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.
NV center is initialized in the |ms=0> ground state (see Figure 1) by an initial green laser pulse that is controlled by a TTL signal applied to an acousto-optic modulator (AOM). In Figure 2, the digital inputs and outputs (DIOs) of the HDAWG send out TTL pulses that pass through a digital buffer before being used to trigger the AOM generating the green laser pulse.
Using the 32 channels of the DIOs or, alternatively, the 4 or 8 marker outputs on the front panel, allows for simpler and more compact experimental setups because there is no need for an external pulse generator.
Spin manipulation is carried out by applying microwave (MW) signals with well-defined amplitude, frequency, and phase. The MW signals are generated 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 2. The I and Q components determine the phase and amplitude of the final MW signal, and any noise in the I and Q components will influence the signal quality and may cause pulsing errors. It is therefore crucial that the I and Q components be fully controllable while also having low amplitude and phase noise. The oscillators of the HDAWG can be set to arbitrary phase values, allowing the phases of the I and Q output signals to be tuned as needed. The low noise of the HDAWG ensures that pulse quality is not limited by the instrument. 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 amplified mixed signal is 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 frequencies components, each with their own pulse shapes, such as in state transfer protocols requiring pulses with two different microwave frequencies or in combined radio-frequency and microwave fields for controlling interactions between nuclear and electron spins. Thanks to the 4 or 8 wave output channels of the HDAWG, multiple sets of pulse envelopes can be generated in concert, making it easy to coordinate pulses with different frequencies.
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.
In either case, the HDAWG-CNT option allows the HDAWG to count the APD pulses and measure the fluorescence rate, including the ability to time-tag fluorescence photons with a timing resolution on the order of nanoseconds.
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 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 HDAWG can be programmed using freely available APIs for MATLAB® and Python, making it straightforward to integrate the HDAWG 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 low noise and low timing jitter.
- To generate the pulse shape that is optimal for the experiment you want to perform, take advantage of the HDAWG sequencer for waveform handling.
- You can simplify your setup thanks to the DIOs and marker channels coordinating the instruments across your experiment.