Subsequently, we investigated the efficiency
and speed of few-photon control. In the first set
of measurements, the depletion of a high-power
pump seeded by a small continuous wave (CW)
signal was characterized. To eliminate any effect
of Brillouin scattering, the high-power CW pump
was converted into quasi-CW, 1-ns pulse with
1.59-W peak power. The pump depletion map
(Fig. 4A) illustrates the static transfer characteristics of the photon gate when the pump was
centered at 1554 nm. The static measurement
accuracy (T0.065 dB) imposed the minimum
observable depletion level, limiting the smallest
input signal power to 30 nW during the measurement. To estimate the maximum bandwidth
of the control signal pulse, we consider a 1-dB
pump depletion level that is ~10 times above the
measurement accuracy. For an input control
power of 178 n W, the signal can be tuned over
520 GHz (line N1 in Fig. 4A) while maintaining
the minimum 1-dB pump depletion level. This
underestimates the true pump depletion level:
The 178-n W signal positioned in the middle of
this frequency range (point P in Fig. 4A) will lead
to higher (∼2 dB) pump depletion. However, even
with this constraint, it is possible to estimate the
minimum photon number in the control pulse
allowed by this fiber: A 2.5-ps-long pulse with
peak power of 178 n W contains less than three
photons, indicating the feasibility of few-photon
switching at a 500-GHz rate.
The same experimental architecture was used
to test the nonreciprocity hypothesis of this work
by inverting the physical direction of the FPM
interaction. Indeed, if a unique core variation
function leads to optimum photon switching,
then a directionally inverted process would be
suboptimal. Consequently, the input and output
ports of the fiber were swapped, and the CW
depletion map was measured again (Fig. 4B). We
note the striking differences between the responses shown in Fig. 4, A and B, with respect to
both the maximum pump depletion and the
Although useful in estimating the performance
of few-photon control, these static depletion
measurements alone cannot accurately quantify
the ultrafast switching response because wide
signal bandwidth precludes the assumption of
monochromatic phase matching. Consequently,
in the second set of measurements, we inves-
tigated the pump depletion induced by an ultra-
fast signal. To approach the maximum bandwidth
predicted by the 1-dB static depletion measure-
ment, a 2.45-ps pulse was generated using a
continuously tunable, cavity-less source (see the
supplementary materials), attenuated, and used
to deplete a 1.45-W pump centered at 1554 nm.
Instantaneous pump depletion was acquired by
a high-sensitivity parametric sampling gate (see
the supplementary materials). The sampling gate
bandwidth was larger than a terahertz, thus
guaranteeing undistorted pump depletion cap-
ture. Figure 4C shows pump depletion traces
corresponding to input signal pulses with an
average of 3, 5, 7, and 15 photons, respectively.
These results prove the basic importance of
nanometer-scale fiber core control in distributed
FPM interactions. Although this report addresses
the feasibility of few-photon switching in locally
controlled fiber, it is not difficult to predict the
broader implications of this approach, specifically
with respect to photon sensors operating in previously forbidden regimes. The most interesting of
these rests on the notion that long-scale, locally
controlled FPM triggers a multifrequency photon
avalanche, inducing massive pump photon annihilation by a few-photon signal stimulus. However, it
is also clear that much work remains before its ultimate limit is reached. To accomplish that, a new
class of fibers that minimize stochastic dispersion
fluctuations (20, 21) while inhibiting parasitic and
noise mechanisms must be created.
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This research is based on work supported by the Office of the
Director of National Intelligence (ODNI), Intelligence Advanced
Research Projects Activity (IARPA).
Materials and Methods
Figs. S1 to S3
10 March 2014; accepted 4 June 2014
Published online 19 June 2014;
Fig. 4. Static and ultrafast photon control response. (A) Measured bandwidth-depletion map for
fiber shown in Fig. 3; 1-dB pump depletion corresponds to a 3-photon pulse (N1), and 3-dB pump
depletion corresponds to an 8-photon pulse (N2). Pump-signal spectral separation was measured with
12-GHz resolution. Control power was varied with 1-dB step. (B) FPM nonreciprocity. Bandwidth-depletion map obtained by reversing the direction of pump and signal propagation. Horizontal axis
indicates power in the input (control) pulse (bottom) and the corresponding average photon count (top).
(C) Pump depletion induced by a 2.45-ps control pulse (top) in the case when it contains an average of
3, 5, 7, and 15 photons. Red curves indicate the average sampling value; blue line indicates the zero-intensity level.