5 research outputs found
Quantum Alternation: Prospects and Problems
We propose a notion of quantum control in a quantum programming language
which permits the superposition of finitely many quantum operations without
performing a measurement. This notion takes the form of a conditional construct
similar to the IF statement in classical programming languages. We show that
adding such a quantum IF statement to the QPL programming language simplifies
the presentation of several quantum algorithms. This motivates the possibility
of extending the denotational semantics of QPL to include this form of quantum
alternation. We give a denotational semantics for this extension of QPL based
on Kraus decompositions rather than on superoperators. Finally, we clarify the
relation between quantum alternation and recursion, and discuss the possibility
of lifting the semantics defined by Kraus operators to the superoperator
semantics defined by Selinger.Comment: In Proceedings QPL 2015, arXiv:1511.0118
Improved quantum data analysis
We provide more sample-efficient versions of some basic routines in quantum
data analysis, along with simpler proofs. Particularly, we give a quantum
"Threshold Search" algorithm that requires only
samples of a -dimensional state . That is, given observables such that for at
least one , the algorithm finds with . As a consequence, we obtain a Shadow Tomography algorithm
requiring only samples, which
simultaneously achieves the best known dependence on each parameter , ,
. This yields the same sample complexity for quantum Hypothesis
Selection among states; we also give an alternative Hypothesis Selection
method using samples
From Symmetric Pattern-Matching to Quantum Control
International audienceOne perspective on quantum algorithms is that they are classical algorithms having access to a special kind of memory with exotic properties. This perspective suggests that, even in the case of quantum algorithms, the control flow notions of sequencing, conditionals, loops, and recursion are entirely classical. There is however, another notion of control flow, that is itself quantum. The notion of quantum conditional expression is reasonably well-understood: the execution of the two expressions becomes itself a superposition of executions. The quantum counterpart of loops and recursion is however not believed to be meaningful in its most general form. In this paper, we argue that, under the right circumstances, a reasonable notion of quantum loops and recursion is possible. To this aim, we first propose a classical, typed, reversible language with lists and fixpoints. We then extend this language to the closed quantum domain (without measurements) by allowing linear combinations of terms and restricting fixpoints to structurally recursive fixpoints whose termination proofs match the proofs of convergence of sequences in infinite-dimensional Hilbert spaces. We additionally give an operational semantics for the quantum language in the spirit of algebraic lambda-calculi and illustrate its expressiveness by modeling several common unitary operations