Nat. Commun. First silicon quantum light source demonstrated!
Quantum technology promises to revolutionize society by enabling entirely new methods of communication, sensing, and computing. For example, quantum cryptography, if implemented, would provide an unparalleled level of data security against hackers: this is because quantum information can be encoded in photons (individual particles of light) and cannot be copied or measured.
Before that can happen, however, scientists must first overcome a major and demanding obstacle to quantum cryptography: the ability to create photons in a way that reliably energizes a quantum network or quantum Internet.
Recently, a research team led by Chenming Hu, an associate professor in the Department of Electrical Engineering and Computer Science at the University of California, Berkeley, and Boubacar Kanté, a scientist in the Materials Science Division at Lawrence Berkeley National Laboratory (Berkeley Lab, LBNL or LBL), has demonstrated the first quantum light source using silicon.
Silicon, the material on which millions of tiny electronic devices are built every day, is the most 'scalable' optoelectronic material known," Kanté said. Using silicon as a quantum light source means that the large-scale complementary metal oxide semiconductor (CMOS) chip fabrication process at the heart of today's optoelectronics and artificial intelligence devices may be directly applicable to future quantum systems."
The related research was published on June 7 in Nature Communications under the title All-silicon quantum light source by embedding an atomic emissive center in a nanophotonic cavity.
However, the most 'scalable' optoelectronic material, silicon, does not produce classical or quantum light directly and efficiently on a chip. Therefore, scaling and integrating it has become the most fundamental challenge facing quantum science and technology.
Since the late 1970s, many promising single-photon emitting quantum devices have been demonstrated for use in quantum cryptography. They include frontiers in materials science such as quantum dots, color centers in wide band gap materials, nonlinear crystals and "atomic vapor cells".
Despite decades of research, the quantum light sources that power the quantum Internet have yet to emerge as clear winners.
It was only in the last two years that individual centers in silicon were isolated. Only since then have emission centers in silicon been coupled to waveguides, and later, centers have been integrated into ring resonators. Recently, efforts are being made to fabricate such high probability controllable single-emission centers in silicon. However, deterministic single-photon sources based on silicon emission centers remain elusive due to the lack of controlled fabrication methods and the complexity of material interfaces after device fabrication.
Schematic diagram of a silicon wafer (left) and a schematic diagram of an all-silicon photonic crystal cavity containing a single atomic emission center (right).
This time, Kanté et al. developed an all-silicon quantum light source based on atomic emission centers in a silicon nanophotonic cavity. The centers were fabricated in silicon-on-insulator (SOI) on insulator substrate with controlled density and preferred dipole orientation, which increases their overlap probability with the designed nanophotonic cavity. They demonstrate the successful alignment of the quantum defect and the nanophotonic cavity dipole moment, and tune the nanophotonic cavity to resonate with the zero-phonon line overlap of the silicon-based quantum defect. Ultimately, an enhancement of the luminescence intensity by more than 30 times, an atom-cavity coupling efficiency close to 1, and an acceleration of the single-photon emissivity by 8 times were achieved. The results open the door to large-scale integration of all-silicon quantum optical devices and systems for applications in quantum communications and networking, sensing, imaging and computing.
The silicon quantum light source developed by the UC Berkeley, or Berkeley Lab, team is the first experimental work to demonstrate the integration of a single silicon atomic emission center, called a G-center, directly into a silicon nanophotonic cavity, Kanté said. "In this work, we succeeded for the first time in embedding an atomic-sized silicon atomic defect into a silicon photonic cavity less than one-tenth the size of a human hair (1 micron). The cavity forces the atoms to be brighter and emit photons at a faster rate. These are the necessary ingredients for future (quantum) Internet scalable quantum light sources."
The proposed all-silicon atomic cavity system shown in Figure 1a consists of a single defect in silicon embedded in a photonic crystal (PhC) defect cavity. the PhC cavity consists of three missing holes in a suspended triangular hole lattice. The atomic defects (i.e., G-centers in silicon) consist of two replacement carbon atoms (black spheres) bound to the same silicon self-interstitial (blue spheres).
The fabrication process starts with the injection of carbon (13C) at an energy of 36 keV into a commercially available 230 nm thick wafer on insulator (SOI). The implantation is followed by electron beam lithography, dry etching, thermal annealing and wet etching. Rapid thermal annealing is an important step for the broad luminescence of the W and G centers induced by the thermally cured dry etching process. Secondary ion mass spectrometry (SIMS) measurements show that the carbon and atomic centers implanted during the annealing process are located in the middle of the silicon layer.
Figure 1: Individual atomic emission centers embedded in a silicon-photonic cavity. a The silicon quantum interface with "atomic defects" is located in a triangular photonic crystal (PhC) with three "photonic defect" cavities with missing holes. The atomic defects are G-centers in silicon, consisting of two substituted carbon atoms (black spheres) bound to the same silicon self-filling gap (blue spheres). The red arrow indicates the direction of the dipole moment of the G-center, which is one of the various emission centers recently observed in silicon, the most scalable optoelectronic material. The calculated electromagnetic pattern of the cavity is superimposed on the sketch of the PhC, demonstrating the high confinement of the electromagnetic field in the missing cavity region of the triangular lattice. The cavity is fabricated so that its dipole moment is co-linear with the dipole moment of the defect. The electric field intensity peaks at the center of the cavity and decays exponentially in the body of the PhC. b Energy level diagram of the G-center in silicon, including the ground state single-linear state, the dark excited triplet state and the excited single-linear state. The cavity can be tuned to resonate with the radiative leap between the excited and ground state single-line states to enhance the light-matter interaction. c Scanning electron microscope (SEM) image of a fabricated silicon-based atomic cavity system suspended in air. Successful embedding of a single G-center in a photonic cavity involves the use of a commercially 230 sequence of controlled fabrication steps nm-thick silicon on insulator (SOI) wafers, carbon injection followed by electron beam lithography, dry etching, thermal annealing, and wet etching. The fabrication steps compatible with standard complementary metal oxide semiconductor (CMOS) processes are optimized to increase the likelihood of monochromatic centers in the cavity.
The dipole moment of the center is calculated by density flooding theory as shown by the red arrow inserted in Figure 1a. G centers are one of the recently observed wide diversity of silicon emission centers and their electronic structures, as shown in Figure 1b, including ground mono-states, dark excited ternary states, and excited mono-states. The computational electromagnetic pattern used for the lateral electrodepolarization of the cavity is superimposed on the sketch of the PhC, demonstrating the high confinement of the electromagnetic field in the missing hole region in the triangular lattice. This polarization matches the direction of the atomic defect dipole moment. Deterministic localization of atomic-scale defects in photonic cavities has been challenging for most platforms and has not yet been achieved for the silicon emission center - it requires not only the overlap of quantum defects with highly confined optical modes, but also the alignment of the dipole moments of the atoms and the cavity.
To increase the overlap probability of the platform, they first investigated scalable fabrication of a single emission center with controlled density and non-uniform broadening. (They determined an annealing time window under which only the central ensemble is created and all individual centers are destroyed outside of that time window.)
The team also found that the shorter annealing time within this window minimizes the non-uniform broadening of the quantum emitter zero-phonon line (ZPL), a key requirement for overlapping the ZPL with the designed nanophotonic resonance to enhance light-light-matter interactions. The controlled density and inhomogeneous broadening of the quantum center increases the probability of overlap with finite-size photonic crystal cavity arrays. The team then investigated the polarization response of the created emission centers, and the statistical analysis provided in Supplementary Note 1 shows the preferred orientation of the emitters in silicon. The PhC cavities were then fabricated so that the dipole moments of the cavities and centers were aligned. Figure 1c shows a scanning electron microscopy (SEM) image of the artificial silicon-based atomic cavity system. The volume of the embedded cavity mode is 0.66(λcav/n).3 The successful embedding of a single center into the cavity involved a controlled sequence of CMOS-compatible fabrication steps.
Figure 2a shows a photoluminescence (PL) grating scan of the device at 4 K temperature from the center of color within the cavity boundary. The dashed white line indicates the boundary of the finite PhC, and the suspended PhC is surrounded by a silicon on insulator (SOI) wafer. The photoluminescence of the photonic device shown in Figure 2b exhibits a sharp peak at ~1275 nm and a blueshifted broader peak at ~1272 nm, corresponding to the color center and the ZPL of the cavity resonance, respectively. the photoluminescence of the cavity originates from the broad spectrum of the background center. The reflectance of the cavity was further characterized using the resonant scattering measurements in Figure 2c. The cavity is illuminated with a linearly polarized white light source (white arrow) at 45° relative to the cavity axis along the x-direction. The polarization signal perpendicular to the excitation (red arrows) was collected to probe the cavity mode and resonances were observed at ~1272 nm, in full agreement with the PL measurements. The reflectivity is equipped with a Fano resonance line shape with an intrinsic quality factor (Q) of 3209. The experimental value is comparable to the theoretical Q of 6000 and the difference is attributed to fabrication defects. Figure 2d shows the polarization diagram of the cavity mode detuned from the ZPL (orange) and the quantum emitter (black). The polarization is in excellent agreement with the dipole model (solid line) and has been successfully aligned.
Figure 2: Experimental characterization of the silicon-based quantum emitter and cavity. a Photoluminescence (PL) grating scan of the device with a single emitter in the cavity at 4K temperature. The boundary of the finite photonic crystal (PhC) is indicated by the white dashed line, and the suspended PhC is surrounded by a silicon on insulator (SOI) wafer. The photoluminescence signal shows bright emission from the color center within the boundary of the cavity. b The photoluminescence of the photonic device exhibits a sharp peak nm at ~1275 and a blueshifted broad peak nm at ~1272, corresponding to the zero phonon line of the color center and the cavity resonance, respectively. c The reflectance of the photonic crystal cavity obtained by resonant scattering measurements. The cavity is illuminated by a linearly polarized white light source (white arrow) at an angle of 45° with respect to the cavity axis along the x-direction. The polarization signal perpendicular to the excitation is collected (red arrow) to probe the cavity mode. d The polarization map of the cavity mode detuned from the ZPL is shown in orange. The polarization map of the quantum emitter alone is shown in black. The polarization is very consistent with the dipole model (solid line) and is well aligned.
In Figure 3a, the spectrum of the quantum emitter over a wide energy range shows the zero phonon line (ZPL) and its phonon side band at the center of the silicon emission. Figure 3b shows the spectrum of the quantum emitter using a high-resolution grating. the ZPL is located at 972.43 meV with a line width of 6.8 GHz (obtained after deconvolution with the spectrometer response function). To demonstrate that the bright emission from the middle of the cavity corresponds to a single emission center, the research team performed quantum coherence measurements of the emitters in the cavity. The autocorrelation measurements shown in Figure 3c were performed using a Hanbury-Brown and Twiss interferometer with a superconducting nanowire single-photon detector (see Supplementary Note 3). Second-order correlation measurements of cavity emission under continuous excitation exhibit anti-divergence, confirming the successful spatial overlap of a single silicon emission center with a nanophotonic cavity having an anti-divergence layer at zero delay g2(0) = 0.30 ± 0.07. The value at zero delay is mainly limited by the emission from the background center. Figure 3d shows the autocorrelation measurements at a repetition rate of 10 MHz under pulsed excitation.
The enhancement of a single center within the cavity requires spatial and spectral overlap. Spatial overlap is achieved in Figures 2 and 3. To achieve spectral overlap, the nanophotonic cavity is tuned using an argon injection cycle. The injected gas condenses on the PhC surface and modifies the effective index of the cavity mode, tuning the cavity to shift the resonance wavelength from ~1269 nm to ~1275 nm. In Fig. 4a, the photoluminescence is enhanced to a maximum at ~1275 nm as the cavity resonance shifts toward the quantum center of the ZPL, resulting in a spectral overlap. the ZPL intensity as a function of cavity detuning shows an enhancement of >30 at resonance (Fig. 4b). For cavity tuning from δ = 2.40 nm to δ = 0.00 nm, the excitation lifetime is reduced from 53.6 ns to 6.7 ns. Compared to the non-resonant case, an 8-fold reduction in lifetime is experimentally observed when overlap is achieved. The light-matter interaction in the cavity is typically quantified using the Purcell factor (Fp), which measures the decay rate enhancement of the atoms from free space to the cavity (γcav = Fpγ0). It can be estimated by Fp = (τbulk/τon-τbulk/τoff)/η, where τbulk is the lifetime of the quantum emitter outside the photonic crystal (dark yellow dot in Fig. 4c) and τoff is the lifetime of the 2.4 nm detuning. The lifetime measured at non-resonance is slightly longer than the lifetime in the bulk due to the reduced density of states in the PhC gap.
Figure 4: Spectral tuning of nanocavities and enhanced atom-cavity interactions. a The enhancement of individual centers in the cavity requires spatial and spectral overlap. The spatial overlap is achieved in Fig. 3 and the spectral overlap is achieved here by tuning the nanophotonic cavity. The cavity tuning is done as a function of the argon gas injection cycle. The gas injection changes the effective refractive index of the cavity mode and tunes the resonant wavelength of the cavity, which is shifted from ~1269 nm to ~1275 nm. As the cavity resonance shifts toward the quantum center of the ZPL, photoluminescence is enhanced, reaching a maximum at ~1275 where a spectral overlap is achieved. b Zero phonon line intensity as a function of cavity detuning. An enhancement of >30 is achieved at the resonance. cδ excitation lifetimes for cavity detuning = 2.40 nm, δ = 0.23 nm and δ = 0 nm. The lifetime of the emitted photon was shortened from 53.6 ns to 6.7 ns. The instrumental response function of the pulsed laser is also provided in Supplementary Note 3. Compared to the non-resonant case, an 8-fold reduction in lifetime was experimentally observed when overlap was achieved. These results constitute the first all-silicon quantum light source to use a silicon emission center in a cavity, and the center can be further accelerated by designing a cavity with a higher quality factor and a more deterministic positioning method to further improve the spatial overlap of the emission cavity. The results will enable scalable quantum optics for all silicon quantum optical interfaces with silicon emission centers.
The team further confirmed that the Purcell enhancement is due to cavity-emitter interactions and presents a dysregulation-dependent Purcell factor. The percentage of photons emitted at the ZPL wavelength of the emitter (Debye-Waller) is η = 15%, which is measured by comparing the count rate with and without the ZPL bandpass filter. The experimental Purcell factor for the defected cavity is Fp ~ 29.0. The coupling efficiency of the center-to-cavity mode (β-factor) can be estimated as 1/τon/[1/τon + 1/τoff], yielding a value of β ~ 89%. The emission of the ZPL from unenhanced centers is about 700 c/s. They measured over 30 Purcell enhancements with a count rate of 20,000 c/s. Enhancement of single centers was observed in several other cavities (see Supplementary Note 2). Systematic enhancement of single emitters in cavities was observed, but there was no direct correlation between enhancement and Q-factor, since the localization of centers in cavities is probabilistic. The lifetime reduction and Purcell acceleration observed in its single-center operation suggest close to unit quantum efficiency. The mechanism leading to the formation of various single G-centers from tethers is currently an open question.
As a result, they developed an all-silicon quantum light source based on atomic emission centers in silicon-based nanophotonic cavities. The quantum center was fabricated directly in silicon using a series of complementary metal oxide semiconductor (CMOS)-compatible nanofabrication steps that simultaneously control the implantation depth, density, nonuniform broadening, and dipole orientation of the emitter. The control of these parameters enables the successful embedding of atomic defects into silicon photonic crystal defect nanocavities. The nanocavities are spectrally tuned using gas condensation to overlap with the resonance of the zero phonon line of the quantum emitter, thereby enhancing the light-matter interaction.
This research work will enable all-silicon quantum optical interfaces for scalable and integrated quantum optoelectronics.
The Molecular Foundry at Lawrence Berkeley National Laboratory. From left to right: Boubacar Kanté, Christos Papapanos, Kaushalya Jhuria, Walid Redjem, Thomas Schenkel, Wayesh Qarony, Vsevolod Ivanov, Yertay Zhiyenbayev, Wei Liu, Liang Tan, Prabin Parajuli, and Scott Dhuey
Using silicon is a bit counterintuitive," said Walid Redjem, a postdoctoral researcher in Kanté's team. Because silicon is what is known as an indirect bandgap semiconductor, this means it is not conducive to luminescence. For example, there are no efficient lasers that use silicon."
But it turns out that the above limitations only apply to classical light sources. "That's not a problem for quantum light sources," Kanté says. He and his team are already hard at work to further refine their all-silicon quantum light source.
Reference links:
[1] https://engineering.berkeley.edu/news/2023/06/new-all-silicon-quantum-light-source-developed-by-berkeley-researchers/
[2]https://www.nature.com/articles/s41467-023-38559-6
[3] https://eecs.berkeley.edu/news/2023/06/boubacar-kante-and-eecs-researchers-develop-all-silicon-quantum-light-source
