Scaling of Spin Qubits

Monday 29 Aug 2022, 08:30 → 22:30 Europe/Copenhagen

A1-05.01 (Copenhagen University- Frederiksberg campus)

Anasua Chatterjee (Center for Quantum Devices - Niels Bohr Institute), Fabio Ansaloni (Quantum Machines)

Scaling of Spin Qubits:

This one-day workshop aims at bringing together scientists from world leading research groups focused on scaling of semiconductor spin qubit arrays. Scaling to intermediate-scale arrays represents a next big challenge for a spin-based quantum computer, and many areas of research will have to be co-integrated. Talks will cover the different stages required for a fully functional small-scale spin-base processor, ranging from materials science, to spin qubit characterization and operation, to automated algorithm implementation. 

Topics to be considered at the workshop include:

• Spin qubit processors in silicon and germanium
• Material development and large scale integration of spin qubit devices
• Qubit connectivity and architectures for long range interactions
• Tuning, readout and noise spectroscopy in small-scale qubit devices

                   

Speakers include:

• Keynote: Jason Petta (Princeton University, USA)
• Stephan Philips (TU Delft, Netherlands)
• Leon Camenzind (RIKEN, Japan)
• Fernando Gonzalez-Zalba (Quantum Motion Technologies & University of Cambridge, UK)
• Philipp Mutter (University of Konstanz, Germany)
• Will Lawrie (University of Copenhagen. Denmark and TU Delft, Netherlands)
• Natalia Ares (University of Oxford, UK)
• Lars Schreiber (RWTH Aachen University, Germany)
• Juan Rojas-Arias (RIKEN, Japan)
• Holly Stemp (UNSW, Australia)
• Peter Krogstrup (University of Copenhagen, Denmark)
• Cécile Yu (CEA-IRIG, France)
• Clément Godfrin (IMEC, Belgium)

Organizers:

• Anasua Chatterjee, Assistant Professor, University of Copenhagen
• Fabio Ansaloni, Research Scientist, Quantum Machines

Sponsors:

We thank the JP-DK international network program, promoted by the Ministry of Higher Education and Science, for sponsoring the 2022 Scaling of Spin Qubit workshop.

Venue:

The workshop will take place at the University of Copenhagen's green and leafy Frederiksberg campus. The auditorium is A1-05.01, and the address is:

Dyrlægevej 100

1870 Frederiksberg

For practicalities (getting to the conference venue, others), please select the "Practical Information" link to the left. If you have questions, email the organisers at the email listed to the left and we will try our best to help you.

Download the conference flyer here

08:30 → 09:00 Registration

09:00 → 09:10 Welcome and opening remarks

09:10 → 10:40 Qubit connectivity and architecture

09:10 Conveyor-mode single-electron shuttling in Si/SiGe

We demonstrate shuttling of a single electron by a propagating wave-potential in an electrostatically defined 420 nm long Si/SiGe quantum-channel [1]. This conveyor-mode shuttling approach requires only four sinusoidal control signals independent from its length. The tuning of the signal parameters is straightforward and we observe a high single-electron shuttling fidelity of 99.42 % including a reversal of direction. We show numerical device simulations including charged defects and discuss spin-dephasing mechanisms expected during conveyor-mode shuttling in Si/SiGe and the perspective for spin-coherent shuttling with a transfer fidelity of at least 99.9 % across a distance of 10 µm [2].
[1] I. Seilder et al., arXiv:2108.00879 (2021); accepted for npj Qant. Inf.
[2] V. Langrock et al., arXiv:2202.11793 (2022).

Speaker: Lars Schreiber (RWTH Aachen University)

09:50 Tomography of universal two-qubit logic operations in exchange-coupled donor electron spin qubits

Scalable quantum processors require high-fidelity universal quantum logic operations, in a manufacturable physical platform, along with the capacity to couple multiple qubits together over a variable range of length scales. The spin of an electron bound to a single donor atom in silicon has shown coherence times of almost a second [1], with single qubit quantum operation fidelities of over 99.9% [2]. In addition, donors in silicon possess a number of intrinsic coupling mechanisms that can be utilised in a scalable quantum processor architecture. One such coupling mechanism is the exchange interaction between donor-bound electrons.
Here we present the experimental demonstration and tomography of universal 1- and 2-qubit gates in a system of two weakly exchange-coupled electrons, with each electron bound to a single donor phosphorus nucleus. By deterministically preparing the two nuclear spins in opposite directions, each electron spin resonance pulse constitutes a native conditional two-qubit gate [3]. We carefully benchmark the fidelity of these native operations using the technique of gate set tomography (GST), achieving qubit gate fidelities above 99% for both electrons separately. The GST method provides precious insights into the nature of the residual errors, and informs strategies for further improvement. Adding to the recent demonstration of universal 2-qubit gates for nuclear spins, and electron-nuclear entanglement [4], these electron two-qubit gates complete the toolbox for constructing a scalable spin-based quantum processor in silicon.

[1] Muhonen, J. T. et al. Storing quantum information for 30 seconds in a nanoelectronic device. Nature nanotechnology 9, 986 (2014).
[2] Dehollain, J. P. et al. Optimization of a solid-state electron spin qubit using gate set tomography. New Journal of Physics 18, 103018 (2016).
[3] Mądzik, M. T. et al. Conditional quantum operation of two exchange-coupled single-donor spin qubits in a MOS-compatible silicon device. Nature Communications 12, 181 (2021).
[4] Mądzik, M. T. et al. Precision tomography of a three-qubit electron-nuclear quantum processor in silicon. arXiv:2106.03082 (2021)

Speaker: Holly Stemp (UNSW)

10:15 Strong hole spin-photon coupling in silicon

Recently, hole spins in silicon and germanium have shown increasing interest for quantum information processing owing to the advantage of manipulating their state with electric instead of magnetic microwave fields. This is possible due to the strong spin-orbit interaction intrinsically present in the valance band of these materials. Spin-orbit coupling offers as well the possibility to couple a hole spin to the electric field component of a microwave photon. Here we show a strong hole spin-photon interaction on a CMOS compatible platform. We find a coupling strength up to 330 MHz, accompanied by a cooperativity reaching 1600. Moreover, the dominating Rashba spin-orbit coupling allows us to tune the spin-photon coupling strength by more than one order of magnitude by simply varying the magnetic field orientation with respect to the spin-orbit field. This largely coupled spin-photon system opens the door to the achievement of high-fidelity two qubits gate with distant spins.

Speaker: Cécile Yu (CEA-IRIG)

10:40 → 10:55 Coffee break

10:55 → 12:35 Materials and large scale integration

10:55 A Quantum Foundry

There is a need to develop atomic scale precision and ultra pure fabrication methods for quantum tech. I will briefly touch upon plans to get there.

Speaker: Peter Krogstrup (Niels Bohr Institute)

11:10 Silicon-based quantum computing: Scaling strategies

Speaker: Fernando Gonzalez-Zalba (Quantum Motion Technology & University of Cambridge)

11:45 Hole spin qubits in Silicon finFETs

The greatest challenge in quantum computing is achieving scalability. Classical computing, which previously faced such issues, currently relies on silicon chips hosting billions of fin field-effect transistors (finFETs). These devices are small enough for quantum applications: an electron or hole trapped under the gate can serve as a spin qubit at low temperatures. This approach allows quantum hardware and its classical control electronics to be integrated on the same chip. However, this requires qubit operation at temperatures above 1 K, where the cooling overcomes heat dissipation of this control electronics. Here, we show that our industry-compatible silicon finFET devices [1] can host hole spin qubits above 4 K [2]. We achieve fast electrical spin control with operation speeds up to 150 MHz, single-qubit gate fidelities at the fault-tolerance threshold, and controllable exchange interaction, allowing a fast CROT (CX) gate with a conditional spin-flip in 32 ns. The strong spin-orbit interaction in these devices leads to anisotropies and coherence hotspots in the single and two-qubit gates, which we investigate to improve the quality of our qubits.
[1] Geyer et al., Appl. Phys. Lett. 118 (2021).
[2] Camenzind et al., Nature Electronics 5 (2022).

Speaker: Leon Camenzind (RIKEN)

12:10 Low charge noise in SiMOS quantum dots with full 300mm CMOS processes

Clement Godfrin [1], Asser Elsayed [1,2], Mohamed Shehata [1,2], Ruoyu Li [1], Stefan Kubicek [1], Shana Massar [1], Yann Canvel [1], Julien Jussot [1], Massimo Mongillo [1], Danny Wan [1], Pol van Dorpe [1,2], Kristiaan De Greve [1,3]
1 IMEC, Kapeldreef 75, B-3001 Leuven, Belgium
2 Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
3 Department of Electrical Engineering, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium

Silicon spin qubits have been considered as one of the most promising candidates for large scale quantum computers due to their long coherence times and compatibility with CMOS technology. However, the Si/SiO2 interface, where the qubit stand, has been widely identified as the source for charge noise and disorder sites, which limits the qubit fidelity and scalability. We address this challenge by optimizing the gate stack with 300mm fabrication processes. On the fully integrated qubit structures, we characterize single electron transistors (SETs) across multiple devices and over large gate voltage range at milli-Kelvin temperatures and report notably low levels of charge noise below 1 μeV\/√Hz. Moreover, the SET barriers show smooth pinch-off curves with highly uniform threshold voltages. These results underpin Si quantum dot qubit systems for large-scale quantum computing.

Speaker: Clément Godfrin (IMEC Leuven)

12:35 → 13:35 Lunch

13:35 → 15:05 Tuning, readout and noise spectroscopy

13:35 Real time control of quantum devices using machine learning

Machine learning has been the enabler of well-known breakthroughs in computer science, such as the victory of Alpha Go over a Go world champion and superhuman face recognition. We can direct this potential to the characterisation and tuning of quantum devices in real time. As in Go, where a player must carefully balance short and long-term goals and devise actions accordingly, we have demonstrated a deep reinforcement learning algorithm that devises efficient policies to find desired measurement features. Our algorithm divides the parameter space of a semiconductor quantum device into blocks and finds target measurement features by performing a minimum number of block measurements. In this way, we reduce the long characterisation times required due to device variability. We have also developed an algorithm that measures bias triangles, important features for qubit operation, and gives them a score. The device parameters are then updated to optimise this score in real-time. The algorithm, using a disentangling variational auto-encoder, proves capable of fine-tuning several device parameters at once. I will also show automatic identification of Pauli spin blockade. To conclude, I will demonstrate an algorithm able to tune a double quantum dot device regardless of the semiconductor realisation. These approaches are widely applicable, opening the way to a completely automatic and efficient route to quantum device tuning and characterisation, and thus taking a crucial step towards the scalability of quantum circuits.

Speaker: Natalia Ares (Oxford University)

14:15 Transmission based qubit noise spectroscopy

Noise spectroscopy is an important first step towards mitigating the detrimental effects of noise on qubits. In this talk I will speak about the transient and long-time transmission through a resonator containing a generic noisy qubit and show that characteristic features of the noise are imprinted in the fluctuations of the averaged resonator response. I will present analytical expressions for the transmission amplitude and speak about the possibility of extracting the power spectral density of arbitrary Gaussian noise with an exponentially decaying error due to finite measurement times [1].

[1] P. M. Mutter and G. Burkard, Phys. Rev. Lett. 128, 236801 (2022)

Speaker: Philipp Mutter (University of Konstanz)

14:40 Correlated charge noise in Si/SiGe quantum dot spin qubits

Electron spin qubits in silicon quantum dots are particularly promising candidates for the implementation of a large-scale quantum computer due to their small physical footprint. However, this compactness raises concerns on the existence of spatial noise correlations that can be severely detrimental for quantum error correction. In this talk, I will show our results on noise spectroscopy of two neighboring qubits in a device made of isotopically purified silicon, where we detect strong noise cross-correlations. These inter-qubit correlations arise from the dominant effect of charge noise in our system, verified by the presence of noise correlations between the bare qubit frequencies and exchange interaction. We devised a simple model of electric fields shifting the position of the qubits within a magnetic field gradient, that allows us to quantitatively reproduce the measured behavior. Our work shows that electrical noise not only limits single-qubit dephasing but can also be a source of correlated phase errors.

Speaker: Juan Rojas-Arias (RIKEN)

15:05 → 15:20 Coffee break

15:20 → 16:50 Spin qubit processors

15:20 Qubits in Planar Germanium

Scalability is becoming an increasingly important metric against which to assess the suitability of qubit candidates for large scale quantum computing implementations. Spin qubits, specifically electron spins in silicon quantum dots, are often considered a highly scalable candidate due to their inherent similarities to transistors. However, complications arise when considering the need for qubit manipulation, capable of individual qubit addressability. Conventionally, bulky structures such as microwave antennas, or micromagnets introduced to allow for control of a spin state. Additionally, to reduce interconnects, a large degree of device uniformity is required in order to facilitate shared control, and high temperature operation will likely be necessary for even modest sized spin qubit-based quantum computers.
In this talk, I will examine the scalability of hole spin qubits in germanium quantum wells, which have recently emerged as a promising candidate for large scale quantum computing [1]. Ge/SiGe is a very clean material with high mobilities and low percolation densities [2]. The intrinsic spin orbit coupling present for holes circumvents the need for additional device infrastructure for qubit manipulation, already simplifying the design of large-scale architectures [3]. Additionally, hole spins show a remarkable robustness to classical crosstalk errors and can achieve single qubit fidelities above 99.99% [4]. Furthermore, hole spins in SiMOS have been operated at elevated temperatures [5], and can exhibit strong spin-photon coupling [6]. These attributes position hole spins as strong contenders for spin qubit scale-up.

[1] G. Scappucci et al. The Germanium Quantum Information Route, Nat Rev Mater 6, 926–943 (2021)
[2] M. Lodari et al. Low percolation density and charge noise with holes in germanium, Mater. Quantum. Technol. 1 011002 (2021)
[3] W.I.L. Lawrie et al. Quantum Dot Arrays in Silicon and Germanium, Appl. Phys. Lett. 116, 080501 (2020)
[4] W.I.L. Lawrie et al. Simultaneous driving of semiconductor spin qubits at the fault-tolerant threshold, arXiv:2109.07837 [cond-mat.mes-hall] (2021)
[5] L.C Camenzind et al. A hole spin qubit in a fin field-effect transistor above 4 kelvin. Nat Electron 5, 178–183 (2022).
[6] C.X. Yu et al. Strong coupling between a photon and a hole spin in silicon arXiv:2206.14082 [cond-mat.mes-hall] (2022)

Speaker: Will Lawrie (Copenhagen University)

15:45 Universal control of a six-qubit quantum processor in silicon

Future quantum computers capable of solving relevant problems will require a large number of qubits that can be operated reliably[1]. However, the requirements of having a large qubit count and operating with high-fidelity are typically conflicting. Spins in semiconductor quantum dots show long-term promise but demonstrations so far use between one and four qubits and typically optimize the fidelity of either single- or two-qubit operations, or initialization and readout [2,3,4,5,6,7,8]. Here we expand the number of qubits and simultaneously achieve respectable fidelities for universal operation, state preparation and measurement. We design, fabricate and operate a six-qubit processor with a focus on careful Hamiltonian engineering, on a high level of abstraction to program the quantum circuits and on efficient background calibration, all of which are essential to achieve high fidelities on this extended system. State preparation combines initialization by measurement and real-time feedback with quantum-non-demolition measurements. These advances will allow testing of increasingly meaningful quantum protocols and constitute a major stepping stone towards large-scale quantum computers.

1. Vandersypen, L. M. K., et al., npj Quantum Information, vol. 3.1, pp. 1-10, 2017.
2. Veldhorst, M., et al, Nature nanotechnology, vol. 9.12, pp. 981-985, 2014.
3. Yoneda J., et al., Nature Nano, vol. 13, pp. 102-106, 2018.
4. Xue X., et al, Nature 601, 343–347, 2022
5. Noiri, A.et al., Nature 601, 338–342, 2022
6. Mills, A.et al., arXiv:2111.11937, 2021
7. Takeda K., et al., Nature Nano, pp. 1-5, 2021.
8. Hendrickx N. W., et al., Nature, vol. 591, pp. 580–585, 2021

Speaker: Stephan Philips (TU Delft)

16:10 High Fidelity Readout and Quantum Control of Si/SiGe Spin Qubits (REMOTE)

Speaker: Jason Petta (Princeton University)

16:50 → 17:00 Farewell and closing remarks

19:30 → 22:00 Conference dinner