Quantum state engineering with photonic qubits

Virginia O. Lorenz

Qi-Ping Su, Liang Bin, Yu Zhang +1 more

In recent years, quantum state engineering and quantum information processing using microwave fields and photons have received increasing attention. In addition, multiqubit gates play an important role in quantum information processing. In this work, we propose to encode a photonic qubit via two arbitrary orthogonal eigenstates (with eigenvalues $\ifmmode\pm\else\textpm\fi{}1$, respectively) of the photon-number parity operator. With such encoding, we then present a single-step method to realize a multi-target-qubit controlled-phase gate with one photonic qubit simultaneously controlling $n\ensuremath{-}1$ target photonic qubits, by employing $n$ microwave cavities coupled to one superconducting flux qutrit. This proposal can be applied not only to implement nonhybrid multi-target-qubit controlled-phase gates using photonic qubits with various encodings, but also to realize hybrid multi-target-qubit controlled-phase gates using photonic qubits with different encodings. The gate realization requires only a single-step operation. The gate operation time does not increase with the number of target qubits. Because the qutrit remains in the ground state during the entire operation, decoherence from the qutrit is greatly suppressed. As an application, we show how to apply this gate to generate a multicavity Greenberger-Horne-Zeilinger (GHZ) entangled state with general expression. Depending on the specific encodings, we further discuss the preparation of several nonhybrid and hybrid GHZ entangled states of multiple cavities. We numerically investigate the circuit-QED experimental feasibility of creating a three-cavity spin-coherent hybrid GHZ state. This proposal can be extended to accomplish the same tasks in a wide range of physical systems, such as multiple microwave or optical cavities coupled to a three-level natural or artificial atom.

Duncan G. Steel

Abstract Chapter 15 derived the fundamental theory and eigenstates for the quantized radiation field and then showed how the quantum vacuum gives rise to spontaneous emission. This chapter now goes more deeply into the meaning and implications of the quantized field. The polarization of a photon can be used as a qubit and the photonic qubit is called a flying qubit. It enables transmission of information from one node to another. Spontaneous emission is shown to enable creation of an entangled state between a photonic qubit and the spin of an electron. Spontaneous emission can also degrade the performance of some device designs and in other devices it can enhance performance such as for a single photon emitter. In this we show how to engineer the vacuum to control spontaneous emission.

R. B. Dalton, Geoff Gillett, Geoff J. Pryde +4 more

Peter Maunz, Steven Olmschenk, David Hayes +3 more

Shalini Devendrababu, Vaishnavi Chandilkar

Superconducting and photonic qubits are the two promising platforms for quantum computation. This review investigates different materials that are being used to advance these platforms, detailing the structural, electrical, and thermal properties that influence qubit performance. The discussion on superconducting qubits covers material properties critical for achieving low-loss circuits, the impact of two-level system defects, and the engineering of Josephson junctions. In contrast, photonic qubits, which leverage light for quantum information processing, are examined through the lens of material platforms such as silicon-based photonics and III–V semiconductors. The chapter also highlights fabrication challenges, qubit coherence improvements, and recent advances in single-photon sources for scalable quantum systems. By analyzing the intersection of materials science, quantum mechanics, and device engineering, this review provides insights into the optimization of quantum hardware for future large-scale quantum computing applications.