The overall goal of this protocol is to generate and coherently control pulsed quantum frequency combs which may be used for quantum information processing. This method can help us provide the tools for fundamental investigations of quantum mechanics and for realizing non-classical technologies, such as quantum communications and information processing. The main advantage of this technique is that well-established telecommunications components and protocols are used to generate and control complex optical quantum states in a reconfigurable and scalable way.
We have this idea when we talk with my colleague Jose Azana who is an expert in telecommunication processing, and we realized that we could transfer these tools to the world of quantum objects. Set up the components of the system on an optical table. The system has stages for generation, control, and processing.
This depiction provides an overview. All elements are connected with polarization maintaining optical fiber. For now, focus on the generation stage.
This stage features an on chip micro-ring resonator embedded in a larger external cavity. The external cavity is made of an electro-optic amplitude modulator driven by a signal generator, an isolator, and an erbium-doped fiber amplifier providing optical gain. There is a narrow filter with a pass band selecting a single microcavity resonance and a notch filter routing the generated photons to the control section.
With the cavity disconnected, initiate the erbium-doped fiber amplifier. Use an optical power meter after the amplitude modulator to measure the transmitted power. At the modulator, use a power supply to apply a DC offset to have the transmitted power.
Reconnect the cavity and insert a fast photo diode into a different ring port. Use an oscilloscope to observe its signal in the time domain. Without the modulator activated, the output will show unstable pulse operation with a low quality pulse train.
Return to the amplitude modulator and connect the function generator. Set the output frequency of the generator to a multiple of the approximate external cavity mode spacing. Selecting a rectangular or sine waveform and turn on the function generator.
Tune the function generator frequency and DC offset to optimize and stabilize the pulse train on the oscilloscope. Manually adjust the gain to set the pulse intensity. The non-linear four wave mixing process inside the ring resonator now creates signal and idler photon pairs which will leave the external cavity at the notch filter.
Connect the synchronization signal from the function generator to the timing electronics. The generation rate of this pulsed quantum frequency comb may be increased by driving the modulator a higher harmonics of the external cavity frequency spacing, thus entering a harmonic mode locking machine. The amplifier gain must then also be increased to ensure the same power per pulse.
After the notch filter in the generation stage, the entangled photons enter the control stage. There, in order, they encounter a programmable filter, a phase modulator, and a second programmable filter. First, program the filters.
Set the amplitude of the desired frequency mode channels and attenuate the others. Similarly, decide on the desired phase mask. Do this for each filter.
This image represents the amplitude and phase for a coincidence measurement. To prepare the phase modulator, connect the signal generator to the RF amplifier with low loss cables. Then connect the RF amplifier to the phase modulator.
Once all the RF ends are connected and terminated, bias the amplifier. Set the RF signal generator to create sidebands which overlap with the desired frequency modes, then turn it on. As a test, set up the system to send a continuous wave laser beam through the phase modulator.
Use an optical spectrum analyzer to check that the input spectrum produces an output spectrum that corresponds to the intended modulation for the experiment. To continue, connect the phase modulator with the programmable filters. When setting the phase modulation pattern, it is important to determine what pattern will create the targeted frequency sidebands for the chosen operation.
This will depend on the dimension of the quantum state used and on microcavity resonance spacing. The photon pairs from the generation stage enter the control stage. The first filter sets the programmed amplitude and phase.
Next, the phase modulator creates the necessary frequency sidebands. The second filter selects the signal and idler pair. On input to the processing stage, separate the entangled photons with a dense wavelength division multiplexer.
From there, they each go to single photon detectors. Record the arrival time of each photon using timing electronics set up for coincidence measurement. The setup allows coherent control over the photon states.
Here frequency sidebands are generated via phase modulation. The sideband spacing and amplitudes are determined by the modulating signal. Coherent control of the photon state can be used for quantum state tomography.
This is an example reconstruction of the state density matrix for a two photon system with a dimensionality of three for each photon. The fidelity between the measured and maximally entangled states is about 81%The pulsed high repetition rate generation technique is not limited to micro-ring resonators and may be adopted for other optical cavities, such as photonic crystal waveguides, microdisks, and second-order nonlinear cavities. With its development, the platform combines established techniques in telecommunications with quantum technologies towards potential applications, like secure communications at high rates over multiplex channels or high-dimensional quantum operations.
