Tripartite interactions between two phase qubits and a resonant cavity

Abstract

Multipartite entanglement is essential for quantum computation 1 and communication [2][3][4] , and for fundamental tests of quantum mechanics 5 and precision measurements 6 . It has been achieved with various forms of quantum bits (qubits), such as trapped ions 7,8 , photons 9 and atoms passing through microwave cavities 10 . Quantum systems based on superconducting circuits, which are potentially more scalable, have been used to control pair-wise interactions of qubits [11][12][13][14][15][16] and spectroscopic evidence for three-particle entanglement was observed 17,18 . Here, we report the demonstration of coherent interactions in the time domain for three directly coupled superconducting quantum systems, two phase qubits and one resonant cavity. We provide evidence for the deterministic evolution from a simple product state, through a tripartite W state, into a (bipartite) Bell state. The cavity can be thought of as a multiphoton register or an entanglement bus, and arbitrary preparation of multiphoton states in this cavity using one of the qubits 19 and subsequent interactions for entanglement distribution should allow for the deterministic creation of another class of entanglement, a GreenbergerHorne-Zeilinger state.With the development of quantum information science 1 , entanglement of multiparticle systems has become a resource for a new information technology. In particular, three-particle or tripartite entanglement allows for teleportation 2 , secret sharing 4 and dense coding 20 , with connections to cosmology 21 . Over the past decade, the development of exquisite control over quantum systems has led to various demonstrations of tripartite entanglement [8][9][10] . Genuine tripartite entanglement is delineated by two inequivalent classes of states 22 : Greenberger-Horne-Zeilinger and W, where the W state involves only a single photon shared amongst three systems. Using multipartite entanglement in a solid-state-qubit system has only recently received theoretical attention [23][24][25] . Thus far in superconducting systems, bipartite entanglement has been verified by two-qubit quantum state tomography 13 and used to carry out a quantum algorithm 15 . Spectroscopic evidence for three-particle entanglement was observed for two current-biased phase qubits coupled to a lumped element consisting of an inductor-capacitor circuit and a cavity as well as for transmon qubits 17,18 . In the experiments described below, we first verified the spectroscopic signature of three coupled systems. Next, we demonstrated coherent interactions. Frequency detuning of the third system was used to verify the proper change in the time evolution of two versus three coupled systems. Finally, we describe a free-evolution process as a means of deterministically preparing arbitrary single-photon tripartite entangled states and a corresponding visualization technique. We present evidence for the proper operation of this protocol by measuring the time-dependent behaviour of the two phase qubits. Here, entanglement is not verified di...

Computing prime factors with a Josephson phase qubit quantum processor

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

“…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].…”

“…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.…”

“…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.…”

“…We begin by characterizing the device with spectroscopy. Next, we produces coherent interactions between five qubits and verify bi-and tripartite entanglement via quantum state tomography (QST) [8,12,14,15]. In the final experiment, we run a three-qubit compiled version of Shor's algorithm to factor the number 15, and successfully find the prime factors 48 % of the time.…”