Research Bits: May 30

Improving qubits
Researchers from QuTech say they have improved the ‘Andreev spin qubit’ by taking the two most promising qubits — the spin qubits in semiconductors and transmon qubits in superconducting circuits — and finding a hybrid way that uses the best of both qubit types. “Spin qubits are small and compatible with current industrial technology, but they struggle with interacting over long distances,” said Marta Pita-Vidal, co-first author, in a press release. “On the other hand, transmon qubits can be controlled and read out efficiently over long distances, but they have a built-in speed limit for operations and are relatively large. The researchers in this study aim to harness the advantages of both types of qubits by developing a hybrid architecture that combines them.”

The team of researchers embedded the Andreev spin qubit in a superconducting transmon qubit, and found the qubits had strong coherent qubit–qubit coupling — crucial for a hybrid architecture that has superconducting and semiconductor qubits. They were also able to control the interaction. “We demonstrated the first direct strong coupling between a spin qubit and a superconducting qubit, meaning that they could get the two qubits to interact in a controlled way. This suggests that the Andreev spin qubit can become a key element to interconnect quantum processors based on radically different qubit technologies: semiconducting spin qubits and superconducting qubits,” said Arno Bargerbos, co-first author.

The next step is to demonstrate multi-qubit operations and improve the coherence time.

QuTech is a collaboration between the Delft University of Technology, the Netherlands, and the Netherlands Organisation for Applied Scientific Research (TNO), founded in 2015. Other team members came from institutions in Europe and the U.S.

Pita-Vidal, M., Bargerbos, A., Žitko, R. et al. Direct manipulation of a superconducting spin qubit strongly coupled to a transmon qubit. Nat. Phys. (2023). https://doi.org/10.1038/s41567-023-02071-x

Microwave qubits entangling with optical photons
A new way for entangling microwave and optical photons may be key to overcoming a major obstacle to the quantum internet. Researchers at the Institute for Science and Technology Austria in Klosterneuburg have found a way to entangle microwave photons with optical photons by pumping a lithium niobate optical resonator that was part of a microwave resonator with a high-power laser at telecom wavelengths. They filtered out most of the returned light and found that one photon per pulse split into two entangled photons — one microwave and the other at a wavelength just slightly longer than the pump photons.

“We verified this entanglement by measuring the covariances of the two electromagnetic field fluctuations. We found microwave-optical correlations that are stronger than classically allowed, which signifies that the two fields are in an entangled state,” said Liu Qiu, a postdoctoral researcher and joint first author on the paper describing the work.

The researchers now hope to extend this entanglement to qubits and room temperature fibres, implement quantum teleportation and entangle qubits in separate dilution refrigerators.

If microwave frequency circuits can exchange quantum information through optical fiber, it could provide a quantum teleportation, where the fragile superconducting qubits could transfer data via easily disrupted microwave frequencies to photons that could then move along on a network. Cryogenic cooling the quantum computers makes the microwaves more stable and less noisy, but the entangled quibits and photons could lead to warmer conditions for quantum computer.

Sahu, L. Qiu, et al. “Entangling microwaves with light,” SCIENCE, 18 May 2023, Vol 380, Issue 6646, pp. 718-721. DOI: 10.1126/science.adg3812

Measuring microwave radiation
Researchers in Finland have created a nanodevice — a bolometer —that measures microwave radiation’s absolute power a trillion times lower and more accurately than previously seen. The bolometer, a type of thermometer, uses a heater to help it measure down to the femtowatt level at ultra-low temperatures, but the device can also still measure a wide range of temperatures.

“We added a heater to the bolometer, so we can apply a known heater current and measure the voltage. Since we know the precise amount of power we’re putting into the heater, we can calibrate the power of the input radiation against the heater power. The result is a self-calibrating bolometer working at low temperatures, which allows us to accurately measure absolute powers at cryogenic temperatures,” Mikko Möttönen, associate professor of quantum technology at Aalto University and VTT, a research institutions owned by the Finnish state, said in a press release. The bolometer can reliably measure at one femtowatt or below.

Fig. 01: Image of the power sensor on a silicon chip. Source: Jean-Philippe Girard/Aalto University

The purpose of the bolometer was to improve the performance of quantum computers. “Measuring microwaves happens in wireless communications, radar technology, and many other fields. They have their ways of performing accurate measurements, but there was no way to do the same when measuring very weak microwave signals for quantum technology. The bolometer is an advanced diagnostic instrument that has been missing from the quantum technology toolbox until now,” said Russell Lake, director of quantum applications at Bluefors, which also worked on the project.

Some off the shelf power sensors measure to the scale of one milliwatt, the new bolometer is a significant step forward in measuring microwave power.

J.-P. Girard, R. E. Lake, W. Liu, R. Kokkoniemi, E. Visakorpi, J. Govenius, M. Möttönen. Cryogenic sensor enabling broad-band and traceable power measurements. Review of Scientific Instruments, 2023; 94 (5) DOI: 10.1063/5.0143761