Quantum Emitters Used to Generate Multiphoton Qudit States, Expanding Quantum Computing Potential

Researchers from Paderborn University and Virginia Tech have proposed a deterministic protocol to generate qudit photonic graph states from quantum emitters. The team demonstrated that their approach could be used to generate any qudit graph state, broadening the range of multiphoton entangled states that can be produced. The research also showed that photons can naturally encode not only qubits but also multidimensional qudit states. This development could lead to novel approaches in quantum computing, communication, sensing, and error correction, where quantum information is stored more compactly. The only additional resource required is the ability to control multilevel emitters.

What is the Deterministic Generation of Qudit Photonic Graph States from Quantum Emitters?

Quantum emitters are a type of quantum system that can emit and absorb photons, the basic units of light. In a recent study by Zahra Raissi from the Department of Computer Science and Institute for Photonic Quantum Systems at Paderborn University, along with Edwin Barnes and Sophia E Economou from the Department of Physics at Virginia Tech, a deterministic protocol to generate qudit photonic graph states from quantum emitters was proposed and analyzed.

The researchers showed that their approach could be applied to generate any qudit graph state. They exemplified this by constructing protocols to generate one and two-dimensional qudit cluster states, absolutely maximally entangled states, and logical states of quantum error-correcting codes. Some of these protocols make use of time-delayed feedback, while others do not. The only additional resource requirement compared to the qubit case is the ability to control multilevel emitters.

This research significantly broadens the range of multiphoton entangled states that can be produced deterministically from quantum emitters. The deterministic generation of multiphoton entangled states has been a focus of theoretical and experimental efforts, primarily using photonic qubits. However, this research shows that photons can naturally encode not only qubits but also multidimensional qudit states.

Why is Entanglement Important in Quantum Information Science?

Entanglement is a uniquely quantum property that plays an important role in almost all aspects of quantum information science. This includes quantum computing, quantum error correction, quantum sensing, and quantum networks. Many of these applications require the generation of large multiphoton entangled resource states upfront, especially in the context of measurement or fusion-based quantum computing and quantum communication.

However, the creation of entangled states of many photons is challenging because photons do not interact directly. Standard ways of circumventing this issue make use of either nonlinear media or quantum interference and measurement. The former approach is made challenging by low coupling efficiencies, while the latter is intrinsically probabilistic. Despite a number of conceptual and technological advances, the probabilistic nature of this approach continues to severely limit the size of multiphoton entangled states constructed in this way.

How Can Quantum Emitters Generate Multiphoton Entanglement?

An alternative approach to generating multiphoton entanglement is to use coupled controllable quantum emitters with suitable level structures. There now exist several explicit protocols for creating entangled states of many photonic qubits either by using entangling gates between emitters and transferring entanglement to the photons in the photon-emission stage, or by sending the photons to interact again with emitters to potentially create entanglement beyond what is generated from the emission process.

Proof-of-principle experimental demonstrations of such deterministic protocols have been performed in both the optical and microwave domains. To date, the vast majority of these efforts have focused on photonic qubits. However, the research by Raissi, Barnes, and Economou shows that photons can naturally encode not only qubits but also multidimensional qudit states.

What are the Benefits of Using Qudit States?

Qudit states, which can be encoded by using more than two spatial paths or time bins, can allow for novel approaches to quantum computing, communication, sensing, and error correction in which quantum information is stored in a more compact way. Such states can, in particular, provide benefits in quantum networks and repeaters.

Although there have been recent experimental demonstrations of entangled qudit state creation, these have been limited thus far to two photons. An outstanding question is whether multiphoton entangled qudit states can be generated deterministically from a small number of quantum emitters.

What are the Future Implications of this Research?

In their paper, Raissi, Barnes, and Economou propose and evaluate deterministic methods to generate multiphoton qudit graph states from multilevel quantum emitters. They present several different explicit protocols that can produce various states either using a single emitter together with time-delayed feedback or using multiple coupled quantum emitters.

The researchers show that any qudit graph state can be produced from multilevel quantum emitters with an appropriate level structure using a small set of gates on the emitters. They then focus on constructing one and two-dimensional cluster states, as well as highly entangled multiparty states called absolutely maximally entangled states. These states are defined by the property that they maximize the entanglement entropy for any bipartition and they have applications in quantum error-correcting codes and secret sharing.

In addition, the researchers present protocols for constructing logical states of quantum error-correcting codes, the code spaces of which are spanned by absolutely maximally entangled states of qutrits. Their approach to qudit photonic graph-state generation incurs only a small resource overhead compared to the qubit case, primarily in the requirement of multilevel control of quantum emitters, which has been achieved experimentally in atomic and defect-based systems.