Abstract
In a recent remarkable experiment [Sci. Adv. 2, e1501531 (2016)], a 3-qubit quantum Fredkin (i.e., controlled-SWAP) gate was demonstrated by using linear optics. Here we propose a simple experimental scheme by utilizing the dispersive interaction in superconducting quantum circuit to implement a hybrid Fredkin gate with a superconducting flux qubit as the control qubit and two separated quantum memories as the target qudits. The quantum memories considered here are prepared by the superconducting coplanar waveguide resonators or nitrogen-vacancy center ensembles. In particular, it is shown that this Fredkin gate can be realized using a single-step operation and more importantly, each target qudit can be in an arbitrary state with arbitrary degrees of freedom. Furthermore, we show that this experimental scheme has many potential applications in quantum computation and quantum information processing such as generating arbitrary entangled states (discrete-variable states or continuous-variable states) of the two memories, measuring the fidelity and the entanglement between the two memories. With state-of-the-art circuit QED technology, the numerical simulation is performed to demonstrate that two-memory NOON states, entangled coherent states, and entangled cat states can be efficiently synthesized.
Highlights
The Fredkin gate is a three-qubit controlled-SWAP gate
Conditioned on the state of the control qubit, the gate can enable the two target qubits swap their quantum states [1]. It has played an important role in quantum computation and quantum information processing (QCQIP) such as error correction [2, 3], quantum cloning [4], quantum fingerprinting [5, 6], and quantum digital signatures [7]
The numerical simulation shows that the high-fidelity generation of above entangled states of two quantum memories is feasible with the state-of-the-art circuit quantum electrodynamics (QED) technology
Summary
The Fredkin gate is a three-qubit controlled-SWAP gate. Conditioned on the state of the control qubit, the gate can enable the two target qubits swap their quantum states [1]. We propose a method for the direct realization of a general hybrid tripartite Fredkin gate by using superconducting resonators as two quantum memories coupled to a superconducting flux qubit. The two proposals have the following distinct advantages: (i) The Fredkin gate can be realized by employing a single unitary operation without need of any microwave pulse; (ii) Our method and experimental setup are simple because only a single qutrit and two target quantum memories are used; (iii) The experimental scheme is based on the superconducting resonator or the NV ensemble which has a long coherence time; (iv) Each controlled target qudit of this gate can be in an arbitrary state (discrete-variable or continuous-variable state) which can further lead to the wide applications such as (a) preparing an arbitrary entangled state of two superconducting resonators or NV ensembles, (b) directly measuring the fidelity between the two quantum memories as well as the entanglement between them without any information on the initial states required. It is obvious that the Hamiltonian Eq (13) has the same form as Eq (5), so the above demonstrated Fredkin gate can be prepared with the NV ensembles as memories
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