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A high brightness source of entangled photons

Leo Wallin Sonesson

Published Jan 1, 2010• 0 citations

Abstract

In the field of quantum optics and its wide variety of applications it is essential to have high brightness sources of entangled photons. In this Bachelor project, the aim has been to produce such a source based on recent experimental setups [1] [2] with a so-called Sagnac interferometer to generate polarization-entangled photons. The source built shows high quality entanglement. High visibility polarisation coincidence fringes (99.5% visibility) were observed. Moreover a Bell’s inequality [6] in a CHSH-form[36] was violated by more than 44 standard deviations. Also the wavelength tuning of the source with respect to temperature and pump wavelength was analysed, revealing shifts in the degeneracy temperature of 0.020 nm/K and a shift of the signal and idler wavelengths of 0.046 nm/K. This thesis begins with an introduction which reviews the fundamental ideas of entanglement and it’s role in quantum mechanics in section 1. This section is continued by a general discussion of photonic entanglement, different types of photonic entanglement and a short discussion of how to measure this entanglement is presented. This is followed by a review of different tecniques of generating entangled photons in sections 2 and 3. In section 4, the experimental process of assembling the Sagnac interferometer is presented. This is followed by a review of experimental results, which includes a characterisation of the source in relation to temperature and pump wavelength tunability in section 5. Then, in section 6, a presentation and an experimental test of Bell’s inequalities are presented. Thereafter, concluding remarks are made regarding the outcome of this project and the potential of further investigations utilizing the source in sections 7 and 8.

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•Jun 19, 2007•2 citations

In the field of quantum optics and its wide variety of applications it is essential to have high brightness sources of entangled photons. In this Bachelor project, the aim has been to produce such a source based on recent experimental setups [1] [2] with a so-called Sagnac interferometer to generate polarization-entangled photons. The source built shows high quality entanglement. High visibility polarisation coincidence fringes (99.5% visibility) were observed. Moreover a Bell’s inequality [6] in a CHSH-form[36] was violated by more than 44 standard deviations. Also the wavelength tuning of the source with respect to temperature and pump wavelength was analysed, revealing shifts in the degeneracy temperature of 0.020 nm/K and a shift of the signal and idler wavelengths of 0.046 nm/K. This thesis begins with an introduction which reviews the fundamental ideas of entanglement and it’s role in quantum mechanics in section 1. This section is continued by a general discussion of photonic entanglement, different types of photonic entanglement and a short discussion of how to measure this entanglement is presented. This is followed by a review of different tecniques of generating entangled photons in sections 2 and 3. In section 4, the experimental process of assembling the Sagnac interferometer is presented. This is followed by a review of experimental results, which includes a characterisation of the source in relation to temperature and pump wavelength tunability in section 5. Then, in section 6, a presentation and an experimental test of Bell’s inequalities are presented. Thereafter, concluding remarks are made regarding the outcome of this project and the potential of further investigations utilizing the source in sections 7 and 8.

Abstract We designed and implemented a novel combination of a Sagnac-interferometer with a Mach–Zehnder interferometer for a source of polarization-entangled photons. The new versatile configuration does not require multi-wavelength polarization optics, yet it performs with a good polarization quality and phase-stability over a wide wavelength range. We demonstrate the interferometer using only standard commercial optics to experimentally realize the pulsed generation of polarization-entangled photon-pairs at wavelengths of 764 nm and 1221 nm via type-I spontaneous four-wave mixing in a polarization-maintaining fiber. Polarization entanglement was verified by a polarization-correlation measurement with a visibility of 95.5% from raw coincidence counts and the violation of the Clauser–Horne–Shimony–Holt (CHSH) inequality with S = 2.70 ± 0.04. The long-term phase-stability was characterized by an Allan deviation of 8° over an integration time of about 1 h with no active phase-stabilization.

Entangled photons are crucial for the development of quantum technologies and especially important in quantum communications. Hence it is paramount to have a reliable, high-fidelity source of entangled photons. Here we describe the construction and characterization of a polarization-entangled photon source. We generate maximally-entangled Bell states using a type-II PPKTP nonlinear crystal inside a Sagnac interferometer. We characterize the source in terms of brightness, visibility, Bell-CHSH test and we perform quantum-state tomography of the density matrix. Our source violates Bell-CHSH inequality

|S|\le 2

by

n=22

standard deviations. With visibilities up to

{\cal V}= 98.9\%

and fidelity

{\cal F}= 97\%

, our source is highly competitive, with state-of-the-art performance. To the best of our knowledge, this is the first source of entangled-photons designed and build in Romania and as such, represents an important step for the development of quantum technologies in our country. We envisage that our results will stimulate the progress of quantum technologies in Romania and will educate the future generation of quantum engineers.

We demonstrate a wavelength-tunable, fiber-coupled source of polarization- entangled photons with extremely high spectral brightness and quality of entanglement. Using a 25 mm PPKTP crystal inside a polarization Sagnac interferometer we detect a spectral brightness of 273000 pairs (s mW nm)(-1), a factor of 28 better than comparable previous sources while state tomography showed the two-photon state to have a tangle of T = 0.987. This improvement was achieved by use of a long crystal, careful selection of focusing parameters and single-mode fiber coupling. We demonstrate that, due to the particular geometry of the setup, the signal and idler wavelengths can be tuned over a wide range without loss of entanglement.

We describe an achromatic, phase-stable, broadband source of polarization-entangled photon pairs with high spectral brightness that uses four-wave mixing in a fiber Sagnac interferometer. We achieved a polarization-entangled two-photon coincidence rate of

7\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}

per

0.5\phantom{\rule{0.3em}{0ex}}\mathrm{THz}

(0.9\phantom{\rule{0.3em}{0ex}}\mathrm{nm})

of bandwidth per $300\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{W}$ of average pump power. At this rate, we observed two-photon fringe interference visibilities greater than 97%, over a

10\phantom{\rule{0.3em}{0ex}}\mathrm{THz}

$(\ensuremath{\approx}21\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ spectral range. We measured violations of Bell's inequality by more than 22 standard deviations for each of the four Bell states in less than

3\phantom{\rule{0.3em}{0ex}}\text{minutes}

per state. The high spectral brightness $(26\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}\phantom{\rule{0.2em}{0ex}}{\mathrm{nm}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{mW}}^{\ensuremath{-}1})$, large tunable wavelength range, single spatial mode, and phase stability make this source a promising candidate for a wide range of quantum-information applications.

Entangled photon source is a basic building block of various quantum technologies such as quantum communication, QKD, quantum metrology, etc. Spontaneous parametric down conversion (SPDC) process is widely reported method for generation of entanglement due to its ease of implementation and stable performance. In this paper, we report design and development of type-II Sagnac loop based entangled photon source with a high brightness of

8.42\times 10^{4}

pairs/s/mW with 98.18% visibility, a CHSH inequality violation of S=2.777±0.006 and state fidelity of 97.8%. An automated test setup has also been developed for carrying out the measurements. The essential formulated design procedure to achieve optimal performance has been discussed and presented. The measured performance of developed source is also compared to other reported sources having the same configuration. In this work, higher brightness is achieved with comparable fidelity to previously reported works. This methodology can be extended to any collinear Sagnac loop based entangled photon sources.

We demonstrate a wavelength-tunable, fiber-coupled source of polarization-entangled photons with extremely high spectral brightness and quality of entanglement. Using a 25 mm PPKTP crystal inside a polarization Sagnac interferometer we detect a spectral brightness of 273000 pairs/(s mW nm), a factor of 28 better than comparable previous sources while state tomography showed the two-photon state to have a tangle of T=0.987. This improvement was achieved by use of a long crystal, careful selection of focusing parameters and single-mode fiber coupling. We demonstrate that, due to the particular geometry of the setup, the signal and idler wavelengths can be tuned over a wide range without loss of entanglement.