Periodic Reporting for period 2 - aCryComm (attojoule Cryogenic Communication)
Periodo di rendicontazione: 2021-10-01 al 2023-09-30
All envisaged practical implementations of cryogenic processors, including both quantum computers (QCs) and classical processors based on single flux quantum (SFQ) signals, require massive data transfer from and to classical high performance computers (HPCs). The EU-funded aCryComm project aims to develop building blocks for cryogenic photonics interconnects and eventually enable this challenging data transfer. The long-term goal is the development of an open-access platform to integrate classical optical interfaces based on low-loss silicon photonics, plasmonics, and nano light sources together with superconducting photonic and electronic devices, including SFQ-based co-processors for HPCs and for QCs. These developments will lead to more efficient, more powerful and greener supercomputers to help tackling relevant problems e.g. in modelling climate change or developing new drugs.
VTT has been adapting the SFQ fabrication process to the new process for Josephson junction (JJ) fabrication adopted in recent years. Meanwhile, VTT, ETH, and Polariton have been studying possible interfaces that will allow to drive plasmonic modulators with SFQ (EO conversion) despite the large mismatch in energy per bit and operation voltage.
On the electro-optic conversion side, cryogenic characterisations of the plasmonic modulators has been delayed by the long delivery time of the cryostat ordered by ETH, but could be conducted in 2022. Besides the MZM validation with an electro-optic bandwidth above 100 GHz, low on-off and driving voltages, data transmission experiments proved data rates up to 128 Gbit/s, all at 4 K. These results were presented at the European Conference on Optical Communication (ECOC 2022). Further, a second generation of plasmonic devices for evaluation at 20 mK were developed and fabricated.
In parallel, TAU has been developing their alternative approach based on high efficiency microcavity light sources following two paths: vertical cavity surface emitting lasers and quantum dot nano light sources. A remarkable breakthrough is the development of quantum dots for operation in the telecom wavelength range around 1550 nm. Also TAU have been setting up a cryostat for characterization of the fabricated light sources. The development by TAU and KTH of light sources modulated by surface acoustic waves shows promising preliminary results, and following fabrication runs are on-going.
On the opto-electric conversion front, we have been studying ultra-short SNSPD devices, aiming at higher speed, trying to tackle the well-known latching problem. This is a necessary first step to then move to plasmonically enhanced SNSPDs, which are under fabrication.
In order to achieve ultra-high speeds, we have also been developing more traditional plasmonic detectors. We have reported on a free-space illuminated photodetector based on a metamaterial-graphene architecture. We have demonstrated that these devices have at room temperature a flat frequency response exceeding 500 GHz – making these devices the fastest photodetectors to date. These results have been published in Science. We have further also characterized the devices for cryogenic operation. We find that the photodetectors retain their high speed of >100 GHz (measurement setup limitation) and further also show an improved responsivity when cooled to 4 K. The devices operate without any external bias, making them a promising candidate to serve as in-cryo electrical high-speed electrical signal generators. We validate the devices further in a data communication scenario both at room temperature (presented at ECOC 2022) and cryogenic temperatures (presented at ECOC 2023).
In parallel, we have been developing packaging and assembly technologies for the future demonstrators, to allow both spatial division multiplexing and wavelength division multiplexing, therefore addressing the bandwidth/footprint targets of the project. In addition to the ongoing development of sources (using OE conversions), modulators, and multiplexers, a fully fiber coupled electro-optic (EO) broadband detection system working at cryogenic temperatures has been developed at PTB. Apart from the broad band detection capability, this detection system also ensures reduced heat insertion into the cryogenic environment and reduced high-frequency signal distortion. These developments also paved the way to new concrete exploitation plans.
On the electro-optic conversion side, cryogenic characterisations of the plasmonic modulators has been delayed by the long delivery time of the cryostat ordered by ETH, but could be conducted in 2022. Besides the MZM validation with an electro-optic bandwidth above 100 GHz, low on-off and driving voltages, data transmission experiments proved data rates up to 128 Gbit/s, all at 4 K. These results were presented at the European Conference on Optical Communication (ECOC 2022). Further, a second generation of plasmonic devices for evaluation at 20 mK were developed and fabricated.
In parallel, TAU has been developing their alternative approach based on high efficiency microcavity light sources following two paths: vertical cavity surface emitting lasers and quantum dot nano light sources. A remarkable breakthrough is the development of quantum dots for operation in the telecom wavelength range around 1550 nm. Also TAU have been setting up a cryostat for characterization of the fabricated light sources. The development by TAU and KTH of light sources modulated by surface acoustic waves shows promising preliminary results, and following fabrication runs are on-going.
On the opto-electric conversion front, we have been studying ultra-short SNSPD devices, aiming at higher speed, trying to tackle the well-known latching problem. This is a necessary first step to then move to plasmonically enhanced SNSPDs, which are under fabrication.
In order to achieve ultra-high speeds, we have also been developing more traditional plasmonic detectors. We have reported on a free-space illuminated photodetector based on a metamaterial-graphene architecture. We have demonstrated that these devices have at room temperature a flat frequency response exceeding 500 GHz – making these devices the fastest photodetectors to date. These results have been published in Science. We have further also characterized the devices for cryogenic operation. We find that the photodetectors retain their high speed of >100 GHz (measurement setup limitation) and further also show an improved responsivity when cooled to 4 K. The devices operate without any external bias, making them a promising candidate to serve as in-cryo electrical high-speed electrical signal generators. We validate the devices further in a data communication scenario both at room temperature (presented at ECOC 2022) and cryogenic temperatures (presented at ECOC 2023).
In parallel, we have been developing packaging and assembly technologies for the future demonstrators, to allow both spatial division multiplexing and wavelength division multiplexing, therefore addressing the bandwidth/footprint targets of the project. In addition to the ongoing development of sources (using OE conversions), modulators, and multiplexers, a fully fiber coupled electro-optic (EO) broadband detection system working at cryogenic temperatures has been developed at PTB. Apart from the broad band detection capability, this detection system also ensures reduced heat insertion into the cryogenic environment and reduced high-frequency signal distortion. These developments also paved the way to new concrete exploitation plans.
Ultrafast plasmonic photodetectors and modulators operating at cryogenic temperatures are a major step beyond the state of the art. In addition, the consortium has demonstrated an ultrafast sampling oscilloscope operating at cryogenic temperatures. All of these breakthroughs demonstrate the opportunities of combining optical and electronic technologies at cryogenic temperatures. The aCryComm project studies technologies at an early level of readiness, but eventually we expect that optical communication will enable a step change in the scalability and energy-efficiency of cryogenic technologies. For example, the power consumption per qubit is intolerable in present superconducting quantum computers, largely due to heat conduction from electrical cabling. The ultimate impact will be on high performance computing, which we expect to lead to significant benefit to major problems of our society, including modelling of climate changes and development of drugs. We have also identified the possible application of aCryComm developments to boost quantum key distribution systems, which will support the development of the European quantum communication infrastructure.