Periodic Reporting for period 2 - NANOMEQ (Nano-mechanical quantum photonic circuits)
Reporting period: 2022-07-01 to 2023-12-31
Photonic integrated circuits constitute a promising platform to develop quantum information processing units, capable of running quantum algorithms at the physical level. Unlike other physical realizations of qubits, photons provide an excellent carrier of quantum information that can travel over long distances, enabling secure communication between distant parties. As the classical Internet is currently heavily relying on optical technologies, it is expected that quantum communication based on quantum light, will soon impact the way we communicate, securing our digital transactions, identity and private conversations. Yet, much needs to be done to develop integrated photonic processors capable of producing and manipulating many single photons with sufficiently low loss, low noise and high speed. The ambitious goal of NANOMEQ is to develop the key hardware to build such advanced quantum photonic resources in an efficient way.
The project investigates a novel class of integrated devices for controlling quantum light at the nanoscale. In traditional quantum photonic integrated circuits, the light is controlled using thermal devices, which are slow and incompatible with quantum emitters or detectors, which operate at cryogenic temperatures of a few degrees Kelvin above the absolute zero. To gain speed and cryogenic compatibility, NANOMEQ sets out to develop nano-opto-electromechanical systems, or NOEMS in short, which employ nanometer-size motion to achieve optical control over the photonic qubits and the relevant light-matter interaction processes. This new approach allows us to co-integrate quantum emitters, such as quantum dots, directly with the photonic quantum gates, with unprecedented system efficiency.
The project develops over three main areas:
1) Implementing a universal, reconfigurable, and on-chip unitary gate,
2) Controlling light-matter interaction with quantum emitters in a chip to scale the number of photon sources in the chip, and
3) Performing frequency conversion to produce photons at the telecom wavelength, for long distance communications.
In all three areas, the key functionality will be provided by nanomechanical devices, which provide an ultra-compact, low-power, low-loss, and novel approach to programmable photonic circuits. By integrating the quantum emitters with the quantum logic gates, the project aims at demonstrating small-scale quantum photonic circuits, which can be readily used for entanglement-based communication, or, in the longer run, as resources for quantum computing.
The project investigates a novel class of integrated devices for controlling quantum light at the nanoscale. In traditional quantum photonic integrated circuits, the light is controlled using thermal devices, which are slow and incompatible with quantum emitters or detectors, which operate at cryogenic temperatures of a few degrees Kelvin above the absolute zero. To gain speed and cryogenic compatibility, NANOMEQ sets out to develop nano-opto-electromechanical systems, or NOEMS in short, which employ nanometer-size motion to achieve optical control over the photonic qubits and the relevant light-matter interaction processes. This new approach allows us to co-integrate quantum emitters, such as quantum dots, directly with the photonic quantum gates, with unprecedented system efficiency.
The project develops over three main areas:
1) Implementing a universal, reconfigurable, and on-chip unitary gate,
2) Controlling light-matter interaction with quantum emitters in a chip to scale the number of photon sources in the chip, and
3) Performing frequency conversion to produce photons at the telecom wavelength, for long distance communications.
In all three areas, the key functionality will be provided by nanomechanical devices, which provide an ultra-compact, low-power, low-loss, and novel approach to programmable photonic circuits. By integrating the quantum emitters with the quantum logic gates, the project aims at demonstrating small-scale quantum photonic circuits, which can be readily used for entanglement-based communication, or, in the longer run, as resources for quantum computing.
During the first period, we have successfully implemented the first building blocks of a scalable quantum photonic platform with integrated quantum emitters in the Gallium Arsenide (GaAs) material platform and demonstrated some initial application of the technology developed in the project.
The devices have been fabricated in our cleanroom at the Niels Bohr Institute (Denmark), on GaAs wafers grown epitaxially by our collaborators at the University of Bochum (Germany).
A large effort has been devoted in investigating the design and fabrication process of flexible suspended nanophotonic devices, which are key in the NOEMS technology. A major hurdle in this sense was to avoid mechanical failure after fabrication, given the very small nano-scale dimensions involved. We have therefore developed a novel fabrication process that integrates thin metallic strips to stiffen the nanostructures and developed numerical models to simulate the mechanical behaviour of devices prior to fabrication. Given the increasing complexity of the circuits and devices, we have developed a computer program to automatize the design of the chips. Such code is publicly available on the GitHub platform. Based on this preliminary work, we have recently designed and fabricated a compact nanomechanical phase shifter, which is currently being characterized (results are not publicly available yet).
The optical characterization of the chips is performed at low temperature (~5 K) on a specially designed cryostation that enables direct coupling between the chip and optical fibers for long-distance communication. The cryostation was designed, purchased and installed during the first two years and it is now fully operational.
Another major achievement is the realization of the first integrated chip which allows us to control and drive two quantum dots simultaneously. We have shown the possibility to adjust the emission wavelength of the emitted photons and perform two-photon interference from two dots within the same circuit with (79 ± 2)% visibility, which is currently only limited by fabrication imperfections, but expected to improve before the end of the project. These results are essential towards developing entanglement-based communication schemes and the first steps in scaling up the sources further.
To achieve long-distance communication, we have demonstrated a frequency-conversion scheme based that converts the photons from our chips to telecommunication wavelength (1550 nm) with >50% efficiency, where optical fibers have the lowest propagation loss. This was used, in collaboration with colleagues at the Technical University of Denmark (DTU), to demonstrate a quantum-secured communication link between two labs in the Copenhagen metropolitan area. The photons generated from our chips are coupled with high efficiency into an 18 km field-installed telecom fiber leading to a ~3 kbit/s secret key rate, suitable for encryption of, e.g. short text. This is an important step forward to the development of secure communication infrastructure based on the laws of quantum mechanics.
The devices have been fabricated in our cleanroom at the Niels Bohr Institute (Denmark), on GaAs wafers grown epitaxially by our collaborators at the University of Bochum (Germany).
A large effort has been devoted in investigating the design and fabrication process of flexible suspended nanophotonic devices, which are key in the NOEMS technology. A major hurdle in this sense was to avoid mechanical failure after fabrication, given the very small nano-scale dimensions involved. We have therefore developed a novel fabrication process that integrates thin metallic strips to stiffen the nanostructures and developed numerical models to simulate the mechanical behaviour of devices prior to fabrication. Given the increasing complexity of the circuits and devices, we have developed a computer program to automatize the design of the chips. Such code is publicly available on the GitHub platform. Based on this preliminary work, we have recently designed and fabricated a compact nanomechanical phase shifter, which is currently being characterized (results are not publicly available yet).
The optical characterization of the chips is performed at low temperature (~5 K) on a specially designed cryostation that enables direct coupling between the chip and optical fibers for long-distance communication. The cryostation was designed, purchased and installed during the first two years and it is now fully operational.
Another major achievement is the realization of the first integrated chip which allows us to control and drive two quantum dots simultaneously. We have shown the possibility to adjust the emission wavelength of the emitted photons and perform two-photon interference from two dots within the same circuit with (79 ± 2)% visibility, which is currently only limited by fabrication imperfections, but expected to improve before the end of the project. These results are essential towards developing entanglement-based communication schemes and the first steps in scaling up the sources further.
To achieve long-distance communication, we have demonstrated a frequency-conversion scheme based that converts the photons from our chips to telecommunication wavelength (1550 nm) with >50% efficiency, where optical fibers have the lowest propagation loss. This was used, in collaboration with colleagues at the Technical University of Denmark (DTU), to demonstrate a quantum-secured communication link between two labs in the Copenhagen metropolitan area. The photons generated from our chips are coupled with high efficiency into an 18 km field-installed telecom fiber leading to a ~3 kbit/s secret key rate, suitable for encryption of, e.g. short text. This is an important step forward to the development of secure communication infrastructure based on the laws of quantum mechanics.
The main advancement, so far, compared to the state-of-the-art was the demonstration of the integration and simultaneous control of two emitters within the same chip. Previously, such control was only demonstrated between remote quantum dots from distant cryostats. Achieving control on the same chip is an important milestone in further scaling to higher number of emitters that can all be integrated in a single processing unit and thus constitutes an important goal of NANOMEQ. By the end of the project, we expect to demonstrate pulsed resonant excitation of the quantum dots to measure their degree of indistinguishability, which is an essential feature to produce entangled photons. It would also allow us to potentially scale the architecture to an even larger number of emitters, by following the same architecture and control scheme.
While the characterization is still in progress, the first phase shifters and nanomechanical devices implemented in the project show remarkable stability and low noise. In particular, the use of metal strips to reinforce the nanostructure has allowed us to fabricate ultra-compact phase shifters, with less than 15 um linear footprint, among the smallest fabricated so far. In comparison, thermo-optic phase shifters are commonly 10-20 times larger. We expect to demonstrate soon a full phase rotation over 2π, and, consequently, to realize a fully ultra-small programmable unitary gate, as originally planned.
In the second half of the project, we expect to be able to combine the two quantum dots directly with the unitary gate, which constitutes an important building block towards measurement device-independent quantum key distribution schemes. Furthermore, the frequency conversion will be implemented directly in GaAs, with the goal of increasing the total conversion efficiency beyond the current state of the art.
While the characterization is still in progress, the first phase shifters and nanomechanical devices implemented in the project show remarkable stability and low noise. In particular, the use of metal strips to reinforce the nanostructure has allowed us to fabricate ultra-compact phase shifters, with less than 15 um linear footprint, among the smallest fabricated so far. In comparison, thermo-optic phase shifters are commonly 10-20 times larger. We expect to demonstrate soon a full phase rotation over 2π, and, consequently, to realize a fully ultra-small programmable unitary gate, as originally planned.
In the second half of the project, we expect to be able to combine the two quantum dots directly with the unitary gate, which constitutes an important building block towards measurement device-independent quantum key distribution schemes. Furthermore, the frequency conversion will be implemented directly in GaAs, with the goal of increasing the total conversion efficiency beyond the current state of the art.