An integrated magneto-optic modulator for cryogenic applications

Abstract

Superconducting circuits can operate at higher energy efficiencies than their room-temperature counterparts and have the potential to enable large-scale control and readout of quantum computers. However, the required interface with room-temperature electronics creates difficulties in scaling up such cryogenic systems. One option is to use optical fibres as a medium in conjunction with fast optical modulators that can be efficiently driven by electrical signals at low temperatures. However, as superconducting circuits are current operated with low impedances, they interface poorly with conventional electro-optical modulators. Here we report an integrated current-driven modulator that is based on the magneto-optic effect and can operate at temperatures as low as 4 K. The device combines a magneto-optic garnet crystal with a silicon waveguide resonator and integrates an electromagnet to modulate the refractive index of the garnet. The modulator offers data rates of up to 2 Gbps with an energy consumption below 4 pJ per bit of transferred information, which could be reduced to less than 50 fJ per bit by replacing dissipative electrodes with superconductors and optimizing the geometric parameters.

A Sub‐Picojoule per Bit Integrated Magneto‐Optic Modulator on Silicon: Modeling and Experimental Demonstration

“…Characterization of the device is needed to assess the actual achievable bandwidth of the device. Similar characterization results have demonstrated a modulation rate of 2 Gbps, [9] therefore we believe the model results at 10 Gbps to be infeasible.…”

“…To assess the efficiency of the modulator, the energy consumed per bit is calculated. Assuming a random bit sequence with an equal distribution of ones and zeros, on average a single bit is estimated to consume [ 9 ] : $$$$\begin{equation} E_{\rm b} = \frac{1}{2}LI_{\rm p}^2 + I_{\rm p}^2 R T_{\rm b} \end{equation}$$$$where L is the coil's inductance, R the equivalent resistance, I p the peak current and T b the bit time. The first half describes the charging and discharging of the magnetic field, halved by the random distribution of ones and zeroes.…”

“…The second half describes the power dissipated per bit by resistive losses in the coil. [ 9 ] The inductance and resistance values found in the hfss model are used. Figure 4d shows the energy per bit E b for various bitrates.…”

“…However, using a similar device, a 2 Gbps modulation rate has been established. [ 9 ] Further material characterization is needed to conclusively assess the switching limitations of Ce:YIG.…”

“…This can be achieved by making use of efficient MO materials, by exploiting the efficiency improvements of these materials at low temperatures [7,[10][11][12] and by co-integration with superconductive circuits. [9] This makes such a device a compelling solution not only for future classical networking architectures, but also for solving the interconnect bottleneck in quantum computing architectures. [13] "fJ bit −1 " interfaces are needed to transfer information from cryogenic environments to room temperature, a challenge which currently hinders the scalability of such architectures beyond tens of qubits.…”