Patent Description:
Entangled photon sources are well known for example as a BBO source from <CIT> or as Sagnac ppKTP type-II source from "A space-suitable, high brilliant entangled photon source for satellite base quantum key distribution" or as a pulsed type-<NUM> PPLN source from "Pulsed Sagnac source of polarization-entangled photon pairs in telecommunication band" or as fiber Sagnac configuration from <CIT>. From <NPL> a polarization-entangled photon source with a Sagnac configuration is known. From <NPL> a space-suitable entangled photon source with a Sagnac-interferometer is known. For a free-space entangled photon source the alignment of the Sagnac loop is of crucial importance.

An objective of the present invention is to provide an improved and more precise method to align a Sagnac loop for an entangled photon source and to provide a Sagnac loop for an entangled photon source with an apparatus for the alignment.

The method to align a Sagnac loop for an entangled photon source is achieved according to claim <NUM> by the steps of providing:.

According to the invention the second and/or third polarizing beam splitter is arranged in the third and/or fourth output path of the first polarizing beam splitter, and the observation in the output path of the second and/or third polarizing beam splitter is performed by a measurement of the optical power of the clockwise and counter-clockwise photon beams with a power-meter, and/or a detection of the interference pattern of the clockwise and counter-clockwise photon beams with a camera or an optical component with a surface to monitor the interference.

This object is further achieved by a Sagnac loop with an apparatus to provide an aligned Sagnac loop for an entangled photon source according to claim <NUM>,.

A polarizing beam splitter comprises four sides, on each side a corresponding output and input path for the polarized photon beam and the down conversion photon beam is assigned. Input path means that a beam enters the polarizing beam splitter on this side. Output path means that a beam exits the polarizing beam splitter on this side. The polarizing beam splitter splits a photon beam into two, whereas the reflected photon beam is vertically polarized (V) and the transmitted photon beam is horizontally polarized (H).

The two mirrors of the Sagnac loop are arranged in a first and second output path of the first polarizing beam splitter. The polarized photon beam enters the first polarizing beam splitter on a third input path and can be split in two polarized photon beams with vertical polarization and horizontal polarization. The transmitted polarized photon beam (horizontal polarization) exits the first polarizing beam splitter on the first output path and travels in counter-clockwise direction and is guided by the mirrors into the second input path of the first polarizing beam splitter. The reflected polarized photon beam (vertical polarization) exits the first polarizing beam splitter on the second output path and travels in clockwise direction and is guided by the mirrors into the first input path of the first polarizing beam splitter. For an aligned Sagnac loop the first and second input and output paths (meaning the clockwise and counter clockwise directions) of the polarized photon beams overlap spatially. To pump the Sagnac loop in both directions (clockwise and counter clockwise) the polarized photon beam in front of the first polarizing beam splitter can have any other polarization but not pure linear horizontal or vertical polarized. This can be achieved and can be controlled by a half-wave plate and/or a quarter-wave plate and/or a polarizer in front of the first polarizing beam splitter.

Tip-tilt or so called two axes tilt means a movement of the mirror around two axes (preferably horizontal and vertical) where each movement around one axis can be performed independently. By a tip-tilt movement of the two mirrors in the Sagnac loop the polarized photon beams of the clockwise and counter-clockwise direction can be brought to a spatial overlap.

The first arrangement of the optical components in a Sagnac loop configuration means that the clockwise and counter-clockwise paths of the polarized photon beams in the Sagnac loop are guided at least to meet the first polarizing beam splitter in the first and second input path. It means at least one Sagnac loop should be formed by the components. In the optimization steps the clockwise and counter-clockwise path are successive brought to a spatial overlap by the tip-tilt movement of the mirrors.

In a preferred embodiment of the method a polarization flip element, preferably a half-wave plate or a dual wavelength half-wave plate is arranged inside the loop. In this configuration, which is the operational setting for producing entanglement, any light entering the Sagnac loop through a particular input path of the first polarizing beam splitter, will exit through the same path. As a result, the pump light will emerge again from the port it initially entered via but with its polarization state flipped. To facilitate the first alignment of the Sagnac loop, the polarization flip element can be removed or placed in a polarization neutral setting. In this case the light will exit through the initially un-used output path (i.e. the port that is not facing the input path or the two-forming part of the Sagnac loop). In the operational setting, a polarizing element in the pump input path can also be used to inspect the pump light.

In a preferred embodiment of the method the tip/tilt movement of the mirrors and a detection and/or minimization of the interference in the reflection output of the second and/or third polarizing beam splitter arranged at the third or fourth side of the first polarizing beam splitter is performed in order to optimize the spatial overlap of the clockwise and counter-clockwise propagation beams in the Sagnac loop.

One important finding is, that the measurement or detection of the counter-clockwise and clockwise propagating polarized photon beams (which correspond to two different polarizations) provides information on the alignment of the loop. This means if the loop is strongly misaligned, there will be two spots on the camera or any other optical component with a surface to monitor the beams and/or the interference; one for each of the two propagation paths (i.e. clockwise / counter-clockwise). These two beams can be brought to overlap by adjustment of the mirrors inside the loop. Once the beams overlap, the procedure proceeds with tests of the interference visibility. This interference is in the polarization state of the two beams; i.e. imperfect overlap results in a spatially varying polarization state of the beam emerging from the Sagnac loop. To make the spatial variation of the polarization state visible, the phase between the horizontal and vertical pump polarization components is translated to an intensity signal. This can be accomplished by placing a polarizing beam splitter outside of the Sagnac loop in the path of the polarized photon beams which are leaving the Sagnac loop, e.g. a polarizer, is added to the setup. This polarizer can be oriented at <NUM>°, so that the linear horizontal beam (corresponding to clockwise / or counter-clockwise propagation inside the loop) and the linear vertical beam (corresponding to counter-clockwise / or counter propagation inside the loop) interfere to produce a spatially varying intensity pattern (i.e. interference pattern) after the polarizer.

The characteristic interference pattern after the polarizing beam splitter also provides information on the nature of the misalignment of the loop. For example, the number of fringes relates to the amount of angular misalignment. If these fringes are oriented vertically (horizontally), then the misalignment is mainly in the horizontal (vertical) plane. Typically, at the start of the alignment procedure, there will be many interference minima and maxima, which indicates periodically repeating polarization states across the beam. The alignment procedure thus aims to minimize the number of fringes. The goal of the alignment procedure is to have a single interference minimum that extends across the whole beam. In the case of optimal beam overlap, the superposition of the counter-clockwise and clockwise propagation pump beam components results in a uniform polarization state across the beam diameter. The shape and form of these fringes may also indicate other types of misalignment: for example, if the fringes are concentric rings then this may indicate transverse misalignment of the focus, that the focus of the beams is not at the centre of the Sagnac loop.

In a preferred embodiment of the method the tip/tilt movement of the two mirrors and a minimization or maximization of the measured optical power in the reflection output of the second and/or third polarizing beam splitter arranged at the third or fourth side of the first polarizing beam splitter is performed in order to optimize the spatial overlap of the clockwise and counter-clockwise propagation beams in the Sagnac loop.

In a preferred embodiment of the method the tip/tilt movement of the mirrors and a maximization of the measured optical power in the reflection output of the third polarizing beam splitter arranged at same side as the light source is performed in order to optimize the spatial overlap of the clockwise and counter-clockwise propagating beams in the Sagnac loop, wherein the third beam splitter is arranged between the light source and the first polarizing beam splitter, and wherein in addition between the third polarizing beam splitter and the first polarizing beam splitter a non-reciprocal optical device, preferably a Faraday-rotator, and a polarizer at <NUM>° is arranged. Preferably the non-reciprocal optical device and the polarizer at <NUM>° is a Faraday isolator or optical isolator.

A combination of a non-reciprocal optical device and polarizers may be used as an optical isolator. An optical isolator is a device that allows light to pass only in one direction, but deflects or absorbs light in the opposite direction. Optical isolators are frequently used in optical setups involving lasers in order to avoid detrimental - and potentially damaging - back-propagation of laser light from the optical setup into the laser cavity. Optical isolators based on the Faraday effect consist of a Faraday-rotator placed between two polarization filters oriented at a relative angle of <NUM>°. These devices, also called Faraday isolators, allow light of a particular polarization to pass in one direction, while deflecting or absorbing light of any polarization that propagates in the opposite direction.

In a preferred embodiment of the method the Sagnac loop comprises a first polarized photon beam to align the Sagnac loop, whereby the Sagnac loop is formed by a first polarizing beam splitter, and two mirrors arranged in a first and second output path of the first polarizing beam splitter, forming a clockwise and counter-clockwise path for the photon beam behind the first polarizing beam splitter, wherein the polarized photon beam of the first light source enters the first polarizing beam splitter on a third or fourth output path, wherein the entangled photon source comprises at least the following optical components:.

wherein the Sagnac loop is aligned by a method according to one method above.

In a preferred embodiment of the method an aligned entangled photon source with an aligned Sagnac loop is provided comprising the steps.

In a preferred embodiment of the method, the first and the second polarized photon beams are emitted by one light source, whereas the fist and the second polarized photon beams are the same.

In a preferred embodiment of the method, the first polarized photon beam is emitted by an auxiliary light source and the second polarized photon beam is emitted by a pump laser, whereas the first and the second polarized photon beams overlap spatially in the Sagnac loop.

In a preferred embodiment the first and the second polarized photon beams are emitted by one light source, or the same light source, whereas the first and the second polarized photon beams are the same.

In a preferred embodiment, the method includes a further step between step IV) and step V): performing one or more alignment steps of the first coupling means by a tip/tilt movement and focus of the first coupling means in order to maximize the measured power of an additional alignment beam detected in the first coupling means, wherein the additional alignment beam enters the entangled photon source by the second coupling means, and/or performing one or more alignment steps of the second coupling means by a tip/tilt movement and focus of the second coupling means in order to maximize the measured power of an additional alignment beam detected in the second coupling means, wherein the additional alignment beam enters the entangled photon source by the first coupling means.

In a preferred embodiment a control device is capable to provide an alignment method, wherein the control device is connected.

In a preferred embodiment a control device is provided wherein the control device is additional connected with the first and second coupling means in order to perform the tip/tilt movement and/or focus adjustment and a first and second detection means in order to monitor the collected light in the first and second coupling means.

In a preferred embodiment a computer device with a microprocessor with a nonvolatile memory is provided, wherein the nonvolatile memory comprises an executable program in order to provide one of the above-mentioned methods, preferably wherein the computer device is the control device.

In a preferred embodiment an apparatus to provide a method according to one of the above-mentioned methods is provided.

In a preferred embodiment of the apparatus an entangled photon source comprising a Sagnac loop and an apparatus to provide an aligned Sagnac loop is provided, wherein the entangled photon source comprises at least the following optical components:.

In the following, the invention will be explained by way of preferred embodiments illustrated in the drawings, yet without being restricted thereto. In the drawings:.

<FIG> shows an entangled photon source <NUM> in Sagnac configuration. The Sagnac loop <NUM> is formed by a first polarizing beam splitter <NUM> and two mirrors <NUM>. In the middle of the Sagnac loop <NUM> a crystal <NUM> is arranged.

The Sagnac loop <NUM> is pumped by the polarized photon beam <NUM> emitted from a laser <NUM> (not shown in <FIG>). A lens <NUM> or a lens system focus the polarized photon beam <NUM> into the crystal <NUM> in the Sagnac loop. In this embodiment of <FIG> the crystal <NUM> is a type-II down conversion crystal <NUM> for example a ppKTP (Potassium titanyl phosphate) crystal oriented so that when pumped by a horizontal polarized photon beam <NUM> pairs of down conversion photons with orthogonal polarization, e.g. horizontal (H) and vertical (V) polarization are produced. In addition, a dual wavelength half-wave plate <NUM> is arranged in the Sagnac loop to change the polarization of all incoming photons of the polarized photon beam <NUM> and the down conversion photon beam <NUM> linearly by <NUM>°, e.g. from horizontal (H) to vertical (V). This dual wavelength half-wave plate <NUM> is not always necessary for the alignment of the Sagnac loop but to ensure the entanglement in the entangled photon source.

The first polarizing beam splitter <NUM> comprises four sides, on each side a corresponding output and input path for the polarized photon beam <NUM> and the down conversion photon beam <NUM> is assigned. Input path means that beams enter the first polarizing beam splitter <NUM> on this side. Output path beams exit the first polarizing beam splitter <NUM> on this side. This also applies for all the following polarizing beam splitters.

The first polarizing beam splitter <NUM> splits a photon beam into two, whereas the reflected photon beam is vertically polarized (V) and the transmitted photon beam is horizontally polarized (H). The two mirrors <NUM> of the Sagnac loop <NUM> are arranged in a first and second output path of the first polarizing beam splitter <NUM>. The polarized photon beam in <FIG> enters the first polarizing beam splitter <NUM> on a third input path (right side of the first polarizing beam splitter <NUM>). The transmitted photon beam exits the first polarizing beam splitter <NUM> on the first output path (left side of the first polarizing beam splitter <NUM>) and travels in counter-clockwise direction (counter-clockwise arrow in <FIG>) and is guided by the mirrors <NUM> into the second input path (bottom side of the first polarizing beam splitter <NUM>) of the first polarizing beam splitter <NUM>. The reflected photon beam exits the first polarizing beam splitter <NUM> on the second output path (bottom side of the first polarizing beam splitter <NUM>) and travels in clockwise direction (clockwise arrow in <FIG>) and is guided by the mirrors <NUM> into the first input path (left side of the first polarizing beam splitter <NUM>) of the first polarizing beam splitter <NUM>. For an aligned Sagnac loop the first and second input and output paths (meaning the clockwise and counter-clockwise directions) of the polarized photon beams overlap spatially.

The down conversion photon pairs are split by the first polarizing beam splitter <NUM>, are guided by mirrors <NUM>, a dichroic mirror <NUM> and focused by a lens or lens system <NUM> and a coupling means <NUM> into single mode fibers <NUM>, which are connected to photon-detectors <NUM>. The signal of the photon-detectors <NUM> is counted in a coincidence logic <NUM> to register single photon events and coincidence events detected on the photon-detectors <NUM>.

To adjust the pump direction of the polarized photon beam <NUM> a half-wave-plate <NUM> and a quarter-wave-plate <NUM> are arranged in the polarized photon beam <NUM> before the first polarizing beam splitter <NUM>.

<FIG> shows the Sagnac loop <NUM> with counter-clockwise pump direction and a first embodiment of the alignment components comprising a second polarizing beam splitter <NUM>, a first light source <NUM> and a power-meter <NUM>. The first light source <NUM> emits the polarized photon beam <NUM>. The polarized photon beam <NUM> transmitted in the second polarizing beam splitter <NUM> is horizontal polarized. The horizontal polarization is not changed by the half-wave-plate <NUM> at <NUM>° leading to a counter-clockwise pump direction in the Sagnac loop. The polarized photon beam <NUM> in the Sagnac loop is rotated to a vertical polarization by the dual wavelength half-wave-plate <NUM> so that the polarized photon beam <NUM> is reflected in the first polarizing beam splitter <NUM>, not rotated on the way towards the second polarizing beam splitter <NUM> by the half-wave-plate <NUM> leading to a reflection of the polarized photon beam <NUM> on the second polarizing beam splitter <NUM>. The polarized photon beam <NUM> is detected by a power-meter <NUM> on the reflection output of the second polarizing beam splitter <NUM>.

<FIG> shows the Sagnac loop <NUM> with clockwise pump direction and the first embodiment of the alignment components corresponding of <FIG>. The first light source <NUM> emits the polarized photon beam <NUM>. The polarized photon beam <NUM> transmitted in the second polarizing beam splitter <NUM> is horizontal polarized. The horizontal polarization can be changed to a linear vertical polarization by the half-wave-plate <NUM> at <NUM>° leading to a clockwise pump direction in the Sagnac loop. The polarized photon beam <NUM> in the Sagnac loop is rotated to a horizontal polarization by the dual wavelength half-wave-plate <NUM> so that the polarized photon beam <NUM> is transmitted in the first polarizing beam splitter <NUM>, rotated again to a vertical polarization on the way towards the second polarizing beam splitter <NUM> by the half-wave-plate <NUM> leading to a reflection of the polarized photon beam <NUM> on the second polarizing beam splitter <NUM>. The polarized photon beam <NUM> is detected by a power-meter <NUM> on the reflection output of the second polarizing beam splitter <NUM>.

<FIG> shows the Sagnac loop <NUM> and the first embodiment of the alignment components corresponding to <FIG> for both, the clockwise and counter-clockwise pump direction. In this embodiment the half-wave-plate <NUM> is arranged at <NUM>,<NUM>° to rotate the horizontal polarized photon beam <NUM> transmitted by the second polarizing beam splitter <NUM> to a <NUM>° linear polarization. The Sagnac loop <NUM> is then pumped in both directions and the polarized photon beams <NUM> from both directions can be detected on the power-meter <NUM>. The spatial overlap of the polarized photon beams <NUM> in the Sagnac loop <NUM> can be aligned by the measurement of the power of the polarized photon beams <NUM> on the power-meter <NUM>.

The spatial overlap of the clockwise and counter-clockwise polarized photon beams <NUM> can be aligned by so called tip-tilt (or two axes tilt) movement of the two mirrors <NUM> in the Sagnac loop <NUM>. Tip-tilt means a movement of the mirror around two axes where each movement around one axis can be performed independently. By a tip-tilt movement of the two mirrors <NUM> the polarized photon beams <NUM> in the Sagnac loop can be brought to a spatial overlap. This can be monitored by the increase of the power on the power-meter <NUM>.

<FIG> shows a second embodiment of the alignment components comprising a second polarizing beam splitter <NUM>, a first light source <NUM> and a monitor <NUM>. The monitor <NUM> can be a camera or any other optical component with a surface to monitor the polarized photon beam <NUM>. On the monitor <NUM> the spatial overlap of the polarized photon beams <NUM> in the Sagnac loop <NUM> can be detected by the overlap or the interference pattern of the two the polarized photon beams <NUM> (clockwise and counter-clockwise) on the monitor <NUM>.

<FIG> shows the first embodiment of the alignment components from <FIG>.

<FIG> shows a third embodiment of the alignment components comprising a second polarizing beam splitter <NUM>, a first light source <NUM> and a power-meter <NUM>, a non-reciprocal optical device <NUM> and a polarizer <NUM>. On the power-meter <NUM> the spatial overlap of the polarized photon beams <NUM> in the Sagnac loop <NUM> can be detected by a maximization of the optical power of the polarized photon beams <NUM> on the power-meter <NUM>.

<FIG> shows a fourth embodiment of the alignment components. These differ from the components in <FIG> only in additional coupling means <NUM>, lenses <NUM> and a fiber <NUM> to connect the Sagnac loop <NUM> (not shown in <FIG>) and the pump and alignment components section <NUM>.

Claim 1:
A method to align a Sagnac loop (<NUM>) for an entangled photon source (<NUM>) comprising the steps of providing:
I) first arrangement of optical components in a Sagnac loop (<NUM>) configuration, wherein the optical components comprise of
- a first light source (<NUM>) emitting a polarized photon beam (<NUM>), and
- the Sagnac loop (<NUM>), formed by a first polarizing beam splitter (<NUM>), and two mirrors (<NUM>) arranged in a first and second output path of the first polarizing beam splitter (<NUM>), forming a clockwise and counter-clockwise path for the photon beam (<NUM>) behind the first polarizing beam splitter (<NUM>), wherein the polarized photon beam (<NUM>) of the first light source (<NUM>) enters the first polarizing beam splitter (<NUM>) on a third or fourth output path,
II) one or more optimization steps performed by a tip/tilt movement of the two mirrors (<NUM>) and an observation of the clockwise and counter-clockwise beams in an output path of a second and/or third polarizing beam splitter (<NUM>), in order to optimize the spatial overlap of the clockwise and counter-clockwise photon beams (<NUM>) in the Sagnac loop (<NUM>),
characterized in that
the second and/or third polarizing beam splitter (<NUM>) is arranged in the third and/or fourth output path of the first polarizing beam splitter (<NUM>), and
the observation in the output path of the second and/or third polarizing beam splitter (<NUM>) is performed by
- a measurement of the optical power of the clockwise and counter-clockwise photon beams (<NUM>) with a power-meter (<NUM>), and/or
- a detection of the interference pattern of the clockwise and counter-clockwise photon beams (<NUM>) with a camera (<NUM>) or an optical component with a surface to monitor the interference.