Researchers from Harvard University have made significant progress in the field of quantum networking by developing a novel laser-based technique for creating single-atom defects on the surface of materials. These defects, known as color centers, can serve as qubits, the basic units of quantum computing. The team also devised a real-time method for measuring and characterizing the formation of optical emitters within nanoscale cavities.
Led by Evelyn Hu, the Tarr-Coyne Professor of Applied Physics and of Electrical Engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), the researchers’ breakthrough could provide better control over the timing and strength of qubit outputs.
Color centers are vacancies in the crystal structure of a material where an atom is missing. These vacancies possess their own electronic states, spin properties, and the ability to emit photons of specific wavelengths. In photonic materials with nanoscale cavities, these defects can act as optical emitters, transmitting information.
The research team aims to investigate the behavior of these defects and their potential as qubits in a quantum network. By entangling an array of defects within nanophotonic cavities, they could enable the transmission of quantum information. Aaron Day and Jonathan Dietz, co-first authors of the paper and Ph.D. candidates in Hu’s lab, share a keen interest in the formation and behavior of these defects.
However, until now, controlling the precise location of optical emitters within nanoscale cavities without damaging the crystal structure has been challenging. Previous techniques involved disrupting the crystal structure using ions or below-band-gap lasers. Unfortunately, ion implantation equipment is not readily available in most laboratories, and these conventional approaches are inefficient and difficult to control, comparable to sandblasting rather than precise drilling.
Recognizing the need for highly precise instruments, Hu and her team set out to develop a laser-based method that could overcome these limitations. The new technique enables the controlled creation of optical emitters within nanoscale cavities, which are approximately 100 times smaller than the width of a human hair. By utilizing lasers, the researchers achieved the desired precision without compromising the integrity of the material’s crystal structure.
This breakthrough opens up new possibilities for quantum networking and paves the way for the development of more advanced quantum technologies. The ability to control the formation and properties of optical emitters in nanoscale cavities brings researchers one step closer to harnessing the full potential of quantum computing and quantum information processing.
The researchers draw an analogy between their solution and a stylus and template system. They utilize a laser as the stylus, which writes, and a nanoscale cavity as the template, into which they create and analyze the material defects. Unlike traditional below-band-gap lasers, they use above-band-gap pulses of light, which contain more photon energy, for more efficient energy transfer from the laser to the material.
The team began by fabricating nanophotonic cavity devices using commercial-grade silicon carbide in a painstaking process within a clean room. They then conducted experiments to precisely create optical emitters within the cavities. Initially, the laser pulses caused the cavities to explode, necessitating a significant reduction in laser energy.
Through trial and error, the researchers determined the optimal laser energy required to create the desired optical emitter without damaging the cavity. They also incorporated a read-out laser into their system, enabling them to assess the photonic signals emitted by the cavity before and after the defect-forming laser pulse.
A remarkable finding was the ability to monitor the cavity and immediately observe changes upon the laser pulse that formed the optical emitter. This real-time feedback mechanism proved invaluable in selecting cavities with suitable properties and reliably transforming them into hosts for quantum information.
The team highlights the scalability of their approach for creating a large number of qubits, which is crucial for the advancement of quantum technologies. Furthermore, they believe their method has broad applicability for various fundamental inquiries. By forming defects within cavities, they can gain instant insight into the local material environment, effectively using the cavity as a nanoscope to probe atomic defects’ characteristics.
Combining the precision of the laser stylus with the feedback provided by cavity resonances, the researchers can seamlessly write and enhance devices. This synergistic approach enhances the capabilities of each tool individually, resulting in a more powerful and efficient system.