Recently there has been much interest in non-classical light both in theoretical and experimental study[1, 2]. Considerable efforts have been devoted to generate quadrature-phase squeezed states using nonlinear optical processes and to generate photon-number squeezed states or sub-Poissonian light using semiconductor devices. With these non-classical states of light, it is possible to improve the sensitivity of various measurements and the performance of communication systems beyond the shot noise limit.
The scheme utilizing semiconductor devices is especially attractive because of its simple experimental configuration and small energy consumption. A tenth noise reduction (10dB) with a laser diode (LD) was reported a few years ago[3]. Generation of ``quantum correlated twin beams"[4, 5] and realization of a ``quantum optical tap (or repeater)"[6, 7] by using light emitting diodes (LED) were also reported. In the most of the experiments, however, the light intensity was about a few milliwatts, and there is no experimental investigation of generating weak sub-Poissonian light with semiconductor devices to the best of author's knowledge.
Generation of weak sub-Poissonian light is important both from the
viewpoint of fundamental research and application .
This is because the lower the light intensity is, the more prominent the
quantum effects become.
For example,
it is interesting to study nonclassical
properties of sub-Poissonian light that is characterized by, e.g.,
the phenomenon of photon antibunching. Because the magnitude of this
effect (the value of the second-order correlation function) is determined
by the inverse average number of photons[10],
the photon number should be small.
In a photon-counting communication system using Poissonian
photons (coherent light) the error probability 
is given by
, where 
denotes the mean photon
number in one optical pulse[8]. can be, therefore,
suppressed sufficiently when the light is intense enough.
However it is necessary to decrease the intensity in order to
reduce the energy consumption for such application as optical
interconnection.
Thus in order to make the sub-Poissonian generation meaningful
in a practical use, the photon number in an optical pulse must be
lowered[9].
Moreover, one may obtain information about the interaction between
photons and carriers in semiconductor as well as behavior of the
carriers by investigating the properties of sub-Poissonian light
at various intensities.
One avenue to the generation of weak sub-Poissonian light is the fabrication of a microcavity laser: by using a technique based on cavity quantum electrodynamic effect, a ``thresholdless" laser can be realized[11]. However, in the case of LD, it is known that weak optical reflection [12] and/or weak side modes [13] will degrade the amount of noise reduction. Thus care must be taken to reduce such effects. Another avenue is the use of LED. LED is intrinsically thresholdless moreover free from above-mentioned problems. It is significant, therefore, to investigate the possibility of generating weak sub-Poissonian light using conventional LED and to clarify problems associated with this scheme.
In this paper we report generation of weak sub-Poissonian light with a high-efficiency LED. The light intensity generated in our experiment is about thousand times as small as those reported in the previous experiments[4, 5, 6, 7, 16, 17]. The minimum obtainable intensity in the experiment is limited by the decrease of emission efficiency at room temperature and by the worsening of frequency response of LED at liquid nitrogen temperature. This behavior is qualitatively explained in terms of trap states.
The generation scheme of sub-Poissonian light in our experiment is based
on the principle of high-impedance suppression of pump-amplitude
fluctuation[14]. In this scheme, the sub-Poissonian
statistics of electron current are converted directly into nonclassical
photon statistics. It is convenient to use the normalized noise power
(Fano factor, W) which is defined as 
. If we take the emission efficiency into account, the
Fano factor of the generated light, 
, is related with that
of the pump current, 
, as [15],
where 
is the normalized number fluctuation within the
observation time 
( 
gives the measurement frequency), and
is the probability that an injected electron will
emit a photon within the delay time .
When the pump current has the Poissonian statistics ( 
),
the statistics of photon is also Poissonian ( 
) regardless
of the value of 
.
When an LED is driven by a Johnson-noise limited source and the
Johnson-noise is negligible compared to quantum noise , is
effectively zero and is given by
.
The amount of noise reduction is equal to the emission efficiency.
The Johnson-noise is smaller than quantum noise
if 
, where 
is the Boltzman energy, e is the
electric charge, and 
is the voltage across the resistor that is
series-connected to the LED[16].
At room temperature 
is 52mV.
Therefore it is easy to reduce the photon number fluctuation
substantially if the emission efficiency is high enough.