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Revision #1 to TR19-182 | 1st March 2020 04:18

#### Lower Bounds on the Running Time of Two-Way Quantum Finite Automata and Sublogarithmic-Space Quantum Turing Machines

Revision #1
Authors: Zachary Remscrim
Accepted on: 1st March 2020 04:18
Keywords:

Abstract:

The two-way finite automaton with quantum and classical states (2QCFA), defined by Ambainis and Watrous, is a model of quantum computation whose quantum part is extremely limited; however, as they showed, 2QCFA are surprisingly powerful: a 2QCFA with only a single-qubit can recognize the language $L_{pal}=\{w \in \{a,b\}^*:w \text{ is a palindrome}\}$ with bounded error in expected time $2^{O(n)}$, on inputs of length $n$.

We prove that their result essentially cannot be improved upon: a 2QCFA (of any size) cannot recognize $L_{pal}$ with bounded error in expected time $2^{o(n)}$. To our knowledge, this is the first example of a language that can be recognized with bounded error by a 2QCFA in exponential time but not in subexponential time. Moreover, we prove that a quantum Turing machine (QTM) running in space $o(\log n)$ and expected time $2^{n^{1-\Omega(1)}}$ cannot recognize $L_{pal}$ with bounded error; again, this is the first lower bound of its kind.

Far more generally, we establish a lower bound on the running time of any 2QCFA or $o(\log n)$-space QTM that recognizes any language $L$ in terms of a natural hardness measure" of $L$. This allows us to exhibit a large family of languages for which we have asymptotically matching lower and upper bounds on the running time of any such 2QCFA or QTM recognizer.

Changes to previous version:

Updated paper to include new results concerning lower bounds on the running time of sublogarithmic-space quantum Turing machines and further dequantumization" results. Title changed accordingly. Removed material concerning lower bounds on one-way quantum finite automata, which will appear as a separate paper.

### Paper:

TR19-182 | 9th December 2019 13:28

#### The Limitations of Few Qubits: One-way and Two-way Quantum Finite Automata and the Group Word Problem

TR19-182
Authors: Zachary Remscrim
Publication: 15th December 2019 08:18
Keywords:

Abstract:

The two-way finite automaton with quantum and classical states (2QCFA), defined by Ambainis and Watrous, is a model of quantum computation whose quantum part is extremely limited; however, as they showed, 2QCFA are surprisingly powerful: a 2QCFA with only a single-qubit can recognize the language $L_{pal}=\{w \in \{a,b\}^*:w \text{ is a palindrome}\}$ with bounded-error in expected exponential time. We prove that their result essentially cannot be improved upon: a 2QCFA (of any finite size) cannot recognize $L_{pal}$ with bounded-error in expected time $2^{o(n)}$, on inputs of length $n$. To our knowledge, this is the first example of a language that can be recognized with bounded-error by a 2QCFA in exponential time but not in subexponential time. A key tool in our result is a generalization to 2QCFA of a technical lemma that was used by Dwork and Stockmeyer to prove a lower bound on the expected running time of any two-way probabilistic finite automaton that recognizes a non-regular language with bounded-error.

Furthermore, we prove strong lower bounds on the expected running time of any 2QCFA that recognizes a group word problem with bounded-error. In a recent paper, we showed that 2QCFA can recognize, with bounded-error, a broad class of group word problems in expected exponential time, and a more narrow class of group word problems in expected polynomial time. As a consequence, we can now exhibit a large family of natural languages that can be recognized with bounded-error by a 2QCFA in expected exponential time, but not in expected subexponential time. Moreover, we obtain significant progress towards a precise classification of those group word problems that can be recognized with bounded-error in expected polynomial time by a 2QCFA.

We also consider the one-way measure-once quantum finite automaton (1QFA), defined by Moore and Crutchfield, as well as a natural generalization to one-way measure-once finite automata with quantum and classical states (1QCFA). We precisely classify those groups whose word problem may be recognized with positive one-sided error (for both the bounded-error and unbounded-error cases) by a 1QFA or 1QCFA with any particular number of quantum states and any particular number of classical states; we also obtain partial results in the negative one-sided error case. As an immediate corollary, we show that allowing a 1QFA or 1QCFA to have even a single additional quantum or classical state enlarges the class of languages that may be recognized with positive one-sided error (of either type).

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