Two-Photon Interferometer
Content:
1. Introduction ..................................................................................................... 2
1.1 Underlying Physics ...................................................................................... 2
1.2 Basic Principles of Operation ........................................................................ 4
1.3 Description ................................................................................................. 4
2 Operation.......................................................................................................... 7
2.1 Handling Guidelines ..................................................................................... 7
2.2 Starting Up ................................................................................................. 7
2.3 Alignment ................................................................................................... 8
2.4 Improving Fiber-to-Fiber Coupling .............................................................. 10
2.5 Restoring Fiber-to-Fiber Coupling ............................................................... 10
3 Searching for the interference dip..................................................................... 12
A Control and Read-out Unit ............................................................................... 13
A.1 Handling advices and tips .......................................................................... 13
A.1.1 Laser module ...................................................................................... 13
A.1.2 Counter module .................................................................................. 13
A.1.3 Avalanche Photodiodes........................................................................ 13
Two-Photon Interferometer
1
1. Introduction
This lab experiment is about letting 2 photons interfere at a non-polarizing beamsplitter and demonstrate how bosons like to stick together. The system used here is
the “Two-Photon Interferometer” (qu2PI) from the company qutools.
The qu2PI is a two-photon interferometer designed to operate together with the
qutools quED (“Entanglement Demonstrator”) as a source of time correlated photon
pairs. It is an education-oriented experimental module capable of demonstrating a
quintessential quantum mechanical phenomenon: the interference of two
indistinguishable photons at a non-polarizing beam-splitter. This phenomenon is
often called Hong-Ou-Mandel interference.
The experimental sign of the two-photon interference - the V-shaped drop or the
“dip” in the number of detected photon pairs towards zero - provides a measure of
the time separation between the photon arrivals with femtosecond precision.
Moreover, the depth of the dip is directly related to the degree of indistinguishability
of the two photons.
The interferometer is fiber-based and manually driven. At its heart there is the
polarization-maintaining fiber coupler ensuring high spatial and polarization overlap
of the interfering photons. The all-fiber solution ensures not only a high quantuminterference visibility, but also greatly simplifies the search and alignment
procedures. The procedures break down into separate deterministic steps, which are
easily accomplished and do not require a lot of technical expertise.
To record the interference dip, the path difference between the interferometer arms
has to be scanned in micrometer-resolved steps. This is accomplished using the
integrated manual translation stage equipped with a differential micrometer screw.
1.1 Underlying Physics
For the single photon impinging on a balanced non-polarizing beam-splitter there is
an equal chance of being reflected or transmitted. If two photon-counting detectors
are positioned at the output ports of the beam-splitter, the photon is registered at
one or the other detector with equal probability, but never in coincidence at both
detectors. The observed perfect anti-correlations between the detection events
provide a simple experimental test of the photon indivisibility.
In the experimental scenario with the two photons simultaneously entering the input
ports of the beam-splitter, one would expect the four possibilities for the output:
both are transmitted (Fig.1a), both are reflected (Fig.1b) and one is transmitted
while the other reflected (Fig.1c and Fig.1d). The detectors at the beam-splitter
outputs should thus register half of the photons in coincidence while the other half is
not detected.
Two-Photon Interferometer
2
This intuitive explanation however fails to account for two-photon interference, which
occurs if the two photons are indistinguishable in all degrees of freedom, i.e. the
photons have the same wavelength, polarization and spatio-temporal mode. In such
a case the first two possibilities - when both photons are transmitted (Fig.1a) or
reflected (Fig.1b) - cannot be distinguished from one another because there is
always one photon in either output mode. As a result, the two possibilities are
coherently superposed. Due to unitarity of the beam-splitter transformation there is
always an overall  -phase shift between the two possibilities and therefore they
completely cancel each other. The involved  -phase shift is universal and
independent of a specific beam-splitter and therefore the described destructive
interference appears for any practical realization.
For the other two possibilities left (Fig.1c and Fig.1d), the photons always exit the
same output port of the beam-splitter. This will manifests itself in the absence of the
coincidence counts. By scanning the relative time delay between the photon arrivals
at the beam-splitter, the degree of temporal distinguishability of the two photons is
effectively changed, and therefore the dip in the coincidence count rate can be
observed. For zero delay, the perfect photon overlap in the time domain is ensured,
and the coincidence rate should theoretically drop to zero.
This is however never the case in practice because of experimental imperfections
such as the deviation from the ideal 50:50 beam-splitter splitting ratio, imperfect
spatial-mode or polarization-mode overlap. Therefore the dip with a limited visibility
is always recorded experimentally. The measured visibility corrected for the imperfect
splitting ratio gives a direct measure of indistinguishability of the input photons.
Unlike the interference effects in conventional Mach-Zehnder or Michelson
interferometers, the Hong-Ou-Mandel effect does not require the phase stability of
the interferometer arms. The path differences of the arms need not be kept constant
to within a fraction of wavelength, but only to within a fraction of the photon
coherence length. From practical point of view the Hong-Ou-Mandel effect can be
viewed as a method to gauge the femtosecond time intervals (corresponding to
micrometer length scales) between the photons and by implication the length of the
photon wave packets. It is relatively straightforward to calculate the coherence
length of the photon wave packets from the measured interference dip.
Figure 1: Four alternative paths for the passage of two photons through a balanced non-polarizing
beam-splitter.
Two-Photon Interferometer
3
1.2 Basic Principles of Operation
As pointed out in the previous section, the qu2PI is not an interferometer in the
traditional sense, i.e. the light is not first split and later recombined like in standard
Michelson or Mach-Zehnder interferometers. Rather the time-correlated photons
provided by a photon-pair source enter the two input ports of the balanced beamsplitter. In the qu2PI, the fused 50:50 polarization maintaining fiber coupler is used
as the beam-splitter. This all-fiber solution ensures a good spatial-mode overlap of
the interfering photons, and provided that the linear polarization with correct
orientation is launched into the input fibers of the coupler, also a good polarizationmode overlap of the photons. The latter is ensured due to the use of polarizationmaintaining fibers, which preserve the linear-polarization of the light that is launched
into the fibers. Moreover, the coupler is designed such that for linear polarization the
splitting ratio is very close to the ideal 50:50 at the operating wavelength.
Either arm of the two-photon interferometer contains a short free-space optical line,
i.e. the fiber-coupled photons coming from the source are collimated using a lens
and after a short free-space transmission collected again into the input fibers of the
coupler. The free-space lines have a double role. First, the two rotatable wave plates
inserted in either arm allow to adjust the polarization of the incoming photon pairs as
required. Second, one of the lines has a variable length, which makes the tuning of
the path difference between the two arms possible. The tuning is controlled by the
manual translation stage equipped with the differential micrometer screw. This
enables the required micrometer-resolved steps for achieving the precise time
overlap of the photons and scanning through the interference dip. For maximum
spectral overlap the qu2PI contains two removable interference filters with
transmission bandwidth significantly smaller compared to the natural spectral width
of the photons. The insertion of filters thus leads not only to higher interference
visibility but also to a wider interference dip. This is particularly useful when
searching the dip, because larger scanning steps are possible.
1.3 Description
This section shows the individual optical components of the qu2PI. The description
follows the notation shown in the photo of the interferometer module in Fig.2.
Two-Photon Interferometer
4
Figure 2: Architecture of the qu2PI, the two-photon interference experimental module.
All the components of the qu2PI are mounted on a solid aluminium base plate. The
four kinematic mounts with adjustable-focus collimators build up the two free-space
lines; one of the lines has a variable length (1), whereas the length of the other is
fixed (2). All the fiber collimators are FC/PC compatible and contain aspheric lenses
with adjustable focus. The focus can be adjusted by rotating the aspheric lens in the
aluminium housing of the collimator. The single-mode fibers from the quED are to be
plugged into FC/PC receptacles of the transmitting collimators (3). These are prealigned to collimate the light at the operating wavelength of 810 nm. The focuses of
aspheric lenses in the receiving collimators (4) are pre-aligned for maximum fiber-tofiber transmission.
The input polarization-maintaining fibers (5) of the fused fiber coupler (6) are
connected to the receiving collimators. The coupler is made for the operation at the
slow axis. This axis defines the unique direction of the linear polarization, which is
maintained during the propagation, and for which the splitting ratio close to 50:50 is
ensured. The connector keys of the polarization-maintaining fibers are oriented with
the slow axis and therefore they define the polarization direction to be coupled into
the fibers. Since the FC/PC receptacles of the receiving collimators are rotated to
accept the connector keys at the vertical orientation, it is the vertical linear
polarization which is aimed to be coupled into the couplers. The one-meter-long
output polarization maintaining fibers (7) of the coupler are connected to the readout unit of the quED during operation.
Either free-space line contains a quarter-wave retarder (8) and a half-wave retarder
(9) mounted in rotation mounts and the interference band-pass filter (10) fixed
inside a removable tube adapter. The inner diameter of the tube adapters is
compatible with the size of the collimators. The adapters can therefore easily be
mounted onto the collimators and secured with the locking screw. Additional M6 taps
Two-Photon Interferometer
5
are available for mounting the polarizers (provided from the quED), which are
required during the alignment procedure.
The mirror mount with the transmitting collimator is attached to the linear manual
positioning stage (11) in the variable-length free-space line. The stage can be
secured with the side-positioned locking screw (12). The linear stage is equipped
with the compact differential micrometer screw (13) (see Fig.3), offering both long
travel and high-resolution adjustments. The coarse-adjustment knob is used for rapid
positioning (500 µm per revolution) of the linear stage over 25 mm travel range. The
coarse adjustment is lockable by tightening the locking nut against the stage body.
The fine-adjustment knob provides 50 µm per revolution of travel with graduation
marks every 1 µm.
Figure 3: The differential micrometer with two knobs for coarse and fine adjustment
The positioning stage allows adjusting the distance between the collimators to
between 87.5 mm and 112.5 mm. Given the 110 mm collimator-to-collimator spacing
in the fixed-length free-space line, the qu2PI allows to tune the interferometer path
difference in the range of [-2.5, 22.5] mm. Two additional taps are provided to shift
the mirror mount with the transmitting collimator in the fixed-length free-space line.
This allows to enlarge the path-length difference range to [17.5, 42.5] mm and
[37.5, 67.5] mm, respectively.
Two-Photon Interferometer
6
2 Operation
You should have already read the operations manual of the quED. All the safety
precautions and guidelines apply here as well.
2.1 Handling Guidelines









Never expose the single photon detectors to intense light fluxes. Use the
protective caps when no fiber is connected and especially don’t shine with the
fiber checker into the detectors. Disconnect the fibers from the detector when
aligning the system.
Keep the room clean and wash your hands. Most of the optical components
are very sensitive to dirt – don’t touch them directly.
The positioning stage has a locking screw on the side. Unlock it before trying
to move the stage via the micrometer.
The coarse adjustment of the micrometer has a locking nut. Unlock it before
turning the knob.
Remove rings, bracelets, watches, etc… from your hands. This ensures that
the laser cannot be reflected.
Do not turn on any screws of the kinetic mounts unless you know what your
doing (especially the ones inside the pump beam enclosure). The alignment of
the entire setup can easily be lost.
Do not remove any of the parts (except the polarisation filters) from the
breadboard. This would destroy the alignment of the setup.
The rotation mounts have a locking screw, be sure to unlock them before you
try to turn them.
Handle the equipment with care, do not use any force.
2.2 Starting Up
1. Before making any connections of the interferometer module to the source
and the controller, please verify first that the source delivers a steady stream
of photon pairs with pre-defined polarization. To this end, please:
(a) Rotate the polarization of the pump beam to the vertical (or horizontal)
direction via the half-wave plate. Consequently, only photon pairs with
horizontal (or vertical) polarization are generated in the non-linear crystal
via type I spontaneous down-conversion.
(b) Remove the polarizing filters from the quED. They will later be used for the
alignment of the photon polarization at the input to the polarizationmaintaining fiber coupler in the qu2PI.
(c) Align the coupling of the photon pairs in the single-mode fibers by
maximizing the single- and coincidence-count rates. The achieved rates
should be noted for reference.
Two-Photon Interferometer
7
2. Unplug the connectors of the single mode fibers from the control unit and
insert them into the transmitting collimators of the qu2PI. Gently fasten the
connectors in the FC/PC receptacles of the collimators.
3. The single-mode fibers connecting the source with the interferometer module
should be precluded from any movements. Please take any appropriate
measures for fixing the fibers to a place, such as winding the fibers to spools
or taping them to the table. Any movement of the (non polarizationmaintaining) fibers rotates the transferred polarization state due to stressinduced birefringence changes and thus makes the proper alignment of the
module impossible!
4. To connect the qu2PI to the control unit, please plug the output polarizationmaintaining fibers of the fiber coupler into the optical inputs of the control
unit. On completion of this step, the interferometer module is prepared for the
operation.
2.3 Alignment
Due to the use of a polarization maintaining fiber coupler for the photon overlap the
whole alignment procedure of the qu2PI is fairly simple. There are basically just two
tasks for the user to be accomplished - align the fiber-to-fiber coupling in both freespace lines and then adjust the photon polarization state at the input to the
polarization-maintaining fiber coupler. The alignment of the two arms is independent
and breaks down into separate independent steps:
1. We advise the user to remove first the interference band-pass filters from the
interferometer. This dramatically increases the available photon-pair counting
rates, and thereby simplifies the alignment of the fiber-to-fiber coupling. To
remove the adapters with the filters from the collimators please loosen the
locking screws first. Make sure the preparatory steps from the previous
section have been carried out, especially item 1.
2. Block both free-space links with a screen and register the displayed idle
single-count rates. They should be close to dark count rates of detectors. If
significantly higher count rates are observed, the appropriate measures has to
be taken, e.g. the level of stray light in the room has to be lowered.
3. Choose the free-space line to be aligned first and unblock it.
4. The displayed single-count rates in both channels should increase above the
idle single-count rates after unblocking the coupling. When correctly aligned,
the aggregate number of detected photons in both channels should reach
between 50-70 % of the single-count rate measured directly in the
corresponding arm of the source. If this is not the case, the fiber-to-fiber
coupling is misaligned and it has to be readjusted. Please choose one of the
following possibilities and act accordingly:
Two-Photon Interferometer
8
(a) If the displayed single-count rates reach the level specified above, please
proceed to the step 5.
(b) If the displayed single-count rates increased, but did not reach the level
specified above, please try improving the fiber-to-fiber coupling according
to the description given in section 2.4.
(c) If the displayed single-count rates in both channels did not change after
unblocking the free-space line please try finding the signal by angle tuning
the receiving collimator. With this aim, please scan angles in a systematic
way around the initial position (which should be always remembered),
while observing single-count rates 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 rates do
not raise above the level of idle counts, please follow the alignment steps
given in section 2.5.
5. After achieving a sufficient fiber-to-fiber coupling efficiency the polarization of
photons at the input to the polarization-maintaining fiber coupler has to be
aligned along the vertical direction. To this end please insert first the
polarization filter into the free-space line behind the quarter- and half-wave
plate. Ensure that the filter is centered on the optical path of the photons.
6. Rotate the polarizing filter for the transmission of horizontal polarization.
7. Adjust the quarter- and half-wave plate till the minimum in the single-count
rates is achieved. The observed rates should be close to the idle count rates
detected when both free-space lines are blocked. Please note that usually
more local minima can be found. The user should adjust the wave retarders to
the positions corresponding to the global minimum.
8. The wave retarders are adjusted now to transform the incoming arbitrary
polarization state of photons to the desired linear vertical polarization. This
can be readily proved by rotating the polarizing filter for transmission of the
vertical polarization and observing maximum single-count rates. The polarizing
filter can be removed from the free-space line.
9. After adjusting the fiber-to-fiber coupling and the orientation of the wave
retarders in one free-space line, the same procedure starting from the step 3
should be repeated for the second free-space line. The free-space line, which
has already been adjusted, should be blocked now.
After aligning both free-space lines, the adapters with the band-pass interference
filters can be mounted back onto the receiving collimators and the search for the
interference dip can be undertaken. Please note that that the mounting of adapters
will lead to a slight misalignment of the fiber-to-fiber coupling and the corresponding
correction has to be performed. Also note that the count rates drop drastically
because of the band-pass filters (about a factor of 10).
Two-Photon Interferometer
9
2.4 Improving Fiber-to-Fiber Coupling
The alignment of coupling in the given free-space line of the interferometer is
performed by angle-tuning the transmitting and receiving collimator. The angular
movements are controlled using a pair of fine-thread screws on the kinematic
mounts. One of the adjustment screws provides the control in the vertical and the
other in the horizontal direction.
The adjustments of the transmitting and receiving collimator are coupled - each
adjustment of the transmitting collimator has to be accompanied by a corresponding
adjustment of the receiving collimator. For example, if you tilt the transmitting
collimator in the horizontal direction and the count rates drop, the receiving
collimator has to be tilted in the horizontal direction as well, such that the single
count rates reach their maximum again. This “angular walking” of the collimators has
to be iteratively performed in both horizontal and vertical directions. If done properly
the angular setting with the maximum observed single-count rates is found after a
while.
Alignment steps have to be sufficiently small to avoid complete loss of the photoncoupling. For example, if you tilt the transmitting collimator allow the count rates to
drop not more than to a quarter of their initial values and then compensate for this
drop by an appropriate movement of the receiving collimator. As mentioned in the
section 1.3 the focuses of the collimators are pre-aligned for maximum fiber-to-fiber
transmission at the operating wavelength of 810 nm.
2.5 Restoring Fiber-to-Fiber Coupling
After transport or after changing the distance between the collimators in the fixedlength free-space line, the fiber-to-fiber coupling can be lost completely. To restore
the coupling, please proceed according to the following steps:
1. Disconnect the single-mode fibers from the twin detection module (always use
protective caps to cover fiber connectors and input optical receptacles).
2. If not already done, unplug the single-mode fiber from the transmitting
collimator and remove the adapter with the long-pass filter from the receiving
collimator.
3. Plug the delivered single mode fiber to the transmitting collimator.
4. Connect the fiber checker to the other end of the single-mode fiber and switch
it on. The red laser beam coming from the transmitting collimator is now
visible. The beam intensity can be changed by adjusting the coupling of the
fiber checker to the single-mode fiber.
Two-Photon Interferometer
10
5. Locate the position of the laser beam on the receiving collimator. Align the
angular pointing of the transmitting collimator using the two adjustment
screws on the kinematic mount such that the laser beam hits the center of the
aspheric lens in the receiving collimator.
6. This step aims to align the receiving collimator. It can be omitted if the fiberto-fiber coupling has been lost because of shifting the transmitting collimator
in the fixed-length line (the receiving collimator should have stayed aligned).
Switch off the fiber checker, unplug it and connect it to one of the output
fibers of the polarization maintaining fiber coupler. The red laser beam coming
from the receiving collimator and propagating towards the lens in the
transmitting collimator is now visible. If aligned correctly, the beam should hit
the center of lens. Please correct the angular pointing of the receiving
collimator using the two adjustment screws on the kinematic mount if
necessary.
7. If aligned correctly, the red laser light coming from the transmitting collimator
should be partly coupled into the input single mode fiber of the polarization
maintaining fiber coupler. To check this, remove the protective cap from one
of the output single mode fibers and put in front of the fiber tip a piece of
blank paper. The faint red spot should be clearly visible. Never look into the
fiber directly! Permanent eye damage could result!
8. Switch off the fiber checker, unplug it and connect the output polarizationmaintaining fibers of the fiber coupler into the optical inputs of the twin
detection module. Reconnect the single-mode fiber from the quED to the
transmitting collimator.
Upon completing the last step, the procedures of sections 2.3 and 2.4 should be
followed again to align the qu2PI and observe the interference pattern.
Two-Photon Interferometer
11
3 Searching for the interference dip
The search of the interference dip is performed by changing the length of the freespace line using the linear translation stage and observing changes in the measured
coincidence count rate. As soon as a significant drop in the coincidence count rate is
observed, the two interferometer arms are adjusted to have the same length and the
interference dip is found.
Please note that the detected coincidence count rate C always fluctuates about its
mean value C with a Poisson standard deviation given by  C  C The interference
should manifest itself by the drop of coincidence count rate deeply below the Poisson
noise. If the input polarization state to the polarization maintaining fiber coupler is
correctly aligned, the coincidence count rate should decrease to at least half of the
value C outside the interference dip.
The scanning of the translation stage is controlled with the coarse adjustment knob
on the differential micrometer screw. The knob is normally secured using the locking
nut, and has to be loosened before usage. The scanning step angle should be chosen
according to the expected width of the interference pattern. This width is inversely
proportional to the spectral bandwidth of photons. If the band-pass interference
filters are removed from the qu2PI, the Hong-Ou-Mandel dip with the width of only
~10µm should be expected due the natural photon spectral bandwidth of ~50µm
(applies for quED I). Given the resolution of the coarse adjustment screw of 500µm
per revolution, the scanning step angle should be restricted to no more than 5° in
such a case.
To allow larger scanning steps it is convenient to insert the delivered 10-nm-wide
band-pass interference filters into the free-space lines of the interferometer. The
filters determine the full width at half maximum of the dip to about 50µm, thereby
allowing to increase the scanning steps to 30-40°. The searching procedure with the
inserted band-pass interference filters is faster and lowers the risk of missing the
interference pattern.
When the dip is found start a measurement and plot your results. Choose
appropriate values for start, stop and step yourself. After remove the band-pass
filters and realignment measure and plot the dip again. Again choose appropriate
values for start, stop and step yourself. Interpret your measurements.
Two-Photon Interferometer
12
A Control and Read-out Unit
The control and read-out unit consist of a laser controller, 2 avalanche photodiodes
and a counter module with display. In this section you can find a few tips and
handling advices that should be enough for the experiments. For further information
please refer to the “quED Control and Read-out Unit operation manual”
A.1 Handling advices and tips
A.1.1 Laser module
The knob can be pushed to select a setting and change it. The interesting setting is
I_set. The red button on the laser module turns the laser on and off.
A.1.2 Counter module
With the “up” and “down” buttons you can change the integration time between 0,1s
and 10s. When performing alignment you might want the setting at about 0,5s so
you have a good compromise between statistical errors in the count rate and the
speed of the alignment procedure.
By pressing the “mode” button you can change between continous and single shot
mode. In the single shot mode you can start a new measurement with the “shift”
button. Until then the value from the previous measurement is displayed.
Pressing “up” or “down” while holding the “shift” button changes the number of
averages in the range from 1 to 15. The display shows a running average. In the
single shot mode the counter stops after n intervals (as set in the number of
averages).
A.1.3 Avalanche Photodiodes
Be careful when plugging in or unplugging the fibers. Most stray light is collected
through the fibers in the vicinity to the plugs. Cover them with a black tissue or
something alike.
The APDs are very sensitive. Don’t expose them to high light fluxes.
Two-Photon Interferometer
13