Baxter T. Flor

Figure 1. A material’s electronic structure determines its emission properties. Change in electronic structure of monolayer WSe2 before (left) and after (right) electron beam exposure. Electron beam irradiation leads to strong coupling between electronic states and the generation of single photon emitters.

Next generation photon emitters
Quantum technologies have the potential to revolutionize conventional computing and communications technologies. By utilizing quantum mechanical principles, quantum computers can perform calculations that are impossible for classical computers, and quantum communication networks can transmit information with increased security. The essential nature of these emerging technologies relies on the transfer and manipulation of quantum information (quantum transduction) between different parts within a quantum network. Photons, the fundamental particle of light, are an ideal platform for facilitating quantum transduction because they interact minimally with the environment, allowing them to carry quantum information over long distances and times. A material capable of emitting identical single photons on demand, known as a single photon emitter (SPE), is required for ensuring reliable and controlled quantum transduction throughout a quantum network. Single photon emission is a quantum mechanical effect that works differently from light emission from sources such as incandescent light bulbs, which emit streams of photons with different energies at random time intervals. Think of a single photon emitter like a printer. When you send an image to a printer, you expect the same image to come out each time.

At the Center for Molecular Quantum Transduction (CMQT), established on August 1, 2020, researchers are focused on uncovering the possibilities of atomic and molecular structures in revolutionizing quantum technologies. Their goal is to unlock the potential for creating, converting, and storing quantum information. However, despite the progress made so far, a perfect SPE, which is crucial for advanced quantum technologies, has yet to be achieved. To address this challenge, researchers in CMQT have set their sights on understanding the pathway to SPE generation and finding ways to control them for practical applications. To fully appreciate their groundbreaking work, it’s important to explore some fundamental quantum mechanical concepts that underpin SPEs. To fully appreciate the groundbreaking work of the CMQT researchers, it is important to explore some fundamental quantum mechanical concepts that underpin SPES. Understanding these fundamentals helps to shed light on a remarkable achievement in the field - the conversion of atomically thin sheets of tungsten diselenide (WSe2) into a functional SPE. This transformation marks a significant milestone in harnessing the potential of novel materials for quantum applications, opening exciting avenues for future advancements in this field.

Quantum principles move beyond classical limitations
A key quantum mechanical feature is the wavefunction, whose name came about because quantum mechanical objects possess both wave- and particle-like characteristics. A wavefunction is a mathematical equation that represents how likely it is to find a quantum system in different states. It can exist in multiple states at the same time, a phenomenon known as superposition. Let’s take a classical computer bit as an example. A classical computer bit has two states available to it: 0 and 1. Our everyday experiences tell us that when the bit is a 0, it cannot also be a 1. However, in the counterintuitive world of quantum mechanics, a quantum computer bit, or qubit, can be a 0 or 1 but can also exist in a superposition (mixture) of 0 and 1. This means that the qubit is both a 0 and 1 at the same time, with each state having a certain probability. Qubits can represent an infinite number of possible values while a classical bit can only represent two. Imagine a qubit like a spinning coin. In the classical world, a coin can only show either heads or tails, but for a spinning coin, it’s both and neither at the same time. At any moment, the chances of landing heads-up or tails-up varies. This is a property of quantum mechanics that allows quantum computers to perform certain tasks that are impossible for classical computers. In principle, a classical computer would need 8192 bits to achieve the same computing speed as a quantum computer operating with 13 qubits.

In the realm of quantum computing, information is stored in the superposition of the wavefunction describing the qubit. However, in order for quantum information to be transmitted in quantum networks, the stationary qubits that make up the network must interact with one another. To achieve reliable quantum transduction, photons play a pivotal role. Photons have minimal interactions with the environment, making them ideal for reliably carrying quantum information within the network. To preserve the quantum state of the information, it is essential that the conversion from stationary qubit to photon occurs the same way each time. SPEs are perfect candidates for this role because they produce identical photons, or print the same image, for every emission event.

Figure 2. Emission from monolayer tungsten diselenide (WSe2) is converted from classical to quantum emission as a single photon emitter (SPE) by application of strain and electron beam irradiation. Single photon emission is characterized by high intensity well-isolated emission peaks.

Transforming classical to quantum emission
Creating materials that emit single photons is an important area of research for quantum technologies. Researchers have explored a range of potential SPE candidates, from diamond defects to zero-dimensional quantum dots and even carbon nanotubes. But one material, in particular, has caught the attention of the scientists at CMQT: a super thin, two-dimensional (2D) material called tungsten diselenide (WSe2).[1,2] Despite being incredibly thin, consisting of only a few atoms, 2D materials can extend laterally by thousands of atoms. This material is intriguing because it interacts strongly with light, making it easy to generate electronically excited states that relax to emit photons. Moreover, it can easily be incorporated into solid-state devices. Usually, when WSe2 emits light, it’s like a chaotic fireworks display, with photons of different energies flying everywhere. This effect is a result of excited electrons transitioning from the conduction band (CB) to the valence band (VB) through different pathways (Figure 1, left). This results in a complex emission spectrum with multiple overlapping peaks, similar to light emitted by incandescent light bulbs (Figure 2, left). To create single-energy photons, excited electrons need to relax from the CB to VB through a single pathway. To achieve this feat, researchers at CMQT applied strain to the WSe2 material, stretching and bending it. Then, they bombarded it with an intense beam of electrons. These combined forces changed the way the material’s electrons move after they are excited, resulting in a stunning transformation of its emission behavior. Instead of a jumble of overlapping peaks, the emission spectrum now exhibited a small number of well-defined, isolated emission peaks, which is a characteristic of SPEs (Figure 2, right).

Future outlook for single photon emitters
The field of single photon emitters is still in its early stages, but there is a great deal of promise for future applications. To fully unlock the power of SPEs, there are a few important challenges that need to be addressed. Increasing their brightness and efficiency is the first major step. Additionally, researchers are actively investigating how to control where and when single photon emission occurs in these materials. And finally, it is necessary to develop methods for easily integrating SPEs into existing advanced technologies that utilize the power of light for various applications in the field of quantum technology. The groundbreaking transformation of tungsten diselenide (WSe2) into a functional SPE at the Center for Molecular Quantum Transduction (CMQT) signifies a significant leap forward in harnessing novel materials for quantum applications. With each step closer to perfecting SPEs, we move closer to a transformative era where quantum technologies become an indispensable part of our everyday lives revolutionizing computing, communication, and information processing on an unprecedented scale.

More Information

REF 1: D.D.X. and A.F.V. contributed equally to this work. This research was primarily supported by the Center for Molecular Quantum Transduction, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0021314.

REF 2: This research was primarily supported by the Center for Molecular Quantum Transduction, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0021314.

About the author(s):

Baxter T. Flor is a PhD student under Dr. George C. Schatz at Northwestern University and Dr. Xuedan Ma at Argonne National Laboratory, working in the Center for Molecular Quantum Transduction (CMQT) EFRC. His research focuses on the photo-physics and electronic structure of 2D nanomaterials for quantum applications. He has studied the wavelength dependence of spin injection with circularly polarized light in cadmium selenide nanoplatelets and is currently investigating the formation of long-lived spin qubits via triplet energy transfer in donor/acceptor complexes. His undergraduate research took place at the Center for Advanced Microstructures and Devices (CAMD) in Baton Rouge, Louisiana under Dr. Gary L. Findley. There he studied electron ionization energies and electron mobility in polar and non-polar fluids. Orcid ID: 0000-0001-8926-9601.

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