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Controlling the spectrum of photons generated on a silicon nanophotonic chip.

Kumar R, Ong JR, Savanier M, Mookherjea S - Nat Commun (2014)

Bottom Line: Here we design a photon-pair source, consisting of planar lightwave components fabricated using CMOS-compatible lithography in silicon, which has the capability to vary the JSI.By controlling either the optical pump wavelength, or the temperature of the chip, we demonstrate the ability to select different JSIs, with a large variation in the Schmidt number.Such control can benefit high-dimensional communications where detector-timing constraints can be relaxed by realizing a large Schmidt number in a small frequency range.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California 92093, USA.

ABSTRACT
Directly modulated semiconductor lasers are widely used, compact light sources in optical communications. Semiconductors can also be used to generate nonclassical light; in fact, CMOS-compatible silicon chips can be used to generate pairs of single photons at room temperature. Unlike the classical laser, the photon-pair source requires control over a two-dimensional joint spectral intensity (JSI) and it is not possible to process the photons separately, as this could destroy the entanglement. Here we design a photon-pair source, consisting of planar lightwave components fabricated using CMOS-compatible lithography in silicon, which has the capability to vary the JSI. By controlling either the optical pump wavelength, or the temperature of the chip, we demonstrate the ability to select different JSIs, with a large variation in the Schmidt number. Such control can benefit high-dimensional communications where detector-timing constraints can be relaxed by realizing a large Schmidt number in a small frequency range.

No MeSH data available.


Related in: MedlinePlus

Deconvolution of JSI.(a) The optical pump was a tunable-wavelength continuous-wave (CW) laser diode (LD) modulated into pulses (~4 ns) using an electro-optic modulator (EOM). The device temperature was controlled using a thermoelectric controller (TEC). Tunable filters were used in front of the SPADs to measure the JSI. (b) The filter point-spread function (PSF) had a full-width at half-maximum of 0.6 nm along the axis for photon 1 and 1.0 nm along the axis for photon 2, which resulted in the measurement of a blurred JSI. The colourbar represents transmission in dB. (c,g) Examples of two different raw (blurred) JSIs, which were deconvoluted from the PSF using the iterative Richardson–Lucy algorithm, with (d,h) 20 iterations and with e,i 50 iterations. (f,j) A classical four-wave mixing experiment was performed to identify the phase-matching points, which form one component of the JSI expression equation (1). Each JSI was normalized to unit area, consistent with its definition as a probability density. In panels c–j the horizontal axes are in units of normalized wavelength (one unit equals 0.6 nm), measured relative to the respective band centres (‘Photon 1’: 1,548.8 nm, ‘Photon 2’: 1,578.7 nm).
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f3: Deconvolution of JSI.(a) The optical pump was a tunable-wavelength continuous-wave (CW) laser diode (LD) modulated into pulses (~4 ns) using an electro-optic modulator (EOM). The device temperature was controlled using a thermoelectric controller (TEC). Tunable filters were used in front of the SPADs to measure the JSI. (b) The filter point-spread function (PSF) had a full-width at half-maximum of 0.6 nm along the axis for photon 1 and 1.0 nm along the axis for photon 2, which resulted in the measurement of a blurred JSI. The colourbar represents transmission in dB. (c,g) Examples of two different raw (blurred) JSIs, which were deconvoluted from the PSF using the iterative Richardson–Lucy algorithm, with (d,h) 20 iterations and with e,i 50 iterations. (f,j) A classical four-wave mixing experiment was performed to identify the phase-matching points, which form one component of the JSI expression equation (1). Each JSI was normalized to unit area, consistent with its definition as a probability density. In panels c–j the horizontal axes are in units of normalized wavelength (one unit equals 0.6 nm), measured relative to the respective band centres (‘Photon 1’: 1,548.8 nm, ‘Photon 2’: 1,578.7 nm).

Mentions: Experimentally, there is no rapid measurement instrument for JSI at this time. The peaks and valleys that distinguish one JSI from another are separated by a frequency interval of ~20 GHz (only one-tenth of a nanometre), and whereas classical optical spectrum analysers are capable of providing such a high resolution, they are not sensitive at the single-photon level. On the other hand, quantum photon detectors such as single-photon avalanche diodes (SPADs) are not wavelength-selective. While 2D arrays of SPADs now being developed3738 will be beneficial in the future, here, we use high-contrast tunable telecommunication-grade optical filters in front of the SPADs, as shown Fig. 3a to measure the JSI by scanning over the 2D frequency grid. The measured data, of which two examples are shown in Fig. 3c,g, represent the convolution of the actual JSI and the point-spread function of the filters shown in Fig. 3b. There are a number of different ways of deconvolving blurred images and as a representative method, we used the Richardson–Lucy (RL) algorithm39. The RL deconvolution is an iterative procedure for recovering, in a maximum-likelihood sense, a latent image that has been blurred by a known point-spread function. The end point of the iteration generally needs to be determined by the user, and we use our prior knowledge of typical JSIs (as shown in Fig. 2b) as a guideline, and confirm our choice by a classical four-wave mixing experiment24. In Fig. 3d,e, we show the results of deconvolution on the measured data (Fig. 3c) for 20 and 50 iteration steps, respectively. Similarly, for the measured data shown in Fig. 3g, we show the results of deconvolution with 20 and 50 iteration steps in Fig. 3h,i, respectively. In each case, we stopped at 50 iterations because the general shape and ‘sharpness’ of the JSI was then similar to the phase-matching function measured by a classical four-wave mixing experiment, shown in Fig. 3f,j for the two cases. The following discussion and insights do not depend on the exact stopping point of the de-blurring algorithm.


Controlling the spectrum of photons generated on a silicon nanophotonic chip.

Kumar R, Ong JR, Savanier M, Mookherjea S - Nat Commun (2014)

Deconvolution of JSI.(a) The optical pump was a tunable-wavelength continuous-wave (CW) laser diode (LD) modulated into pulses (~4 ns) using an electro-optic modulator (EOM). The device temperature was controlled using a thermoelectric controller (TEC). Tunable filters were used in front of the SPADs to measure the JSI. (b) The filter point-spread function (PSF) had a full-width at half-maximum of 0.6 nm along the axis for photon 1 and 1.0 nm along the axis for photon 2, which resulted in the measurement of a blurred JSI. The colourbar represents transmission in dB. (c,g) Examples of two different raw (blurred) JSIs, which were deconvoluted from the PSF using the iterative Richardson–Lucy algorithm, with (d,h) 20 iterations and with e,i 50 iterations. (f,j) A classical four-wave mixing experiment was performed to identify the phase-matching points, which form one component of the JSI expression equation (1). Each JSI was normalized to unit area, consistent with its definition as a probability density. In panels c–j the horizontal axes are in units of normalized wavelength (one unit equals 0.6 nm), measured relative to the respective band centres (‘Photon 1’: 1,548.8 nm, ‘Photon 2’: 1,578.7 nm).
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4263184&req=5

f3: Deconvolution of JSI.(a) The optical pump was a tunable-wavelength continuous-wave (CW) laser diode (LD) modulated into pulses (~4 ns) using an electro-optic modulator (EOM). The device temperature was controlled using a thermoelectric controller (TEC). Tunable filters were used in front of the SPADs to measure the JSI. (b) The filter point-spread function (PSF) had a full-width at half-maximum of 0.6 nm along the axis for photon 1 and 1.0 nm along the axis for photon 2, which resulted in the measurement of a blurred JSI. The colourbar represents transmission in dB. (c,g) Examples of two different raw (blurred) JSIs, which were deconvoluted from the PSF using the iterative Richardson–Lucy algorithm, with (d,h) 20 iterations and with e,i 50 iterations. (f,j) A classical four-wave mixing experiment was performed to identify the phase-matching points, which form one component of the JSI expression equation (1). Each JSI was normalized to unit area, consistent with its definition as a probability density. In panels c–j the horizontal axes are in units of normalized wavelength (one unit equals 0.6 nm), measured relative to the respective band centres (‘Photon 1’: 1,548.8 nm, ‘Photon 2’: 1,578.7 nm).
Mentions: Experimentally, there is no rapid measurement instrument for JSI at this time. The peaks and valleys that distinguish one JSI from another are separated by a frequency interval of ~20 GHz (only one-tenth of a nanometre), and whereas classical optical spectrum analysers are capable of providing such a high resolution, they are not sensitive at the single-photon level. On the other hand, quantum photon detectors such as single-photon avalanche diodes (SPADs) are not wavelength-selective. While 2D arrays of SPADs now being developed3738 will be beneficial in the future, here, we use high-contrast tunable telecommunication-grade optical filters in front of the SPADs, as shown Fig. 3a to measure the JSI by scanning over the 2D frequency grid. The measured data, of which two examples are shown in Fig. 3c,g, represent the convolution of the actual JSI and the point-spread function of the filters shown in Fig. 3b. There are a number of different ways of deconvolving blurred images and as a representative method, we used the Richardson–Lucy (RL) algorithm39. The RL deconvolution is an iterative procedure for recovering, in a maximum-likelihood sense, a latent image that has been blurred by a known point-spread function. The end point of the iteration generally needs to be determined by the user, and we use our prior knowledge of typical JSIs (as shown in Fig. 2b) as a guideline, and confirm our choice by a classical four-wave mixing experiment24. In Fig. 3d,e, we show the results of deconvolution on the measured data (Fig. 3c) for 20 and 50 iteration steps, respectively. Similarly, for the measured data shown in Fig. 3g, we show the results of deconvolution with 20 and 50 iteration steps in Fig. 3h,i, respectively. In each case, we stopped at 50 iterations because the general shape and ‘sharpness’ of the JSI was then similar to the phase-matching function measured by a classical four-wave mixing experiment, shown in Fig. 3f,j for the two cases. The following discussion and insights do not depend on the exact stopping point of the de-blurring algorithm.

Bottom Line: Here we design a photon-pair source, consisting of planar lightwave components fabricated using CMOS-compatible lithography in silicon, which has the capability to vary the JSI.By controlling either the optical pump wavelength, or the temperature of the chip, we demonstrate the ability to select different JSIs, with a large variation in the Schmidt number.Such control can benefit high-dimensional communications where detector-timing constraints can be relaxed by realizing a large Schmidt number in a small frequency range.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California 92093, USA.

ABSTRACT
Directly modulated semiconductor lasers are widely used, compact light sources in optical communications. Semiconductors can also be used to generate nonclassical light; in fact, CMOS-compatible silicon chips can be used to generate pairs of single photons at room temperature. Unlike the classical laser, the photon-pair source requires control over a two-dimensional joint spectral intensity (JSI) and it is not possible to process the photons separately, as this could destroy the entanglement. Here we design a photon-pair source, consisting of planar lightwave components fabricated using CMOS-compatible lithography in silicon, which has the capability to vary the JSI. By controlling either the optical pump wavelength, or the temperature of the chip, we demonstrate the ability to select different JSIs, with a large variation in the Schmidt number. Such control can benefit high-dimensional communications where detector-timing constraints can be relaxed by realizing a large Schmidt number in a small frequency range.

No MeSH data available.


Related in: MedlinePlus