Abstract Efficient detection of single optical centres in solids is essential for quantum information processing, sensing and single-photon generation applications. In this work, we use radio-frequency (RF) reflectometry to electrically detect the photoionisation induced by a single Er3+ ion in Si. The high bandwidth and sensitivity of the RF reflectometry provide sub-100-ns time resolution for the photoionisation detection. With this technique, the optically excited state lifetime of a single Er3+ ion in a Si nano-transistor is measured for the first time to be
0.49±0.04 μs. Our results demonstrate an efficient approach for detecting a charge state change induced by Er excitation and relaxation. This approach could be used for fast readout of other single optical centres in solids and is attractive for large-scale integrated optical quantum systems thanks to the multi-channel RF reflectometry demonstrated with frequency multiplexing techniques.
Single rare-earth ions in solids show great potential for quantum applications, including single-photon emission, quantum computing, and high-precision sensing. However, the linewidths of single rare-earth ions are often broadened due to perturbations associated with the detection methods. Identifying the dominant broadening sources is key to reduce the linewidths for practical applications. We report a spectral broadening study on a single
Er3+ ion in a
Si nanotransistor. The single ion spectra display a Lorentzian lineshape at all light intensities considered. The linewidth remains nearly constant at 32\ifmmode\pm\else\textpm\fi{}2 MHz in the low-intensity regime, and shows a monotonic increase with the intensity in the high-intensity regime. The power broadening does not persist over the microsecond time scales considered after resonant excitation. Nor does it depend on the resonant excitation intensity or the Zeeman shift. These observations and temperature-dependent measurements suggest that charge fluctuations are likely to be a dominant broadening source. Laser heating may also contribute to the power broadening. Charge suppression in the
Er3+-doped region and coupling
Er3+ ions to a confined optical mode could be implemented to reduce the spectral linewidth and to enhance the sensing precision of single
Er3+ ions in
Si.
In spite of a small and indirect bandgap, silicon features properties that are potentially very attractive for optical applications. The unsurpassed level of impurity control leads to suppression of nonradiative recombination paths and provides for a very long minority carrier lifetime. Up to the present, observation of an optical memory effect has not been reported for crystalline silicon (c-Si). In contrast, it has been observed in GaAs and GaN. In this study, we have used c-Si doped with erbium (Er) ions, since the trivalent Er3+ ion emits at a wavelength of 1.54 μm that is suitable for telecommunication applications. In a two-color experiment, utilizing a Nd:YAG pulse excitation as a writing beam and a free-electron laser (FEL) as a reading beam, we revealed that Si exhibits optical memory and afterglow effects at low temperatures T<50 K. Both effects are, in fact, manifestations of the properties of Si itself, conveniently revealed by the optical dopant. Detailed investigations of the FEL-induced emission at 1.54 μm show that this excitation mechanism is governed by the release of a single type of charge carrier. This implies that hole and electron capture events necessary for the Er excitation could, in principle, be separated in time. Therefore it appears possible to combine optical and electrical write/read procedures for the reported memory effect.
This thesis investigates the potential of Er:YSO and Er:Si for quantum communication and computation applications. Erbium uniquely possess optical transitions in the 1.5 um region, making it suitable for both fibre telecommunication and silicon photonics. The properties of the
I15/2↔I13/2 optical transition in Er:YSO have already been extensively studied. Over two decades ago, improvements in
Er3+ dephasing time at 1.5 um were achieved by applying a 5T field along the D1 axis. More recently, a record 4.4 ms coherence time on the same optical transition was achieved using a 7T field. These investigations, among others, illustrate that large Er electron spins become thermally polarised with sufficient magnetic field. However, no long lived and coherent spin transitions associated with the Er ions had previously been identified, and such transitions are necessary for on-demand quantum state storage. To address this requirement, the optical and hyperfine transition properties of 167-Er:YSO were investigated in large magnetic fields. In a field of 7T, spectral hole lifetimes of 1 minute and hyperfine population lifetimes of 12 minutes were observed. These measurements illustrate the effect of spin-lattice relaxation in this system, and how it can be mitigated. Efficient spin-polarisation of the entire 167-Er hyperfine ensemble is also demonstrated. This is the first such demonstration in rare earth systems, and a key requirement for broadband optical storage. Moreover, a 1.3 second coherence time was recorded for an 167-Er:YSO hyperfine transition at 7T and 1.4 K. This is an improvement of several orders-of-magnitude over previous coherence measurements on spin-transitions in Er doped solids. This is also sufficient for maximal entanglement rates in quantum repeater networks that span distances of 1000 km or greater. With an optical transition at 1.5 um, Er is also an ideal candidate to connect silicon based quantum computers to the future quantum Internet. In particular, single Er:Si ions could be used to develop an optical-spin bus between P:Si qubits and fibre based quantum networks. Presented here is the first spectroscopic investigation of single Er:Si ions. This required a novel opto-electronic approach to single ion detection, where the Er ions are implanted into a nanometre scale fin-shaped Field Effect Transistor. With this approach it was possible to develop high resolution optical spectra, where both the electronic and hyperfine levels of individual Er ions were resolved. Long optical and spin coherence times are also important requirements for an optical-spin bus. To address the first requirement, an investigation of the optical lineshape was undertaken. Here it was determined that sources of Stark noise external to the transistor channel contribute a significant amount to optical homogeneous linewidth. However, the dominant noise contribution was determined to be short-range (from within the 30 nm wide channel) and the total homogeneous linewidth was measured to be 50 MHz. The site structure of an individual Er:Si ion was then analysed, using magnetic field rotation patterns and optical transitions between multiple crystal field levels. This site was determined to have approximately axial (
C3) symmetry. The purpose of this study was to determine a magnetic field regime in which the Er electrons spin can be polarised, which is necessary for realising of long hyperfine lifetimes and coherence times.
Single quantum emitters embedded in solid-state hosts are an ideal platform for realizing quantum information processors and quantum network nodes. Among the currently investigated candidates, Er^{3+} ions are particularly appealing due to their 1.5 μm optical transition in the telecom band as well as their long spin coherence times. However, the long lifetimes of the excited state-generally in excess of 1 ms-along with the inhomogeneous broadening of the optical transition result in significant challenges. Photon emission rates are prohibitively small, and different emitters generally create photons with distinct spectra, thereby preventing multiphoton interference-a requirement for building large-scale, multinode quantum networks. Here we solve this challenge by demonstrating for the first time linear Stark tuning of the emission frequency of a single Er^{3+} ion. Our ions are embedded in a lithium niobate crystal and couple evanescently to a silicon nanophotonic crystal cavity that provides a strong increase of the measured decay rate. By applying an electric field along the crystal c axis, we achieve a Stark tuning greater than the ion's linewidth without changing the single-photon emission statistics of the ion. These results are a key step towards rare earth ion-based quantum networks.