Quantum Machines Explores Direct Digital Synthesis in Advancing Quantum Computing

In the development of the OPX1000, a controller designed for quantum processors with 1,000 qubits and beyond, scalability was a key consideration. The recently launched OPX1000 module for microwave generation (MW-FEM) is able to generate pulses up to 10.5 GHz directly, without the need for analog oscillators or mixers. The choice of technology for reaching microwave frequencies is not a simple one, and in this blog post, we will explore the decision to use direct digital synthesis (DDS) and compare it to other alternatives.

Many Qubit labs have traditionally used mixing-based solutions to handle control signals for qubit drive and readout in the microwave range. These solutions often involve analog local oscillators (LOs) that are multiplied by the signal of a controller or an arbitrary waveform generator (AWG). However, these mixer-based solutions come with their own set of challenges. IQ-mixers, for example, suffer from LO leakage and mixer image issues that require calibration to remove. Other schemes, like double super-heterodyne (DSH) conversion, offer a zero-calibration solution but require more components.

Furthermore, mixing schemes require a separate LO source for each mixer when different drive frequencies are used. This can be costly and lead to phase differences between channels, which is not ideal for multi-qubit systems. While mixers can be useful, it’s important to consider the pros and cons involved.

Direct Digital Synthesis (DDS) is a more recent technique that offers clear advantages over analog counterparts. DDS simplifies the system by eliminating the need for mixers, LOs, and calibration. It provides better frequency agility, improved phase noise, and precise control over the phase of multiple channels playing synchronously. In summary, DDS offers a simpler and more precise solution compared to other alternatives.

One important performance indicator for microwave generation or up-conversion modules is the spurious-free dynamic range (SFDR). This measures the power difference between the desired signal and unwanted signals (spurs) that may be generated. IQ-mixers require calibration for good SFDR due to LO leakage and image signal generation. Quantum Machines’ Octave, for example, achieves plug-and-play operation up to 18 GHz with an SFDR of 50 dBc, although the calibration process is not trivial.

More advanced schemes, like DSH, are engineered for high SFDR and can achieve up to 70 dBc on narrowband and 60 dBc on operational bandwidth without calibration. However, these schemes require more components. In contrast, the OPX1000’s MW-FEM module, operating with DDS without conversion or mixers, achieves an SFDR exceeding 60 dBc on the entire 0.05-10 GHz spectrum. DDS offers ideal performance with better phase coherence and fewer components.

It’s worth noting that most qubit chips won’t see a significant difference between 50 dBc and 70 dBc SFDR. The fidelity of qubits is impacted by various sources of noise and spurs, so the need for better SFDR saturates around 50 dBc. With the OPX1000, you can be confident that your control system won’t be the fidelity bottleneck anytime soon.

For more information on “Phase Coherence” and “Scalability Considerations,” you can follow this link to read further.

Technology Explained

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Qubit: Qubit is a unit of quantum information that is used in quantum computing. It is the smallest unit of information that can be stored and manipulated in a quantum computer. A qubit can represent a 0, 1, or both 0 and 1 simultaneously, which is known as a superposition. This allows quantum computers to process and store information much faster than traditional computers. The applications of qubits in the computer industry are vast, ranging from cryptography and artificial intelligence to drug discovery and financial modeling. By harnessing the power of quantum computing, businesses can solve complex problems faster and more efficiently than ever before.

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