DDS Technology for Microwave Quantum Ion Control. As the foundation of extremely flexible signal generators for quantum computing and tiny microwave sources for quantum sensing, DDS technology is presently transforming important facets of quantum systems. The basic requirement for quick, accurate, and highly integrated control systems needed for qubit manipulation and system resonance analysis is met by this technology.
Integrated DDS Microwave Source for Quantum Magnetic Resonance
Magnetic resonance is a fundamental method used to manipulate qubits in spin-based quantum information processing. Finding the system’s resonance by frequency sweeping is a crucial challenge in this field. Conventional techniques, which measure the resonance spectrum point by point, take a lot of time and frequently miss resonance offsets brought on by changes in the environment. A signal source with quick frequency sweeping is necessary to greatly increase efficiency.
To meet this need, an integrated microwave (MW) source based on a Direct Digital Synthesiser (DDS) has been created. With a sub-microsecond (μs) latency, this source offers hardware-synchronized frequency sweeping. On a single printed circuit board (PCB) that is only 100 × 60 mm2, this DDS-based device is extremely integrated, in contrast to commercial vector signal generators, which are usually large and inappropriate for integrated quantum devices. The engineering of miniature quantum sensors, such those based on nitrogen-vacancy (NV) centres in diamond, which have numerous uses in temperature, magnetic field, and electric field sensing, depends on this small design.
A Field-Programmable-Gate-Array (FPGA) chip, a DDS chip (AD9914) with a 1.4 GHz frequency bandwidth, and a number of external peripherals form the basis of the device’s fundamental design. These peripherals consist of frequency filters, a power amplifier, a frequency mixer (Mini-Circuits ZX05-42MH-S+), and two wideband Phase-Locked Loops (PLLs). A low-frequency baseband signal is produced by the DDS chip when it is controlled by an FPGA. At the same time, a high-frequency local oscillation (LO) signal is produced by another PLL. The frequency mixer then combines these two signals, amplifies, filters, and sends them to the experimental setup.
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Hardware Implementation and Performance Metrics
The digital control and synchronisation features of the integrated DDS MW source are responsible for its high-speed operation and adaptability. The FPGA sets up the parameters of the DDS chip, such as the 32-bit frequency tuning words (FTWs) and 12-bit amplitude scale factor (ASF), and enables connection with a host over a Serial Peripheral Interface (SPI). The DDS can have a maximum frequency tuning resolution of 190 pHz.
Synchronised fast frequency sweeping is where the device shines. A timing sequence shows that the baseband signal’s frequency sweeping is exactly in time with arriving trigger pulses. The system’s pointer goes back to the beginning of the stored frequency list upon receiving a synchronisation pulse, and it then moves forward through the list with each succeeding trigger pulse. Extremely precise timing is guaranteed by this synchronisation priority. The time lag between the appearance of the trigger pulse and the anticipated baseband signal output is just 245 ns, according to experiments that validated the time sequence.
With a three-order gain in sweeping speed over the SMIQ03B, this latency is far lower than that of commercial equivalents such as the Rohde & Schwarz SMIQ03B (480 μs) and the Rigol DSG3065B (4.5 ms). With an output latency of roughly 250 ns, the latency performance is similar to that of other FPGA-DDS commercial radio frequency sources.
Additionally, in order to correct for signal amplitude fluctuations that could otherwise introduce mistakes into quantum experiments, the system integrates the essential power stabilisation. The power fluctuation of the mixed MW signals is significantly decreased by modifying the ASF and applying particular compensation depending on detected maximum output power values.
Although this range can be expanded up to 7.4 GHz by modifying the LO frequency and replacing components, the adjusted output frequency range currently covers 1.4 to 3 GHz. The source’s strong performance on frequency sweeping and power stability has been confirmed by successful demonstrations in Rabi oscillation and pulsed-ODMR studies on a single NV centre.
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DDS Technology for Microwave Ion Qubit Control
Innovation in quantum computing hardware, especially for trapped-ion systems, is also being fuelled by DDS technology. DDS is used in the ground-breaking quantum computers of the German start-up eleQtron, which manage individual trapped-ion qubits using microwave radiation rather than lasers. This technology requires roughly one-fifth of the power required for conventional laser-based techniques, resulting in a simpler design and significant cooling and power consumption reduction.
The proprietary MAGIC (MAgnetic Gradient Induced Coupling) quantum processors are used in EleQtron’s quantum computers. A string of Ytterbium ions ( 171 Yb + ) is first produced by laser ablation in a high vacuum. These ions can form a register of up to 30 ions, each of which functions as a qubit. A Paul trap, which is created by combining an oscillating electric field with a magnetic field, holds qubits in place.
Using a single-sideband (SSB) mixer, the control mechanism creates signals at a frequency of about 12.64 GHz by mixing the output of a specialised DDS card with a high-frequency oscillator source. Importantly, the multi-tone signal needed for controlling and manipulating individual qubits is produced by the DDS card. By modulating the signal in 3–5 MHz increments, the magnetic field produces the Zeeman effect, allowing individual ion addressing while maintaining low crosstalk and integrating well with chip-based ion traps.
Spectrum Instrumentation’s Arbitrary Waveform Generators (AWGs), which frequently use the M4i.66xx-series, enable this capability. These strong AWGs may produce up to 20 separate sine wave carriers per channel when operating in DDS mode.
To get the required Rabi frequency, which determines the pace of quantum operations, the signals must be properly adjusted in terms of amplitude, phase offset, pulse length, and frequency. The DDS firmware offers the flexibility required to create intricate quantum circuits and address additional qubits by enabling ultrafast modifications to the sine wave cores with a resolution of 6.4 ns. The DDS solution was acknowledged by the eleQtron team as being essential to their idea.
Conclusion and Prospects for the Future
The development of quantum systems depends on DDS technology‘s vital features, which include fast sweeping, high integration, multi-channel synchronisation, and highly resolved frequency tuning. DDS streamlines hardware design while improving performance, whether it is used in sophisticated AWGs for intricate trapped-ion qubit control or as a quick, integrated microwave source for NV sensing applications. In the future, the development of the integrated MW source seeks to improve integration even more by substituting on-board chips for external peripherals.
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