adversary method ($ADV$), and overcomes the two barriers that ...
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1answer
205 views

Computational Library to compute Quantum Cluster States

I want to write a simulator for a quantum computing model that I am working on and I was wondering what would be the correct library / implementation strategy to implement quantum cluster states? ...
6
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1answer
260 views

Proof that Entanglement Cannot Increase the Capacity of a Noiseless Classical Channel

I am aware that quantum entanglement cannot increase the asymptotic capacity of a noiseless classical channel. However, can anyone provide some type of reference in the literature that contains a ...
1
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1answer
262 views

Branch prediction in quantum algorithms

Are there any good examples of branching efficiency / prediction in quantum algorihms? Specifically suppose I have a set of CNOT gates one after the other that have the control line on the same line ...
6
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3answers
620 views

Why does the Complexity Class PostBQP makes proving PP greater than or equal to QMA easier?

PP was proved greater than QMA in by Kitaev and Watrous 2000 - Parallelization, Amplification, and Exponential Time Simulation of Quantum Interactive Proof Systems. Later Aaronson proved that PostBQP ...
10
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1answer
415 views

Why must QMA complete problems be promise problems?

I'm reading Watrous's excellent survey paper on paper on quantum complexity theory. In it he states that it would be surprising if a QMA -complete problem were found to have a vacuous promise (I.e. Be ...
11
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2answers
502 views

Nonlocal Games and Quantum Communication

I'm currently on the look out for some good reference material relating non-local games with beneficial aspects in quantum communication. For instance, I am aware that non-local games are good at ...
10
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4answers
447 views

Quantum Bell-Type Inequalities

I'm curious if someone could recommend some supplementary material for gaining a deeper understanding of the paper : "Some Results and Problems on Quantum Bell-Type Inequalities - Tsirelson". ...
4
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1answer
656 views

Qubits and permutation symmetry

To put it straight - are qubits fermions, bosons or else? For example, the Bell states that are frequently used in quantum computations have different symmetry (00 + 11 is symmetric, 10 - 01 is ...
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0answers
866 views

Are quantum computers turing complete? [closed]

I have gained some interest in quantum computing ever since I have been reading Scott Aaronson's blog. The fact that using this computational model, you would be able to factor integers in polynomial ...
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4answers
2k views

Von Neumann architectures and quantum computing

Can you do quantum computing in a von neuman architecture? If not, why? What is the constraint? If you control a qbit, can a quantum computer use the von neuman architecture? Thanks.
14
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1answer
663 views

Reading up on $BQP = BPP^{BQNC}$

What should I read to understand this problem? The power of small-depth quantum circuits. Is $BQP = BPP^{BQNC}$? In other words, can the "quantum" part of any quantum algorithm be compressed ...
9
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1answer
386 views

What is the proper role of verification in quantum sampling, simulation, and extended-Church-Turing (E-C-T) testing?

Since no answer was given, a flag has been set requesting that this question be converted to a community wiki. The comments by Aaron Sterling, Sasho Nikolov, and Vor have been synthesized into the ...
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1answer
504 views

Is there any problem which is in AWPP but conjectured to be not in BQP?

Is there any known problem which is in $\mathsf{AWPP}$ but conjectured to be not in $\mathsf{BQP}$? What about relative to an oracle? Is there any known problem in $\mathsf{MQ^2}$ which is ...
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2answers
380 views

Restricting entries of unitary operators to real numbers and universal gate sets

In Bernstein and Vazirani's seminal paper "Quantum Complexity Theory", they show that a $d$-dimensional unitary transformation can be efficiently approximated by a product of what they call "near-...
5
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1answer
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Quantum Channel Decoding

Let a quantum channel $\Phi(\cdot)$ between two Hilbert spaces $\mathcal{H}_{in}$ and $\mathcal{H}_{out}$. What is the quantum channel $\Phi_{inv}(\cdot)$ that best reverses $\Phi(\cdot)$ ? $\forall $...
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1answer
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Does cryptography have an inherent thermodynamic cost?

Reversible computing is a computational model that only allows thermodynamically reversible operations. According to Landauer's principle, which states that erasing a bit of information releases $kT ...
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319 views

Approximation of Quantum Channels

Background: In quantum information theory, a wide class of processes acting on stochastic quantum states can be described using the formalism of Quantum Channels: A quantum channel is a linear, ...
15
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2answers
614 views

Quantum PAC learning

Background Functions in $AC^0$ are PAC learnable in quasipolynomial time with a classical algorithm that requires $O(2^{log(n)^{O(d)}})$ randomly chosen queries to learn a circuit of depth d [1]. If ...
5
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1answer
354 views

How efficiently can a 1-sparse Hamiltonian be simulated (quantum mechanically)?

In quantum computation there is a fair amount of interest in the task of simulating quantum physics. One instance of this is the problem of simulating the evolution of a system under the action of ...
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440 views

Is there a standard definition of Quantum Randomness?

I hope this question is not too vague. For classical bit generators there is the classical statistical definition which (informally) states that a source is ideally random if its output $X_1,X_2,\...
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1answer
312 views

Reference request: number-theory-free proof that maximal stabilizer groups determine unique states

Context. I am writing on topics such as the Gottesman-Knill theorem, using Pauli stabilizer groups, but in the case of d-dimensional qudits — where d may have more than one prime factor. (I ...
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2answers
1k views

Consequences of $SAT \in BQP$

As a TCS amateur, I'm reading some popular, very introductory material on quantum computing. Here are the few elementary bits of information I've learned so far: Quantum computers are not known to ...
27
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1answer
4k views

Shor's factoring algorithm help

I'm having a little trouble fully understanding the final steps of Shor's factoring algorithm. Given an $N$ we want to factor, we choose a random $x$ which has order $r$. The first step involves ...
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4answers
635 views

Bounding the gap between quantum and deterministic query complexity

Although exponential separations between bounded-error quantum query complexity ($Q(f)$) and deterministic query complexity ($D(f)$) or bounded-error randomized query complexity ($R(f)$) are known, ...
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1answer
431 views

Duality computers

There is a lot on the internet about quantum computers and how they could factor integers. However, there is a type of computer which also uses the principles of quantum mechanics, which can be used ...
10
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1answer
423 views

Span programs, witness size, and certificate complexity

A span program is a linear-algebraic way of specifying a boolean function introduced here. Recently, this model was used to show that the negative adversary method provides a tight characterization (...
9
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3answers
549 views

Interactive Proofs via Postselection?

Define the computational model MPostBQP to be identical to PostBQP except we allow polynomially many qubit measurements before the post-selection and final measurement. Can we give any evidence ...
9
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2answers
240 views

Polynomial algorithms for UPB (Unextendable Product Bases)

Consider a Hilbert space $H = H_1 \otimes \dots \otimes H_n$. An Unextendable Product Basis (UPB) is a set of product vectors $\vert v_i \rangle = \vert v_i^1 \rangle \otimes \dots \otimes \vert v_i^n ...
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2answers
353 views

Quantum evolutions

I was reading Quantum Computation Explained to my Mother. While considering the following problem: Problem 1 Suppose we are given a mysterious boolean operator F (a black box) which takes one ...
7
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2answers
449 views

Quantum query complexity and certificate complexity

A certificate for an input $x$ is a subset of bits $S \subseteq \{1,...,n\}$ such that for all inputs $y$, $(\forall i \in S \quad y_i = x_i) \rightarrow f(y) = f(x)$. Then $C_x(f)$ is the minimum ...
2
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2answers
368 views

Time-entanglement phenomenon

Please let me mention certain idea here, although it is probably vague (and new, at least as related to experiment mentioned below, as far as I know). The general notion of algorithm is model of ...
17
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2answers
367 views

Is PARITY in QAC_0 (if that even makes sense)

As is well known PARITY cannot be done in poly-sized constant-depth circuits, and in fact const-dept circuits require EXP number of gates. What about QUANTUM circuits? a) Can PARITY be done with a ...
9
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1answer
628 views

Lower bounds on the Threshold function

In decision tree complexity of a boolean function, a very well know lower bound method is to find a (approximate) polynomial that represents the function. Paturi gave a characterization for symmetric ...
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3answers
5k views

Is there any connection between the diamond norm and the distance of the associated states?

In quantum information theory, the distance between two quantum channels is often measured using the diamond norm. There are also a number of ways to measure distance between two quantum states, such ...