Quantum Query Complexity of Boolean Functions with Small On-Sets

Transcription

1 Quantum Query Complexity of Boolean Functions with Small On-Sets Seiichiro Tani TT/JST ERATO-SORST. Joint work with Andris Ambainis Univ. of Latvia Kazuo Iwama Kyoto Univ. Masaki akanishi AIST. Harumichi ishimura Osaka Pref. Univ Rudy Raymond IBM Shigeru Yamashita AIST.

2 Motivation Want to test some properties of huge data X, Or, compute some function f(x). e.g. WWW analysis, Experimental data analysis. X Reading all memory cells of X costs too much. Can we save the number of accessing X when computing certain functions f(x)?

3 Oracle Computation Model Can know the value of one cell by making a query to X. Description of f (e.g. truth table) What is value x i of the i th cell? 0/1 X x 1 x i x f(x) output Cost measure:= of queries to be made. (All other computation is free.) R(f): Query complexity of f := of queries needed to compute f for the worst input X

4 Oracle Computation Model Can know the value of one cell by making a query to X. Description of f (e.g. truth table) What is value x i of the i th cell? 0/1 X x 1 x i x f(x) output Cost measure:= of queries to be made. (All other computation is free.) Bounded error R(f): Query complexity of f with error probability < 1/3 := of queries needed to compute f for the worst input X

5 Quantum Computation Qubit: A unit of quantum information. A quantum state φ of one qubit : a unit vector in 2- dimensional Hilbert space. For an orthonorma l basis '' 1$, ' 0$ $ = 0 1 φ = α 0 + β 1 where α, β C and α ( 0, 1 ) 2 + β, 2 = 1. A quantum state ϕ of n qubits : n a unit vector in 2 - dimensiona l Hilbert space. ϕ n 2 1 = α for orthonormal basis { }. i 0 i i i = i Quantum operation: only unitary operation H ϕ ϕ

6 Oracle Computation Model (Quantum) A quantum query is a linear combination of classical queries. Can know a linear combination of the value of cells per query. Description of f (e.g. truth table) f(x) output α i α i i i 0 x i 2 ( αi = 1) i X x 1 x i x Q(f): (Bounded-error) Quantum query complexity of f := of quantum queries needed to compute f with error probability < 1/3 for the worst input X

7 Fundamental Problems What is the quantum/classical query complexity of function f? For what function f, is quantum computation faster than classical one? In particular, Boolean functions are major targets. This talk focuses on Boolean functions in bounded-error setting (constant error probability is allowed).

8 Previous Works (Almost) o quantum speed up against classical. PARITY, MAJORITY [BBCMdW01]. Ω() quantum queries are needed. Polynomial quantum speed up against classical OR [Gro96], AD-OR trees [HMW03,ACRSZ07] Quantum O( ) v.s. Classical Ω (). k-threshold functions for k<< /2 [BBCMdW01] Quantum Θ( (k)) v.s. Classical Ω (). Testing graph properties (=n(n-1)/2 variables) Triangle: Quantum O(n 1.3 ) [MSS05] Star: Quantum Θ(n 1.5 ) [BCdWZ99] Connectivity: Quantum Θ(n 1.5 ) [DHHM06] Classical Ω(n 2 ) But much less is known except for the above typical cases. We investigate the query complexity of the families defined a natural parameter.

9 On-set of Boolean Functions We consider the size of the on-set of a Boolean function as a parameter. On-set S f of a Boolean function f: The set of input X {0,1} for which f(x)=1. Ex.) On-set S f of f=(x 1 x 2 ) x 3 : (x 1,x 2,x 3 )=(1,1,0), (1,1,1), (0,0,1), (0,1,1), (1,0,1). The size of S f is 5.

10 Our Results (1/2) F,M : family of -variable Boolean functions f whose on-set is of size M. Query complexity of the functions ( poly() M 2 with 0 < d < 1) d in F, M Quantum Q(f) (1) Hardest case : Θ$ (2) Easiest case : Θ (3) Average Case : ( ) O( M + ), Ω$ M M + << Classical R(f) Ω() F,M easy (2) (3) (1) complexity hard

11 Our results (2/2) Our hardest-case complexity gives the tight complexity of some graph property testing. Does G have property P? Is there edge (i,j)? Yes/o a b c d e a $ b $ c $ $ d $ e $ Adjacency matrix of G = b n(n-1)/2 variables c a d e Unknown G (Planarity testing) Is G planar? : Q(f)=Θ(n 1.5 ). R(f)=Ω(n 2 ) (For a given adjacency list, O(n) time complexity [Hopcroft-Tarjan74]) (Graph Isomorphism testing) Is G isomorphic to a fixed graph G? : Q(f)=Θ(n 1.5 ). (R(f)=Ω(n 2 ) [DHHM06]) By setting M = of graphs with property P.

12 OUTLIES OF PROOFS

13 Our Results(1/2) F,M : family of -variable Boolean functions f whose on-set is of size M. Query complexity of the functions ( poly() M 2 with 0 < d < 1) d in F, M (1) Hardest case : Θ$ (2) Easiest case : Θ (3) Average Case : Quantum Q(f) F,M ( ) O( M + ), Ω$ M M + << Classical R(f) Ω() easy (2) (3) (1) complexity hard

14 Hardest-case Bound Theorem :For any function f F, M, Q( f ) ' = Θ M $ if poly( ) M 2 d for some constant d(0 < d < 1). Proof. Lower Bound: By showinga function for every M Upper bound: which has O$ M (The function is similar to t - threshold function for t =.) M complexity. Use the algorithm [AIKMRY07] for Oracle Identification Problem.

15 Oracle Identification Problem (OIP) Given a set of M candidates, identify the -bit string in the oracle.. Oracle (=8) i x i ???????? Candidate Set (=8, M=4) Can see the contents w/o making queries. i Candidate 1 Candidate 2 Candidate 3 Candidate

16 Hardest-case Bound Proof (Continued) Theorem[AIKMRY07]: ' OIP can be solved with bounded - error by making O if poly( ) M 2 d for some constant d(0 < d < 1). M $ quantum queries, Idea: Set the onset S f to the candidate set of OIP and run the algorithm for OIP to get an estimate Y S f of X. By definition, Y=X (with high probability) iff f(x)=1. Test if X=Y, which can be done with quantum query complexity O( ).

17 Our Results (1/2) F,M : family of -variable Boolean functions f whose on-set is of size M. Query complexity of the functions ( poly() M 2 with 0 < d < 1) d in F, M (1) Hardest case : Θ$ (2) Easiest case : Θ (3) Average Case : Quantum Q(f) ( ) O( M + ), Ω$ M M + << Classical R(f) Ω() F,M easy (2) (3) (1) complexity hard

18 Easiest-case Bound Theorem :If Q( f 2+ ε M 2 for any positive constant ε, ) = Θ( ) for any f F., M Proof: Use sensitivity argument. Th.[Beals et al. 2001] Q( f ) = Ω( s( f )) Assuming s(f)=o(), we can conclude a contradiction by simply counting, f 1 (1) 2+ ε > 2 M We can construct a function with such quantum query complexity.

19 Our Results (1/2) F,M : family of -variable Boolean functions f whose on-set is of size M. Query complexity of the functions ( poly() M 2 with 0 < d < 1) d in F, M (1) Hardest case : Θ$ (2) Easiest case : Θ (3) Average Case : Quantum Q(f) ( ) O( M + ), Ω$ M M + << Classical R(f) Ω() F,M easy (2) (3) (1) complexity hard

20 Average-case Bound Theorem :Averageof Q( f ) over all f F, is O( M M + ). Proof. Claim: For almost all functions f in F,M, every element in the on-set S f differs from any other in the first O( M) bits. Y 1 Y M-1 Y M O( M) bits Make queries to the first O( M) bits to identify a unique string Y in S f (If there is no such Y, we are done: f(x)=0.) 2. Test if Y=X with O( ) quantum queries. Y=X if and only if f(x)=1.

21 Average-case Bound M Theorem : Average of Q( f ) over all f F, M is O$ +. c + M Proof 1 xi With one quantum query, ϕ X = ( 1) i. i= 1 Claim: For almost all functions in F, M, every X,Y intheonset 1 M ϕ X ϕ Y = ( 2Ham( X, Y )) > 2. S f satisfy (Proof is by bounding Hamming distance with coding-theory argument and Chernoff-like bound.) ϕ X ϕ Y is large enough to identify X in S f with ' M O c + M $ copies of ϕ X according toquantum state discriminationtheorem [HW06].

22 Average-case Bound Theorem : Average of Q( f ) over all f F, M is Ω( M / + ). Actually, we prove stronger statement.

23 Average-case Bound Theorem : Average of unbounded - error query complexity over all f F is Ω( M / )., M + Unbounded-error: error probability is 1/2-ε for arbitrary small ε Proof: Use the next theorem. Theorem[Anthony ext Talk] The number of Boolean functions f whose unbounded query complexity is d/2 is For d = M 2 T(, d), 2 D 1 k = 0 we can prove ( 2 1 ' k $ for D = d i= 0 ( ' i. $ T$, M 2 is much smaller than $ 2 M, i.e., the sizeof F,M.

24 Our Quantum Complexity F,M : family of -variable Boolean functions f whose on-set is of size M. Quantum query complexity of the functions in F, M For poly() M 2 with 0 < d < 1, M (1) Hardest case : Θ$ (2) Easiest case : Θ (3) Average Case : d ( ) O( M + ), Ω$ M + M Θ$ c + M (1 M 2 M Θ$ + c + M (1 M 2 /( ) 2+ε / 2) ) F,M easy (2) (3) (1) complexity hard

25 Theorem: Application: Planarity Testing R(f planarity )= Θ(n 1.5 ), while R(f planarity )=Θ(n 2 ). Proof. Since the planar graph has at most 3n-6 edges. of possible edges M = (of planar graphs) ' $ 3n 6 = ' n( n 1) / 2$ 3n 6 2 6nn By the hardest - case complexity, M, we can obtain the upper bound. For the lower bound, we carefully prepare a set of planar graphs and a set of non-planar graphs, and then apply the quantum/classical adversary method [Amb01,Aar04].

26 Summary Proved the tight quantum query complexity of the family of Boolean functions with fixed on-set size M. Functions with on-set size M have various quantum query complexity, while their randomized query d complexity is Ω() for poly( ) M 2. (For large M, the functions may have small randomized query complexity.) On-set size is a very simple and natural parameter, which enables us to easily analyze the query complexity of some Boolean functions with our bounds. In particular, we proved the tight quantum query complexity of some graph property testing problems.

27 Thank you