REVIEW
New upper and lower bounds for randomized and quantum Local Search
Quantum Physics · Submitted 6 Mar 2006 · 23 references
Pith's verdict: Tight bounds for local search on the hypercube and grids are likely right but the block-threading proof in §4.2.2 needs a referee's hard look before the paper's claims are trusted.
T0 · AI review
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Referee report
The paper
The paper claims
Tight query bounds for Local Search on hypercubes and grids.
The reading
What carries the argument
The key machinery is the product-graph clock: decompose a graph \(G\) as \(G_w \times G_c\), run a regular random walk in \(G_w\), and pair each step with two forward steps along a fixed self-avoiding path in \(G_c\). The \(G_c\) part acts as a one-way clock, so each vertex of the combined path is associated with a unique time; this converts passing probabilities into hitting probabilities and makes the adversary weight analysis tractable. The lower-bound proof combines this construction with the quantum adversary method and the relational adversary method, choosing weights so that the clock's one-wayness does the work. The upper-bound algorithm instead uses slow expansion: it samples in exponentially shrinking balls around a best-so-far vertex, tests whether the boundary of the current ball has high function values, and then follows a decreasing path inside the final small ball, using quantum minimum-finding for the boundary tests.
Extended reading notes
Core claim
The paper's central claim is that Local Search on a graph containing a product graph \(G_w \times G_c\) has query complexity controlled by a random walk in one factor timed by an irreversible 'clock' walk along a self-avoiding path in the other factor. This clock makes the troublesome 'passing probability' of earlier analyses become a much smaller 'hitting probability': a path can pass through a vertex only at one precise clock reading. Applying the resulting lower-bound theorem gives \(\mathrm{RLS}(\{0,1\}^n) = \Theta($2^{{n/2}}$ $n^{{1/2}}$)\) and \(\mathrm{QLS}(\{0,1\}^n) = \Theta($2^{{n/3}}$ $n^{{1/6}}$)\); on grids, \(\mathrm{RLS}([n]^d) = \Theta($n^{{d/2}}$)\) for \(d \geq 4\) and \(\mathrm{QLS}([n]^d) = \Theta($n^{{d/3}}$)\) for \(d \geq 6\), with improved lower bounds for the remaining small dimensions. The companion upper-bound theorem says that when the number of vertices within distance \(k\) of any vertex grows like \(O(k^\$\alpha$)\), Local Search can be solved with \(O($d^{{\alpha-1}}$ \log\log d)\) randomized queries or \(O($d^{{(\alpha-1)/2}}$ (\log\log d)^{1.5})\) quantum queries, where \(d\) is the graph diameter; on \([n]^2\) this gives \(\mathrm{QLS}([n]^2) = O(\sqrt{n} (\log\log n)^{1.5})\).
Load-bearing premise
The grid lower bounds assume that cutting a grid into blocks and threading them with disjoint block-changing segments produces exactly the same hitting probabilities as a product walk on \([\$\alpha$]^{d-1} \times [L]\), apart from reflections inside each block; if those segments introduce correlations or shorten the effective clock, the low-dimensional bounds do not follow.
Editorial extensions
If this is right
- On the Boolean hypercube, the exact exponent is now known: no randomized algorithm can beat \(\Theta(2^{n/2} n^{1/2})\) queries and no quantum algorithm can beat \(\Theta(2^{n/3} n^{1/6})\), while matching algorithms exist.
- On grids of dimension at least 4 (classically) or at least 6 (quantum), the generic random-sampling plus steepest-descent method is optimal up to constant factors.
- On the 2D grid, quantum Local Search is now known to require at most \(O(\sqrt{n} (\log\log n)^{1.5})\) queries, improving the previous \(O(n^{2/3})\) bound; the \(\Omega(n^{2/5})\) lower bound shows a genuine gap remains.
- On graphs with polynomial volume growth, \(c(k)=O(k^\alpha)\), Local Search admits diameter-based poly-logarithmic algorithms; on the line this yields \(O(\log n \log\log n)\) randomized queries.
- The paper's Section 6 lists the remaining gaps: for example, \(\mathrm{QLS}([n]^2)\) is between \(\Omega(n^{2/5})\) and \(O(\sqrt{n})\), and \(\mathrm{RLS}([n]^3)\) is between \(\Omega(n^{3/2}/\sqrt{\log n})\) and \(O(n^{3/2})\).
Reading between the lines
- The same clock technique should transfer to other product-containing graphs—Hamming graphs, tori, or grids with unequal side lengths—by choosing a different random walk in the first factor; the paper's Theorem 9 is general, and only the applications are specialized.
- The 2D gap between \(\Omega(n^{2/5})\) and \(O(\sqrt{n} (\log\log n)^{1.5})\) suggests the true quantum complexity may lie at an intermediate power of \(n\); a more tailored random walk or a sharper upper bound using two-scale growth might close it.
- The recursive shrinking-ball algorithm is phrased for any graph with slow volume growth, so it may also give improved bounds on trees, graphs of bounded genus, or other structured families, even though the paper only works out the grid and line cases.
- If the lower bounds transfer to arbitrary graphs containing a product subgraph, then Local Search complexity would be governed by the best random walk in the largest product factor; this suggests a general principle connecting expansion, hitting times, and Local Search hardness.
Original abstract
Local Search problem, which finds a local minimum of a black-box function on a given graph, is of both practical and theoretical importance to combinatorial optimization, complexity theory and many other areas in theoretical computer science. In this paper, we study the problem in the randomized and quantum query models and give new lower and upper bound techniques in both models. The lower bound technique works for any graph that contains a product graph as a subgraph. Applying it to the Boolean hypercube {0,1}^n and the constant dimensional grids [n]^d, two particular product graphs that recently drew much attention, we get the following tight results: RLS({0,1}^n) = \Theta(2^{n/2}n^{1/2}), QLS({0,1}^n) = \Theta(2^{n/3}n^{1/6}), RLS([n]^d) = \Theta(n^{d/2}) for d \geq 4, QLS([n]^d) = \Theta(n^{d/3}) for d \geq 6. Here RLS(G) and QLS(G) are the randomized and quantum query complexities of Local Search on G, respectively. These improve the previous results by Aaronson [STOC'04], Ambainis (unpublished) and Santha and Szegedy [STOC'04]. Our new algorithms work well when the underlying graph expands slowly. As an application to [n]^2, a new quantum algorithm using O(\sqrt{n}(\log\log n)^{1.5}) queries is given. This improves the previous best known upper bound of O(n^{2/3}) (Aaronson, [STOC'04]), and implies that Local Search on grids exhibits different properties in low dimensions.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
Local Search problem, which finds a local minimum of a black-box function on a given graph, is of both practical and theoretical importance to combinatorial optimization, complexity theory and many other areas in theoretical computer science. In this paper, we study the problem in the randomized and quantum query models and give new lower and upper bound techniques in both models. The lower bound technique works for any graph that contains a product graph as a subgraph. Applying it to the Boolean hypercube {0,1}^n and the constant dimensional grids [n]^d, two particular product graphs that recently drew much attention, we get the following tight results: RLS({0,1}^n) = \Theta(2^{n/2}n^{1/2}), QLS({0,1}^n) = \Theta(2^{n/3}n^{1/6}), RLS([n]^d) = \Theta(n^{d/2}) for d \geq 4, QLS([n]^d) = \Theta(n^{d/3}) for d \geq 6. Here RLS(G) and QLS(G) are the randomized and quantum query complexities of Local Search on G, respectively. These improve the previous results by Aaronson [STOC'04], Ambainis (unpublished) and Santha and Szegedy [STOC'04]. Our new algorithms work well when the underlying graph expands slowly. As an application to [n]^2, a new quantum algorithm using O(\sqrt{n}(\log\log n)^{1.5}) queries is given. This improves the previous best known upper bound of O(n^{2/3}) (Aaronson, [STOC'04]), and implies that Local Search on grids exhibits different properties in low dimensions.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Local Search problem, which finds a local minimum of a black-box function on a given graph, is of both practical and theoretical importance to combinatorial optimization, complexity theory and many other areas in theoretical computer science. | background 1.00 | 0.04 | 0.02 | asserted 1.00 |
| s1 | In this paper, we study the problem in the randomized and quantum query models and give new lower and upper bound techniques in both models. | contribution 1.00 | 0.94 | 0.13 | asserted 1.00 |
| s2 | The lower bound technique works for any graph that contains a product graph as a subgraph. | method 0.85 | 0.84 | 0.06 | asserted 1.00 |
| s3 | Applying it to the Boolean hypercube {0,1}^n and the constant dimensional grids [n]^d, two particular product graphs that recently drew much attention, we get the following tight results: RLS({0,1}^n) = \Theta(2^{n/2}n^{1/2}), QLS({0,1}^n) = \Theta(2^{n/3}n^{1/6}), RLS([n]^d) = \Theta(n^{d/2}) for d \geq 4, QLS([n]^d) = \Theta(n^{d/3}) for d \geq 6. | result 1.00 | 0.94 | 0.78 | asserted 1.00 |
| s4 | Here RLS(G) and QLS(G) are the randomized and quantum query complexities of Local Search on G, respectively. | background 0.71 | 0.26 | 0.04 | asserted 0.96 |
| s5 | These improve the previous results by Aaronson [STOC'04], Ambainis (unpublished) and Santha and Szegedy [STOC'04]. | contribution 0.50 | 0.46 | 0.09 | asserted 1.00 |
| s6 | Our new algorithms work well when the underlying graph expands slowly. | limitation 0.49 | 0.82 | 0.05 | asserted 0.98 |
| s7 | As an application to [n]^2, a new quantum algorithm using O(\sqrt{n}(\log\log n)^{1.5}) queries is given. | result 0.67 | 0.95 | 0.06 | asserted 1.00 |
| s8 | This improves the previous best known upper bound of O(n^{2/3}) (Aaronson, [STOC'04]), and implies that Local Search on grids exhibits different properties in low dimensions. | interpretation 0.66 | 0.86 | 0.11 | asserted 1.00 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:quant-ph/0603034 by the authors.
References
How the literature uses it
Papers whose main result relates to this one's
States the same result 1 paper
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In this paper, we show that for the Boolean hypercube $\B^n$, the randomized query complexity of Local Search is $\Theta(2^{n/2}n^{1/2})$ and the quantum query complexity is $\Theta(2^{n/3}n^{1/6})$.
(Almost) tight bounds for randomized and quantum Local Search on hypercubes and grids · 0.98
Discussion
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