Deterministic Entanglement of Photons in Two Superconducting Microwave Resonators

Abstract

Quantum entanglement, one of the defining features of quantum mechanics, has been demonstrated in a variety of nonlinear spin-like systems. Quantum entanglement in linear systems has proven significantly more challenging, as the intrinsic energy level degeneracy associated with linearity makes quantum control more difficult. Here we demonstrate the quantum entanglement of photon states in two independent linear microwave resonators, creating N -photon NOON states as a benchmark demonstration. We use a superconducting quantum circuit that includes Josephson qubits to control and measure the two resonators, and we completely characterize the entangled states with bipartite Wigner tomography. These results demonstrate a significant advance in the quantum control of linear resonators in superconducting circuits.Quantum superposition and entanglement have been demonstrated experimentally using spin-like physical systems ranging from atoms to electronic circuits [1][2][3][4][5][6][7]. These systems all display strong nonlinearity, and are used because this nonlinearity allows straightforward quantum control by classical means. The quantum control of linear systems, exemplified by the harmonic oscillator, is by contrast more difficult, and has only been achieved using nonlinear intermediaries: Atoms [1,8] to control optical cavities, ions to control ion motion [9,10], and superconducting qubits to control photons in microwave resonators [11][12][13][14]. Quantum entanglement of cavity photons still presents a significant challenge: Experiments have demonstrated maximally-entangled photons in different polarization modes of the same cavity [15,16], but the entanglement of photons in two physically distinct cavities [17][18][19] has proven more elusive.Here we show the deterministic generation of entangled photon states in two spatially-separated microwave resonators, achieved by manipulating the photon states with a pair of superconducting phase qubits. We use as a benchmark the generation of NOON states [20][21][22][23][24], comprising a total of N photons in the two resonators (A and B), entangled in the quantum statewith N photons in resonator A and zero in B, superposed with the state with the occupation numbers reversed. Such a state has the same degree of entanglement as the Bell state, but with N excitations. We also generate MOON states, in which, e.g., resonator A has M or zero quanta, entangled with resonator B with zero or N quanta.We fully characterize the two-resonator photon states using bipartite Wigner tomography, which represents a non-trivial extension of single-cavity Wigner to- mography [1,9,[12][13][14], and allows us to distinguish entanglement from an incoherent ensemble.To accomplish this goal, we developed a new quantum circuit comprising two superconducting phase qubits [25] and three microwave resonators. A sketch of the circuit topology is shown in Figs. 1(a). The circuit includes a coupling resonator C, connected to both qubits, and two state storage resonators A and B, each coupled to one q...

Microwave quantum optics and electron transport through a metallic dot strongly coupled to a transmission line cavity

“…[8][9][10][11][12][13][14] A parallel development concerned the possibilities to perform fundamental quantum optics experiments with microwave photons in cavities. Experiments on microwave quantum optics range from arbitrary photon state preparation 15 and entanglement of cavity photons 16 to single photon generation, 17 microwave lasing 18 and fast tuning of cavity photon properties. 19,20 An important recent development is the efforts to reach the ultrastrong coupling regime, where the strength of the coupling between the qubit and the cavity becomes comparable to the frequency of the fundamental cavity mode.…”

Correlations, indistinguishability and entanglement in Hong–Ou–Mandel experiments at microwave frequencies

“…The created states are also referred to as NOON states. In contrast to the NOON states of propagating photons investigated here, NOON states have also been investigated in superconducting circuits with photons localized in resonators 28,29 .…”

Engineering two-mode entangled states between two superconducting resonators by dissipation

“…Particularly, plenty of works have focused on the extension from single to more versatile multi-resonator architectures [19][20][21][22], where the resonators are interconnected by superconducting qubits. These multi-resonator systems allow manipulation of spatially separated photon modes and engineering quantum states between physically distant cavities [23][24][25][26][27][28][29][30][31][32][33][34][35].…”