Electron Beams Manipulate Quantum Entanglement and Correlations in Semiconductors

Nahid Talebi and colleagues at Kiel University present a new approach to probing and potentially manipulating quantum systems within semiconductors and two-dimensional materials using electron microscopy. The research offers new methods for examining and controlling the delicate interactions between quantum qubits and their surrounding environment, representing a significant step towards realising table-top quantum computing hardware. Understanding and tailoring these interactions is key for building stable and scalable quantum devices, as decoherence, the loss of quantum information, is a major obstacle in quantum technology development.

Electron beam measurement of coherence in boron nitride and semiconductor qubits

Ramsey interferometry, a technique for precisely measuring quantum state stability, analogous to determining how long a spinning top spins before toppling, formed the basis of this investigation. This method relies on applying a series of precisely timed pulses to a quantum system and observing the resulting oscillations in the probability of finding the system in a specific state. A coherent superposition of quantum states is established within a nitrogen-vacancy (NV) defect in hexagonal boron nitride, and these defects function as qubits, the basic units of quantum information. NV defects are particularly attractive as qubits due to their relatively long coherence times, even at room temperature, and their optical addressability. An electron beam then acts as a sensitive probe, interacting with this superposition to generate a measurable signal that reveals how long the delicate quantum coherence persists before disruption. Experiments utilise electron beams to investigate quantum coherence in solid-state qubits, specifically nitrogen-vacancy defects in hexagonal boron nitride and semiconductor quantum dots, employing electron energies around 30 keV with energy spreads of approximately 0.2 eV, resulting in temporal broadening of around 3.3 femtoseconds. This energy spread represents the uncertainty in the electron beam’s energy, which limits the precision of the measurements. The temporal broadening defines the minimum timescale over which coherence can be accurately assessed.

Electron beams reveal quantum coherence via enhanced Ramsey interferometry

Optimised collection optics now achieve a signal-to-noise ratio of approximately 20, a substantial improvement over previous detection methods. This threshold is important because it allows clear observation of Ramsey interference fringes, a phenomenon previously obscured by noise and preventing detailed examination of quantum coherence. These fringes arise from the constructive and destructive interference of the quantum states, providing a direct measure of the coherence time. Talebi and colleagues at Kiel University have, for the first time, demonstrated Ramsey interferometry using both electron-driven photon sources and electron beams interacting with a nitrogen-vacancy defect in hexagonal boron nitride. Using photons to excite the NV defect provides a well-established benchmark for comparison, validating the use of electron beams as an alternative excitation source.

Electron beams now function as potential tools for manipulating quantum states and generating entanglement between qubits, a key step towards more complex quantum systems, extending beyond simple probing. Entanglement, where two or more qubits become linked and share the same fate, is crucial for performing quantum computations that are impossible for classical computers. The experiments use cathodoluminescence spectroscopy, which resolves spectral features of individual qubits with nanometer precision, allowing detailed examination of their behaviour. This technique involves analysing the photons emitted by the qubit when excited, providing information about its energy levels and quantum state. Theoretical studies suggest that manipulating sequential interactions between electron beams and two qubits coupled to a photonic resonator could generate entanglement, surpassing typical electron-photon entanglement schemes. This proposed scheme leverages the strong interaction between the electron beam and the qubits, mediated by the photonic resonator, to create a more efficient entanglement process. Although a signal-to-noise ratio of approximately 20 was achieved, these experiments currently focus on individual emitters and do not yet demonstrate scalable entanglement or control necessary for complex quantum computations. Experiments utilise electron beams to investigate quantum coherence in solid-state qubits, specifically nitrogen-vacancy defects in hexagonal boron nitride and semiconductor quantum dots, employing electron energies around 30 keV with energy spreads of approximately 0.2 eV, resulting in temporal broadening of around 3.3 femtoseconds. This scaling up to multiple qubits presents significant challenges in terms of addressing each qubit individually and minimising cross-talk between them.

Electron beam control of nanoscale nitrogen-vacancy defects for scalable quantum computation

Precise control over qubits and their interactions with the surrounding environment is essential for advancing semiconductor quantum technologies. Environmental factors, such as electromagnetic noise and temperature fluctuations, can cause decoherence and limit the performance of quantum devices. Recent work demonstrates electron beams can both probe and manipulate these quantum states, but scaling up these techniques remains a vital challenge. Current approaches largely focus on individual nitrogen-vacancy defects in hexagonal boron nitride, and achieving entanglement between multiple qubits, essential for complex computations, necessitates overcoming limitations in signal clarity and fabrication precision. Fabricating identical NV defects with consistent properties is a significant hurdle, as variations in defect size and surrounding crystal structure can affect their coherence times.

Acknowledging the hurdles in fabricating and controlling multiple qubits is important for practical quantum devices. This detailed examination of quantum coherence, despite current limitations, advances fundamental understanding and supports progress in semiconductor quantum computing. Demonstrating Ramsey interferometry with both electron and photon excitation establishes electron beams not only as sensitive probes of quantum coherence, but also as potential tools for manipulating quantum states within materials. This capability opens up new avenues for exploring and optimising qubit performance.

This dual capability transcends traditional observation, offering a pathway towards actively controlling qubits, the fundamental units of quantum information, and their interactions. Consequently, this investigation shifts focus from merely detecting quantum phenomena to engineering them, raising questions regarding the feasibility of scalable entanglement schemes using sequential electron-beam interactions. This approach provides a complementary method to existing techniques, offering a unique pathway to investigate and manipulate these systems at the nanoscale. Further research will need to address the challenges of scaling up these techniques and demonstrating robust quantum control over multiple qubits to realise the full potential of electron-beam-based quantum technologies.

The research demonstrated that electron beams can both examine and manipulate quantum states in semiconductors like hexagonal boron nitride. This is important because controlling these states is crucial for developing functional quantum computing hardware. The study focused on nitrogen-vacancy defects and highlights the challenges of creating consistent qubits and achieving entanglement between them. Researchers suggest further work is needed to scale up these techniques and improve quantum control over multiple qubits.