Quantum
Optical
Devices
Introduction
Since its development around the turn of the 20th century, quantum mechanics has profoundly impacted our lives through the improved understanding of the microscopic world, the rules governing it, and the technologies resulting from this knowledge. However, despite there being ample convincing evidence that this is the correct theory to describe the world, its most elusive and intriguing phenomena such as quantum superposition and entanglement are absent in our everyday lives as they seem just too fragile to exist in an ambient environment.
However, there are many reasons why it is worthwhile to try to build many-body systems which can experience such quantum effects at large scales. On the fundamental side it opens up the possibility to test emergent features in quantum mechanics that may be absent at smaller scales and if the nature of quantum correlations implies different macroscopic properties. On the more practical side, such systems can be used as machines to process quantum information, enabling applications such as quantum computing and simulation, provably-secure communication or quantum-enhanced metrology, all of which will outperform machines governed by classical physics.
Solid-state platforms are highly desirable due to their potential for realizing highly-scalable and compact chip-integrated devices. However, realizing such systems in the solid state is particularly complicated due to often very fast decoherence processes caused by interactions with e.g. electron or nuclear spin baths in the solid-state matrix.
To overcome this and to build useful solid-state quantum technologies in the future consolidated efforts of studying novel quantum systems and developing novel quantum materials are of utmost importance. The QuOD laboratory achieves this by combining the expertise of currently two research groups described below.
Solid State Quantum Optics Group (S²QO)
Novel Quantum Bits in Synthetic Diamond
Westudy novel qubits in diamond for matter-based quantum information processing. The goal is to use our now improved knowledge of diamond defect physics to find replacements for current-generation contestants such as the NV or the SiV, which have been used successfully in a number of proof-of-principle experiments but face a variety of challenges making it difficult and/or costly to scale these technologies.
We apply various defects in diamond, in particular the nitrogen vacancy center, as microscopic quantum sensors for example for magnetic field sensing with high spatial resolution and sensitivity. For example we are currently working towards a device combining NV centers with electrons trapped on superfluid helium to use the NV centers to sense and access the electron spin degree of freedom. This also includes research towards improving the fabrication of these centers in a deterministic and highly localized fashion. To this end, we have recently commissioned an aberation-corrected multiphoton femtosecond laser fabrication system.
Optical Quantum Memories in Diamond
A second research line focuses on the development of optical devices which can be used to store or process quantum information encoded in photons. This project draws on our study of novel defects in diamond. However, to achieve efficient storage of single photons, we here use dense ensembles of such color centers as a storage medium. We employ off-resonant two-photon/Raman absorption protocols to achieve large memory acceptance bandwidths, enabling the storage of temporally short photons to achieve high data rates. Applications of such quantum memories include single-photon source synchronization, the generation of highly-entangled large photonic cluster resource states for one-way quantum computing, or single-photon-level nonlinearities for deterministic multi-photon gates.
Quantum Thermodynamics
A more fundamental research topic is the use of diamond defects, specifically the nitrogen vacancy, as a testbed to study the thermodynamic properties of microscopic systems in the presence of quantum agents such as superposition or entanglement. In a landmark study we recently and for the first time demonstrated that individual microscopic heat engines can break the classical power-bound using quantum coherence between their energy levels as a resource. Building upon this concept we intend to study the thermodynamic influence of more complex quantum agents such as entanglement in many-body quantum systems in the future. This will help to improve our thermodynamic understanding of large entangled quantum systems as they will be used for quantum computing and perhaps, in the distant future, lead to quantum-enhanced energy storage solutions.
Quantum Sensing with Defects in Diamond
Diamond & Quantum Materials Group (DQM)
Growth of Doped Diamond
We study the microwave plasma assisted chemical vapor deposition (CVD) of single crystal and polycrystalline films of diamond, and in particular the growth of doped diamond for quantum applications including sensing, quantum communication, quantum memories, and quantum computing. Control of the charge state is crucial to optimization and utilization of a variety of defects in synthetic diamond, including the widely studies negatively charged nitrogen vacancy center (NV⁻) and the the neutral silicon vacancy center (SiV⁰). By careful incorporation of dopant atoms during the CVD growth we can optimize the stability of a desired charge state for these and other novel defects.
Novel Material Development
Single crystal diamond is a highly attractive material for nanophotonic applications, including quantum computing and optical quantum memory applications, due to its high transparency and the ability to host a wide range of technologically interesting color centers due to its large bandgap. In a strong collaboration with the S²QO group, we are using our experience and capabilities in doped diamond growth to investigate the incorporation and charge control of novel defects in diamond as potential qubit candidates. We are also developing the growth and processing of other crystalline materials for quantum applications, including stoichiometric rare earth crystals.
Device Fabrication and Nanofabrication
Optical elements such as optical cavities and waveguides rely upon light confinement due to Bragg reflection or total internal reflection, which can be achieved straightforwardly through the etching of small features to create index contrast with the surrounding air. Furthermore the photon collection efficiency from defects can be substantially increased by utilizing nanopillar structures. The processing of thin films of diamond and the etching of relevant features are complicated however, due to diamond’s exceptional hardness and chemical inertness. We investigate nanofabrication processes using Reactive Ion Etching (RIE), in which a chemically reactive plasma is used to remove diamond at a rate of tens of nanometers per minute.