S. Zheng et al. / Theoretical Computer Science 666 (2017) 48–64 51
2.2. Quantum and semi-quantum finite automata basic models and working modes
Quantum finite automata were introduced by Kondacs and Watrous [21] and also by Moore and Crutchfields [25]. It has
been proved that one-way quantum finite automata (1QFA) with unitary operations and projective measurements are less
powerful than one-way classical finite automata (1FA) [2,22]. However, 1QFA can be more succinct in recognizing languages
or solving promise problems [2–5,7,8,6,12,18,39,43–45].
Definition
8. A measure-once quantum finite automaton (MO-1QFA) M is specified by a 5-tuple
M = (Q ,,{U
σ
|σ ∈
}, |0, Q
a
) (9)
where:
• Q is a finite set of orthonormal quantum (basis) states, denoted as {|i | 0 ≤ i < |Q |};
• is a finite alphabet of input symbols and
= ∪{| c, $} (where | c will be used as the left end-marker and $as the
right end-marker);
•|0 ∈ Q is the initial quantum state;
• Q
a
⊆ Q denotes the set of accepting basis states;
• U
σ
’s (σ ∈
) are unitary operators.
The
quantum state space of this model will be the |Q |-dimensional Hilbert space denoted H
Q
.
Each
quantum basis state |i in H
Q
can be represented by a column vector with the (i + 1)th entry being 1 and other
entries being 0. With this notational convenience we can describe the above model as follows:
1. The initial state |0 is represented as |q
0
= (1,
|Q |−1
0, ···, 0)
T
.
2. The accepting set Q
a
corresponds to the projective operator P
acc
=
|i∈Q
a
|ii|.
The
computation of an MO-1QFA M on an input string x = σ
1
σ
2
···σ
n
∈
∗
goes as follows: M “reads” the input string
from the left end-marker to the right end-marker, symbol by symbol, and the unitary matrices U
|c
, U
σ
1
, U
σ
2
, ···, U
σ
n
, U
$
are
applied, one by one, always on the current state, starting with |0 as the initial state. Finally, the projective measurement
{P
acc
, I − P
acc
} is performed on the final state, in order to accept or reject the input. Therefore, for an input string w =
σ
1
σ
2
···σ
n
, M has the accepting probability
Pr[M accepts w]=P
acc
U
$
U
σ
n
···U
σ
2
U
σ
1
U
|c
|0
2
(10)
and the rejecting probability
Pr[M rejects w]=1 − Pr[M accepts w]. (11)
Definition 9. A promise version of a measure-once quantum finite automaton (pvMO-1QFA) M is specified by a 6-tuple
M = (Q ,,{U
σ
|σ ∈
}, |0, Q
a
, Q
r
) (12)
where: Q , ,
, |0, Q
a
, U
σ
are as defined in an MO-1QFA, Q
r
⊆ Q (Q
r
∩ Q
a
=∅) denotes the set of rejecting basis states.
The set Q
r
corresponds to the projective operator P
rej
=
|i∈Q
r
|ii|.
For
an input string w = σ
1
σ
2
···σ
n
, M has the accepting probability
Pr[M accepts w]=P
acc
U
$
U
σ
n
···U
σ
2
U
σ
1
U
|c
|0
2
(13)
and the rejecting probability
Pr[M rejects w]=P
rej
U
$
U
σ
n
···U
σ
2
U
σ
1
U
|c
|0
2
. (14)
Another interesting (important) model of two-way finite automata with quantum and classical states (2QCFA) – was intro-
duced
by Ambainis and Watrous [1] and explored in [17,24,39,43,41,42,44]. If restricting the read-head in a 2QCFA to be
one-way, then it is natural to get one-way finite automata with quantum and classical states (1QCFA). That is, 1QCFA are one-way
versions of 2QCFA, studied by Zheng and Qiu et al. [43]. It is worth mentioning that more previously a different but more
practical model called as one-way quantum finite automata together with classical states (1QFAC) was proposed and studied by
Qiu et al. [34]. Informally, a 1QCFA can be seen as a DFA which has an access to a quantum memory of a constant size
(dimension), upon which the automaton performs quantum transformations and projective measurements. Given a finite set
of quantum basis states Q , we denote by H(Q ) the Hilbert space spanned by Q . Let U (H(Q )) and O(H(Q )) denote the
sets of unitary operators and projective measurements over H(Q ), respectively.