Coherent Control of NV Centers
Nitrogen vacancy (NV) centers in diamond offer a prime opportunity for coherently controlling the state of a quantum system. The spin state of the NV center can be manipulated with a sequence of optical and microwave pulses, and exhibits long coherence times even at room temperature. It can either be isolated from the environment for quantum information processing tasks or be used as a sensor of external electric or magnetic fields. Shifting the NV center's energy levels with a vector magnet allows it to operate at frequencies ranging from DC up to 20 GHz. The NV center's high degree of tunability, both in terms of its frequency range as well as its sensitivity to the environment, make it a versatile system when coupled with an experimental setup that can fully exploit its properties.
The NV center is initialized in the |ms=0> ground state (see Figure 1) by a green laser pulse that is controlled by a TTL signal applied to an acousto-optic modulator (AOM). Figure 2 shows how a marker output of the Zurich Instruments SHFSG Signal Generator can send out TTL pulses to trigger the AOM that generates the green laser pulse; Figure 3 illustrates an alternative setup based on the digital inputs and outputs of the HDAWG Arbitrary Waveform Generator.
Using the 4 or 8 marker outputs on the front panel or, alternatively, the 32 channels of the DIOs allows for simpler and more compact experimental setups because there is no need for an external TTL pulse generator.
Spin manipulation is carried out by applying microwave (MW) signals with well-defined amplitude, frequency, and phase. The SHFSG combines the ability of an AWG to generate IQ signals with built-in frequency upconversion based on a double superheterodyne technique: this makes it possible to output spectrally clean and complex sensing sequences directly at the NV center spin transition frequency. The oscillators of the SHFSG can be set to arbitrary phase values, so that the phases of the output signals can be tuned as needed.
The amplified output signal is then sent to a MW antenna, which generates microwave magnetic fields at the NV center and thereby transmits sequences of pulses to manipulate the spin state. Some measurements require combinations of multiple frequency components, each with their own pulse shapes: this is the case in state transfer protocols requiring pulses with two different microwave frequencies, for example, or in combined radio-frequency and microwave fields that control interactions between nuclear and electron spins. Multiple sets of pulse envelopes can be generated in concert thanks to the 4 or 8 output channels of the SHFSG, making it easy to coordinate pulses with different frequencies.
Alternatively, the HDAWG can generate MW signals by using an IQ mixer to combine the frequency of a local oscillator (LO) with two outputs from the HDAWG, labelled I and Q in Figure 3. The I and Q components determine the phase and amplitude of the final MW signal. Any noise in the I and Q components influences the signal quality and may cause pulsing errors: the low noise of the HDAWG ensures that pulse quality is not limited by the instrumentation. Some IQ mixers suffer from LO leakage, which can drive unwanted transitions and reduce measurement quality; if needed, the marker outputs of the HDAWG can be used to control MW switches and prevent leaked LO from reaching the NV center. The MW signal is then amplified and sent to a MW antenna, just as with the SHFSG.
The quantum state readout of the NV center spin is carried out by illuminating the system with the green laser and measuring the fluorescence rate on an avalanche photodiode (APD). Characterization or control of the NV center can also be achieved with a red laser (for resonant excitation) or a yellow laser (for charge state readout); the result can be monitored through counts on the APD. The counts on the APD can be recorded either with an external counter card that can be gated by the SHFSG or with the HDAWG-CNT Pulse Counter option on the HDAWG. With the HDAWG-CNT upgrade option it is also possible to use the detected count rates to make real-time decisions within a sequence, for example for charge state initialization.
Quantum state readout of an ensemble of NV centers can be achieved with a normal photodiode and lock-in detection. This application page presents the benefits of using Zurich Instruments' lock-in amplifiers for this use case.
Improving the isolation from the environment or the sensing resolution often requires complicated sequences featuring a long series of pulses or a few short pulses separated by long evolution times. With the LabOne® AWG Sequencer it is possible to optimize waveform handling so that the SHFSG or the HDAWG can generate long signals with a short upload time while maintaining a high timing precision with a jitter below 10 ps. For setups with their own control software, the SHFSG and the HDAWG can both be programmed using freely available APIs for MATLAB® and Python or the LabOne Q software framework, making it straightforward to integrate the instruments into existing systems.
The Benefits of Choosing Zurich Instruments
- Improve the sensitivity or coherence protection with long and complex pulse sequences that are not limited by memory or upload time.
- You can boost the quality of your measurements by generating pulses with clean spectra and low timing jitter.
- To generate the pulse shape that is optimal for the experiment you want to perform, take advantage of the AWG Sequencer for waveform handling.
- You can simplify your setup thanks to the marker channels and DIOs coordinating the instruments across your experiment.