Research & Facilities
The MQO group investigates fundamental physics and develops quantum applications with mesoscopic-sized quantum states of light, all underpinned by enabling technology. Mesoscopic quantum optical states comprise 10s, 100s and 1000s of nonclassically-distributed photons. From a fundamental perspective, they can be used to explore quantum physics at energy scales that are visible to the human eye. In addition, this scale is crucial for demonstrating a genuine quantum enhancement over classical schemes in many areas including metrology, computation and communication.
To study these systems, we use integrated optical circuits based on titanium in-diffused waguides in lithium niobate, in combination with superconducting detectors. This means that much of the nonlinear optical properties of lithium niobate must be re-optimised to function in a low-temperature environment.
In this context, the research of the MQO group can be divided into three interrelated areas: detector architectures for mesoscopic quantum optics, fundamentals of low-temperature nonlinear optics and device development for low-temperature integrated optical quantum technologies.
Our research fields
As the number of photons in quantum optical systems increases, suitable detectors need to be built and characterised to measure these systems. We develop integrated detector solutions based on in-line arrays of superconducting detectors, as well as other detector multiplexing strategies, to measure bright photonic states. Furthermore, we develop tools to characterise the outputs of such detectors in a way that reveals the quantum optical characteristics of the detected light.
When combining integrated nonlinear optics with superconducting components, these devices must be run at very low temperatures. We investigate how the nonlinear optical properties of lithium niobate behave at a temperature approaching absolute zero, and develop functional components that function reliably under these challenging operating conditions. This includes the necessary packaging of such devices, which is the efficiect, low-temperature compatible interfacing of the integrated device and optical fibre.
A lot of experiments in our group require measurements on single photons, the quantum particles of light. Over the last decades, a new type of single-photon detectors based on the breakdown of superconductivity in a material has opened up the field for new and exciting quantum experiments due to their high efficiency and low dark counts. In our closed-cycle helium sorption fridge from Photonspot, we can operate these detectors at sub-1-Kelvin-temperatures. In addition, we can use the cryostat to investigate new detectors and photonic integrated circuits.
To integrate on-chip optical components with the detectors, are we interested in realising a variety of optical components that operate at cryogenic temperatures. The optical components are made on lithium niobate chips and their function ranges from frequency converters over directional couplers to active devices like electro-optical phase shifters. A second closed-cycle cryostat enables us to test these devices in a temperature range from room temperature to about 4 Kelvin. For the optical access we can couple the light into the cryostat with free-space access via lenses which enables us to test multiple devices on a single chip. In comparison to the Photonspot cryostat, this cryostat cannot reach sub-1-Kelvin-temperatures to operate superconducting single photon detectors (SNSPDs). This Cryostat from Attocube enables to make the main optimisation for our optical components for the operation at cryogenic temperatures. Afterwards, we can integrate the passive and active components together with SNSPDs.
To build integrated quantum circuits a variety of passive and active components need to be developed and fabricated. The patterning of these structures is realized with your laser lithography system. Therefore, a UV-sensitive photoresist is deposited and patterned by the 375 nm Laser of our system. The application of direct laser writing down to 300 nm small structures opens a broad window of possibilities for fabrication.
