qutools quED - Switching on

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14 quED Manual www.qutools.com

Switching On

1. Make yourself familiar with the operation of the quCR Controller for the quED setup

which comes with its own “Down-conversion Controller Manual”. Switch on the control

unit.

2. Switch to the count rate panel. Once stabilized, compare the actual displayed single

count rates to dark count rates speciﬁed in the inspection data sheet, which is included

in the delivery notiﬁcation. If the actual measured values are signiﬁcantly higher, the

level of stray light in the room has to be lowered. Especially, please try covering the ﬁber

inputs at the control unit by a piece of a black tissue.

3. Switch the laser diode on and increase the input current on the laser driver tab. The

laser diode starts lasing at the threshold current speciﬁed in the inspection data sheet.

By further increasing the input current the source starts generating photon pairs and

the coincidence count rate (green ”01” number in the counter tab) becomes nonzero.

The single count rates rapidly increase above the level of dark counts. After shipping or a

longer period of storage there is probably a slight misalignment of the photon coupling

into the single-mode ﬁbers. Please choose one of the following possibilities and act

accordingly:

a) If the displayed single and coincidence count rates reach approximately the values

speciﬁed in the inspection data sheet, the quED is still perfectly aligned and it is ready to

be used for entanglement tests and other experiments.

b) If the displayed coincidence count rate is well above zero and single count rates are well

above the level of dark counts, but all of them do not reach the regime speciﬁed in the

inspection data sheet, please try improving the coupling of the down-conversion

photons according to description given in section 3.4.2.

c) If the displayed coincidence count rate is zero or close to zero and single count rate in

one of the source arms is at the dark-count level but in the other arm well above this

level, please:

• Try improving the coupling of down-conversion photons in the arm, which

shows single count rate above the dark-count level. This arm can be easily

identiﬁed by blocking the path of down-conversion photons and observing the

drop in single count rate. To this end, please follow the instructions given in

section 3.4.2.

• Try ﬁnding down-conversion photons by angle tuning the adjustable coupler in

the arm, which shows single-count rate at the dark-count level. With this aim,

please scan angles in a systematic way around the initial position (which should

be always remembered), while observing the respective single-count rate at the

control unit. As the scanning is performed blindly without any reference, the

success comes only if the initial misalignment is small. If the scanning fails and

single-count rate does not increase above the level of dark counts, please align

the coupling mode in the arm according to steps of the full alignment procedure

described in section 3.4.3.

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