Optimizing the Electrical Interface for Large-Scale Color-Center Quantum Processors

TL;DR

This work presents a high-level, system-engineering study of an electrically interfaceable color-center quantum processor. It derives detailed specifications for DC and AC magnetic-field control, photon-detector interfaces, and 3D-integrated cryo-CMOS controllers, all aimed at achieving fidelity of $F\geq 0.9999$ in a scalable unit-cell architecture. The proposed solution favors dedicated unit-cell drivers and coils to maximize qubit addressability and minimize crosstalk, and provides a quantitative power-estimation framework showing that >100 unit cells could operate at 1 K with sub-watt total dissipation. Overall, the paper establishes a concrete baseline for scalable, power-efficient electrical interfaces for large-scale color-center quantum computers and offers design guidance transferable to related qubit platforms.

Abstract

Quantum processors based on color centers in diamond are promising candidates for future large-scale quantum computers thanks to their flexible optical interface, (relatively) high operating temperature, and high-fidelity operation. Similar to other quantum-computing platforms, the electrical interface required to control and read out such qubits may limit both the performance of the whole system and its scalability. To address this challenge, this work analyzes the requirements of the electrical interface and investigates how to efficiently implement the electronic controller in a scalable architecture comprising a large number of identical unit cells. Among the different discussed functionalities, a specific focus is devoted to the generation of the static and dynamic magnetic fields driving the electron and nuclear spins, because of their major impact on fidelity and scalability. Following the derived requirements, different system architectures, such as a qubit frequency-multiplexing scheme, are considered to identify the most power efficient approach, especially in the presence of inhomogeneity of the qubit Larmor frequency across the processor. As a result, a non-frequency-multiplexed, 1-mm$^2$ unit-cell architecture is proposed as the optimal solution, able to address up to one electron-spin qubit and 9 nuclear-spin qubits within a 3-mW average power consumption, thus establishing the baseline for the scalable electrical interface for future large-scale color-center quantum computers.

Figures (10)

• Figure 1: Atomic structure and level structure for the electron and nuclear spins as a function of magnetic field for a) a nitrogen-vacancy center Bradley2019 and b) a tin-vacancy center nguyen2019quantum. The Larmor frequency of the carbon-13 spins depends on the state of the electron spin and their interaction, with the blue and pink levels related to the electron-spin state in blue and pink, respectively. Parameters $A_\parallel$ and $A_\perp$ indicate the coupling of the nuclear spin to the electron spin, $\gamma_e$ and $\gamma_c$ indicate the gyromagnetic ratio of the electron and carbon spin and $B_z$ indicates the magnetic field along the color center axis, these are further discussed in Section \ref{['sec:mf_specs']}.
• Figure 2: a) Typical free-space setup for nitrogen-vacancy centers in diamond. The diamond is mounted on a cold finger inside a cryostat, while the permanent magnet induces the Zeeman splitting. The MW line introduces an AC magnetic field and the DC bias electrodes tune the electric field to change the wavelength of the emitted photons. A solid immersion lens fabricated around the color center improves the light collection. Optical signal sources and filters are omitted for clarity. b) The NV center hyperfine interactions split the electron spin state energy levels. By driving all energy levels equally, unconditional rotations are performed. Driving a specific energy level results in conditional qubit gates. c) Initialization and readout of the electron spin state occurs through optical transitions between the ground state (GS) and excited state (ES). thesis_connor. d) Overview of the qubit gates that can be performed for the electron (E) and nuclear (N) spin qubits in NV centers. Nuclear spin operations can be performed by either 'periodically coupling' the electron spin to the nuclear spin or by 'directly driving' the Larmor frequency of the nuclear spin. Colors represent different frequencies for electron spin (blue) and nuclear spins (orange, pink). $\tau_1$ and $\tau_2$ resonantly couple with different nuclear spins, while $\tau$ decouples the electron from the environment. Two-qubit Operations are conditional rotations where the control qubit is listed in the respective header of the table column, except for the E to E operation where the two qubits are projected in an entangled state through measuring the photons. Nuclear spins can be measured and initialized with measurement-based initialization (MBI) or SWAP sequences thesis_connorthesis_pfaff. Architecture compatibility refers to the use of a shared driver and coil or separate driver and coil, which is discussed in Section \ref{['sec:sys_arch']}.
• Figure 3: Illustration of a quantum processor based on color centers in diamond, showing the 3D-integrated cryo-CMOS chip, the qubits and the photonics on the left and the components present on the photonics chip on the right.
• Figure 4: Readout infidelity of an NV center versus perpendicular magnetic fields ($B_\perp$) for various permanent magnet strengths, computed by taking the overlap between the excited and ground state Hamiltonian's, assuming no strain and $N=100$ readout cycles.
• Figure 5: Unit-cell architectures for multiplexing the AC driving signal: a) A shared coil distributes the MW fields to all the color centers, whose Larmor frequencies are spaced by using local magnetic biasing via the DC-coils; b) Similar to a), but with multiple MW coils, each addressing a subset of the unit cells; c) An individual MW coil is used per unit cell, removing the need for frequency spacing; d) Dedicated MW coils per unit cells are employed as in c) but a checkerboard pattern facilitated by local DC-coil biasing is employed for the Larmor frequency to reduce the effect of crosstalk.