Researchers demonstrate new type of laser
Lasers
Lasers are unique due to the emission of perfectly synchronised coherent light. This means that the linewidth (which corresponds with the colour spectrum) can be very narrow. A typical laser consists of a large number of emitters (such as atoms, molecules, or charge carriers in semiconductors) in a fabricated cavity. Such conventional lasers are often inefficient and generate a lot of heat. This makes it difficult to combine them in low temperature applications, such as quantum technology.
Superconducting Josephson junction
In 1911, the Dutch physicist Heike Kamerlingh Onnes discovered that certain materials at low temperatures transition into a superconducting state in which electrical current experiences no loss. One of the most important applications of superconductivity is the Josephson effect: if a piece of superconducting material is very briefly interrupted then the charge carriers can tunnel through that barrier in accordance with the laws of quantum mechanics. This happens with a very specific frequency that depends on an externally applied direct voltage. The Josephson junction is therefore perfect for the conversion of voltage into light (frequency).
QuTech researchers developed an on-chip microwave laser based on a fundamental aspect of superconductivity: the AC Josephson effect. On the chip there is a nanoscale Josephson junction connected to a superconducting cavity. When a small direct voltage from a battery is applied across the junction, Cooper pairs tunnel through the junction as a result of which microwave light is released. The superconducting cavity ensures an amplification of this light. This gives rise to a coherent beam of microwave light that is ultimately emitted by the chip. Such a chip is highly promising for applications such as a scalable quantum computer.
Josephson junction laser
The QuTech researchers produced a single Josephson junction in a very precisely produced superconducting microcavity, smaller than an ant. In this case the Josephson junction functions as a single atom, whereas the microcavity functions as two mirrors for microwave light. By applying a small direct voltage across the Josephson junction, microwave particles arise with a wavelength that coincides with the microcavity. While the light particles resonate between the superconducting mirrors, the Josephson junction is forced to generate more light particles that are synchronous with the light particles in the microcavity. When they cooled the chip to ultralow temperatures (<1 Kelvin) the researchers observed a coherent beam of microwave light at the exit of the microcavity when they applied a small voltage. The on-chip laser, which consists entirely of superconducting material, is very energy efficient and far more stable than the previously demonstrated semiconductor lasers. Less than a picoWatt is needed to produce light, which is more than 100 billion times less energy than a light bulb needs.
Quantum control
Efficient, coherent microwave sources are vital for all current proposals for the quantum computer. Microwave light is used for the readout and transfer of quantum information, for the correction of errors and for controlling individual quantum particles. Unlike the current expensive and inefficient microwave sources, this Josephson junction laser is not only efficient but also easy to control and to modify due to the design of the chip. The researchers are now expanding the design to include tuneable Josephson junctions made from nanowires so that short pulses can be generated to quickly control several quantum particles. In the future, such a chip could be used for so-called 'amplitude-squeezed' light with even smaller intensity fluctuations. That is important in most of the quantum communication protocols proposed. This work is an important step towards realising large-scale quantum control for quantum information technology.
This work was carried out by QuTech and the Kavli Institute of Nanoscience at Delft University of Technology. It was funded by the collaboration between FOM and Microsoft in the area of Topological Quantum Computation and by the European Research Council (ERC).
Paper: Demonstration of an AC Josephson junction laser. M. C. Cassidy1, A. Bruno1, S. Rubbert2, M. Irfan2, J. Kammhuber1, R. N. Schouten1,2, A. R. Akhmerov2, L. P. Kouwenhoven1,2. (DOI 10.1126/science.aah6640)
1QuTech, Delft University of Technology, PO Box 5046, 2600 GA Delft, the Netherlands.
2Kavli Institute for Nanoscience, Delft University of Technology, PO Box 5046, 2600 GA Delft, the Netherlands.
Contact
Dr M. Cassidy, QuTech TU Delft, +61 47 905 7729
Dr J. Cramer, Outreach Coordinator QuTech TU Delft, +31 6 2492 8665