Periodic Reporting for period 1 - SILEQS (Spins Interfaced with Light for Quantum Silicon technologies)
Berichtszeitraum: 2022-10-01 bis 2025-03-31
Due to its superior maturity in large-scale nanofabrication, silicon is the flagship material from both the microelectronics and integrated photonics industries. As a logical consequence, this semiconductor is one of the most desirable platforms to develop new technologies based on the control of physical systems obeying not to classical but to quantum mechanics laws. These systems, commonly called quantum bits or qubits, can be used as quantum information carriers. Up to recently, qubits in silicon detactable and controllable at individual scale were split into two types. On one hand, single electrons trapped in the silicon crystal using either metal electrodes or impurities can locally store quantum information for long time. However, they are lacking an optical interface required for long-distance communications. On the other hand, particles of light called photons, can be generated inside silicon with laser using optical non-linear processes. But unfortunately, they are not coupled to memory qubits to locally store information.
Lately, a new type of individual quantum systems has been detected in silicon: fluorescent point defects. These systems, also called color centers, can be excited by lasers and in response emit fluorescent light, like artificial atoms trapped in the silicon crystal. They can be detected at single-defect scale using advanced microscopy technics at cryogenic temperatures, typically bellow -240°C. These color centers have the benefit of emitting single photons in the telecom band, i.e. in the range of light that can propagate over the greatest distances in optical fibers used for the Internet. Furthermore, some fluorescent defects do possess an additional quantum degree of freedom associated with a non-zero spin state that can encode quantum information. As a consequence, optically-active spin defects could combine a spin memory qubit interfaced with telecom single photons.
The goal of the SILEQS project is to explore the quantum properties of these single color centers recently detected in silicon. A first objective relates to the control of their luminescence in order to have the defects generating photons one after the other with identical properties. A second one concerns the control of their spin states, down to the single-defect level. Like other quantum systems, particular attention will be given to the impact of their environment on the defects' quantum properties.
Lately, a new type of individual quantum systems has been detected in silicon: fluorescent point defects. These systems, also called color centers, can be excited by lasers and in response emit fluorescent light, like artificial atoms trapped in the silicon crystal. They can be detected at single-defect scale using advanced microscopy technics at cryogenic temperatures, typically bellow -240°C. These color centers have the benefit of emitting single photons in the telecom band, i.e. in the range of light that can propagate over the greatest distances in optical fibers used for the Internet. Furthermore, some fluorescent defects do possess an additional quantum degree of freedom associated with a non-zero spin state that can encode quantum information. As a consequence, optically-active spin defects could combine a spin memory qubit interfaced with telecom single photons.
The goal of the SILEQS project is to explore the quantum properties of these single color centers recently detected in silicon. A first objective relates to the control of their luminescence in order to have the defects generating photons one after the other with identical properties. A second one concerns the control of their spin states, down to the single-defect level. Like other quantum systems, particular attention will be given to the impact of their environment on the defects' quantum properties.
So far, the SILEQS project has been mainly focused on a specific fluorescent defect in silicon made of carbon impurities and called the G center. This color center, known for more than half a century, has only been isolated at single defect scale recently. Individual G centers are appealing for quantum applications as they emit telecom single photons and have a non-zero metastable electron spin state. Furthermore, these defects possess an additional degree of freedom associated to the motion of one of their atoms.
By investigating single G centers in silicon samples adapted for integrated photonics, we do observe an optical signature of the motion of a single atom inside a macroscopic crystal. In particular, the pattern of emission lines produced by the G defects is linked to the motion dynamics of their mobile atom, which is impacted by the crystal environment. In a perfect silicon crystal, the G center's mobile atom is perfectly delocalized between 6 sites. For a G center in a strained crystal, for instance due to the presence of a silicon oxide layer below the silicon layer, the behavior is drastically different. In this case, at each optical cycle, the defect center-of-mass randomly hops between the different sites, as if it were a ball in a 6-slot roulette wheel.
These results open the way to the tuning of the G center emission lines by strain engineering of the silicon crystal. Furthermore, as the G defect is sensitive to strain, we could think of using it as an optical sensor of strain with sub-micrometer spatial resolution. In parallel, we are also investigating the control of its spin states, as well as the integration of single G centers inside photonic cavities to boost the collection and the emission of single photons.
By investigating single G centers in silicon samples adapted for integrated photonics, we do observe an optical signature of the motion of a single atom inside a macroscopic crystal. In particular, the pattern of emission lines produced by the G defects is linked to the motion dynamics of their mobile atom, which is impacted by the crystal environment. In a perfect silicon crystal, the G center's mobile atom is perfectly delocalized between 6 sites. For a G center in a strained crystal, for instance due to the presence of a silicon oxide layer below the silicon layer, the behavior is drastically different. In this case, at each optical cycle, the defect center-of-mass randomly hops between the different sites, as if it were a ball in a 6-slot roulette wheel.
These results open the way to the tuning of the G center emission lines by strain engineering of the silicon crystal. Furthermore, as the G defect is sensitive to strain, we could think of using it as an optical sensor of strain with sub-micrometer spatial resolution. In parallel, we are also investigating the control of its spin states, as well as the integration of single G centers inside photonic cavities to boost the collection and the emission of single photons.
Achieving the SILEQS project's objectives will open up many new prospects in silicon-based quantum technologies. First, it could enable the fabrication of efficient single photon sources at telecom wavelengths and directly integrated in a platform adapted to large-scale nanofabrication. Secondly, combining the control over the spin states and the optical emission of the color centers could provide a spin qubit interfaced with light for long-distance quantum communications. Given the advanced maturity of silicon nanofabrication, other paths could be explored such as the development of hybrid quantum systems with single color centers interacting with silicon-based mechanical resonators. Beyond quantum technologies, research on single fluorescent defects in silicon could lead to advances in the field of defect physics in this key material.