Brief Overview of Semiconductor-Based Qubits
Qingyun Xie, Nadim Chowdhury
Microsystems Technology Laboratories, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
E-mail: qyxie@mit.edu, nadim@mit.edu
The field of microelectronics has witnessed explosive growth since the mid-Twentieth Century, offering unprecedented convenience, connectivity and added functionality to our daily lives. A crucial factor in propelling this revolution is the advancement of micro-fabrication and nano-fabrication, which has resulted in spillover effects in adjacent fields like silicon photonics, micro-electro-mechanical systems (MEMS), microfluidics and increasingly, in semiconductor-based quantum computing hardware.
1. Materials
1.1 Silicon
Silicon has been the dominant semiconductor material. Several types of qubits have realized in Silicon, including silicon spin qubit and silicon quantum dot qubit. A major advantage of Si-based qubits is the maturity and scalability of the fabrication process, allowing it to be precisely fabricated at commercial foundries. Furthermore, though not straightforward, efforts have been made to integrate silicon qubits with control/readout electronics, highlighting the potential of Si-based qubits on CMOS platform for scalable quantum processing units [1].
1.2 III-V materials
To achieve high-quality III-V materials (e.g. low defect density), molecular beam epitaxy (MBE) is often required and preferred over metal organic chemical vapor deposition (MOCVD). Expensive (non-Si) substrates (e.g. InP) are preferred for minimal lattice mismatch. The issue is that these substrates and MBE growth technology are often not scalable, hindering the advancement towards larger scale devices and integration of various functionalities.
2 Device
2.1 Device structure
The figure above illustrates a few different types of superconducting qubit structures [2]. An area of recent advancement is voltage-controlled devices, or “gatemons.” From a device design perspective, an important requirement, similar to transistors, is the confinement of charge carriers. Two device structures are proposed, namely the nanowire and two-dimensional electron gas.
2.2. Contacts
Another challenge is to form high-quality contact to the channels to form the semiconductor-superconductor junction. For example, MBE-grown aluminum was used to form contact to the AlGaAs/GaAs channel [3].
2.3 Device Fabrication
The device fabrication process is relatively straightforward as it employs the established fabrication techniques used in semiconductor devices and that the device structure is planar. For example, a high-quality gate dielectric (low leakage current, traps etc.) is important. In fact, it is precisely because of the availability of processing techniques that places semiconductor-based qubits at an advantage. In every semiconductor material (Si, III-V etc.), sub-micron features have been successfully achieved, because of the demand in scaling from logic and RF electronics.
3 Promising Potential for Integration
A highlight of semiconductor-based qubit technology is the enormous potential for integration. This section briefly discusses two aspects of integration, namely RF electronics and MEMS, which would greatly enhance the functionality of the integrated microsystem.
3.1 RF electronics
In order to achieve integration between the qubit and control/readout circuit, it is necessary to bring qubits to work properly at higher temperatures, and to bring the circuit to work at deep cryogenic temperatures. Thanks to advanced VLSI technology, the latter option (circuits working at deep cryogenic temperatures) seems to be the significantly more viable option. Silicon CMOS, thanks to its high degree of integration, is preferred for making the complex digital and analog electronics [4]. For the low noise readout of weak qubit signals, which involves ultra-low noise amplifiers, III-V (e.g. InP) is preferred, primarily because of their extremely high electron mobility. Heterogeneous integration of silicon and III-V would need to be achieved, for example using “flip chip” and other packaging technologies. GaN electronics is considered as a promising candidate thanks to the high mobility channel, high frequency devices, and availability of large-area substrate (300 mm MOCVD on Si substrate) [5–6].
3.2 MEMS Resonators
MEMS resonators have been widely used as filters in communication systems, and even for a wide variety of sensing applications [7]. With the maturation of various MEMS resonator technologies, e.g. FBAR, SAW, Lamb Wave, there has been increasing research in the area of coupling MEMS resonators to qubits. In an early demonstration by O’Connell et al. (2010), the resonator used was an AlN FBAR with Al electrodes at a frequency of ~ 6GHz [8]. In another demonstration by A. Bienfait et al., LiNbO3 SAW resonator was inductively coupled to the qubit through flip-chip bonding between the LiNbO3 substrate and sapphire substrate [9]. There would be a possibility to achieve quantum control of acoustic vibrations, or even to link hybrid quantum systems (e.g. between superconducting qubit and another two-level system)
4 Conclusion
In summary, semiconductor-based qubit technology could pave the way for high performance and scalable quantum computing technology. Taking advantage of the Si+X heterogeneous integration platform, a wide range of functionalities could be realized on the same chip.
References
[1] L. Le Guevel et al., “A 110mK 295μW 28nm FDSOI CMOS Quantum Integrated Circuit with a 2.8GHz Excitation and nA Current Sensing of an On-Chip Double Quantum Dot,” ISSCC 2020, pp. 306–308, Feb. 2020.
[2] S. J. Weber, “Gatemons get serious,” Nat. Comm., vol. 13, pp. 877–878, Oct. 2018.
[3] L. Casparis, “Superconducting gatemon qubit based on a proximitized two-dimensional electron gas,” Nat. Nanotech., vol. 13, pp. 915–919, Oct. 2018.
[4] B. Patra et al., “Cryo-CMOS Circuits and Systems for Quantum Computing Applications,” IEEE J. Solid-State Circuits, vol. 53, no. 1, pp. 309–321, Jan. 2018.
[5] Q. Xie, “Gallium Nitride electronics for cryogenic and high frequency applications,” S.M. Thesis, Massachusetts Institute of Technology, May 2020.
[6] N. Chowdhury et al., “Regrowth-Free GaN Complementary Logic on a Si Substrate,” IEEE Electron Device Lett., vol. 41, no. 6, pp. 820–823, Jun. 2020.
[7] Q. Xie et al., “Effectiveness of oxide trench array as a passive temperature compensation structure in AlN-on-silicon micromechanical resonators,” Appl. Phys. Lett., vol. 110, no. 8, pp. 083501, Feb. 2017.
[8] A. D. O’Connell, et al., “Quantum ground state and single-phonon control of a mechanical resonator,” Nature, vol. 464, pp. 697–703, Apr. 2010.
[9] A. Bienfait et al., “Phonon-mediated quantum state transfer and remote qubit entanglement,” vol. 364, pp. 368–371, Apr. 2019.
