We study superfluorescence (SF) from spherical and cigar-shaped clouds of laser-cooled Rubidium atoms from the $5{D}_{5/2}$ level through the $6{P}_{3/2}$ level to the $5{S}_{1/2}$ ground level. The atomic system is initially excited to the $5{D}_{5/2}$ level from the ground state via two-photon excitation through the intermediate $5{P}_{3/2}$ level. The fluorescence on the $6P\text{\ensuremath{-}}5S$ transition at 420 nm is recorded using time-resolved measurements. The time delays of the observed SF emission peaks typically scale as $\ensuremath{\sim}{N}^{\ensuremath{-}1}$, where $N$ is the atom number, and are much smaller than the time delay expected for uncorrelated cascade fluorescence. Since $N$ is significantly smaller than the threshold number for SF on the 420 nm transition, and larger than the threshold number for the $5D\text{\ensuremath{-}}6P$ transition at $5.2\text{ }\ensuremath{\mu}\text{m}$, our observations suggest that the 420 nm SF emission is triggered by rapid deexcitation of the $5D$ to the $6P$ level via SF at $5.2\text{ }\ensuremath{\mu}\text{m}$. The observed SF time delays for 420 nm emission agree with SF time-delay estimates for the $5.2\text{ }\ensuremath{\mu}\text{m}$ transition. For spherical clouds, the SF is isotropic. For cigar-shaped clouds, the SF is highly anisotropic. Along the long axis of cigar-shaped atom clouds, SF and incoherent cascade fluorescence produce temporally well-resolved peaks in the detected signal. In this case, the SF component of the signal is highly concentrated along a direction in between the directions of the two almost parallel excitation beams. The observed SF intensities scale as $N$ , suggesting that the $5D$ level is regeneratively pumped during the SF decay.We study superfluorescence (SF) from spherical and cigar-shaped clouds of laser-cooled Rubidium atoms from the $5{D}_{5/2}$ level through the $6{P}_{3/2}$ level to the $5{S}_{1/2}$ ground level. The atomic system is initially excited to the $5{D}_{5/2}$ level from the ground state via two-photon excitation through the intermediate $5{P}_{3/2}$ level. The fluorescence on the $6P\text{\ensuremath{-}}5S$ transition at 420 nm is recorded using time-resolved measurements. The time delays of the observed SF emission peaks typically scale as $\ensuremath{\sim}{N}^{\ensuremath{-}1}$, where $N$ is the atom number, and are much smaller than the time delay expected for uncorrelated cascade fluorescence. Since $N$ is significantly smaller than the threshold number for SF on the 420 nm transition, and larger than the threshold number for the $5D\text{\ensuremath{-}}6P$ transition at $5.2\text{ }\ensuremath{\mu}\text{m}$, our observations suggest that the 420 nm SF emission is triggered by rapid deexcitation of the $5D$ to the $6P$ level via SF at $5.2\text{ }\ensuremath{\mu}\text{m}$. The observed SF time delays for 420 nm emission agree with SF time-delay estimates for the $5.2\text{ }\ensuremath{\mu}\text{m}$ transition. For spherical clouds, the SF is isotropic. For cigar-shaped clouds, the SF is highly anisotropic. Along the long axis of cigar-shaped atom clouds, SF and incoherent cascade fluorescence produce temporally well-resolved peaks in the detected signal. In this case, the SF component of the signal is highly concentrated along a direction in between the directions of the two almost parallel excitation beams. The observed SF intensities scale as $N$, suggesting that the $5D$ level is regeneratively pumped during the SF decay.

Physical Review AApr 25, 200843 citations

調和トラップ中の1次元ボース系の理論(3)

正人大矢

Sep 1, 20070 citations

Simplified Symbol Flipping Algorithms for Nonbinary Low-Density Parity-Check Codes

Jianyong Zhang

Based on the symbol-flipping algorithm with multiple-votes (MV-SF), this paper presents two simplified algorithms, symbol flipping with truncated and multiple votes (T-MV-SF) and symbol flipping with simplified and multiple votes (S-MV-SF), for nonbinary low-density parity-check(NB-LDPC) codes. Unlike previous symbol-flipping based algorithms (SFBAs), the T(S)-MV-SF uses the truncated voting information in each check and variable node update. The proposed algorithms also approximate the sorting function in each check update. Therefore, their complexity and memory consumption are remarkably reduced as compared with existing SFBAs. Furthermore, we introduce several parameters to compensate the information loss caused by the truncated voting information. After optimizing all the parameters, the proposed algorithms even have better performance than the MV-SF. As compared with the MV-SF, the maximum performance gain among all numerical examples is about 0.11 dB for the S-MV-SF on the (63, 37) code. The T-MV-SF has the largest reduction of the computational complexity on the (255, 175) code, which is about 80% of the complexity of the MV-SF. Moreover, the performance of the T-MV-SF is better than the MV-SF on the (255, 175) code. As a result, the proposed algorithms can effectively balance the error performance and complexity for decoding of NB-LDPCs.Based on the symbol-flipping algorithm with multiple-votes (MV-SF), this paper presents two simplified algorithms, symbol flipping with truncated and multiple votes (T-MV-SF) and symbol flipping with simplified and multiple votes (S-MV-SF), for nonbinary low-density parity-check(NB-LDPC) codes. Unlike previous symbol-flipping based algorithms (SFBAs), the T(S)-MV-SF uses the truncated voting information in each check and variable node update. The proposed algorithms also approximate the sorting function in each check update. Therefore, their complexity and memory consumption are remarkably reduced as compared with existing SFBAs. Furthermore, we introduce several parameters to compensate the information loss caused by the truncated voting information. After optimizing all the parameters, the proposed algorithms even have better performance than the MV-SF. As compared with the MV-SF, the maximum performance gain among all numerical examples is about 0.11 dB for the S-MV-SF on the (63, 37) code. The T-MV-SF has the largest reduction of the computational complexity on the (255, 175) code, which is about 80% of the complexity of the MV-SF. Moreover, the performance of the T-MV-SF is better than the MV-SF on the (255, 175) code. As a result, the proposed algorithms can effectively balance the error performance and complexity for decoding of NB-LDPCs.

IEEE Transactions on CommunicationsJan 1, 20175 citations

Superfluorescence from optically trapped calcium atoms

Anantharaman Kumarakrishnan, Xianming L. Han

We have studied superfluorescence (SF) under highly unfavorable conditions of rapid collisional and radiative distribution in a Doppler-broadened medium. Nanosecond SF pulses at 5.5 \ensuremath{\mu}m were generated on the Ca $4s4p{}^{1}{P}_{1}--3d4s{}^{1}{D}_{2}$ transition from a column of calcium vapor buffered with Ar by optically pumping the ${4s}^{2}{}^{1}{S}_{0}--4s4p{}^{1}{P}_{1}$ transition. The Rabi frequency associated with the intense pump pulse prevents the occurrence of SF while the pump laser is on. As a result, the predicted scaling laws that describe the properties of SF in a transversely excited system, such as peak heights, pulse widths, and delay times, are shown to apply in our situation in which the conditions resemble swept excitation. The delay times were found to be in agreement with a fully quantum mechanical calculation which describes the initiation of SF. Measurements of the densities of the three levels, the absolute SF photon yield, and the spatial distribution of the excited states indicate that the system has a quantum yield of unity. The SF intensity increases with an increase in Ar pressure due to collisional redistribution until the collisional dephasing rate inhibits SF. The conditions describing the transition of SF to amplified spontaneous emission allow us to measure the collisional broadening rate for the SF transition.We have studied superfluorescence (SF) under highly unfavorable conditions of rapid collisional and radiative distribution in a Doppler-broadened medium. Nanosecond SF pulses at 5.5 \ensuremath{\mu}m were generated on the Ca $4s4p{}^{1}{P}_{1}--3d4s{}^{1}{D}_{2}$ transition from a column of calcium vapor buffered with Ar by optically pumping the ${4s}^{2}{}^{1}{S}_{0}--4s4p{}^{1}{P}_{1}$ transition. The Rabi frequency associated with the intense pump pulse prevents the occurrence of SF while the pump laser is on. As a result, the predicted scaling laws that describe the properties of SF in a transversely excited system, such as peak heights, pulse widths, and delay times, are shown to apply in our situation in which the conditions resemble swept excitation. The delay times were found to be in agreement with a fully quantum mechanical calculation which describes the initiation of SF. Measurements of the densities of the three levels, the absolute SF photon yield, and the spatial distribution of the excited states indicate that the system has a quantum yield of unity. The SF intensity increases with an increase in Ar pressure due to collisional redistribution until the collisional dephasing rate inhibits SF. The conditions describing the transition of SF to amplified spontaneous emission allow us to measure the collisional broadening rate for the SF transition.

Physical Review ANov 1, 199829 citations

新量子力學の發展(1)

ペヨルダン

Dec 25, 19270 citations

We study superfluorescence (SF) from spherical and cigar-shaped clouds of laser-cooled Rubidium atoms from the

5{D}_{5/2}

level through the

6{P}_{3/2}

level to the

5{S}_{1/2}

ground level. The atomic system is initially excited to the

5{D}_{5/2}

level from the ground state via two-photon excitation through the intermediate

5{P}_{3/2}

level. The fluorescence on the $6P\text{\ensuremath{-}}5S$ transition at 420 nm is recorded using time-resolved measurements. The time delays of the observed SF emission peaks typically scale as $\ensuremath{\sim}{N}^{\ensuremath{-}1}$, where

N

is the atom number, and are much smaller than the time delay expected for uncorrelated cascade fluorescence. Since

N

is significantly smaller than the threshold number for SF on the 420 nm transition, and larger than the threshold number for the $5D\text{\ensuremath{-}}6P$ transition at $5.2\text{ }\ensuremath{\mu}\text{m}$, our observations suggest that the 420 nm SF emission is triggered by rapid deexcitation of the

5D

to the

6P

level via SF at $5.2\text{ }\ensuremath{\mu}\text{m}$. The observed SF time delays for 420 nm emission agree with SF time-delay estimates for the $5.2\text{ }\ensuremath{\mu}\text{m}$ transition. For spherical clouds, the SF is isotropic. For cigar-shaped clouds, the SF is highly anisotropic. Along the long axis of cigar-shaped atom clouds, SF and incoherent cascade fluorescence produce temporally well-resolved peaks in the detected signal. In this case, the SF component of the signal is highly concentrated along a direction in between the directions of the two almost parallel excitation beams. The observed SF intensities scale as

N

, suggesting that the

5D

level is regeneratively pumped during the SF decay.

4s4p{}^{1}{P}_{1}--3d4s{}^{1}{D}_{2}

transition from a column of calcium vapor buffered with Ar by optically pumping the

{4s}^{2}{}^{1}{S}_{0}--4s4p{}^{1}{P}_{1}

transition. The Rabi frequency associated with the intense pump pulse prevents the occurrence of SF while the pump laser is on. As a result, the predicted scaling laws that describe the properties of SF in a transversely excited system, such as peak heights, pulse widths, and delay times, are shown to apply in our situation in which the conditions resemble swept excitation. The delay times were found to be in agreement with a fully quantum mechanical calculation which describes the initiation of SF. Measurements of the densities of the three levels, the absolute SF photon yield, and the spatial distribution of the excited states indicate that the system has a quantum yield of unity. The SF intensity increases with an increase in Ar pressure due to collisional redistribution until the collisional dephasing rate inhibits SF. The conditions describing the transition of SF to amplified spontaneous emission allow us to measure the collisional broadening rate for the SF transition.