Hybrid quantum devices, spin-based quantum information technology
The Hybrid Quantum Device team is pursuing the following projects, all of which are hardware physics and engineering on microwave quantum technology devices based on spins in gem crystals.
Quantum transducer with spins in diamond
Superconducting qubits (SCQs) have shown promise in their scalability [Arute et al., Nature (2019)]. However, The information produced by superconducting qubits cannot be sent out of the millikelvin environment (i.e., the dilution refrigerator), as it will be quickly killed by room-temperature thermal noise. Optical photons, on the other hand, have high energy and can propagate over long distances at room temperature. Therefore, for the quantum internet with superconducting quantum computers, a way to convert the information between microwave and optical frequencies is needed but still missing. This has led to the need for a quantum device that can perform the task, a device so-called "quantum transducer". Such a conversion scheme could enable long-range communication between quantum processor nodes, in a sort of "quantum internet" [Kimble, Nature (2008)].
The Hybrid Quantum Device team is trying to realize a quantum transducer, in principle following the proposal [Williamson et al. PRL (2014)], where a conversion scheme is theoretically proposed assuming an ensemble of erbium ions in an optical crystal. We are also trying but with silicon-vacancy (SiV) centers in diamond. SiV centers have shown outstanding optical properties [Rogers et al., Nature Com 5, 4739 (2014)].
By coupling an ensemble of SiV centers to both a microwave and an optical resonator, we will attempt to achieve coherent, bidirectional conversion between microwave and optical photons, taking advantage of the narrow optical linewidths and large electric dipole moment of SiV centers.
A cartoon of the spin-based quantum transducer. A diamond crystal is placed in a microwave resonator and an optical cavity.
Ultra-low noise microwave amplification by means of maser
Because of the tiny energy of microwave signals at the quantum regime, amplification of microwave signals at millikelvin environments is necessary and the key for quantum information experiments or technology applications. The reason for this is that the signal-to-noise ratio of the measurement system is determined by the noise added by the first amplifier; therefore, the first amplifier should have a noise temperature as close as possible to the vacuum fluctuation (quantum noise).
This has been realized by Josephson parametric amplifiers (JPA, Figure 1), which are based on superconducting circuits in which Josephson junctions (superconducting tunnel junctions) are embedded. JPAs have been intensively studied and developed in quantum computing research.
However, JPAs have suffered from a limited dynamic range, i.e., their very low input saturation power. The state-of-the-art JPA reported in 2015 by Maclin et al. [Science 350, 307 (2015)] has a maximum input power of about -100 dBm (0.1 picowatts)
We are trying to realize another type of ultra-low noise microwave amplifier with impurity spins in gem crystals, based on maser (microwave amplification by stimulated emission of radiation), which is the microwave version of the laser.
Cartoon of a spin maser amplifier.
In our research, we primarily use three different impurity centers in diamond: NV centers, nitrogen (P1) centers, and silicon-vacancy (SiV) centers.