# Tag Info

22

The oracle is basically just an implementation of the predicate you want to search for a satisfying solution to. For example, suppose you have a 3-sat problem: (¬x1 ∨ ¬x3 ∨ ¬x4) ∧ (x2 ∨ x3 ∨ ¬x4) ∧ (x1 ∨ ¬x2 ∨ x4) ∧ (x1 ∨ x3 ∨ x4) ∧ (¬x1 ∨ x2 ∨ ¬x3) Or, in table form with each row being a 3-clause, x meaning "this variable false", o ...

19

Two quick clarifications: Adiabatic QC is typically "based on qubits" just as much as circuit-based QC is -- I don't know where you got the idea that it isn't! (Though one could also use qutrits or other building blocks, in either the circuit or the adiabatic models.) As Mateus pointed out, the justly-famous result of Aharonov et al. says that "adiabatic ...

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Adiabatic Quantum Computation is Equivalent to Standard Quantum Computation - Dorit Aharonov, Wim van Dam, Julia Kempe, Zeph Landau, Seth Lloyd, Oded Regev

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Renyi entropy is analogous, in some sense, to $\ell_p$-norms, so let's first recall why those norms are useful. Suppose we have a vector of numbers $a \in \mathbb{R}^n$. We want to have a single number that represents, in some sense, how does the typical element of $a$ look like. One way to do so is to take the average of the numbers in $a$, which roughly ...

17

Here the goal is to construct a decision problem D so that (a) if you can factor you can solve the decision problem in polynomial time and (b) if you can solve the decision problem you can factor in polynomial time. There are a number of ways to do this. To name just two: D: given n and k does n have a divisor d satisfying 1 < d <= k? D: given ...

17

Yes, but most of the work so far (except very recently, see below) has focused on turning irreversible computations into reversible ones, thereby hoping to avoid any entropy generation. (Note: there is an important difference between energy needed to make a computation run, and entropy generated by the computation and put out into the environment, typically ...

16

I would like to add some more references to Scott's comment: Indeed, Clebsch-Gordan transforms (that you can think of as multi-register quantum Fourier transforms) are a useful tool in the design of quantum algorithms for non-Abelian hidden subgroup problems (HSPs). Clebsch-Gordan transforms were used by Greg Kuperberg and Oded Regev to solve the dihedral ...

15

Consider trying to make atomic guesses for an unknown random variable $X$ distributed over some finite set $A.$ In Shannon entropy, it is assumed that you can query bit by bit, i.e., if $A=\{1,\ldots,N\}$ you can ask: Is $X\in \{1,\ldots,N/2\}$ ? (assume $N$ even or use floor/ceiling functions) In crypto and some decoding scenarios this is not realistic. ...

15

This is a great question, Suresh! Our randomness expansion result does not imply any complexity theoretic result. Here's one way to understand the result: we believe that quantum mechanics governs the world, and under this assumption, there are quantum devices that generate genuine, true, information-theoretic randomness. However, imagine that you're ...

12

Sorry I'm late! In quantum computing theory, there are lots of examples of optimization problems over the unitary group that, surprisingly (at least to me), are solvable in (classical) polynomial time by reduction to semidefinite programming. Here was an early example: solving a problem of mine from 2000, in 2003 Barnum, Saks, and Szegedy showed that Q(f), ...

12

I'm not familiar with details of the ellipsoid method specifically for semi-definite programs, but even for linear programs, analysis of the ellipsoid method is pretty subtle. First, one needs to bound the number of iterations of the ideal ellipsoid algorithm. Let $E_i$ be the ellispoid used in the $i$th iteration of the ellipsoid algorithm, and let $c_i$ ...

12

I apologize; I was too glib when I wrote that. While I believe it's possible to prove an oracle separation between $BQP$ and $BPP^{BQNC}$ using current techniques, it hasn't been done (12 years after I first thought about the problem, then put it off!), and would certainly be worth a paper for whoever did it. Maybe your post will help motivate me to ...

11

Renyi entropy (of order 2) is useful in cryptography for analyzing the probability of collisions. Recall that the Renyi entropy of order 2 of a random variable $X$ is given by $$H_2(X) = - \log_2 \sum_x \Pr[X=x]^2.$$ It turns out that $H_2(X)$ lets us measure of the probability that two values drawn i.i.d. according to the distribution of $X$ happen to be ...

9

If by "classically using the solutions of the linear equation" you mean "accessing the information in the exactly same way a classical computer does" or, in other words, "obtaining the classical solution $x$ to a system $Ax=b$" then the answer is no. As you mention, the final quantum state in Harrow, Hassidim, Lloyd's algorithm does not immediately give you ...

9

If you just need any code $E : \{0,1\}^n \to \{0,1\}^m$ where $m=O(n)$ and where the distance is linear in $m$, then what you are looking for is called an "asymptotically good code". There are many explicit constructions of such codes, and you can find the basic ones in lecture notes of courses about coding theory. For example, you can find a description of ...

8

Yes, there is a paper by John Watrous, which formally addresses the concerns raised on Lipton's blog (reported in user2917198's comment). Here is the reference: J. Watrous. An introduction to quantum information and quantum circuits. ACM SIGACT News 42(2): 52–67, 2011. https://cs.uwaterloo.ca/~watrous/Papers/IntroductionQuantumCircuits.pdf As pointed out ...

8

The answer to your question is the contents of section 1.3.2, titled "[w]hen $\mathcal{P}_{p,r}$ is known to be difficult". (Here $\mathcal{P}_{p,r}$ is the problem of computing the norm $\|A\|_{p,r} = \sup_{\|x\|_p=1} \|Ax\|_r$.) According to that section, the only cases which are known to be difficult are $\mathcal{P}_{\infty,1},\mathcal{P}_{\infty,2},\... 8 There are proposals for quantum money where it appears that not even the bank can produce two copies of a quantum money state with the same serial number. See Farhi et al's paper Quantum Money from Knots, Mark Zhandry's paper Quantum Lightning Never Strikes the Same State Twice, Daniel Kane's paper Quantum Money from Modular Forms. In order to make ... 7 Short answer: No. There are really two separate issues to unpack here: Even if it worked exactly as advertised (which, as you might have heard, is somewhat disputed at present... :-) ), D-Wave's machine, by the company's own account, is only for adiabatic optimization. There's no particular reason to think that it could implement (e.g.) Shor's factoring ... 7 Quantum annealing essentially offers a square-root speed-up over classical simulated annealing in many circumstances. So, yes, it is potentially a faster approach for some optimization problems, but the speed-up isn't enough to make most hard problems tractable. Unfortunately, you cannot efficiently simulate quantum annealing classically, because any ... 7 Adiabatic quantum computing (AQC) is a computational model (as Peter said in the comments). Compare AQC with other models of computation such as: circuit-based quantum computing (CBQC) Adleman-Lipton model (a model for computing using DNA) Turing machine model (a model where computations are done with symbols on a tape) One can devise algorithms using ... 7 One can encrypt an n-qubit state using a 2n-bit classical secret key. The idea is to use the key to select a random Pauli operator, and apply that operator to the secret as an encryption. (The inverse operator is applied to decrypt.) The resulting scheme is perfectly secure -- if the key is selected uniformly at random, then even an attacker who know a ... 7 I just ran the first state you suggest (i.e. the GHZ state with negative phase in the Hadamard basis). Basically what I did was to write a circuit which creates that state, apply one of 5 stabilizer generators (XIXII,IXXII,IIXXI,IIXIX or -ZZZZZ) and then inverted the creation process. The circuit is shown below for the -ZZZZZ stabilizer. There seems to be ... 6 I don't know of any function with communication much higher than the$\gamma_2$bound. However, my intuition of why it is not tight is because the$\gamma_2$-norm is also a lower bound for QCMA communication. See this paper by Klauck for the definition of QCMA communication. To prove the lower bound on QCMA communication using the$\gamma_2$-norm you can ... 6 This question seems a little confused. The class of decision problems solvable efficiently on a quantum computer is BQP, while on a classical computer it is either P or BPP depending on exactly how you define things. An interactive proof is something entirely different. It is a protocol which allows a prover to prove, beyond reasonable doubt, the outcome of ... 6 I'm not sure who would suggest that qubits can meaningfully be described this way, or why anyone would do so. There are simply too many missing details, and it falls afoul of no-go theorems for local hidden-variable theories in quantum mechanics. This isn't "an exhaustive description of a digital quantum computer that uses a finite set of pure states to ... 6 We will never be able to prove this statement, because we can never be able to know for sure whether we have the exact laws of physics, or just a very good approximation to them. Even if we had a satisfactory theory of everything which we could use to make good predictions about every experimentally measurable physical system, there would be no way to tell ... 6 Look at John Preskill's Lecture Notes; particularly Section 3.2. As you noted, you can do a NAND gate by using a Toffoli gate and throwing away some of the output qubits. This results in decoherence, so you no longer necessarily have a pure state of your system. For non-unitary quantum operations, if you assume that the environment doesn't remember ... 6 Clearly you can work with abstract compressed representations of circuits. You can reason about them and manipulate them and turn them into concrete lists of gates. We do it all the time. But in context the author is in the middle of explaining the complexity class BQP (bounded-probability quantum polynomial-time). I think they're just making sure that you ... 6 It's not unitary, so it's impossible because all quantum transformations have to be unitary. Consider the states $$\frac{3}{5} |0\rangle + \frac{4}{5} |1\rangle \quad \mathrm{and} \quad \frac{3}{5} |0\rangle - \frac{4}{5} |1\rangle.$$ These get taken to$\$ \frac{1}{\sqrt{2}}\left(e^{3/5 i} |0\rangle + e^{4/5 i} |1\rangle\right) \quad \mbox{and}\quad \...

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