Scalable Optical Links for Controlling Bosonic Quantum Processors

Abstract

Superconducting quantum computing has the potential to revolutionize computational capabilities. However, scaling up large quantum processors is limited by the cumbersome and heat-conductive electronic cables that connect room-temperature control electronics to quantum processors, leading to significant signal attenuation. Optical fibers provide a promising solution, but their use has been restricted to controlling simple two-level quantum systems over short distances. Here, we demonstrate optical control of a bosonic quantum processor, achieving universal operations on the joint Hilbert space of a transmon qubit and a storage cavity. Using an array of cryogenic fiber-integrated uni-traveling-carrier photodiodes, we prepare Fock states containing up to ten photons. Additionally, remote control of bosonic modes over a transmission distance of 15 km has been achieved, with fidelities exceeding 95%. The combination of high-dimensional quantum control, multi-channel operation, and long-distance transmission addresses the key requirements for scaling superconducting quantum computers and enables architectures for distributed quantum data centers.

Figures (4)

• Figure 1: Optical links for controlling superconducting quantum processor units. a and b, Comparison of conventional microwave link (left) and optical link (right) for delivering control signals from room temperature electronics facilities to superconducting devices in a dilution refrigerator. Microwave links employ coaxial cables spanning multiple temperature stages with attenuators at each stage, while optical links transmit modulated optical carrier through optical fibers and recover the microwave signal through a UTC-PD. c, Photograph of a UTC-PD chiplet, which contains 6 elements and two of which are wire-bonded to a printed circuit board for microwave output. d, Packaged UTC-PD array with two separate chiplets, showing glued optical fibers to 4 elements. e, Dependence of transmittance and the ratio of carrier power dissipation $\eta_\mathrm{heat}$ on the transmission distance in coaxial cables (dashed line) and optical fibers (solid line). Inset: the structures of an optical fiber and a coaxial cable.
• Figure 2: Independent optical control of components in a bosonic quantum processor.a, Schematic of the optical links (A and B) and the bosonic quantum processor comprising a transmon qubit, a readout resonator, and a high-quality storage cavity. b, Transmitted microwave signals of the readout resonator with input from microwave link (blue) and optical link (red). Input pulses are $4~\mathrm{\mu s}$ square waves at $7.508~\mathrm{GHz}$, are down-converted to $50~\mathrm{MHz}$ for pulse shape acquisition. c, The excitation probability $\text{P}(\left|e\right\rangle)$ of the transmon qubit driven by optical link A, using Gaussian pulse modulation on the electro-optics modulator with pulse-width $\sigma=60~\mathrm{ns}$ and complex modulation amplitude in the IQ plane. d, Line cut along the red dashed line in c, showing qubit Rabi oscillations as a function of the I modulation amplitude. e, Linear dependence of the qubit Rabi frequency ($\Omega_\text{R}$) on the modulation amplitude. f, Cavity microwave photon number distribution as a function of the modulation amplitude, with the curves are fittings according to Poisson distribution for coherent states with different displacement amplitude. g, Linear relationship between the coherent state displacement amplitude and the modulation amplitude.
• Figure 3: Quantum control of the joint cavity-qubit system via optical links.a, Schematic of arbitrary operations on the bosonic quantum processor through two optical links. Inset: the synchronized pulse sequences on both optical links A and B for implementing encoding and decoding operations between qubit states {$\left|g\right\rangle$, $\left|e\right\rangle$} and cavity Fock states {$\left|0\right\rangle$,$\left|n\right\rangle$}. b, Gallery of measured Wigner functions for prepared cavity states. Top row: Fock states $\left|n\right\rangle$ for $n=1$ to $4$. Bottom row: superposition states $\left|0\right\rangle+i\left|n\right\rangle$. The corresponding state fidelities are $96.9\%(1),90.1\%(2),97.7\%(3),88.3\%(4)$ and $95.0\%(1),90.4\%(2),92.1\%(3),85.6\%(4)$, respectively. c, Process fidelity against the rounds of encoding and decoding operations for Fock states with $n=1$ to $4$. Inset: estimated encoding operation fidelity.
• Figure 4: Scalability of optical links in Hilbert space dimension and transmission distance.a-d, Measured Wigner functions and the reconstructed density matrices of the Fock state $\left|10\right\rangle$ and its superposition state $\left|0\right\rangle + i\left|10\right\rangle$, respectively. e, Schematic of remote control of bosonic quantum processor via optical link over a long transmission distance. f and g, Process fidelity decay and extracted fidelity of encoding operation for Fock state basis {$\left|0\right\rangle$, $\left|1\right\rangle$}, with varying fiber spools lengths ($0-15\,\mathrm{km}$) inserted between room-temperature electronics and the cryogenic processor. h and i, Process fidelity characterizations for binomial code logical state basis {$\left|0_L\right\rangle=(\left|0\right\rangle+\left|4\right\rangle)/\sqrt{2}$, $\left|1_L\right\rangle=\left|2\right\rangle$}. The inset in h shows the results of the Wigner tomography of the state $\left|0_L\right\rangle+i\left|1_L\right\rangle$ at $0\,\mathrm{km}$.