Name: Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source (Video) | JoVE Journal
Uploaded: 2017-04-04T10:30:00.000+00:00
Duration: 12 min 19 s
Description: Learn to characterize integrated photonic photon pair sources through quantum interference measurements using silicon ring resonators. This method enables chip-scale correlated photon generation for quantum circuits.

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5 minutes

This work was performed in part at the Cornell University NanoScale Science and Technology Facility, a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation (Grant ECCS-1542081). We acknowledge support for this work from the Air Force Research Lab (AFRL). This material is based upon work partially supported by the National Science Foundation under Award No. ECCS14052481.

NOTE: This protocol assumes that the photonic chip has already been fabricated. The chip described here (shown in Figure 1A) was fabricated at the Cornell University NanoScale Science &amp; Technology Facility using standard processing techniques for silicon photonic devices11. These include the use of silicon-on-insulator wafers (composed of a 220 nm-thick silicon layer, a 3-µm layer of silicon dioxide, and a 525 µm-thick silicon substrate), electron-beam lithography to define the strip waveguides (500 nm-wide), and the plasma-enhanced chemical vapor deposition of the silicon dioxide cladding (~3 µm-thick). The micro-ring resonators were designed with an inner radius of 18.5 µm and a waveguide-to-ring gap of 150 nm. Figures of merit for this device include loss, quality factor, free spectral range, and dispersion.
1. Photonic Chip Preparation
<ol>
	<li>Place a small amount of wax on a cross-sectioning polishing mount and heat it to ~130 °C. 
	NOTE: The amount of wax to be used depends upon the size of the sample being mounted. There must be enough wax to keep the chip immobile, while too much will result in wax on the chip facets.</li>
	<li>Place the photonic chip on the portion of the polishing mount with the wax. Ensure that the wax is melted completely so that the chip is flat against the mount. Use plastic tweezers when handling the chip to avoid damaging the facets.</li>
	<li>Allow the mount to cool in ambient air so that the wax solidifies. Cooling faster than this may result in damage to the chip.</li>
	<li>Polishing the chip facets. 
	NOTE: It is important to choose the correct lapping pad as starting with a pad that is too aggressive can result in polishing away more of the chip than desired.
	<ol>
		<li>Attach the polishing mount to the polisher and polish for only a few seconds. A pad with a 3-µm roughness has shown to be a good starting point for silicon chips with facet lengths of ~1 cm.</li>
		<li>Remove the polishing mount and inspect the chip facet to determine how level the chip is mounted. 
		NOTE: A microscope is useful for measuring the distance between the ends of the waveguides and the facet of the chip. These measurements allow for the angle between the facet and the waveguide to be determined.</li>
		<li>Make the necessary adjustments to the micrometers on the polisher in order to improve the leveling of the chip.</li>
		<li>Repeat steps 1.4.1-1.4.3 until the facet of the chip and the waveguides are within 0.15° of being orthogonal to each other.</li>
		<li>Polish the chip in steps of ~50 µm, inspecting the chip between each step to monitor the distance remaining, until there is ~100 µm left to polish. If at any point the cladding appears to be delaminating from the surface, ensure that the pad is rotating so as to polish from the top of the chip to the bottom. 
		NOTE: It may also help to use a polishing lubricant instead of water. This delamination is a result of stress in the cladding and is an indication that the fabrication process needs to be optimized.</li>
		<li>Change to a 1-µm lapping pad and polish until there is ~20 µm remaining.</li>
		<li>Change to a 0.5-µm pad and continue polishing for another 15 µm.</li>
		<li>Use a 0.1-µm pad for the final 5 µm to ensure a smooth facet. Microscope images of the facet of a silicon photonic chip before and after polishing are shown in Figure 2.</li>
	</ol>
	</li>
	<li>Heat the mount with the attached chip to ~130 °C to allow the wax to melt.</li>
	<li>Once the wax is completely melted, remove the chip from the mount and allow it to cool slowly.</li>
	<li>Clean any of the remaining wax from the chip using acetone, isopropanol, and water.</li>
</ol>
2. Preparation of Fiber Pigtails
<ol>
	<li>Strip any buffer or coating from the end of a single mode fiber (SMF) pigtail and from one end of an ultra-high numerical aperture (UHNA) fiber.</li>
	<li>Clean the bare ends of the fibers with a mixture of acetone and methanol.</li>
	<li>Cleave the bare ends of both fibers with a commercial fiber cleaver.</li>
	<li>Fusion splice the cleaved end of the fibers. A recipe for splicing SMF to UHNA fiber is shown in Table 1.</li>
	<li>Slide a protective sleeve over the splice and place it in the sleeve oven to permanently attach it to the fiber.</li>
	<li>Repeat steps 2.1-2.5 to prepare a total of three fibers.</li>
</ol>
3. Configuration of the Testing Setup
NOTE: A diagram of the testing setup is shown in Figure 1B. The mount for the chip is a copper pedestal that is in contact with a thermo-electric cooler (TEC). There is a microscope fitted with both visible and infrared (IR) cameras for viewing the photonic chip.
<ol>
	<li>Place a small amount of wax on the chip mount and apply voltage to the TEC to melt the wax.</li>
	<li>Place the chip on the melted wax, ensuring that it is sitting flat on the mount.</li>
	<li>Remove the voltage from the TEC and allow the mount and chip to cool slowly.</li>
	<li>Attach each of the spliced fibers to a fiber v-groove with polyimide tape and mount a single v-groove to each of the 3-axis stages using the manufacturer-supplied mounting hardware.</li>
	<li>Fiber edge coupling.
	<ol>
		<li>Connect the three fibers to their respective components: one to the optical output of the laser and the other two to optical power meters.</li>
		<li>Adjust the microscope so that it is focused on the chip where the waveguides reach the edge.</li>
		<li>Position the fibers close to the chip edge so that they are in the view of the visible camera and adjust their heights so that the core of each fiber is in focus.</li>
		<li>Adjust the horizontal positioning of the fibers with the stage micrometers so that they are lined up with the waveguides.</li>
		<li>Turn on the optical output of the laser and tune the horizontal and vertical micrometer positions of the input fiber until the light is coupling into the waveguide. This will show up on the IR camera as scattering along the input waveguide.</li>
		<li>Tune the wavelength of the laser to a point where the micro-ring resonator is lit up on the camera. This indicates that the resonance condition is being satisfied and that the light is reaching the output waveguides.</li>
		<li>Adjust the horizontal and vertical micrometer positions of the output fibers until there is a measurable amount of light extending from the waveguides to the power meters.</li>
		<li>Maximize the power to both detectors by manipulating the horizontal and vertical micrometer positions of the three fibers.</li>
		<li>Further maximize the power to the detectors by making fine adjustments to the horizontal and vertical fiber positions using the piezo controllers.</li>
		<li>Use the piezo controllers to move the fibers slightly closer to the chip. Make sure not to use the micrometers to push the fibers onto the chip, as doing so will likely damage the cleaved ends of the fibers.</li>
		<li>Repeat steps 3.5.9 and 3.5.10 until the fibers are firmly pressed against the sides of the chip. 
		NOTE: Excessive scattered light from the waveguides, coupled with poor waveguide transmission, can be an indication of waveguide defects. These can include, but are not limited to, material defect sites, stitching boundaries, and excessive waveguide roughness.</li>
	</ol>
	</li>
	<li>Place a fiber-based polarization controller between the laser and the chip. This allows for control of the polarization state that makes it to the chip. The waveguides, being wider than they are tall, aid in minimizing any on-chip polarization rotation.</li>
	<li>Dispersion characterization.
	<ol>
		<li>Tune the polarization of the laser output to maximize the coupling to the chip. This device was designed for transverse-electric (TE) polarization and, as such, the transverse-magnetic (TM) polarization has a much higher loss.</li>
		<li>Scan a tunable laser over the wavelength range of interest (1,510 nm to 1,600 nm in this case) and monitor the power meters. Any artifacts in the transmission spectra are likely due to a combination of the TM component of the polarization and etalon effects from the fiber-chip interface.</li>
		<li>Locate the resonant wavelengths in the spectrum and also extract the bandwidth of each resonance. This particular chip had bandwidths as small as 65 pm, which translates to quality factors (Q) of up to 23,000.</li>
		<li>Determine the free spectral range (FSR), the separation between resonances, for each adjacent pair of resonances. This particular device had an FSR of ~5 nm.</li>
		<li>Calculate the group index (ng) of the guided mode for each value of the FSR using the following equation: 
		<img alt="Effective refractive index formula, \( n_g = \frac{\lambda^2}{FSR \times 2\pi \times r} \), optical analysis." src="/files/ftp_upload/55257/55257eq5.jpg" data-altupdatedat="1788250015000" data-alt="Equation 5"> 
		where λ is the wavelength and r is the radius of the micro-ring resonator. The above equation is a first-order approximation of the group index.</li>
		<li>Use the bandwidth of each resonance to determine a width (Δng) associated with each value of the group index.</li>
		<li>Choose the wavelengths for the two pump lasers such that they coincide with resonances in the spectrum and have an odd number of resonances between them (Figure 1C).</li>
		<li>Determine the wavelength of the degenerate bi-photons using the following equation: 
		<img alt="Bi-photon wavelength equation, λBi-Photons formula for photonic studies, theoretical analysis." src="/files/ftp_upload/55257/55257eq6.jpg" data-altupdatedat="1788250015000" data-alt="Equation 6"> 
		where λPump 1 and λPump 2 are the wavelengths of the pump photons.</li>
		<li>Add a horizontal line on the plot of group index versus wavelength that extends between the two pump wavelengths (Figure 3). If it is possible for the line to sit within ng ± Δng at all three wavelengths of interest simultaneously, the phase-matching condition is satisfied and photons can be generated via SFWM. If it is not possible, try choosing pump wavelengths that are closer together and check again.</li>
	</ol>
	</li>
	<li>Add a second tunable laser source and polarization controller to the setup and combine the optical outputs from both lasers with a 1 x 2 fiber combiner.</li>
	<li>Add a series of fiber-based notch filters (enough of them achieve ~120 dB of attenuation) immediately before the chip. 
	NOTE: The filters allow both pump wavelengths to pass but reject the bi-photon wavelength. They help to remove excess noise (i.e., broadband Raman scattering in the optical fiber) before coupling to the chip. The filter spectrum is shown in Figure 1B.</li>
	<li>Add a series of fiber-based bandpass filters (enough of them to achieve ~150 dB of attenuation) immediately after the chip. 
	NOTE: The filters should be wide enough to allow the bi-photons to pass but narrow enough to reject the pump photons. Two sets of these are needed, one set for each output. The filter spectrum is shown in Figure 1B.</li>
	<li>Send the rejected photons from each set of filters to separate power meters. 
	NOTE: These power meters are used to monitor the optical coupling to the chip and can also be used to determine if the pump lasers are remaining on-resonance.</li>
	<li>Connect the individual optical output from each set of fiber-based filters to a single photon detector (SPD) and connect both electrical signal outputs from the SPDs to a coincidence correlator.</li>
	<li>Cross a pair of tungsten probes and set the tips down on one of the spiral legs (~1 mm in length) of the MZI.</li>
	<li>Connect a power supply to the two crossed probes such that they generate heat when voltage is applied. This will act as the phase-shifter for the MZI. 
	NOTE: See the discussion for a description of the more standardized method for the thermal tuning of photonic devices.</li>
</ol>
4. Measuring Two-photon Interference
<ol>
	<li>Tune both of the pump lasers to the chosen wavelengths. Use the power meters that are monitoring the rejected pump photons to ensure that both lasers are tuned to the resonances. When the lasers are properly tuned to the desired resonances, the rejected signal from the filters will be maximized.</li>
	<li>Set the optical power output from each laser to -3 dBm. 
	NOTE: This will result in &lt;100 µW at the chip. It is important to keep the pump power this low in order to minimize loss (from multi-photon absorption and free-carrier absorption) and maintain stability (by minimizing light-induced thermal shifts). PN junctions can be used to remove carriers from the waveguide to better accommodate higher pump powers.</li>
	<li>Monitor the coincidence counts (synchronous singles across two ports) by integrating over ~220 ps about the peak of the data. A sufficient integration time has passed when a minimum of 100 coincidence counts have been collected. 
	Note: The integration window should be wide enough to account for the timing jitter of the SPDs.</li>
	<li>Set the power supply for the phase-shifter to the initial voltage (e.g., 0 V).</li>
	<li>Scan one of the tunable lasers over the entire wavelength range and use the power meters that are collecting the rejected pump photons to confirm the location of the resonances of interest. Set the pump lasers to the wavelengths corresponding to the desired resonances. 
	Note: It is important to complete this step every time the phase shifter voltage is changed as the thermal tuning can result in small shifts in the resonant wavelengths.</li>
	<li>Collect the resulting data (single-photon counts as well as the coincidence counts) from the coincidence correlator for the previously chosen integration time. Here, an integration time of 90 s was chosen with a timing resolution of 32 ps.</li>
	<li>Increase the voltage applied to the phase-shifter by 5 mV.</li>
	<li>Repeat steps 4.4-4.6 until data has been collected for the desired range of voltages. 
	NOTE: The maximum voltage was limited to 2.4 V due to rapid degradation of the probes above this voltage.</li>
	<li>Integrate the coincidence peaks for each power-supply voltage over ~220 ps to determine the total number of coincidences (Figure 4).</li>
	<li>Integrate over 320 ns away from the coincidence peak to obtain the accidental coincidences. Use this result to calculate the number of accidentals in the coincidence peak.</li>
	<li>Fit the singles counts from each detector with the following modified sine function: 
	<img alt="Mathematical function formula: f(V)=A*sin(B*e^(C*V)+D)*(V+E)+F for data modeling analysis." src="/files/ftp_upload/55257/55257eq7.jpg" data-altupdatedat="1788250015000" data-alt="Equation 7"> 
	where A, B, C, D, E, and F are the fit parameters. This fit is necessary due to the nonlinear relationship between the voltage and the induced thermal shift (relative phase).</li>
	<li>Convert the independent variable to relative phase for all three sets of data (the singles counts from each detector and the coincidence counts) with the following equation: 
	<img alt="Equation for membrane potential, θ(V)=(B*e^(C*V)+D)*(V+E), relevant to electrophysiology." src="/files/ftp_upload/55257/55257eq8.jpg" data-altupdatedat="1788250015000" data-alt="Equation 8"> 
	where B, C, D, and E are the fit parameters from step 4.11. This conversion is possible due to the well-known sinusoidal transfer function of an MZI12.</li>
	<li>Fit the coincidence data (with the relative phase as the independent variable) with the following sine function: 
	<img alt="Trigonometric function, \( f(\theta) = A \cdot \sin(2\theta) + B \), mathematical formula analysis." src="/files/ftp_upload/55257/55257eq9.jpg" data-altupdatedat="1788250015000" data-alt="Equation 9"> 
	where A and B are the fit parameters.</li>
	<li>Calculate the visibility of each interference pattern with the following equation: 
	<img alt="Static equilibrium equation: ν = (f(θ)max - f(θ)min) / (f(θ)max + f(θ)min) formula." src="/files/ftp_upload/55257/55257eq10.jpg" data-altupdatedat="1788250015000" data-alt="Equation 10"> 
	where ƒ(θ)max and ƒ(θ)min are the maximum and minimum values of ƒ(θ), respectively. A visibility of 1 corresponds to a perfect interference pattern.</li>
</ol>

<ol><li>Silverstone, J. W., et al. <a target="_blank" href="http://www.nature.com/nphoton/journal/v8/n2/full/nphoton.2013.339.html">On-chip quantum interference between silicon photon-pair sources.</a> Nat. Photon. 8 (2), 104-108 (2014).</li>
<li>Harris, N. C., et al. <a target="_blank" href="http://journals.aps.org/prx/abstract/10.1103/PhysRevX.4.041047">Integrated Source of Spectrally Filtered Correlated Photons for Large-Scale Quantum Photonic Systems.</a> Phys. Rev. X. 041047, 1-10 (2014).</li>
<li>Grassani, D., et al. <a target="_blank" href="https://www.osapublishing.org/optica/abstract.cfm?uri=optica-2-2-88">Micrometer-scale integrated silicon source of time-energy entangled photons.</a> Optica. 2 (2), 88(2015).</li>
<li>Najafi, F., et al. Scalable Integration of Single-Photon Detectors. Nat. Commun. 6, 1-8 (2015).</li>
<li>Dutt, A., et al. <a target="_blank" href="http://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.3.044005">On-Chip Optical Squeezing.</a> Phys. Rev. Appl. 3 (4), 1-7 (2015).</li>
<li>Azzini, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Ultra-low+power+generation+of+twin+photons+in+a+compact+silicon+ring+resonator%5BTitle%5D)+AND+%22Opt.+Express%22%5BJournal%5D)">Ultra-low power generation of twin photons in a compact silicon ring resonator.</a> Opt. Express. 20 (21), 23100-23107 (2012).</li>
<li>Clemmen, S., et al. <a target="_blank" href="https://www.osapublishing.org/oe/abstract.cfm?uri=oe-17-19-16558">Continuous wave photon pair generation in silicon-on-insulator waveguides and ring resonators erratum.</a> Opt. Express. 17 (19), 16558(2009).</li>
<li>Engin, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Photon+pair+generation+in+a+silicon+micro-ring+resonator+with+reverse+bias+enhancement%5BTitle%5D)+AND+%22Opt.+Express%22%5BJournal%5D)">Photon pair generation in a silicon micro-ring resonator with reverse bias enhancement.</a> Opt. Express. 21 (23), 27826-27834 (2013).</li>
<li>Steidle, J. a, et al. <a target="_blank" href="http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2299737">High spectral purity silicon ring resonator photon-pair source.</a> Proc. of SPIE. 9500, 950015(2015).</li>
<li>Preble, S. F., et al. <a target="_blank" href="http://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.4.021001">On-Chip Quantum Interference from a Single Silicon Ring-Resonator Source.</a> Phys. Rev. Appl. 4, 021001(2015).</li>
<li>Cao, L., Aboketaf, A. A., Preble, S. F. <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/S0030401813004604">CMOS compatible micro-oven heater for efficient thermal control of silicon photonic devices.</a> Opt. Commun. 305, 66-70 (2013).</li>
<li>Chrostowski, L., Hochberg, M. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aChrostowski%2C+L+%22Silicon+Photonics+Design%22">Silicon Photonics Design</a>. , Cambridge University Press. Section 4.3 (2013).</li>
</ol>

We have nothing to disclose.

There are multiple challenges for the field of integrated photonics to overcome in order for complex and scalable systems of photonic devices to be feasible. These include, but are not limited to: tight fabrication tolerances, isolation from environmental instabilities, and minimization of all forms of loss. There are critical steps in the above protocol that help to minimize the loss of photonic devices.
One of the most crucial requirements in minimizing loss is closely matching the optical m...

Silicon is showing great promise as a photonics platform for quantum information processing1,2,3,4,5. One of the vital components of quantum photonic circuits is the photon source. Photon-pair sources have been developed from silicon in the form of micro-ring resonators made via a third-order nonlinear process, spontaneous four-wave mixing (SFWM)6,7,8. These sources are capable of producing pairs of indistinguishable photons, which are ideal for experiments involving photon entanglement9.
It is important to note that ring resonator sources can operate with both clockwise and counter-clockwise propagation, and the two different propagation directions are generally independent from one another. This allows a single ring to function as two sources. When optically pumped from both directions, these sources generate the following entangled state:
<img alt="Quantum state equation, diagram, describing optical cavity modes in ring resonators." src="/files/ftp_upload/55257/55257eq1.jpg" data-altupdatedat="1788250015000" data-alt="Equation 1">
where <img alt="Quantum operator symbol â_cw†; representation in optics; text-based scientific concept." src="/files/ftp_upload/55257/55257eq2.jpg" data-altupdatedat="1788250015000" data-alt="Equation 2"> and <img alt="Counter-clockwise mode operator symbol diagram, Â†ccw, used in quantum optics and photonics." src="/files/ftp_upload/55257/55257eq3.jpg" data-altupdatedat="1788250015000" data-alt="Equation 3"> are the independent creation operators for clockwise- and counterclockwise-propagating bi-photons, respectively. This is a very desirable form of entangled state known as a N00N state (N=2)10.
Passing this state through an on-chip Mach-Zehnder interferometer (MZI) results in the state:
<img alt="Quantum optics equation; Ψ state calculation formula; mathematical representation of optical systems." src="/files/ftp_upload/55257/55257eq4.jpg" data-altupdatedat="1788250015000" data-alt="Equation 4">
This state oscillates between maximum coincidence and zero coincidence at twice the frequency of classical interference in an MZI, effectively doubling the sensitivity of the interferometer10. Here, we present the procedure used to test such an integrated photon source and MZI device.

<table><tbody><tr><td>3-Axis NanoMax Flexure Stage</td><td>Thorlabs</td><td>MAX312D</td><td>Precision 3-axis stages</td></tr><tr><td>Three-Hole Fiber Stripping Tool</td><td>Thorlabs</td><td>FTS4</td><td>buffer stripping tool</td></tr><tr><td>Three Channel Piezo Controller</td><td>Thorlabs</td><td>MDT693B</td><td>Piezo controllers for NanoMax stages</td></tr><tr><td>Fiber Polarization Controller</td><td>Thorlabs</td><td>FPC562</td><td>3-Paddle fiber-based polarization controller</td></tr><tr><td>Fiber Cleaver</td><td>Thorlabs</td><td>XL411</td><td>Fiber cleaver</td></tr><tr><td>Standard V-Groove Fiber Holder</td><td>Thorlabs</td><td>HFV001</td><td>standard v-groove mount</td></tr><tr><td>Tapered V-Groove Fiber Holder</td><td>Thorlabs</td><td>HFV002</td><td>tapered v-groove mount</td></tr><tr><td>Right-Angle Top Plate for NanoMax Stage</td><td>Thorlabs</td><td>AMA011</td><td>right-angle bracket</td></tr><tr><td>50:50 Fiber Optic Coupler</td><td>Thorlabs</td><td>TW1550R5F1</td><td>50/50 combiner</td></tr><tr><td>Optical Fiber Fusion Splicer</td><td>Fujikura</td><td>FSM-40S</td><td>Fusion splicer</td></tr><tr><td>MultiPrep Polishing System - 8&quot;</td><td>Allied High Tech</td><td>15-2100</td><td>Chip polisher</td></tr><tr><td>GreenLube</td><td>Allied High Tech</td><td>90-209010</td><td>Polishing Lubricant</td></tr><tr><td>Cross-Sectioning Paddle with Reference Edge</td><td>Allied High Tech</td><td>15-1010-RE</td><td>Polishing mount</td></tr><tr><td>Lightwave Measurement System</td><td>Keysight</td><td>8164B</td><td>Mainframe for tunable laser</td></tr><tr><td>Tunable Laser Source</td><td>Keysight</td><td>81606A</td><td>Tunable laser</td></tr><tr><td>Optical Power Sensor</td><td>Keysight</td><td>81634B</td><td>Power meter</td></tr><tr><td>NIR Single Photon Detector</td><td>ID Quantique</td><td>ID210</td><td>Single photon detectors</td></tr><tr><td>NIR Single Photon Detector</td><td>ID Quantique</td><td>ID230</td><td>Low noise, free-running single photon detectors</td></tr><tr><td>PicoHarp</td><td>PicoQuant</td><td>PicoHarp 300</td><td>Time-correlated single photon counting</td></tr><tr><td>WiDy SWIR InGaAs Camera</td><td>NIT</td><td>640U-S</td><td>IR Camera</td></tr><tr><td>WDM Bandpass Filter</td><td>JDS Uniphase</td><td>30055053-368-2.2</td><td>pump cleanup filters</td></tr><tr><td>WDM Bandpass Filter</td><td>JDS Uniphase</td><td>1011787-012</td><td>pump rejection filters</td></tr><tr><td>Ultra-High Numerical Aperture Fiber</td><td>Nufern</td><td>UHNA-7</td><td>high index fiber</td></tr><tr><td>Ultra Optical Single Mode Fiber</td><td>Corning</td><td>SMF-28</td><td>standard single mode fiber</td></tr></tbody></table>

measurement of quantum interference in a silicon ring resonator photon source

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Individual photon counts from each detector, as well as the coincidence counts, were collected as the relative phase between the two paths was tuned. The individual counts (Figure 5A) show the classical interference pattern from an MZI with visibilities of 94.5 ± 1.6% and 94.9 ± 0.9%. The coincidence measurements (Figure 5B) show the quantum interference of the entangled state, as is evident by the oscillation at twice the frequency of the classical inter...

Silicon photonic chips have the potential to realize complex integrated quantum systems. Presented here is a method for preparing and testing a silicon photonic chip for quantum measurements.

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit ...

The overall goal of this procedure is to characterize an integrated photonic photon pair source through the measurement of quantum interference. This method can help answer key questions related to integrated quantum photonics, including how to realize chip scale sources of correlated photons and integrate them into quantum integrated photonic circuits. The main advantage of this technique is that it can be applied to a wide variety of integrated quantum photonic circuits.At the heart of the experiment is the photonic chip. This chip is approximately five millimeters on a side, and is fabricated using standard techniques. This image of the chip reveals its components.There is a pump circuit, including the input wave guide, a ring resonator, in which photons will propagate both clockwise and counterclockwise, and a Mach-Zehnder interferometer, which is followed by output wave guides. Metal leads allow for on-chip heating, resulting in a phase shift in the interferometer. To prepare the chip for use in the circuit, polish with a chip polisher.First use the polisher to level the chip, and make all facets orthogonal. Polish the chip with a three-micron lapping pad, in steps of about 50 microns, until within about 100 microns of the end of the polishing marks. After each 50 microns, inspect the chip to determine the remaining distance.When there is about 100 microns remaining, change to a one-micron lapping pad. Continue to polish the chip and to monitor the progress. When there is about 20 microns remaining, change to a 0.5 micron pad.Polish the chip further, until within 15 microns of the end of the polishing marks. At 15 microns, change the lapping pad to one with 0.1 micron roughness. Use this pad to polish the chip until only 10 microns of the polishing marks remain.The last polishing step, with a 0.1 micron lapping pad, ensures a smooth facet. Remove the chip before cleaning it and storing it for later use. Gather the necessary equipment to prepare the optical fibers.This includes a fiber stripper, a fiber cleaver, a fusion splicer, and a sleeve oven. Work with the three single-mode fiber pigtails, and about 20 to 30 centimeters of ultra-high numerical aperture fiber for each. To prepare one pigtail, use the fiber stripper to remove any buffer or coding from its end.Do the same with one end of the length of ultra-high numerical aperture fiber. After cleaning the fibers, use the fiber cleaver to prepare them for fusion splicing. Next, move the fibers to the splicer.Put the fibers in position, and properly align the cleaved ends. Enter the appropriate parameters, and perform the splice. When done, remove the spliced fibers and inspect it.If the splice is acceptable, slide a protective sleeve over the splice site. Then place the sleeve-covered splice into the sleeve oven to permanently secure it to the fiber. Go on to produce three spliced fibers for use in the experiment.The experiment takes place on an optical bench. On the bench are three three-axis translation stages with piezo controllers. They are positioned to allow access to the chip wave guides.The translation stages surround the optical chip that has already been mounted on a copper pedestal. The pedestal is in contact with the thermoelectric cooler. Each translation stage has one of the prepared fibers in a V-groove, and attached with polyimide tape.The region with the chip can be viewed using a microscope, which is equipped with both visible and infrared cameras. At this point, the fibers can be connected to the experiment instruments. Connect the chip input to the optical output of a tunable laser source via a polarization controller.Connect each output of the chip to an optical power meter. Now, adjust the microscope position to work with the chip. Focus the microscope where the wave guides reach the edge of the chip, and use the translation stages to position the fibers near the chip edge.Bring the fibers into view of the visible camera, and adjust their heights, so the core of each fiber is in focus. Before proceeding, ensure the horizontal position of each fiber lines up with its wave guide. Turn on the optical output of the laser, and adjust the position of the input fiber until light couples into the wave guide.On the infrared camera, this will appear as scattering along the input wave guide. Next, tune the wavelength of the laser so the microring resonator is lit up on the infrared camera. This indicates the resonance condition has been satisfied.Continue by manipulating the fiber positions with the micrometers, so as to maximize the output power measured by the power meters. Finely tune the fiber positions, and move each fiber slightly closer to the chip, using the piezo stage controllers. Iterate between fine-tuning all the fiber couplings and moving all fibers closer to the chip.The goal is to have the fibers pressed firmly against the sides of the chip, with the measured power maximized. The next step is to characterize the dispersion. Begin the characterization by tuning the polarization controller to maximize the power reading on the power meters.Now, scan the tunable laser over the wavelength range of interest to find the transmission spectrum. Extract the bandwidth of each resonance, and use the information to find the group indices and corresponding uncertainties. Next, identify the wavelengths of the two pump lasers by finding two resonances that have an odd number of resonances between them.Knowing these wavelengths allows determination of biphoton wavelength. To test whether these three wavelengths are consistent with spontaneous four-wave mixing, plot the group index versus the wavelength. In this case, the blue points are the group indices.The red shading corresponds to the uncertainty of the group indices, as a result of the bandwidth of each resonance. The green horizontal line extends between the candidate pump laser wavelengths. Since the line is entirely within the shaded region, the pump and biphoton wavelengths can be used for the experiment.Once the probe wavelengths have been determined, create the final experiment setup. This has two tunable laser sources, one for each of the pump laser wavelengths. The laser outputs each go to separate polarization controllers.From there, the two laser outputs are combined in a fiber combiner. Next to there is a series of fiber-based notch filters. These filters allow passage of the pump wavelengths, but they achieve approximately 120 decibels of attenuation of the biphoton wavelength.The output of this filter goes into the photonic chip. On each output, after the chip, there is a series of bandpass filters. These filters attenuate the pump wavelengths by about 150 decibels, but pass the biphoton wavelengths.The rejected photons from each set of filters are sent to a dedicated power meter. The output from each of the fiber-based filters goes to a dedicated single photon detector. Each of the single photon detectors provides input to a coincidence correlator.The phase shifter for the Mach-Zehnder interferometer is an on chip resistive heater. Connect a computer-controlled current driver to the chip's contact pads to generate heat when the voltage is set. For two photon interference measurements, start with the pump lasers at the chosen wavelengths.Monitor the power meters to ensure each laser is tuned to its resonance, and the power is maximized. Next, monitor the coincidence counts at the correlator. As indicated in this figure, find the peak of the data and integrate over an approximately 220 picosecond window, centered on it.Keep track of the coincidence counts until there is a total of at least 100. This indicates that a sufficient integration time has passed. Now, turn to the computer to set the voltage control for the phase shifter at zero volts.Once the phase shift is set, go to one of the tunable lasers and scan over the entire wavelength range. Use the power meters for the rejected pump photons to identify the location of the previously-selected resonances which may have drifted. Set the pump lasers to match the previously-chosen resonances.It is important to follow the chosen resonances over time, rather than the wavelengths. is heated, the ring is also heated, but much less efficiently. This shifts the resonances to longer wavelengths.Collect the resulting data from the time correlator using the previously-chosen integration time. This includes the number of photons counted by each detector in the coincidence counts. After collecting the data, adjust the voltage control of the phase shifter, and increment it by five millivolts.Repeat scanning the laser and collecting count data until the desired range of voltages is covered. These classical light interference patterns were obtained using the testing setup by collecting individual photon counts as a function of the relative phase between the two paths. In addition to the measured data, represented by circles and diamonds, the solid lines are fits to the data.The numbers represent the calculated visibility. Coincidence correlation measurements show the quantum interference of the entangled photons. Note that the oscillation is at twice the frequency of the classical pattern.The orange curve is from a test of photon origin that requires entangled photons to be generated at a wavelength not supported by the ring. It demonstrates the coincidences are from photons generated in the ring. This data is from six experiments in which resonance pairs are symmetric in frequency, about the desired biphoton residence.Each set of data demonstrates a period of half that of the relative phase. Once mastered, this technique can be done in 10 to 15 hours, if it is performed properly. The total time is primarily determined by the resolution of the phase shifter voltage increment and the associated integration time of each biphoton coincidence measurement.While attempting this procedure, it is important to remember to take your time while optimizing the couplings of the chip. If not done properly, the fibers may not be stable during the measurements. After watching this video, you should have a good understanding of how to prepare and test integrated photonic photon sources.

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Research

JoVE Journal

Engineering

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