Acquisition of a cryostat for the photonics platform

The Niels Bohr Institute is part of the Faculty of SCIENCE at the University of Copenhagen. The Niels Bohr Institute has a 100 year long tradition of excellence within science, in particular within the area of quantum physics. The institute was inaugurated on March 3, 1921, by Professor Niels Bohr. We are approx. 430 researchers, 70 administrative and technical staff, 100 PhD students and 400 students. The European Physical Society has declared the Niels Bohr Institute a Historic Site of great international importance for the development of physics and research.

The cryostat will be part of an experimental setup intended to enable development system will be part of an experimental setup intended to enable development of high-performance quantum photon emitters for fault tolerant quantum computing hardware based on photons. Initially the cryostat will house the quantum dots used to generate entangled photons with high-quality spin-photon interfaces. It later might also accommodate superconducting and gate-defined quantum dot devices for transduction of solid-state qubits and microwave photons into optical photons, a key goal for NQCP, as well as on-chip photonic circuits for qubit operations and single photon detectors. The cryostat must have low noise DC and RF lines to control spins in quantum dots and superconducting qubits, a >3T vector magnet to tune the quantum dot energy levels, optical fibers for laser access and photon collection, good radial and axial optical access, and a fast exchange sample chamber to enable efficient device characterization routines and fast transitions between Faraday and Voigt geometry. To improve results previously achieved by the HyQ Photonics group at NBI and to be compatible with superconducting and gate-defined solid-state qubits, we require the cryostat to reach 10-100 mK temperatures.

With the requirements for our application listed in 1) in mind, Bluefors was the only company we identified which can provide free-space optical access from the bottom (required to minimise axial path length to sample) and from the side (radially) with a large hole diameter (>25 mm) as standard options. Furthermore, they offer fiber access access in the presence of a vector magnet. These are all important to operate optical devices in the various configurations we target (Voigt and Faraday geometries, and fiber-coupled devices).

As part of their standard selection, Bluefors also offers integration of electronic components (QFilters from industry leader QDevil), which are important to achieve a low noise level on our RF signals. They also offer an off-the-shelf 5-1-1 T vector magnet compatible with our requirements.

Moreover, at NBI there is already experience with optical accessible dilution fridge in the Schliesser’s laboratory, where a Bluefors system is used. Being able to leverage on such pre-existing experience is key to having confidence in the quality and suitability of this equipment, as well as for ensuring good maintenance of the device.

In addition, strong compatibility with other hardware platforms developed in NQCP and NBI is key, as targeting transduction and interconnection between platforms within NQCP is a central goal for purchasing this equipment. For example, the superconducting qubits team at NQCP, led by integrator Kjaergaard, uses exclusively Bluefors systems. Hence, purchasing from Bluefors is also key to ensuring compatibility.

Other key capabilities provided by Bluefors that are key in our choice are: a fully welded heat exchanger, oil free pump and compressor system, and a membrane free compressor, which all reduces the risk of failures (3 years of guarantee without required service.

As described in 1), this cryostat will be used to improve the state-of-the-art photon entangling with a quantum dot done by the Quantum Photonics group at NBI, and thereby contribute to the development of a fault tolerant quantum computer.