Generation of three-qubit entangled states using superconducting phase qubits

Abstract

Entanglement is one of the key resources required for quantum computation, so the experimental creation and measurement of entangled states is of crucial importance for various physical implementations of quantum computers. In superconducting devices, two-qubit entangled states have been demonstrated and used to show violations of Bell's inequality and to implement simple quantum algorithms. Unlike the two-qubit case, where all maximally entangled two-qubit states are equivalent up to local changes of basis, three qubits can be entangled in two fundamentally different ways. These are typified by the states |GHZ>= (|000+ |111>)/ sqrt [2] and |W>= (|001> + |010> + |100>)/ sqrt [3]. Here we demonstrate the operation of three coupled superconducting phase qubits and use them to create and measure |GHZ> and |W>states. The states are fully characterized using quantum state tomography and are shown to satisfy entanglement witnesses, confirming that they are indeed examples of three-qubit entanglement and are not separable into mixtures of two-qubit entanglement.

Computing prime factors with a Josephson phase qubit quantum processor

“…At present, this combination of entangling capabilities has not been demonstrated on a single device. Previous examples have shown: spectroscopic evidence of the increased coupling for up to three qubits coupled to a resonator [14], as well as coherent interactions between two and three qubits with a resonator [12], although these lacked tomographic evidence of entanglement.Here we show coherent interactions for up to four qubits with a resonator and verify genuine bi-and tripartite entanglement including Bell [9] and |W -states [10] with quantum state tomography (QST). This QuP has the further advantage of creating entanglement at a rate more than twice that of previous demonstrations [10,12].…”

“…This QuP has the further advantage of creating entanglement at a rate more than twice that of previous demonstrations [10,12].…”

“…Two advantages of superconducting qubit architectures are the use of conventional microfabrication techniques, which allow straightforward scaling to large numbers of qubits, and a toolkit of circuit elements that can be used to engineer a variety of qubit types and interactions [6,7]. Using a number of recent qubit control and hardware advances [7][8][9][10][11][12][13], here we demonstrate a nine-quantum-element solid-state QuP and show three experiments to highlight its capabilities. We begin by characterizing the device with spectroscopy.…”

“…Qubit operation and calibration are similar to previous works [7][8][9][10]13],with the addition of an automated calibration process [25]. As shown in Fig.1d, we used swap spectroscopy [7] to calibrate all nine of the engineered quantum elements on the QuP, the four phase qubits (Q 1 − Q 4 ), the four quarter-wave CPW quantum memory resonators (M 1 − M 4 ), and one half-wave CPW bus resonator (B).…”

Suppression of Dephasing by Qubit Motion in Superconducting Circuits

“…In general, the protocol provides a diagnostic tool for measurements of the noise correlations. Development of superconducting qubits [1][2][3][4][5][6][7] has reached the stage where it is interesting to discuss possible architectures of the quantum information processing circuits. The common feature of any quantum computation process of even moderate complexity is the requirement of information transfer between different elements of the qubit circuit.…”