Patent Description:
Quantum communication, for example used for quantum key distribution (QKD) known for example from "<NPL>, is using single photons for a secure and robust communication. For the communication, a common reference frame between the emitter and the one or more receivers has to be accomplished to compensate unknown polarization-transformation introduced by the transmission channel. To do so, the alignment was realized by polarization controllers with feedback control known for example from "<CIT>" or by a time-consuming method to ensure a common reference frame by an iteration of several alignment steps and the comparison of at least two non-orthogonal states in the emitter and the receivers. The typical approach is to align two basis vector sets iteratively (e.g. H horizontal and V vertical linearly polarized photons and +<NUM>° and -<NUM>° linearly polarized photons), where each iteration step affects the other basis set less. Also, alignment-free transmission methods are known, for example from "<NPL>.

An objective of the present invention is to provide an improved method to set a common polarization reference between the emitter and one receiver or more receivers.

The method to provide a common polarization reference frame between an emitter and a receiver is achieved by the method according to claim <NUM>.

The emitter comprises a source to produce polarized photons in at least two non-orthogonal polarizations, and the receiver comprises a detection means to determine the polarization of the polarized photons in at least two non-orthogonal polarization states, wherein the emitter and receiver are connected via an optical channel. The optical channel comprises a transmission means to guide the polarized photons from the emitter to the receiver, and a first correction means to correct any unitary transformation of the polarization of the photons caused by the optical channel. The first correction means consists of a first correction component and a second correction component, wherein the first and second correction components are separate components. The method comprises the steps of.

A very useful way to visualize polarization states and their transformations is the geometrical representation on the Poincaré sphere. In this representation, a pure state corresponds to a point on the surface of the sphere with polar coordinates theta and phi. Polarization qubits can be conveniently detected and manipulated using only simple optical devices such as wave plates and polarizers and any unitary (coherence conserving) transformation introduced by birefringent elements can be understood as a rotation of the polarization states on the Poincaré-sphere about a particular axis. A key prerequisite for polarization-mode encoded quantum communication is setting and maintaining a common polarization reference frame between the emitter, preferably the source and the receiver, preferably the detection means. , any polarization vector on the Poincaré-sphere in the source reference frame must be mapped to some known counterpart on the Poincaré-sphere at the receiver. For example, the source reference frame is defined by the labeling of the axes of the non-linear crystal or a polarizer in the emitter. The mapping is equivalent to undoing any unitary transformation introduced from the source to the receiver and matching the computational basis polarizations (H horizontal linear polarization and V vertical linear polarization). However, since quantum information processing protocols usually involve measurements in a second, non-orthogonal measurement basis, any superposition of the modes (for example D = H + V diagonal linear polarization and A = H - V antidiagonal linear polarization) must also be matched from the source to the receiver, thus mapping the coherent measurement basis from the source to the receiver.

Here, a simple two-step alignment method is proposed that ensures complete alignment of the entire Poincaré-sphere from transmitter to receiver without the need for iterative alignment of two mutually unbiased measurement bases. A logic computational basis between the emitter and the receiver is matched first with the first correction component. In a second step, the coherent measurement basis is aligned, decoupled form the adjustment of the logic computational basis. The alignment of the first step ensures that the first basis is set between the emitter and the receiver, and is ambiguous with respect to the relative phase phi. This transformation can be understood as a rotation of the Poincaré-sphere, for example about the axis combining H and V. This remaining Poincaré-sphere transformation is adjusted in the manner that does not introduce an additional rotation of the first basis (as an example here the H/V basis), by introducing only an additional relative variable phase between the basis vectors of the polarization basis used for the alignment in the first step. This can be achieved, for example, using a tilted wave-plate, a quarter-wave/half-wave/quarter-wave combination, a Soleil Babinet Compensator that introduces only a relative phase.

The object of the invention is also achieved by an apparatus to provide a common reference frame between an emitter and one receiver in at least two non-orthogonal polarization states, wherein the emitter comprises a source to produce polarized photons in at least two non-orthogonal polarizations adapted to perform step I), and wherein the receiver comprises a detection means to determine the polarization of the polarized photons in at least two non-orthogonal polarization states, and wherein the emitter and receiver are connected via an optical channel comprising a transmission means to guide the polarized photons from the emitter to the receiver, and a first correction means to correct any unitary transformation of the polarization of the photons caused by the optical channel, and wherein the first correction means consists of at least two separate components, a first correction component adapted to perform step II) and a second correction component adapted to perform step III).

In a preferred embodiment of the methods and the apparatus with one receiver, the first and/or the second correction component of the first correction means comprises a variable wave-plate, or a tilted wave-plate, or a soleil Babinet compensator, or a combination of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate, or a fiber controller creating the behavior of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate by a first, second, and third spools of the fiber, or a rotatable fiber squeezer.

In a preferred embodiment of the methods and the apparatus with one receiver, the local polarization reference means comprises one of a polarizing means, or a polarizing beam splitter, or a polarizer, or down converted photons, or a non-linear crystal, or a laser producing polarized photons. In a preferred embodiment of the methods and the apparatus with one receiver, the local polarization reference means comprise in addition an optical element to change the polarization of the photons in at least two non-orthogonal polarizations, preferably a wave plate.

In a preferred embodiment of the methods with one receiver, the calibration step of the first correction component is performed in order to compensate the unitary transformation of the first emitter polarization state caused by the optical channel.

In a preferred embodiment of the methods and the apparatus with one receiver, the calibration step of the second correction component is performed in order to compensate the unitary transformation of the second emitter polarization state caused by the optical channel.

In a preferred embodiment of the methods and the apparatus with one receiver, whereas each receiver can measure the photons in at least two orthogonal states in its non-orthogonal basis.

In a preferred embodiment of the methods and the apparatus with one receiver, the transmission channel is a free-space channel, preferably air or vacuum or any other atmosphere or a liquid. In a preferred embodiment of the methods and the apparatus with one receiver, the first correction component of the first correction means comprises a variable wave-plate, or a tilted wave-plate, or a Babinet-soleil compensator, or a combination of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate. In a preferred embodiment of the methods and the apparatus with one receiver, the second correction component of the first correction means comprises a variable wave-plate, or a tilted wave-plate, or a Babinet-soleil compensator, or a combination of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate.

In a preferred embodiment of the methods and the apparatus with one receiver the transmission channel is the wave guide, preferably a fiber.

In a preferred embodiment of the methods and the apparatus with one receiver, the transmission means comprises two coupling means and a waveguide, preferably a fiber, connecting the two coupling means, wherein the first coupling means couples the photons from the emitter into the fiber and the second coupling means couples the photons out of the fiber into the first receiver. In a preferred embodiment of the methods and the apparatus with one receiver, the first correction component of the first correction means comprises a first quarter wave-plate, and a half wave-plate, and a second quarter-wave plate, before the first coupling means or after the second coupling means, or a fiber controller creating the behavior of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate by a first, second, and third spools, of the fiber or a rotatable fiber squeezer in the fiber. In a preferred embodiment of the methods and the apparatus with one receiver, the second correction component of the first correction means comprises a variable wave-plate, or a tilted wave-plate, or a soleil Babinet compensator, or a combination of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate, before the first coupling means or after the second coupling means, or a fiber controller creating the behavior of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate by a first, second, and third spools of the fiber, or a rotatable fiber squeezer in the fiber.

In a preferred embodiment of the methods with one receiver, the first or the second emitter polarization state is a linear horizontal/vertical, or linear <NUM>°/-<NUM>° polarization or left/right circular polarization.

As an example, here the alignment and calibration for an emitter, which produces at least horizontal and diagonal polarized photons is explained. The local reference frame can for example be set with a polarizer. Sending horizontal polarized photons (H) from the emitter to the receivers, the detected events at the receiver in the corresponding vertical (V) detector should be minimized by the first correction means. Subsequently, diagonal polarized photons (D) are transmitted to the receiver and the corresponding detected events at the receiver in the corresponding antidiagonal (A) detector should be minimized by the second correction means.

In a preferred embodiment of the methods, the common polarization reference frame is provided between an emitter and two receivers, wherein the emitter is a laser or a photon source producing polarized photons in at least two non-orthogonal polarizations, and wherein the first receiver comprises a first detection means to determine the polarization of the polarized photons in at least two non-orthogonal polarization stats, and the second receiver comprises a second detection means to determine the polarization of the polarized photons in at least two non-orthogonal polarization states. A first optical channel, connecting the emitter and the first receiver, comprising a first transmission means to guide the polarized photons from the emitter to the first receiver, and a second optical channel, connecting the emitter and the second receiver, comprising a second transmission means to guide the polarized photons from the emitter to the second receiver, and wherein the first optical channel comprises a first correction means to correct any unitary transformation of the polarization of the photons caused by the first optical channel, and/or wherein the second optical channel comprises a second correction means to correct any unitary transformation of the polarization of the photons caused by the second optical channel, and wherein the first and/or second correction means each consists respectively of a first correction component and a second correction component, wherein the emitter sends polarized photons in the first optical channel to the first receiver and in the second optical channel to the second receiver, wherein the calibration of the common polarization reference frame between the emitter and the first and/or second receiver, or between the first and the second receiver is according to one of the methods described above.

In a preferred embodiment of the apparatus, the common polarization reference frame is provided between an emitter and two receivers, wherein the emitter comprises a laser or a photon source producing polarization photons in at least two non-orthogonal polarizations adapted to perform step I), wherein the first receiver comprises a first detection means to determine the polarization of the polarized photons in at least two non-orthogonal polarization states and the second receiver comprises a second detection means to determine the polarization of the polarized photons in at least two non-orthogonal polarization states, wherein the laser or the photon source and the first receiver are connected via a first optical channel comprising a first transmission means to guide the polarized photons from the emitter to the first receiver, and a first correction means to correct any unitary transformation of the polarization of the photons caused by the first optical channel, and wherein the laser or the photon source and the second receiver are connected via a second optical channel comprising a second transmission means to guide the polarized photons from the emitter to the second receiver, and a second correction means to correct any unitary transformation of the polarization of the photons caused by the second optical channel, wherein the emitter sends polarized photons in the first optical channel to the first receiver and in the second optical channel to the second receiver, wherein the first and/or second correction means consists respectively of a first correction component adapted to perform step II) and a second correction component adapted to perform step III).

In a preferred embodiment of the methods and the apparatus with two receivers, the first and/or the second correction component of the first and/or the second correction means comprises a variable wave-plate, or a tilted wave-plate, or a soleil Babinet compensator, or a combination of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate, or a fiber controller creating the behavior of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate by a first, second, and third spools of the fiber, or a rotatable fiber squeezer in the fiber.

In a preferred embodiment of the methods and the apparatus with two receivers, the local polarization reference means comprises one of a polarizing means, or a polarizing beam splitter, or a polarizer, or down converted photons, or a non-linear crystal, or a laser producing polarized photons. In a preferred embodiment of the methods and the apparatus with two receivers, the local polarization reference means comprises in addition an optical element to change the polarization of the photons in at least two non-orthogonal polarizations, preferably a wave plate.

In a preferred embodiment of the methods with two receivers, the first or the second emitter polarization state is a linear horizontal/vertical, or linear <NUM>°/-<NUM>° polarization or left/right circular polarization.

In a preferred embodiment of the methods with two receivers, the calibration step of the first correction component of the first correction means is performed in order to compensate the unitary transformation of the first emitter polarization state caused by the first optical channel.

In a preferred embodiment of the methods with two receivers, the calibration step of the first correction component of the second correction means is performed in order to compensate the unitary transformation of the first emitter polarization state caused by the second optical channel.

In a preferred embodiment of the methods with two receivers, the calibration step of the second correction component of the first correction means is performed in order to compensate the unitary transformation of the second emitter polarization state caused by the first optical channel.

In a preferred embodiment of the methods with two receivers, the calibration step of the second correction component of the second correction means is performed in order to compensate the unitary transformation of the second emitter polarization state caused by the second optical channel.

In a preferred embodiment of the methods and the apparatus with two receivers, each receiver can measure the photons in at least two orthogonal states in its non-orthogonal basis.

In a preferred embodiment of the methods and the apparatus with two receivers, the photon source is an entangled photon source producing polarization entangled photon pairs, wherein one photon of each entangled photon pair is sent in the first optical channel to the first receiver and the second photon of each entangled photon pair is sent in the second optical channel to the second receiver.

In a preferred embodiment of the methods with an emitter and two receivers, the method comprises the further steps of.

In a preferred embodiment of the methods with two receivers, the local polarization reference frame in step I) is set by a polarizing beam splitter, or a polarizer, or the polarization of a down converted photon, or the axes of a non-linear crystal, and the propagation of the photon beam through this optical component, or is set by the detection means of the first receiver comprising a polarizing beam splitter, or a polarizer.

In a preferred embodiment of the methods and the apparatus with two receivers, the first and/or second transmission channel is/are free-space channel/s, preferably air or vacuum or any other atmosphere or a liquid. In a preferred embodiment of the methods and the apparatus with two receivers, the first correction component of the first and/or second correction means comprises a variable wave-plate, or a tilted wave-plate, or a Babinet-soleil compensator, or a combination of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate. In a preferred embodiment of the methods and the apparatus with two receivers, the second correction component of the first and/or second correction means comprises a variable wave-plate, or a tilted wave-plate, or a Babinet-soleil compensator, or a combination of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate.

In a preferred embodiment of the methods and the apparatus with two receivers, the transmission channel is a wave guide, preferably a fiber. In a preferred embodiment of the methods and the apparatus with two receivers, the first and/or second optical channel comprises two coupling means and a waveguide, preferably a fiber, connecting the two coupling means, wherein the first coupling means couples the photons from the emitter into the fiber and the second coupling means couples the photons out of the fiber into the first and/or second receiver. In a preferred embodiment of the methods and the apparatus with two receivers, the first correction component of the first and/or second correction means comprises a first quarter wave-plate, and a half wave-plate, and a second quarter-wave plate, before the first coupling means or after the second coupling means, or a fiber controller creating the behavior of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate by a first, second, and third spools, of the fiber or a rotatable fiber squeezer in the fiber.

In a preferred embodiment of the methods and the apparatus with two receivers, the second correction component of the first and/or second correction means comprises a variable wave-plate, or a tilted wave-plate, or a soleil Babinet compensator, or a combination of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate, before the first coupling means or after the second coupling means, or a fiber controller creating the behavior of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate by a first, second, and third spools of the fiber, or a rotatable fiber squeezer in the fiber.

The object of the invention is also achieved by a method to provide an emitter and two receivers with a common polarization reference frame according to claim <NUM>. The emitter is an entangled photon source, producing polarization entangled photon pairs, and wherein the first receiver comprises a first detection means to determine the polarization of the polarized photons in at least two non-orthogonal polarization states and the second receiver comprises a second detection means to determine the polarization of the polarized photons in at least two non-orthogonal polarization states, and a first optical channel, connecting the emitter and the first receiver, comprising a first transmission means to guide the polarized photons from the emitter to the first receiver, and a second optical channel, connecting the emitter and the second receiver, comprising a second transmission means to guide the polarized photons from the emitter to the second receiver, and wherein one photon of each entangled photon pair is sent in the first optical channel to the first receiver and the second photon of each entangled photon pair is sent in the second optical channel to the second receiver, wherein the first optical channel comprises a first correction means to correct any unitary transformation of the polarization of the photons caused by the first optical channel, and/or wherein the second optical channel comprises a second correction means to correct any unitary transformation of the polarization of the photons caused by the second optical channel, and/or wherein the emitter comprises a first and/or second correction means to correct any unitary transformation of the polarization of the photons caused by the first and/or second optical channel and wherein the first and/or second correction means each consists respectively of a first correction component and a second correction component, and wherein the calibration of the common polarization reference frame between the emitter and the first and/or second receiver or the first and the second receiver is according to the steps.

As an emitter, entangled photons can be produced via numerous schemes by an entangled photon source, for example by spontaneous parametric down-conversion in a non-linear crystal. With an entangled photon source, for example, a maximally polarization-entangled Bell state like <MAT> <MAT> can be produced. The unique quantum feature of such entangled Bell states is that they also exhibit correlations in other measurement bases. In order to exploit the polarization correlations, for example, in quantum information processing, the photon pairs must be transmitted to the first and second receivers (Alice and Bob) in a transmission channel. One or both transmission channels can introduce a transformation of the polarization reference frame. Due to that, the emitter and the first and second receivers (Alice and Bob) must set a common polarization reference frame. With common polarization reference frame in the emitter and both receivers, for example, the nonlocal correlations in the entangled state can be verified, exploited and used.

A very useful way to visualize polarization states and their transformations is the geometrical representation on the Poincaré sphere. In this representation, a pure state corresponds to a point on the surface of the sphere with polar coordinates theta and phi. Polarization qubits can be conveniently detected and manipulated using only simple optical devices such as wave plates and polarizers and any unitary (coherence conserving) transformation introduced by birefringent elements can be understood as a rotation of the polarization states on the Poincaré-sphere about a particular axis. A key prerequisite for polarization-mode encoded quantum communication is setting and maintaining a common polarization reference frame between the emitter, preferably the source and the receivers, preferably the detection means. , any polarization vector on the Poincaré-sphere in the source reference frame must be mapped to some known counterpart on the Poincaré-sphere at the receivers. For example, the source reference frame is defined by the labeling of the axes of the non-linear crystal or a polarizer in the emitter. The mapping is equivalent to undoing any unitary transformation introduced from the source to the receivers and matching the computational basis polarizations (H horizontal linear polarization and V vertical linear polarization). However, since quantum information processing protocols usually involve measurements in a second, non-orthogonal measurement basis, any superposition of the modes (for example D = H + V diagonal linear polarization and A = H - V antidiagonal linear polarization) must also be matched from the source to the receivers, thus mapping the coherent measurement basis from the source to the receivers.

Here, a simple two-step alignment method is proposed that ensures complete alignment of the entire Poincaré-sphere from transmitter to receivers without the need for iterative alignment of two mutually unbiased measurement bases. A logic computational basis between the emitter and the receivers is matched first with the first correction component. In a second step, the coherent measurement basis is aligned, decoupled from the adjustment of the logic computational basis. The alignment of the first step ensures that the first basis is set between the emitter and the receivers, and is ambiguous with respect to the relative phase phi. This transformation can be understood as a rotation of the Poincaré-sphere, for example about the axis combining H and V. This remaining Poincaré-sphere transformation is adjusted in the manner that does not introduce an additional rotation of the first basis (as an example here the H/V basis), by introducing only an additional relative variable phase between the basis vectors of the polarization basis used for the alignment in the first step. This can be achieved, for example, using a tilted wave-plate, a quarter-wave/half-wave/quarter-wave combination, a Soleil Babinet Compensator that introduces only a relative phase with respect to the H/V basis of the entangled photon source, or a polarizer or polarizing beam splitter in the receiver. In the case of a polarization-entangled photon source, this phase can also be absorbed into the phase of the entangled photon generation.

In a preferred embodiment of the apparatus, the common polarization reference frame is provided between an emitter and two receivers, wherein the emitter comprises an entangled photon source adapted to perform step I), wherein the first receiver comprises a first detection means to determine the polarization of the polarized photons in at least two non-orthogonal polarization states and the second receiver comprises a second detection means to determine the polarization of the polarized photons in at least two non-orthogonal polarization states, and a first optical channel, connecting the emitter and the first receiver, comprising a first transmission means to guide the polarized photons from the emitter to the first receiver, and a second optical channel, connecting the emitter and the second receiver, comprising a second transmission means to guide the polarized photons from the emitter to the second receiver, and wherein one photon of each entangled photon pair is sent in the first optical channel to the first receiver and the second photon of each entangled photon pair is sent in the second optical channel to the second receiver, wherein the first optical channel comprises a first correction means to correct any unitary transformation of the polarization of the photons caused by the first optical channel, and/or wherein the second optical channel comprises a second correction means to correct any unitary transformation of the polarization of the photons caused by the second optical channel, and/or wherein the emitter comprises a first and/or second correction means to correct any unitary transformation of the polarization of the photons caused by the first and/or second optical channel and wherein the first and/or second correction means each consist respectively of a first correction component adapted to perform step II) and a second correction component adapted to perform step III).

In a preferred embodiment of the methods and the apparatus with two receivers, the first and/or the second correction component of the first and/or the second correction means comprises a variable wave-plate, or a tilted wave-plate, or a soleil Babinet compensator, or a combination of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate, or a fiber controller creating the behavior of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate by a first, second, and third spools of the fiber, or a rotatable fiber squeezer in the fiber or the first and/or the second correction component is realized by changing the pump polarization of the entangled photon source.

In a preferred embodiment of the methods and the apparatus with two receivers, the local polarization reference means comprises one of a polarizing means, or a polarizing beam splitter, or a polarizer, or down converted photons, or a non-linear crystal.

In a preferred embodiment of the methods with two receivers, the calibration step of the first correction component of the first correction means is performed in order to compensate the unitary transformation of the first emitter polarization state caused by the first and/or second optical channel.

In a preferred embodiment of the methods with two receivers, the calibration step of the first correction component of the second correction means is performed in order to compensate the unitary transformation of the first emitter polarization state caused by the first and/or second optical channel.

In a preferred embodiment of the methods and the apparatus with two receivers, the calibration step of the second correction component of the first correction means is performed in order to compensate the unitary transformation of the second emitter polarization state caused by the first and/or second optical channel.

In a preferred embodiment of the methods and the apparatus with two receivers, the calibration step of the second correction component of the second correction means is performed in order to compensate the unitary transformation of the second emitter polarization state caused by the first and/or second optical channel.

In a preferred embodiment of the methods with two receivers, the local polarization reference frame in step I) is set by the polarization of down converted photons, or the axes of a non-linear crystal, and the propagation of the photons through this optical component, or by a polarizing beam splitter in a Sagnac entangled photon source.

In a preferred embodiment of the methods and the apparatus with two receivers, the emitter comprises the entangled photon source and one or two coupling means with a first part of a waveguide. The transmission means consist of the remaining waveguide to the receivers. In this embodiment, the coupling means and a part of the waveguides are part of the emitter, preferably parts of the entangled photon source. This allows, for example, that the first correction component in the emitter can be arranged in the waveguide and the second correction component in the emitter can be arranged in the entangled photon source.

In a preferred embodiment of the methods and the apparatus with two receivers, the first and/or second transmission channel is/are a free-space channel/s, preferably air or vacuum or any other atmosphere or a liquid. In a preferred embodiment of the methods and the apparatus with two receivers, the first correction component of the first and/or second correction means comprises a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate or is realized by changing the pump polarization of the entangled photon source. In a preferred embodiment of the methods and the apparatus with two receivers, the second correction component of the first and/or second correction means comprises a variable wave-plate, or a tilted wave-plate, or a Babinet-soleil compensator, or a combination of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate or is realized by changing the pump polarization of the entangled photon source.

In a preferred embodiment of the methods and the apparatus with two receivers, the transmission channel is a wave guide, preferably a fiber.

In a preferred embodiment of the methods and the apparatus with two receivers, the first and/or second optical channel comprises one coupling means and a waveguide, preferably a fiber, connecting the optical channel to the first and/or second receiver, wherein the first coupling means couples the photons from the emitter into the fiber. In a preferred embodiment of the methods and the apparatus with two receivers, the first and/or second optical channel comprises two coupling means and waveguide, preferably a fiber, connecting the two coupling means, wherein the first coupling means couples the photons from the emitter into the fiber and the second coupling means couples the photons out of the fiber into the first and/or second receiver. In a preferred embodiment of the methods and the apparatus with two receivers, the first correction component of the first and/or second correction means comprises a first quarter wave-plate, and a half wave-plate, and a second quarter-wave plate, before the first coupling means or after the second coupling means, or a fiber controller creating the behavior of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate by a first, second, and third spools, of the fiber or a rotatable fiber squeezer in the fiber or is realized by changing the pump polarization of the entangled photon source.

In a preferred embodiment of the methods and the apparatus with two receivers, the second correction component of the first and/or second correction means comprises a variable wave-plate, or a tilted wave-plate, or a soleil Babinet compensator, or a combination of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate, before the first coupling means or after the second coupling means, or a fiber controller creating the behavior of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate by a first, second, and third spools of the fiber, or a rotatable fiber squeezer in the fiber or is realized by changing the pump polarization of the entangled photon source.

As an example, here the alignment and calibration for a polarization-entangled photon source as an emitter with a local reference frame defined by the axes of the non-linear crystal is explained. The local reference frame can also be set by a polarizing beam splitter in a Sagnac loop source, producing entangled photons or by pumping the photon source to produce, for example, only HH photons. These polarization measurements are performed using polarizers in the transmission channels. For example, the maximally-entangled Psi- state exhibits anti-correlation in any measurement basis. In order to set this state also in the receiver, it is sufficient to ensure that the joint detection probability for H/H coincidence detection of photon pairs between both receivers are minimized (with H as linear horizontal polarization and V as linear vertical polarization). , when the two polarizers are in parallel, or the photon source is pumped to produce only HH photons (denoting H as horizontal linear polarization in both polarizer), the first correction component of the first and/or second correction means has to be set to detect a minimum of coincidence counts (two-photon detection events of photon pairs).

Once this has been achieved (for example by comparison of the contrast of H/H and H/V coincidence counts), the local polarization measurement is changed to another measurement basis in the local reference frame, a non-orthogonal state to the first H/H polarization state. In the example of the Phi- state, a minimum of coincidence counts (two-photon detection events of photon pairs) is also expected for measurement in the <NUM>°/-<NUM>° measurement basis (with <NUM>° as linear polarization at <NUM>° and -<NUM>° as linear polarization at -<NUM>°). This is also expected for left and right circular polarization. This can be achieved by changing the pump polarization of the entangled photon source, specifically the relative phase phi between the horizontal and vertical pump photon components. This can also be achieved by a second correction means comprising a variable wave-plate, or a tilted wave-plate, or a soleil Babinet compensator, or a combination of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate, before the first coupling means or after the second coupling means, or a fiber controller creating the behavior of a first quarter wave-plate, a half wave-plate, and a second quarter-wave plate by a first, second, and third spools of the fiber, or a rotatable fiber squeezer in the fiber.

The alignment of the first step has ensured in this example that the first basis is set between the emitter and the receivers, and is ambiguous with respect to the relative phase phi. This transformation can be understood as a rotation of the Poincaré-sphere about the axis combining H and V. This remaining Poincaré-sphere transformation is adjusted in the manner that does not introduce an additional rotation of the H/V basis, by introducing only an additional relative variable phase between the basis vectors of the polarization basis used for the alignment in the first step. This can be achieved, for example using a tilted wave-plate, a quarter-wave/half-wave/quarter-wave combination, a Soleil Babinet Compensator that introduces only a relative phase with respect to the H/V basis of the entangled photon source, or a polarizer or polarizing beam splitter in the receiver. In the case of a polarization-entangled photon source, this phase can also be absorbed into the phase of the entangled photon generation.

In the same way, the common reference frame can be set with any other entangled state taking the correlations or anti-correlations of the specific stats for any measurement basis into account.

The object of the invention is in addition achieved by a control device adapted to provide and capable of providing a method according to one the methods described above with one receiver, wherein the control device is connected with the first correction means and with the first detection means, and preferably with the local polarization reference means.

The object of the invention is in addition achieved by a control device adapted to provide and capable of providing a method according to one of the methods described above with two receivers, wherein the control device is connected with the first and/or second correction means and with the first and/or second detection means, and preferably with the local polarization reference means.

The object of the invention is in addition achieved by a computer device with a microprocessor with a nonvolatile memory, wherein the nonvolatile memory comprises an executable program in order to provide a method according to one of the methods described above, preferably wherein the computer device is the control device.

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 a schematic setup for an emitter <NUM> connected via a first transmission means <NUM> with a first receiver <NUM>. The emitter <NUM> produces polarized photons and sets a local polarized reference frame in at least two non-orthogonal or complementary polarization states. The polarized photons are transmitted in the first transmission means <NUM> to the first receiver <NUM>. The first receiver <NUM> can measure the photons in at least two non-orthogonal polarization states. To set a common reference frame in the emitter <NUM> and the first receiver <NUM>, a first correction means <NUM> is arranged in the first transmission means <NUM> to correct any unitary transformation of the polarization of the photons caused by the first transmission means <NUM>. To compensate any unitary transformation in at least two non-orthogonal states, the first correction means <NUM> comprises a first correction component <NUM> and a second correction component <NUM>. Both correction components <NUM> and <NUM> can be used independently to set the common reference frame in the emitter <NUM> and the first receiver <NUM> independently for the first and the second non-orthogonal state. For example, the first correction component <NUM> is used to set the first non-orthogonal state of the emitter <NUM> in the first receiver <NUM>, and the second correction component <NUM> is used to set the second non-orthogonal state of the emitter <NUM> in the first receiver <NUM>. With that, a simple two-step alignment method is possible that ensures complete alignment of the entire Poincaré-sphere from transmitter to receiver without the need for iterative alignment of two mutually unbiased measurement basis. The emitter <NUM> in <FIG> can be a laser with an optical element or any other component generating polarized photons in at least two non-orthogonal polarization states.

<FIG> shows a schematic setup for an emitter <NUM> connected via a first transmission means <NUM> with a first receiver <NUM> and via a second transmission means <NUM> with a second receiver <NUM>. <FIG> differs from <FIG> only in that in the second transmission means <NUM>, a second correction means <NUM> is arranged, comprising a first correction component <NUM> and a second correction component <NUM>. The second correction means <NUM> is arranged in the second transmission means <NUM> to compensate any unitary transformation in at least two non-orthogonal states by the second transmission means <NUM>. Both correction components <NUM> and <NUM> can be used independently to set the common reference frame in the emitter <NUM> and the second receiver <NUM> independently for the first and the second non-orthogonal state. The emitter <NUM> in <FIG> can be a laser with an optical element or any other component generating polarized photons in at least two non-orthogonal polarization states and a separation or switching component to transmit the photons to the first and/or the second receiver <NUM> and <NUM>. It is also possible that the emitter <NUM> in <FIG> is an entangled photon source producing photon pairs entangled in the polarization. The first photon of each entangled photon pair is sent via the first transmission channel <NUM> to the first receiver <NUM> and the second photon of each entangled photon pair is sent via the second transmission channel <NUM> to the second receiver <NUM>. In <FIG>, the simple two-step alignment method from <FIG> has to be performed for the first receiver <NUM> with the first correction means <NUM>, and a second time for the second receiver <NUM> with the second correction means <NUM>.

<FIG> shows a first inventive configuration of the first correction means <NUM> in the first transmission means <NUM>. The first correction component <NUM> comprises a first quarter wave-plate <NUM>, a half wave-plate <NUM>, and a second quarter-wave plate <NUM>. The second correction component comprises a tilted wave-plate <NUM>. With the separate first and second correction components <NUM> and <NUM>, the independent two-step alignment method can be performed to set a common reference frame between the emitter <NUM> and the receiver <NUM>.

<FIG> shows a second inventive configuration of the correction means <NUM> in the first transmission means <NUM>. This differs from <FIG> only in that the second correction component <NUM> comprises a Babinet compensator <NUM>. <FIG> shows a third inventive configuration of the correction means <NUM> in the first transmission means <NUM>. This differs from <FIG> only in that the second correction component <NUM> comprises a first quarter wave-plate <NUM>, a half wave-plate <NUM>, and a second quarter-wave plate <NUM>. All inventive examples of the <FIG> can also be implemented as the second correction means <NUM>. It is also possible to use a variable wave-plate as a second correction component <NUM>.

<FIG> shows a setup for providing a common reference frame between the emitter <NUM> and two receivers <NUM> and <NUM>. In this inventive example, the emitter <NUM> is an entangled photon source <NUM>, here as an example in a type-<NUM> SPDC process, a high energy pump photon, emitted from a laser <NUM> on the first photon path <NUM> with polarization H, can produce a pair of photons HH with lower energy traveling on the second photon path <NUM> to coupling means <NUM>. In this case, H denotes some particular optical axis with respect to the nonlinear photon pair source. Then, for example, by placing two down-converters, preferably crystals <NUM> with their optical axes rotated by <NUM> degrees in sequence and pumping this crossed crystal configuration pump photons in a superposition mode |H〉 + |V〉 which can be adjusted by a half-wave plate <NUM> and a quarter-wave plate <NUM> behind the laser <NUM>, a maximally polarization-entangled Bell state <MAT> can be produced. The photons are guided by dichroic mirrors <NUM>, mirrors <NUM>, and lenses <NUM> to the coupling means <NUM> into the first and second transmission channels <NUM> and <NUM>.

The two transmission channels <NUM> and <NUM> guide the photons to the first receiver <NUM>, in the following called Alice, and to the second receiver <NUM>, in the following called Bob. Alice <NUM> and Bob <NUM> each have a detection means, preferably a polarization analyzer that detects the polarization of single photons. The detection means, preferably the polarization analyzer can consist of a polarizing beam splitter cube and a number of wave-plates to perform polarization measurements for arbitrary polarization states, and is shown in detail in Fig. <NUM>. The unique quantum feature of such entangled Bell states is that they also exhibit correlations in other measurement bases. In order to exploit the polarization correlations in quantum information processing, the photon pairs must be transmitted to distant measurement sites (Alice <NUM> and Bob <NUM>). Since the transmission channel can introduce a transformation of the polarization reference frames, Alice <NUM> and Bob <NUM> and the entangled photon source <NUM> must set a common polarization reference frame to verify and exploit the nonlocal correlations present in the entangled state.

In this invention, a simple two-step alignment method is proposed that ensures complete alignment of the entire Poincaré-sphere from emitter <NUM>, both receiver <NUM> and <NUM> without the need for iterative alignment of two mutually unbiased measurement basis. The alignment procedure comprises the following steps:
The entangled photon source is aligned and calibrated to produce a particular polarization-entangled state in the local reference frame, as defined for example by the axes of the non-linear crystal <NUM>, or the Polarizing beam splitter in a Sagnac loop source of entangled photons. The polarization correlations that reflect a particular Bell state are then ensured in the local measurement basis. The measurements of theses polarization correlations are performed using calibrated thin film polarizers <NUM>, inserted before the first and second transmission channels <NUM> and <NUM>. For example, the maximally-entangled Phi+ state exhibits correlation in any measurement basis. Thus, in order to set this particular state, it is sufficient to ensure (I) the maximization of the detection of the H photons in the first receiver <NUM> and the maximization of the detection of the H photons in the second receiver <NUM>, which, for example, can be realized when the joint detection probability for HV or VH coincidence detection of photon pairs is minimized, or the joint detection probability for HH or VV coincidence detection of photon pairs is maximized. The coincidence detection is the detection of two photons in the coincidence window. For that, the detectors (not shown in <FIG>) send a signal, preferably an electrical signal to the coincidence logic <NUM>, which counts a coincidence when two photons, one in each receiver <NUM> and <NUM>, are detected at the same time, taking different distances and cable lengths into account. , when the two thin film polarizers are in parallel, there should be a minimum of two-photon detection events registered by single-photon detectors located in the first and second receivers <NUM> and <NUM>. This first step is realized for the emitter <NUM> and the first and second receivers <NUM> and <NUM>, with the first correction component <NUM> of the first correction means <NUM> and/or the first correction component <NUM> of the second correction means <NUM>.

Once this has been achieved with sufficient contrast between HV, VH, HH and VV, the local polarization measurements are changed to another measurement basis in the local reference frame. In the example of the Phi+ state, a maximum or minimum of two-photon detection events is also expected for measurements in a non-orthogonal state measurement basis. This can be achieved by changing the pump polarization, specifically the relative phase phi between the horizontal and vertical pump photon components (which drive the two SPDC possibilities). The alignment of the HV basis in step (I) has ensured that HV or VH coincidence detection of photon pairs is minimized, or the joint detection probability for HH or VV coincidence detection of photon pairs is maximized, and is ambiguous with respect to the relative phase phi. This transformation can be understood as a rotation of the Poincaré-sphere about the axis combining H and V. This remaining Poincaré-sphere transformation is adjusted in a manner that does not introduce an additional rotation of the HV basis, by introducing only an additional relative variable phase between the basis vectors of the polarization basis used for the alignment in step (I). This can be achieved by the second correction component <NUM> of the first correction means <NUM> and/or the second correction <NUM> of the second correction means <NUM>, for example by first quarter wave-plate <NUM>, a half wave-plate <NUM>, and a second quarter-wave plate <NUM>, or a tilted wave-plate <NUM>, or a Soleil Babinet Compensator that introduces only a relative phase with respect to the HV basis of either the entangled photon source (non-linear crystal axis), or the polarizing beam splitter of the detection means. In the case of a polarization-entangled photon source, this phase can also be absorbed into the phase of the entangled photon generation.

As an emitter <NUM>, an entangled photon source in Sagnac configuration is also possible. These entangled photon sources are well known.

<FIG> shows different positions of the correction means <NUM> for a first transmission channel <NUM>, comprising a free-space transmission section and a fiber transmission section connected via a coupling means <NUM>. The correction means <NUM> can be arranged in the free-space transmission section (<FIG>) or in the fiber transmission section (<FIG>).

Claim 1:
A method to provide an emitter (<NUM>) and a receiver (<NUM>) with a common polarization reference frame
wherein the emitter (<NUM>) comprises a source to produce polarized photons in at least two non-orthogonal polarizations,
wherein the receiver (<NUM>) comprises a detection means to determine the polarization of the polarized photons in at least two non-orthogonal polarization states,
wherein the emitter (<NUM>) and receiver (<NUM>) are connected via an optical channel, wherein the optical channel comprises a transmission means (<NUM>) to guide the polarized photons from the emitter to the receiver, and a first correction means (<NUM>) to correct any unitary transformation of the polarization of the photons caused by the optical channel,
wherein the first correction means (<NUM>) consists of a first correction component (<NUM>) and a second correction component (<NUM>), wherein the first and second correction components (<NUM>, <NUM>) are separate components,
wherein the method comprises the steps of
I) providing a local polarization reference frame with a local polarization reference means in the emitter (<NUM>) with at least two non-orthogonal polarization states, a first local polarization reference state and a second local polarization reference state,
II) performing one or more calibration steps by adjusting the first correction component (<NUM>) of the correction means (<NUM>) and maximization and/or minimization of the detected photons in the detection means in the receiver in the first reference polarization state, in order to set the first local polarization reference state in the receiver,
III) performing one or more calibration steps by adjusting the second correction component (<NUM>) of the correction means (<NUM>) and maximization and/or minimization of the detected photons in the detection means in the receiver in the second reference polarization state in order to set the second local polarization reference state in the receiver.