This research was supported by Research Foundation for Opto-Science and Technology, Japan. We thank to Dr. Tomo Osada for the useful discussions.

The adopted procedure comprises four main stages using the overall experimental setup shown in Figure 2. The first stage was the preparation of the pump laser for SPDC. In the second stage, the optical interferometer - Sagnac interferometer was constructed using a nonlinear crystal and optical polarization components. The coincidence measurement procedure using the electrical components shown in Figure 3 was described in the third stage. Finally, the actual photon correlation data shown in Figure 4 was used to estimate the fidelity and Bell parameters of the generated unconditional polarization-entangled photons.
1. Configuration of the pump laser
<ol>
	<li>Switch on the 405 nm grating-stabilized single-frequency laser diode. Adjust the output power to a few mW by reducing the input electrical current to the laser diode and by neutral density filters.</li>
	<li>Construct an external cavity between the surface of the laser diode and the holographic grating (3,600 mm&#8722;1) to realize a single-frequency operation referred to as a spectrometer. Place the holographic grating about 45o against the laser diode surface and slowly move the screw to adjust the degree, and maximize the output power from the cavity by referring to the image of the beam.</li>
	<li>Couple a laser to the polarization-maintaining optical fiber (PMF) to run a single spatial-mode operation. Adjust the fiber coupler screws to maximize the output power from PMF using a power meter.</li>
	<li>Collimate the output laser from the PMF with a fiber coupler lens. Channel the output laser through an isolator into the center of half-wave plate (HWP), a quarter-wave plate (QWP), and a short-pass dichroic mirror (DM) as shown in Figure 2. For the purpose of generating the polarization-entangled photons with the state as in (6), set the polarization state of the pump laser with diagonal (D) by setting the HWP to 22.5o, and QWP to 0o.</li>
</ol>
2. Construction of the interferometric setup
<ol>
	<li>Place a dichroic mirror (DM), a regular mirror, a PBS, and a ppKTP crystal with dimensions: 10 mm long (crystallographic x-axis), 10 mm wide (y-axis), and 1 mm thick (z-axis) as shown in Figure 2. The PBS operates at both the wavelength of the laser (405 nm) and that of the down-converted photons (810 nm). The poling period of the ppKTP crystal is 3.425 <img alt="Equation 19" src="/files/ftp_upload/59705/59705eq19.jpg" />&#160;which is designed for the collinear type-0 SPDC with 405 nm laser pump and has an anti-reflection coating at both wavelengths.</li>
	<li>Adjust the PBS and mirrors using the pump laser (405 nm) and a reference laser (810 nm). Since the length from the input to output of the interferometer is about 600 mm, make the transmitted and reflected light from PBS parallel for more than 600 mm (desirable for a few meters) to make spatial mode matchings.</li>
	<li>Place HWP1 and HWP2 into the setup. They operate at both 405 nm and 810 nm wavelengths. Adjust the HWPs to be perpendicular to the incident light using the reflected light from the surface. Set the angle of HWP1 to 45o and HWP2 to 22.5o</li>
	<li>Place a retroreflector into the setup. Adjust the position of the retroreflector such that the clockwise (CW) and counter-clockwise (CCW) reference beams are on the same spatial mode. Place charge-coupled device (CCD) cameras on mode 1 and 2 in Figure 2 to refer the beam profiling images from the output of the interferometer. Adjust the mirror and retroreflector to make the spatial mode matching by referring the profiling images on the camera.</li>
	<li>Place a focus lens between QWP for laser and DM. Since the length from the input to output of the interferometer is about 600 mm, select a lens with a focus length of 300 mm. Empirically set the focal point of the input laser pump to not be on the exact middle point of the interferometer but to be around the generation position of the second SPDC to make same-level generation efficiency of down-converted photons between first and second SPDC.</li>
	<li>Remove the CCD camera and place QWPs, polarizers (POLs), interference filters (IFs) with an 810 nm center and 3 nm bandwidth in the mode 1 and 2 as shown in Figure 2. Adjust the optical elements to be perpendicular to the incident light using the reflected light. Couple the reference laser beams to the multimode fibers using fiber couplers for detection.</li>
	<li>Place a 300 mm focus lens between DM and QWP in mode 1 and mode 2. Make the output reference laser beams to collimate for detection.</li>
	<li>Connect the multimode fibers to the single-photon counting modules (SPCMs) constructed from Silicon (Si) avalanche photodiodes. Switch off the reference laser. Switch on the SPCMs in a darkroom condition, and count the down-converted photons.</li>
	<li>Adjust the temperature of ppKTP crystal mounted on a temperature controller by referencing the count rates of down-converted photons. The appropriate temperature is typically 25-30 &#176;C.</li>
	<li>Adjust the tilting angle of HWP1 to maximize the count rates of down-converted photons. If the count rates are too weak, measure the counts without the optical elements in mode 1 and 2.</li>
</ol>
3. Measurement procedure of the coincidence count
<ol>
	<li>Select the polarization bases in mode 1 and 2 to measure the incident polarization-entangled photons using POLs and QWPs as shown in Figure 3. For the measurement of the incident photon with H (V) base, set the QWP to 0o and the POL to 0o (90o). For the measurement of the incident photon with D (A) base, set the QWP to 0o and the POL to 45o (-45o). For the measurement of the incident photon with R (L) base, set the QWP to 45o (-45o) and the POL to 0o.</li>
	<li>Connect the transistor-transistor logic (TTL) signal generated from the SPCM in mode 2 to the start signal input of a time-to-amplitude converter (TAC), and the signal in mode 1 to the stop signal input after it has passed through the electrical delay line (Delay). TAC generates electrical signals from 0 to 10 V corresponding to the time delay between two signals.
	<ol>
		<li>In this experiment, set the time delay &#916;T as 50 ns by selecting the delay line pins. Set the display of PC to show 100 ns time range by setting the dial of TAC. Then TAC generates 5 V signals as 50 ns delay time given by the electrical delay line. Therefore the 5 V signals correspond to the coincidences at 0 ns delay time of actual pulses coming from SPCMs. The coincidences at 0 ns delay time appear in the center of the display time range as shown in Figure 3.</li>
	</ol>
	</li>
	<li>Click the start button of the software, called MAESTRO-32, to measure the pulse height distribution and record the distribution with a computer controlled (PC) multi-channel analyzer (MCA). In this experiment, set the measurement time of TAC for 30 s. Analyze the height distribution of the TAC pulses from 0 to 10 V which corresponded to a -50 to 50 ns delay time between the incident photons and the SPCMs by the setting described in step 3.2.</li>
	<li>After recording the pulse height distribution, obtain the pulse height distribution data for several polarization bases as shown in Figure 4. Select the time window to be considered for coincidence counts for the analyzation of the data. Since the width of the pulse peak is determined by the SPCM&#39;s resolution time of~1 ns, the coincidence time window is necessary to be larger than the resolution time.
	<ol>
		<li>In this experiment, choose the coincidence time window to be 10 ns. Estimate the coincidence counts by integrating the area of the time window.</li>
	</ol>
	</li>
</ol>
4. Estimation procedure of the Fidelity and Bell parameters
<ol>
	<li>Determine the polarized second-order correlations <img alt="Equation 21" src="/files/ftp_upload/59705/59705eq21v2.jpg" />&#160;and cross-polarized second-order correlations <img alt="Equation 22" src="/files/ftp_upload/59705/59705eq22v2.jpg" />, where <img alt="Equation 23" src="/files/ftp_upload/59705/59705eq23v2.jpg" />&#160;refers to the polarization states H, D, and R, and <img alt="Equation 24" src="/files/ftp_upload/59705/59705eq24v2.jpg" />&#160;refers to the cross-polarization states V, A, and L. Obtain these functions by dividing the measured coincidence counts <img alt="Equation 25" src="/files/ftp_upload/59705/59705eq25v2.jpg" />&#160;by the background level <img alt="Equation 26" src="/files/ftp_upload/59705/59705eq26v2.jpg" />. Figure 4 shows the actually measured pulse height distribution of coincidence counts with several polarization bases for 30s. 
	NOTE: For example, the coincidence counts the with polarization base HH gives <img alt="Equation 27" src="/files/ftp_upload/59705/59705eq27v2.jpg" />&#160;count/30 s for coincidence window 10 ns. The average back ground level for the coincidence window is calculated as 4.3 count/30 s. Since second-order correlations are given by <img alt="Equation 28" src="/files/ftp_upload/59705/59705eq28v2.jpg" />, the polarized second-order correlation functions with polarization base HH becomes <img alt="Equation 29" src="/files/ftp_upload/59705/59705eq29v2.jpg" />. Similarly second-order correlation functions with other polarization bases are given as: <img alt="Equation 30" src="/files/ftp_upload/59705/59705eq30v2.jpg" />, and&#160;<img alt="Equation 31" src="/files/ftp_upload/59705/59705eq31v2.jpg" /> and cross-polarized second-order correlation functions as:&#160;<img alt="Equation 32" src="/files/ftp_upload/59705/59705eq32v2.jpg" /> and <img alt="Equation 33" src="/files/ftp_upload/59705/59705eq33v2.jpg" />.</li>
	<li>Determine the degree of polarization correlation between two photons for three polarization bases defined by20,21: 
	<img alt="Equation 34" src="/files/ftp_upload/59705/59705eq34v2.jpg" />&#160; &#160;(7) 
	where&#160;<img alt="Equation 35" src="/files/ftp_upload/59705/59705eq35v2.jpg" /> refers to the polarization bases of the rectilinear (H and V), diagonal (D and A), and circular (R and L) bases. The measured second-order correlation functions give the degree of each polarization bases as follows: <img alt="Equation 36" src="/files/ftp_upload/59705/59705eq36v2.jpg" />, and <img alt="Equation 37" src="/files/ftp_upload/59705/59705eq37v2.jpg" />.</li>
	<li>Determine the fidelity of generated entangled photons. Calculate the fidelity of the polarization-entangled state with respect to the state (6) in three bases20,21: 
	<img alt="Equation 38" src="/files/ftp_upload/59705/59705eq38v2.jpg" /> 
	The measured degrees of polarization correlation was <img alt="Equation 39" src="/files/ftp_upload/59705/59705eq39v2.jpg" />. The number exceeded the classical polarization correlation limit of 0.50.</li>
	<li>Determine the Bell parameters of the generated entangled photons21. Calculate the parameters from the polarization correlations as follows 19,20: 
	<img alt="Equation 40" src="/files/ftp_upload/59705/59705eq40v2.jpg" /> 
	<img alt="Equation 41" src="/files/ftp_upload/59705/59705eq41v2.jpg" /> 
	<img alt="Equation 42" src="/files/ftp_upload/59705/59705eq42v2.jpg" /> 
	The measured bases of polarization correlation were <img alt="Equation 43" src="/files/ftp_upload/59705/59705eq43v2.jpg" />. These numbers exceed the classical parameter limit of 2 and violate the Bell inequality.</li>
</ol>

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W., Bouwmeester, D., Weinfurter, H., Zeilinger, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=experimental+entanglement+swapping:+entangling+photons+that+never+interacted.">experimental entanglement swapping: entangling photons that never interacted.</a> Physical Review Letters. 80, 3891-3894 (1998).</li><li>Bouwmeester, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+quantum+teleportation.">Experimental quantum teleportation.</a> Nature. 390, 575-579 (1997).</li><li>Armstrong, D. J., Alford, W. J., Raymond, T. D., Smith, A. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Absolute+measurement+of+the+effective+nonlinearities+of+KTP+and+BBO+crystals+by+optical+parametric+amplification.">Absolute measurement of the effective nonlinearities of KTP and BBO crystals by optical parametric amplification.</a> Applied Optics. 35, 2032-2040 (1996).</li><li>Shi, B. S., Tomita, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Generation+of+a+pulsed+polarization+entangled+photon+pair+using+a+Sagnac+interferometer.">Generation of a pulsed polarization entangled photon pair using a Sagnac interferometer.</a> Physical Review A. 69, 013803 (2004).</li><li>Kim, T., Fiorentino, M., Wong, F. N. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Phase-stable+source+of+polarization-entangled+photons+using+a+polarization+Sagnac+interferometer.">Phase-stable source of polarization-entangled photons using a polarization Sagnac interferometer.</a> Physical Review A. 73, 012316 (2006).</li><li>Steinlechner, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Efficient+heralding+of+polarization-entangled+photons+from+type-0+and+type-II+spontaneous+parametric+downconversion+in+periodically+poled+KTiOPO4.">Efficient heralding of polarization-entangled photons from type-0 and type-II spontaneous parametric downconversion in periodically poled KTiOPO4.</a> Journal of the Optical Society of America B. 31, 2068 (2014).</li><li>Steinlechner, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Phase-stable+source+of+polarization-entangled+photons+in+a+linear+double-pass+configuration.">Phase-stable source of polarization-entangled photons in a linear double-pass configuration.</a> Optics Express. 21, 11943-11951 (2013).</li><li>Okano, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=0.54+resolution+two-photon+interference+with+dispersion+cancellation+for+quantum+optical+coherence+tomography.">0.54 resolution two-photon interference with dispersion cancellation for quantum optical coherence tomography.</a> Scientific Reports. 5, 18042 (2015).</li><li>Dayan, B., Pe'er, A., Friesem, A. A., Silberberg, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nonlinear+interactions+with+an+ultrahigh+flux+of+broadband+entangled+photons.">Nonlinear interactions with an ultrahigh flux of broadband entangled photons.</a> Physical Review Letters. 94, 043602 (2005).</li><li>Nasr, M. B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultrabroadband+biphotons+generated+via+chirped+quasi-phase-matched+optical+parametric+down-conversion.">Ultrabroadband biphotons generated via chirped quasi-phase-matched optical parametric down-conversion.</a> Physical Review Letters. 100, 183601 (2008).</li><li>Giovannetti, V., Lloyd, S., Maccone, L., Wong, F. N. 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The authors have nothing to disclose.

The critical step within the protocol is how to maximize the fidelity of the generated polarization entangled photons. The estimated fidelity and Bell parameters are currently limited, mainly because we used multimode fibers to collect the generated entangled photons. The tilting of HWP1 affected the height difference of the spatial modes between the photons of the first and second SPDC and caused a spatial-mode mismatch on the output of the Sagnac interferometer. The fidelity is expected to be higher when using single-mode fibers that filter out the spatial-mode-overlapping area of the generated first and second SPDC photons. Moreover, the birefringence effect of the ppKTP crystal affected the mode mismatch between the first and second SPDC photons. In future, we can possibly improve the parameters by using additional compensation crystals.
The significance of the protocol is to realize several properties simultaneously with respect to existing method. The source of the polarization entangled photons with the protocol have a high-emission rate, are degenerate, have a broadband distribution, and are post-selection free. The characteristic advantage of the protocol is based on the multiple quantum interference using a double-pass polarization Sagnac interferometer. The photonic system makes it possible to use the large generation efficiency of polarization entangled photons and to separate degenerate photon pairs into different optical modes with no requirement of postselection. The system of high-performance polarization entangled photons can be applied for novel photonic quantum information technologies1,2,3,4.

The entangled state of photons has attracted considerable interest in the study of local realism in quantum theory and novel applications of quantum cryptography1, quantum dense coding2, quantum repeater3, and quantum teleportation4. Spontaneous parametric down-conversion (SPDC) is a second-order nonlinear process that has been introduced to directly produce entangled photon pairs in the polarization states. Owing to the recent development in quasi-phase-matching techniques, the periodically poled KTiOPO4 (ppKTP) and LiNbO3 (ppLN) have become a standard technique5. Several types of entanglement sources are developed by combining these nonlinear crystals with a Sagnac interferometer6,7,8. In particular, the scheme with orthogonally polarized photon pairs obtained by type-II SPDC makes it possible to generate unconditional polarization-entangled photons and also separate degenerated polarization-entangled photon pairs into different optical modes without postselective detection7.
In contrast, type-0 SPDC has the advantage of a simple setup and a high-emission ratio of photon pairs9. Moreover, the generated photon pairs in type-0 SPDC show a much broader bandwidth than the photons of type-II SPDC. The total photon-pair production rate per unit pump power is two orders of magnitude higher due to its large bandwidth8. A large bandwidth of correlated photon pairs allows a very short coincidence time between the detected photon pairs. This property has led to several potential applications such as quantum optical coherence tomography10, for achieving ultrashort temporal correlations through nonlinear interactions with the flux of entangled photons11, metrology methods using the very narrow dip in quantum interference12, quantum clock synchronization13, time-frequency entanglement measurement14, and multimode frequency entanglement15. However, the scheme with ordinary type-0 SPDC requires conditional detection schemes6 or wavelength filtering8 or spatial-mode filtering to separate the generated polarization-entangled photons16.
We realized a scheme that satisfies the properties of both type-0 and type-II SPDC simultaneously based on multiple quantum interference processes17. The details of the optical system were described and experimentally used to measure the parameters that characterize the generated polarization-entangled photons using a minimum number of experimental data.
The Jones Vector of horizontal (H) and vertical (V) polarization state can be written as <img alt="Equation 1" src="/files/ftp_upload/59705/59705eq1.jpg" />&#160;and <img alt="Equation 2" src="/files/ftp_upload/59705/59705eq2.jpg" style="font-size: 14px;" />. All possible pure polarization states are constructed from coherent superpositions of these two polarization states. For example, the diagonal(D), anti-diagonal(A), right-circular(R), and left-circular(L) light, respectively, are represented by:
<img alt="Equation 3" src="/files/ftp_upload/59705/59705eq3.jpg" style="font-size: 14px; opacity: 0.9;" />,
<img alt="Equation 4" src="/files/ftp_upload/59705/59705eq4.jpg" style="font-size: 14px;" />,&#160; (1)
<img alt="Equation 5" src="/files/ftp_upload/59705/59705eq5.jpg" style="font-size: 14px;" />,&#160; and
<img alt="Equation 6" src="/files/ftp_upload/59705/59705eq6.jpg" style="font-size: 14px;" />,
H and V are called the rectilinear polarization bases. D and A are called the diagonal polarization bases. R and L are called the circular polarization bases. These pure and also mixed states of the polarization can be represented by density matrixes based on the H- and V-polarization bases18.
The operating principle of the scheme is shown in Figure 1a-e. The laser is injected into a polarization Sagnac interferometer comprised of a polarizing beam splitter (PBS), two half-wave plates set to 45o (HWP1) and 22.5o (HWP2), a ppKTP crystal, and mirrors. The polarization optics with this setup work for both the wavelength of the pump laser field and down-converted photons.
The H-component of the pump laser passes through the PBS as shown in Figure 1a and round trips the setup in a clockwise (CW) direction. The polarization of the pump laser was inverted to the diagonal (D) state through HWP2. Here the V-component of the pump laser works for down-conversion, and the generated photons are V-polarized with type-0 SPDC. The SPDC polarization state of generated photon pairs can be represented as:
<img alt="Equation 7" src="/files/ftp_upload/59705/59705eq7.jpg" />.&#160;(2)
The down-converted photon pairs are H-polarized through the HWP1 set to 45o as shown in Figure 1b, and the polarization state becomes:
<img alt="Equation 8" src="/files/ftp_upload/59705/59705eq8.jpg" style="font-size: 14px;" />. (3)
The pump laser beam again injected the inverted photon pairs into the ppKTP. The generated photon pairs from the second SPDC are both V-polarized and superposed with the photon pairs generated by the first SPDC for a collinear optical mode as shown Figure 1c. The polarization state of the photon pairs after the second SPDC is represented as:
<img alt="Equation 9" src="/files/ftp_upload/59705/59705eq9.jpg" style="font-size: 14px;" />&#160;(4)
where <img alt="Equation 10" src="/files/ftp_upload/59705/59705eq10.jpg" />&#160;is the relative phase between the photon pair from the first and second SPDC. The phase does not vary with time because it is determined by the HWP1&#39;s material dispersion between the pump laser and the down-converted photons, and adjustable by tilting HWP1. The H (V)-polarization state of the down-converted photons was inverted to A (D) state as shown in (1). The polarization state of the output photon pair from HWP2 is represented as:
<img alt="Equation 11" src="/files/ftp_upload/59705/59705eq11.jpg" />&#160; (5)
When the phase <img alt="Equation 12" src="/files/ftp_upload/59705/59705eq12.jpg" style="opacity: 0.9;" />&#160;is set by tilting HWP1, only the first term of the state (5) remains as shown in Figure 1d. This is the quantum interference process that corresponds to the reverse Hong-Ou-Mandel (HOM) interference process of the polarization bases19. When the H-photon passes through PBS and the V-photon is reflected by PBS, the polarization state of the output photon pairs from PBS is represented as&#160;<img alt="Equation 13" src="/files/ftp_upload/59705/59705eq13.jpg" style="opacity: 0.9;" /> for optical mode1 and 2 as shown in Figure 1e.
Conversely, the V-component of the pump laser was reflected by PBS as shown in Figure 1f and round tripped in a counter-clockwise (CCW) direction. Through similar multiple type-0 SPDC processes and unitary transformations, the polarization state of the output from PBS becomes <img alt="Equation 14" src="/files/ftp_upload/59705/59705eq14.jpg" style="opacity: 0.9;" />. When the polarization state of the pump laser was prepared in diagonal (D) state, the relative phase between H- and V-components of the pump laser was zero. Therefore, the output state of generated photons from CW and CCW directions are superposed with the same amplitudes and represented as:
<img alt="Equation 15" src="/files/ftp_upload/59705/59705eq15.jpg" />.&#160; (6)
The output state is a polarization-entangled state known as one of the Bell states and can be converted to other three states using the polarization optics elements7. Using the relation shown in (1), the output state <img alt="Equation 16" src="/files/ftp_upload/59705/59705eq16.jpg" style="opacity: 0.9;" />&#160;can be represented by diagonal polarization bases as:
<img alt="Equation 17" src="/files/ftp_upload/59705/59705eq17.jpg" style="opacity: 0.9;" />&#160;and by circular polarization bases as: <img alt="Equation 18" src="/files/ftp_upload/59705/59705eq18.jpg" style="opacity: 0.9;" />.

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			<td height="21" style="height:21px;width:239px;">300mm fous lens</td>
			<td style="width:157px;">Thorlabs. INC.</td>
			<td style="width:165px;">AC254-300-B</td>
			<td style="width:216px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">405nm LD</td>
			<td>Digi-Key Electronics</td>
			<td>NV4V31SF-A-ND</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Delay line</td>
			<td>Ortec INC.</td>
			<td>DB463</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Dichroic mirror (DM)</td>
			<td>Midwest Optical Systems INC.</td>
			<td>SP650-25.4</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Half-wave plate (HWP) for 405nm</td>
			<td>Thorlabs. INC.</td>
			<td>WPH05M-405</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Half-wave plate (HWP) for dual wavelengths</td>
			<td>Meadowlark Co.</td>
			<td colspan="2">DHHM-100-0405/0810&#12288;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Interference filter (IF)</td>
			<td>IDEX Health &#38; Science, LLC</td>
			<td>LL01-808-12.5</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Multi-channel analyzer (MCA)</td>
			<td>Ortec INC.</td>
			<td>EASY-MCA-2K</td>
			<td>MAESTRO-32 software</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Polarization-maintaining fiber</td>
			<td>Thorlabs. INC.</td>
			<td>P1-405BPM-FC-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Polarizer (POL)</td>
			<td>Meadowlark Co.</td>
			<td>G335743000</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">ppKTP crystal</td>
			<td>RAICOL CRYSTAL LTD.</td>
			<td></td>
			<td>Type-0, 3.425 microns period</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Quarter-wave plate (QWP) for 808nm</td>
			<td>Thorlabs. INC.</td>
			<td>WPQ05M-808</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Quarter-wave plate (QWP) for 405nm</td>
			<td>Thorlabs. INC.</td>
			<td>WPQ05M-405</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Retroreflector</td>
			<td>Newport Co.</td>
			<td>U-BER 1-1S</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Single photon counting Module (SPCM)</td>
			<td>Laser Cpmponents LTD.</td>
			<td>Count -100C-FC</td>
			<td>FC connecting</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Time-to-amplitude converter (TAC)</td>
			<td>Ortec INC.</td>
			<td>567</td>
			<td></td>
		</tr>
	</tbody>
</table>

a photonic system for generating unconditional polarization-entangled photons based on multiple quantum interference

We present a high-performance source of unconditional polarization-entangled photons that have a high-emission rate, a broadband distribution, are degenerated and postselection free. The property of the source is based on the multiple quantum interference effect with a round-trip configuration of a Sagnac interferometer. The quantum interference effects make it possible to use the high generation efficiency of the polarization-entangled photons to process parametric down-conversion, and separate degenerated photon pairs into different optical modes without a postselection requirement. The principle of the optical system was described and experimentally used to measure the fidelity and Bell parameters, and also to characterize the generated polarization-entangled photons from a minimum of six combinations of polarization correlated data. The experimentally obtained fidelity and Bell parameters exceeded the classical local correlation limit and are clear evidence of the generation of unconditional polarization-entangled photons.

The optical system to generate unconditional entangled photons for polarization states based on multiple quantum interferences and detection schemes to estimate the experimental fidelity by polarization correlation of generated photon pairs was discussed. The estimated fidelity of the generated photons exceeded the classical local correlation limit of 0.50. The measured Bell parameters exceeded the classical parameter limit of 2 and violated the Bell inequality. In this paper, coincidence measurements obtained from a minimum of six combinations of polarization bases were used to evaluate these parameters. Furthermore, it is possible to completely reconstruct the density matrix of the generated polarization-entangled photons via quantum state tomography, which requires coincidence measurements of 16 combinations of polarization bases18.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59705/59705fig1.jpg" /> 
Figure 1: Schematic of an integrated double-pass polarization Sagnac interferometer. (a) The generation of photon pairs after the first spontaneous parametric down-conversion (SPDC). (b) Polarization rotation of the photon pairs by a half-wave plate (HWP1). (c) The generation of photon pairs after the second SPDC. (d) The quantum interference between photon pairs of the first and second SPDC by HWP2. (e) Output photon pairs produced in the clockwise (CW) direction. (f) Output photon pairs produced in the counter-clockwise (CCW) direction. <a href="https://www.jove.com/files/ftp_upload/59705/59705fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/59705/59705fig2.jpg" /> 
Figure 2:Overall optical system for generating unconditional polarization-entangled photons. The first half-wave plate (HWP) and a quarter-wave plate (QWP) are used to set the polarization state of the pump laser passing through polarization-maintaining optical fiber (PMF). The output photons were passed through lenses, QWPs, polarizers (POLs), and interference filters (IFs) in modes 1 and 2, and detected by the single-photon counting modules (SPCM). <a href="https://www.jove.com/files/ftp_upload/59705/59705fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/59705/59705fig3.jpg" /> 
Figure 3: Overall coincidence detection system for the generated polarization-entangled photons. The electrical signals from the SPCM were used to start and stop the signal of the time-to-amplitude converter (TAC) through an electrical delay line (Delay). The pulse height distribution obtained from time difference was analyzed with a computer controlled (PC) multi-channel analyzer (MCA). <a href="https://www.jove.com/files/ftp_upload/59705/59705fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/59705/59705fig4.jpg" /> 
Figure 4: Measured time difference distributions with parallel and orthogonal polarizer settings. The combinations are horizontal (H), vertical (V), diagonal (D), anti-diagonal (A), right-circular (R), and left-circular (L) polarization bases. <a href="https://www.jove.com/files/ftp_upload/59705/59705fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about A Photonic System for Generating Unconditional Polarization-Entangled Photons Based on Multiple Quantum Interference at JoVE.com

A Photonic System for Generating Unconditional Polarization-Entangled Photons Based on Multiple Quantum Interference

We describe an optical system for the generation of unconditional polarization-entangled photons based on multiple quantum interference effects with a detection scheme to estimate the experimental fidelity of generated entangled photons.

We present a high-performance source of unconditional polarization-entangled photons that have a high-emission rate, a broadband distribution, are ...

a-photonic-system-for-generating-unconditional-polarization-entangled

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JoVE Journal

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8.3K Views.  Tokyo University of Science. We describe an optical system for the generation of unconditional polarization-entangled photons based on multiple quantum interference effects with a detection scheme to estimate the experimental fidelity of generated entangled photons.

Video: A Photonic System for Generating Unconditional Polarization-Entangled Photons Based on Multiple Quantum Interference

Micropunching Lithography for Generating Micro- and Submicron-patterns on Polymer Substrates

Conducting polymers have attracted great attention since the discovery of high conductivity in doped polyacetylene in 19771. They offer the advantages of low weight, easy tailoring of properties and a wide spectrum of applications2,3. Due to sensitivity of conducting polymers to environmental conditions (e.g., air, oxygen, moisture, high temperature and chemical solutions), lithographic techniques present significant technical challenges when working with these materials4. For example, current photolithographic methods, such as ultra-violet (UV), are unsuitable for patterning the conducting polymers due to the involvement of wet and/or dry etching processes in these methods. In addition, current micro/nanosystems mainly have a planar form5,6. One layer of structures is built on the top surfaces of another layer of fabricated features. Multiple layers of these structures are stacked together to form numerous devices on a common substrate. The sidewall surfaces of the microstructures have not been used in constructing devices. On the other hand, sidewall patterns could be used, for example, to build 3-D circuits, modify fluidic channels and direct horizontal growth of nanowires and nanotubes.
A macropunching method has been applied in the manufacturing industry to create macropatterns in a sheet metal for over a hundred years. Motivated by this approach, we have developed a micropunching lithography method (MPL) to overcome the obstacles of patterning conducting polymers and generating sidewall patterns. Like the macropunching method, the MPL also includes two operations (Fig. 1): (i) cutting; and (ii) drawing. The "cutting" operation was applied to pattern three conducting polymers4, polypyrrole (PPy), Poly(3,4-ethylenedioxythiophen)-poly(4-styrenesulphonate) (PEDOT) and polyaniline (PANI). It was also employed to create Al microstructures7. The fabricated microstructures of conducting polymers have been used as humidity8, chemical8, and glucose sensors9. Combined microstructures of Al and conducting polymers have been employed to fabricate capacitors and various heterojunctions9,10,11. The "cutting" operation was also applied to generate submicron-patterns, such as 100- and 500-nm-wide PPy lines as well as 100-nm-wide Au wires. The "drawing" operation was employed for two applications: (i) produce Au sidewall patterns on high density polyethylene (HDPE) channels which could be used for building 3D microsystems12,13,14, and (ii) fabricate polydimethylsiloxane (PDMS) micropillars on HDPE substrates to increase the contact angle of the channel15.

A micropunching lithography approach is developed to generate micro- and submicron-patterns on top, sidewall and bottom surfaces of polymer substrates. It overcomes the obstacles of patterning conducting polymers and generating sidewall patterns. This method allows rapid fabrication of multiple features and is free of aggressive chemistry.

Conducting polymers have attracted great attention since the discovery of high conductivity in doped polyacetylene in 19771. They offer the advantages of ...

Gradient Echo Quantum Memory in Warm Atomic Vapor

Gradient echo memory (GEM) is a protocol for storing optical quantum states of light in atomic ensembles. The primary motivation for such a technology is that quantum key distribution (QKD), which uses Heisenberg uncertainty to guarantee security of cryptographic keys, is limited in transmission distance. The development of a quantum repeater is a possible path to extend QKD range, but a repeater will need a quantum memory. In our experiments we use a gas of rubidium 87 vapor that is contained in a warm gas cell. This makes the scheme particularly simple. It is also a highly versatile scheme that enables in-memory refinement of the stored state, such as frequency shifting and bandwidth manipulation. The basis of the GEM protocol is to absorb the light into an ensemble of atoms that has been prepared in a magnetic field gradient. The reversal of this gradient leads to rephasing of the atomic polarization and thus recall of the stored optical state. We will outline how we prepare the atoms and this gradient and also describe some of the pitfalls that need to be avoided, in particular four-wave mixing, which can give rise to optical gain.

The gradient echo memory is a protocol for storing optical quantum states of light in atomic ensembles. Quantum memory is a key element of a quantum repeater, which can extend the range of quantum key distribution. We outline the operation of the scheme when implemented in a 3-level atomic ensemble.

Gradient  echo memory (GEM) is a protocol for storing optical quantum states of light in  atomic ensembles. The primary motivation for such a technology ...

Analysis of Contact Interfaces for Single GaN Nanowire Devices

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit a strong degradation after annealing due to the occurrence of void formation at the contact/SiO2 interface. This void formation can cause cracking and delamination of the metal film, which can increase the resistance or lead to a complete failure of the NW device. In order to address issues associated with void formation, a technique was developed that removes Ni/Au contact metal films from the substrates to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN NW devices. This procedure determines the degree of adhesion of the contact films to the substrate and NWs and allows for the characterization of the morphology and composition of the contact interface with the substrate and nanowires. This technique is also useful for assessing the amount of residual contamination that remains from the NW suspension and from photolithographic processes on the NW-SiO2 surface prior to metal deposition. The detailed steps of this procedure are presented for the removal of annealed Ni/Au contacts to Mg-doped GaN NWs on a SiO2 substrate.

A technique was developed that removes Ni/Au contact metal films from their substrate to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN nanowire devices.

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit  a strong degradation after annealing due to the  occurrence of void formation at the ...

Using Microwave and Macroscopic Samples of Dielectric Solids to Study the Photonic Properties of Disordered Photonic Bandgap Materials

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). In this article we will describe the methods for constructing and characterizing macroscopic disordered photonic structures using microwaves. The microwave regime offers the most convenient experimental sample size to build and test PBG media. Easily manipulated dielectric lattice components extend flexibility in building various 2D structures on top of pre-printed plastic templates. Once built, the structures could be quickly modified with point and line defects to make freeform waveguides and filters. Testing is done using a widely available Vector Network Analyzer and pairs of microwave horn antennas. Due to the scale invariance property of electromagnetic fields, the results we obtained in the microwave region can be directly applied to infrared and optical regions. Our approach is simple but delivers exciting new insight into the nature of light and disordered matter interaction.

Our representative results include the first experimental demonstration of the existence of a complete and isotropic PBG in a two-dimensional (2D) hyperuniform disordered dielectric structure. Additionally we demonstrate experimentally the ability of this novel photonic structure to guide electromagnetic waves (EM) through freeform waveguides of arbitrary shape.

Disordered structures offer new mechanisms for forming photonic bandgaps and unprecedented freedom in functional-defect designs. To circumvent the computational challenges of disordered systems, we construct modular macroscopic samples of the new class of PBG materials and use microwaves to characterize their scale-invariant photonic properties, in an easy and inexpensive manner.

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). ...

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H2O2/H2O solution did not show measurable change in their initial contact angle (CA) ~75&#176;. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H2O2/H2O solution at 250 and 65 mJ/cm2, respectively, has decreased the CA to near 15&#176;, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafer&#8217;s surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of&#160;the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH3OH) using ArF laser operating at pulse fluence of 65 mJ/cm2 and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103&#176;. The XPS results indicate ArF laser induced formation of Si&#8211;(OCH3)x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH3 radicals.

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with&#160;H2O2/H2O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of ...

Silicon Metal-oxide-semiconductor Quantum Dots for Single-electron Pumping

As mass-produced silicon transistors have reached the nano-scale, their behavior and performances are increasingly affected, and often deteriorated, by quantum mechanical effects such as tunneling through single dopants, scattering via interface defects, and discrete trap charge states. However, progress in silicon technology has shown that these phenomena can be harnessed and exploited for a new class of quantum-based electronics. Among others, multi-layer-gated silicon metal-oxide-semiconductor (MOS) technology can be used to control single charge or spin confined in electrostatically-defined quantum dots (QD). These QD-based devices are an excellent platform for quantum computing applications and, recently, it has been demonstrated that they can also be used as single-electron pumps, which are accurate sources of quantized current for metrological purposes. Here, we discuss in detail the fabrication protocol for silicon MOS QDs which is relevant to both quantum computing and quantum metrology applications. Moreover, we describe characterization methods to test the integrity of the devices after fabrication. Finally, we give a brief description of the measurement set-up used for charge pumping experiments and show representative results of electric current quantization.

The fabrication process and experimental characterization techniques relevant to single-electron pumps based on silicon metal-oxide-semiconductor quantum dots are discussed.

As mass-produced silicon transistors have reached the nano-scale, their behavior and performances are increasingly affected, and often deteriorated, by ...

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...

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.

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 ...

Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation displays and solid-state lighting technologies. The brightness and efficiency of blue QDs-based light-emitting diodes (LEDs) remain inferior to their red and green counterparts, due to the inherently unfavorable energy levels of different colors of light. To solve these problems, a device structure should be designed to balance the injection holes and electrons into the emissive QD layer. Herein, through a simple autoxidation strategy, pure blue QD-LEDs which are highly bright and efficient are demonstrated, with a structure of ITO/PEDOT:PSS/Poly-TPD/QDs/Al:Al2O3. The autoxidized Al:Al2O3 cathode can effectively balance the injected charges and enhance radiative recombination without introducing an additional electron transport layer (ETL). As a result, high color-saturated blue QD-LEDs are achieved with a maximum luminance over 13,000 cd m-2, and a maximum current efficiency of 1.15 cd A-1. The easily controlled autoxidation procedure paves the way for achieving high-performance blue QD-LEDs.

A protocol is presented for fabricating high-performance, pure blue ZnCdS/ZnS-based quantum dots light-emitting diodes by employing an autoxidized aluminum cathode.

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation ...

Uncoupling Coriolis Force and Rotating Buoyancy Effects on Full-Field Heat Transfer Properties of a Rotating Channel

An experimental method for exploring the heat transfer characteristics of an axially rotating channel is proposed. The governing flow parameters that characterize the transport phenomena in a rotating channel are identified via the parametric analysis of the momentum and energy equations referring to a rotating frame of reference. Based on these dimensionless flow equations, an experimental strategy that links the design of the test module, the experimental program and the data analysis is formulated with the attempt to reveal the isolated Coriolis-force and buoyancy effects on heat transfer performances. The effects of Coriolis force and rotating buoyancy are illustrated using the selective results measured from rotating channels with various geometries. While the Coriolis-force and rotating-buoyancy impacts share several common features among the various rotating channels, the unique heat transfer signatures are found in association with the flow direction, the channel shape and the arrangement of heat transfer enhancement devices. Regardless of the flow configurations of the rotating channels, the presented experimental method enables the development of physically consistent heat transfer correlations that permit the evaluation of isolated and interdependent Coriolis-force and rotating-buoyancy effects on the heat transfer properties of rotating channels.

Here, we present an experimental method for decoupling the interdependent Coriolis-force and rotating-buoyancy effects on full-field heat transfer distributions of a rotating channel.

An experimental method for exploring the heat transfer characteristics of an axially rotating channel is proposed. The governing flow parameters that ...