We propose and demonstrate a new implementation of a quantum protocol which can operate deterministically only with linear optics. This protocol, which is called quantum leader election, exhibits the quantum advantages over classical protocol.
Linear optics is a promising candidate to enable the construction of quantum computers. A number of quantum protocols gates based on linear optics have been demonstrated. However, it is well known that these gates are nondeterministic and that higher order nonlinearity is necessary for deterministic operations. We found the quantum leader election protocol can be operated deterministically only with linear optics, and we have demonstrated the nearly deterministic operation which overcomes classical limit.
In this paper we show that sufficient multi-partite quantum entanglement helps in fair and unbiased election of a leader in a distributed network of processors with only linear classical communication complexity. We show that a total of O(logn) distinct multi-partite maximally entanglement sets (ebits) are capable of supporting such a protocol in the presence of nodes that may lie and thus be biased. Here, n is the number of nodes in the network. We also demonstrate the difficulty of performing unbiased and fair election of a leader with linear classical communication complexity in the absence of quantum entanglement even if all nodes have perfect random bit generators. We show that the presence of a sufficient number O(n/logn) of biased agents leads to a non-zero limiting probability of biased election of the leader, whereas, the presence of a smaller number O(logn) of biased agents matters little. We define two new related complexity classes motivated by the our leader election problem and discuss a few open questions.
The anonymous network model was introduced by Angluin in (STOC '80, pages 82-93) to understand the fundamental properties of distributed computing by examining how much each party in a network needs to know about its own identity and the identities of other parties. In this model, all parties with the same number of communication links are identical. When studying the model, the problem of electing a unique leader is the central problem in the sense that once a unique leader is elected, he can assign a unique identity to each party. It was proved in the literature, however, that it is impossible to deterministically solve the leader election problem with any finite amount of communication. This contrasts sharply with the situation in the quantum setting: the present authors gave quantum algorithms that exactly solve the problem on anonymous quantum networks, where quantum communication and computation can be performed (TOCT, vol.4, no.1, article 1). The core of their algorithm consists of somewhat tricky unitary operators, which eliminate the possibility of ties during (quantum) coin flipping and thereby reduce the number of leader candidates with certainty. This paper presents a new quantum leader election algorithm that is based on quantum reduction via exact amplitude amplification to a classically solvable problem, computing a certain symmetric function, which provides more intuitive reasoning behind the existence of exact quantum algorithms for leader election. The algorithm first achieves a round complexity that is linear in the number of parties, i.e., the largest possible diameter plus one of the underlying graph of the network.