OPTICAL PHASE CHANGE APPARATUS AND METHOD OF CHANGING A PHASE OF AN OPTICAL SIGNAL

There is described an optical apparatus having an optical fiber link propagating an optical signal; a first device coupled with a first fiber portion of the link and configured for imparting a phase changing contribution thereto; and a second device coupled with a second fiber portion of the link and configured for imparting a phase changing contribution thereto, the second device operating within a second response frequency range having a maximum value greater than a maximum value of a first response frequency range of the first device; wherein, upon the optical signal experiencing a phase change including a frequency value greater than the first response frequency range, the first device imparts a first phase change at a first response frequency value within the first response frequency range, and the second device imparts a second phase change at a second response frequency value greater than the first response frequency range.

FIELD

The improvements relate to quantum computing and more particularly to photonic fault-tolerant quantum computing systems.

BACKGROUND

A quantum computer can theoretically perform some computations faster than the analogous classical computer. Yet, due to noise and decoherence of the quantum bits (qubits), a small physical error rate is inevitable. Applying quantum logic gates on these qubits can cause said physical errors to quickly spread throughout the quantum computing system. Therefore, even a small physical error rate could mean that quantum computers can only apply a certain number of logic gates to a set of qubits before a longer computation is destroyed by noise. Unexpectedly, the quantum fault-tolerance theorem shows that, if the physical error rate is below some threshold, an appropriate quantum error correction scheme can be used such that one can perform relatively long quantum computations with large numbers of quantum logic gates while maintaining arbitrarily low levels of logical errors.

Quantum computers that comply with the quantum fault-tolerance theorem are referred to as fault-tolerant quantum computing (FTQC) systems. The typical photonic FTQC system (i.e., an FTQC system in which the physical qubits are quantum states of light) includes three sub-systems: a preparation sub-system, a stitching sub-system, and a measurement sub-system. The preparation sub-system prepares one or more optical signals by imparting them with initial quantum states of light and can sometimes involve entangling said quantum states of light. For instance, the quantum states of light can include, but are not limited to, single-photon states, non-Gaussian states, squeezed states, squeezed vacuum states, Gottesman-Kitaev-Preskill (GKP) states, GKP magic states, and cat states, to name a few examples. The quantum state can be represented by a mathematical entity providing a probability distribution for the outcomes of each possible measurement on an optical signal. The stitching sub-system includes a number of optical components (e.g., beam splitters and phase shifters) that are configured to entangle the quantum states of light to form a lattice structure. The measurement sub-system then measures the quantum states of light in order to perform measurement-based quantum computation (MBQC).

These sub-systems and/or components within these sub-systems may be optically connected to one another via optical fiber links whose lengths and/or refractive indices can change due to environmental factors (e.g., temperature changes, vibrations). While such changes may be inconsequential in some optical applications, in photonic FTQC systems these changes can result in adverse effects such as drifts or otherwise changes of the phases of the optical signals propagating along the optical fibre links, which can in turn lead to poorer quantum state preparation, quantum logic gate operation, and measurement, thus making the quantum computing system more susceptible to errors. There remains room for improvement in controlling the phases and timing of optical signals propagating within and between the sub-systems of any photonic FTQC system.

SUMMARY

As photonic FTQC systems involve fragile quantum states of light, it is primordial to appropriately manage losses across the whole system but also ensure that the optical signals' phase and timing be precisely controlled. In some situations, the optical fiber links optically connecting the sub-systems to one another and/or connecting components within the sub-systems can extend along a non-negligible length which, when exposed to varying surrounding conditions, may impart slower phase drifts to the optical signals propagating therealong. Moreover, the optical fiber links may receive a momentary localized or distributed stress (e.g., heat, pressure) which may impart faster phase changes to the optical signals propagating therealong. However, regardless of whether they are slow or fast, these environmentally induced phase changes have to be accounted for. Otherwise, the quantum states of the optical signals may be modified, resulting in an unwanted increase of the error rate.

In this disclosure, the optical fiber links do not only have a desired length and refractive index, but they are also made to respond to environmental changes in order to precisely control the optical signals' timing and phase, which is achieved using one or more optical phase change apparatuses along the optical fiber links. The optical phase change apparatus proposed herein can be used to change a phase of an optical signal propagating along an optical fiber link, for instance if it is detected or otherwise known that the optical signal has experienced an undesirable phase change, whether the phase change is fast or slow, or both. To do so, the optical phase change apparatus has a first phase change device which is optically coupled to a first fiber portion and which imparts a phase changing contribution to the first fiber portion, and a second phase change device which is optically coupled to a second fiber portion and which imparts a phase changing contribution to the second fiber portion. The first and second phase change devices are complementary to one another in the sense that the first phase change device is configured to be more efficient in addressing slower but larger phase changes whereas the second phase change device is configured to be more efficient in addressing smaller but faster phase changes. More specifically, the first phase change device operates within a slow response frequency range while the second phase change device operates within a fast response frequency range. Additionally or alternately, the first phase change device operates within a large phase change range while the second phase change device operates within a smaller phase change range. As such, the optical phase change apparatus can address environmentally induced phase changes appropriately.

In accordance with a first aspect of the present disclosure, there is provided an optical phase change apparatus comprising: an optical fiber link along which an optical signal propagates; a first phase change device coupled with a first fiber portion of the optical fiber link and configured for imparting a phase changing contribution to the first fiber portion via said coupling, the first phase change device operating within a first response frequency range; a second phase change device coupled with a second fiber portion of the optical fiber link and configured for imparting a phase changing contribution to the second fiber portion via said coupling, the second phase change device operating within a second response frequency range, the second response frequency range having a maximum response frequency value greater than a maximum response frequency value of the first response frequency range; and a controller communicatively connected to the first and second phase change devices, the controller configured for: upon determining that said optical signal experiences a given phase change including a given frequency value greater than the first response frequency range, instructing the first phase change device to impart, along the first fiber portion, a first phase change at a first response frequency value within the first response frequency range; and instructing the second phase change device to impart, along the second fiber portion, a second phase change at a second response frequency value within the second response frequency range and greater than the first response frequency range, the first and second phase changes corresponding to the given phase change.

Further in accordance with the first aspect of the present disclosure, the second response frequency value can for example be of at most about 40 GHZ, preferably at most about 100 MHz and most preferably at most about 100 KHz.

Still further in accordance with the first aspect of the present disclosure, the first response frequency value can for example be of at most about 160 Hz, preferably at most about 50 Hz and most preferably at most about 10 Hz.

Still further in accordance with the first aspect of the present disclosure, the first phase change device can for example operate within a first phase range and the second phase change device operates within a second phase range, the first phase range greater than the second phase range, said first phase change imparted within the first phase range and said second phase change imparted within the second phase range.

Still further in accordance with the first aspect of the present disclosure, the second phase range can for example be of at most one hundred π, preferably at most ten π and most preferably at most two π.

Still further in accordance with the first aspect of the present disclosure, the first phase range can for example be of at most fifty thousand π, preferably at most five hundred π and most preferably at most forty π.

Still further in accordance with the first aspect of the present disclosure, the second phase change device can for example have a piezoelectric element mechanically coupled to the second fiber portion.

Still further in accordance with the first aspect of the present disclosure, the first phase change device can for example have a thermoelectric element thermally coupled to the first fiber portion.

Still further in accordance with the first aspect of the present disclosure, the first phase change device can for example have an electro-mechanical actuator mechanically coupled to the first fiber portion.

Still further in accordance with the first aspect of the present disclosure, the first fiber portion can for example extend along at least 1 m, preferably at least 200 m and most preferably at least 250 m.

Still further in accordance with the first aspect of the present disclosure, the controller can for example monitor a current phase signal indicative of a current phase of the optical signal, said instructing being based on said monitored current phase signal.

In accordance with a second aspect of the present disclosure, there is provided a method of changing a phase of an optical signal, the method comprising: propagating the optical signal along an optical fiber link; coupling a first fiber portion of the optical fiber link to a first phase change device, the first phase change device configured for imparting a phase changing contribution to the first fiber portion via said coupling, the first phase change device operating within a first response frequency range; coupling a second fiber portion of the optical fiber link to a second phase change device, the second phase change device configured for imparting a phase changing contribution to the second fiber portion via said coupling, the second phase change device operating within a second response frequency range, the second response frequency range having a maximum response frequency value greater than a maximum response frequency value of the first response frequency range; and upon determining that said optical signal experiences a given phase change including a frequency greater than the first response frequency range: the first phase change device imparting, along the first fiber portion, a first phase change at a first response frequency value within the first response frequency range, and the second phase change device imparting, along the second fiber portion, a second phase change at a second response frequency value within the second response frequency range and greater than the first response frequency range, the first and second phase changes corresponding to the given phase change.

Further in accordance with the second aspect of the present disclosure, the second response frequency value can for example be of at most about 40 GHZ, preferably at most about 100 MHz and most preferably at most about 100 KHz.

Still further in accordance with the second aspect of the present disclosure, the first response frequency value can for example be of at most about 160 Hz, preferably at most about 50 Hz and most preferably at most about 10 Hz.

Still further in accordance with the second aspect of the present disclosure, the first phase change device can for example operate within a first phase range and the second phase change device operates within a second phase range, the first phase range greater than the second phase range, said first phase change imparted within the first phase range and said second phase change imparted within the second phase range.

Still further in accordance with the second aspect of the present disclosure, the second phase range can for example be of at most one hundred π, preferably at most ten π and most preferably at most two π.

Still further in accordance with the second aspect of the present disclosure, the first phase range can for example be of at most fifty thousand π, preferably at most five hundred π and most preferably at most forty π.

Still further in accordance with the second aspect of the present disclosure, said imparting can for example include decreasing the second phase change as the first phase change gradually increases.

Still further in accordance with the second aspect of the present disclosure, the method can for example further comprise monitoring a current phase signal indicative of a current phase of the optical signal, said operating based on said monitored current phase signal.

In accordance with a third aspect of the present disclosure, there is provided an optical apparatus comprising: an optical fiber link along which an optical signal propagates; a first phase change device coupled with a first fiber portion of the optical fiber link and configured for imparting a phase changing contribution to the first fiber portion via said coupling, the first phase change device operating within a first response frequency range; and a second phase change device coupled with a second fiber portion of the optical fiber link and configured for imparting a phase changing contribution to the second fiber portion via said coupling, the second phase change device operating within a second response frequency range, the second response frequency range having a maximum response frequency value greater than a maximum response frequency value of the first response frequency range, wherein, upon determining that said optical signal experiences a given phase change including a given frequency value greater than the first response frequency range: the first phase change device imparts, along the first fiber portion, a first phase change at a first response frequency within the first response frequency range; and the second phase change device imparts, along the second fiber portion, a second phase change at a second response frequency value within the second response frequency range and greater than the first response frequency range.

In accordance with a fourth aspect of the present disclosure, there is provided an optical phase change apparatus comprising: an optical fiber link along which an optical signal propagates; a first phase change device coupled with a first fiber portion of the optical fiber link and configured for imparting a phase changing contribution to the first fiber portion via said coupling, the first phase change device operating within a first phase range; a second phase change device coupled with a second fiber portion of the optical fiber link and configured for imparting a phase changing contribution to the second fiber portion via said coupling, the second phase change device operating within a second phase range greater than the first phase range; and a controller communicatively connected to the first and second phase change devices, the controller configured for: upon determining that said optical signal experiences a given phase change greater than the first phase range, instructing the first phase change device to impart a first phase change within the first phase range along the first fiber portion and instructing the second phase change device to impart a second phase change within the second phase range along the second fiber portion, the first and second phase changes corresponding to the given phase change.

In accordance with a fifth aspect of the present disclosure, there is provided a method of changing a phase of an optical signal, the method comprising: propagating the optical signal along an optical fiber link; coupling a first fiber portion of the optical fiber link to a first phase change device, the first phase change device configured for imparting a phase changing contribution to the first fiber portion via said coupling, the first phase change device operating within a first phase range; coupling a second fiber portion of the optical fiber link to a second phase change device, the second phase change device configured for imparting a phase changing contribution to the second fiber portion via said coupling, the second phase change device operating within a second phase range greater than the first phase range; and upon determining that said optical signal experiences a given phase change greater than the first phase range: the first phase change device imparting a first phase change within the first phase range along the first fiber portion, and the second phase change device imparting a second phase change within the second phase range along the second fiber portion, the first and second phase changes corresponding to the given phase change.

In accordance with a sixth aspect of the present disclosure, there is provided an optical apparatus comprising: an optical fiber link along which an optical signal propagates; a first phase change device coupled with a first fiber portion of the optical fiber link and configured for imparting a phase changing contribution to the first fiber portion via said coupling, the first phase change device operating within a first phase range; and a second phase change device coupled with a second fiber portion of the optical fiber link and configured for imparting a phase changing contribution to the second fiber portion via said coupling, the second phase change device operating within a second phase range greater than the first phase range, wherein upon determining that said optical signal experiences a given phase change greater than the first phase range, operating the first phase change device to impart a first phase change within the first phase range along the first fiber portion and operating the second phase change device to impart a second phase change within the second phase range along the second fiber portion, the first and second phase changes corresponding to the given phase change.

Further in accordance with the fourth aspect, the fifth aspect and/or the sixth aspect, the first phase change device can for instance operate within a first response frequency range and the second phase change device can for instance operate within a second response frequency range having a maximum response frequency value smaller than a maximum response frequency value of the first response frequency range, wherein, upon determining that the given phase change occurs at a given frequency value greater than the second response frequency range, the second phase change device imparts, along the second fiber portion, the second phase change at a second response frequency value within the second response frequency range; and the first phase change device imparts, along the first fiber portion, the first phase change at a first response frequency value within the first response frequency range and greater than the second response frequency range.

All technical implementation details and advantages described with respect to a particular aspect of the present invention are self-evidently mutatis mutandis applicable for all other aspects of the present invention.

DETAILED DESCRIPTION

FIG.1shows an example of a quantum computer and more specifically of a photonic fault-tolerant quantum computing (FTQC) system100, in accordance with an embodiment. As depicted, the photonic FTQC system100includes a preparation sub-system102and stitching and measurement sub-systems105. In this example, the preparation sub-system102has a quantum light source102aand a quantum light delivery device102b. The preparation sub-system102prepares one or more optical signals104by imparting them with initial quantum states of light. For instance, the quantum states of light can include, but are not limited to, single-photon states, non-Gaussian states, squeezed states, squeezed vacuum states, Gottesman-Kitaev-Preskill (GKP) states, GKP magic states, and cat states, to name a few examples. In some embodiments, the optical signal104is provided in the form of a single optical pulse or a train of optical pulses.

Such optical pulses can have a temporal shape similar to the one shown inFIG.1A. More specifically, the optical pulse104has an envelope104ashown in dashed line whereas the solid line represents an amplitude104bof the optical pulse104. Note that there are several oscillations of the amplitude within a single optical pulse. For example, the wavelength, frequency, and period of the oscillations may be A=1550 nm, f=193 THz, and T=5.2 fs, respectively, while the width of the envelope may be in the range of 1-10 ns for some photonic FTQC applications. However, these parameters can change depending on the embodiment. Terms such as “time-of-flight,” “arrival time,” and “timing” of an optical pulse refer to the timing of the envelope104a, whereas the phase of an optical pulse refers to the oscillations of the amplitude104bwithin the envelope104a. The optical signal can also be provided in the form of a continuous wave (CW) signal in some other embodiments. The CW signal can be modulated into pulses.

As shown, the stitching and measurement sub-systems105include the stitching sub-system105aand the measurement sub-system105b, which together would be used to perform measurement-based quantum computation (MBQC). As shown, the photonic FTQC system's sub-systems102and105aare optically connected using one or more optical fiber links106.

As discussed above, as undesirable environmentally induced phase changes can occur along any one of the optical fiber links106, optical phase change apparatuses such as those shown at110are provided along corresponding ones of the optical fiber links106. For instance, in this embodiment, the optical phase change apparatus110is provided along the optical fiber link106extending between the preparation sub-system102and the stitching sub-system104a. However, the optical phase change apparatus110can be positioned at any suitable position along an optical fiber link. For instance, in the depicted embodiment, an optical phase change apparatus can be positioned between the quantum light source102aand the quantum light delivery device102b. Additionally or alternately, an optical phase change apparatus can be positioned between the stitching sub-system105aand the measurement sub-system105b. As described below, the optical phase change apparatus110imparts phase changing contributions (localized or distributed temperature variations, thermo-optic modulations, mechanical stress, electro-optic modulations, light pulses, etc.) along the optical fiber link106in a way which modifies a phase of an optical signal propagating therealong. For instance, upon determining that the optical signal104has experienced a given phase change, the optical phase change apparatus110can be operated to impart a compensating phase change to the optical signal104which cancels out the given phase change. In other words, the optical phase change apparatus110can be used to ensure that the phase of the optical signal104propagating along the optical fiber link106remains as close as possible to a reference phase or reference phase curve at all times.

More specifically, and as shown inFIG.1, the optical phase change apparatus110encompasses the optical fiber link106. In some embodiments, the optical fiber link106can be provided in the form of a single length of optical fiber. In some other embodiments, the optical fiber link106can include a series of optical fiber sections optically connected to one another via splices or fiber connectors, for instance. The type of optical fiber can include, but is not limited to, a single-mode telecommunications fiber such as the Corning® SMF-28® optical fiber, the Corning® SMF-28@ Ultra-Low-Loss (ULL) optical fiber, the Corning® SMF-28@ Ultra optical fiber, the Corning® SMF-28e+@ optical fiber, the Corning® ClearCurve® single-mode optical fiber, the Sumitomo Electric@ Z Fiber™ LL, the Sumitomo Electric® Z-PLUS Fiber™ 150, or any custom single-mode optical fiber designed to increase reliability under stress. It is noted that SMF-28 fibers have a propagation time temperature sensitivity of ˜38 ps/km/K for a wavelength λ of 1550 nm. This equates to a phase temperature sensitivity of ˜48 rad/m/K of which a vast majority (˜95%) of this effect is due to a thermal dependence of the refractive index of the glass medium while the rest (˜5%) is due to changes in the fiber length from thermal expansion/contraction. In some embodiments, the photonic FTQC system100involves fibered systems, in which case the optical phase change apparatus110can include an input port111at a first end of the optical fiber link106, and an output port113at a second end, opposite the first end, of the optical fiber link106. In some embodiments, the photonic FTQC system100involves free-space light propagation, in which case the optical phase change apparatus110can include an injection port at a first end of the optical fiber link where the optical signal propagating in free space is injected into and along a core of the optical fiber, and an output port at a second, opposite end of the optical fiber link where the optical signal can be outputted. In some embodiments, the photonic FTQC system100includes a combination of free-space optical and fibered propagation.

Still referring toFIG.1, the optical phase change apparatus110has a first phase change device112coupled with a first fiber portion106aof the optical fiber link106and configured for imparting a phase changing contribution to the first fiber portion106avia its coupling to the first fiber portion106a. Moreover, the optical phase change apparatus110has a second phase change device114coupled with a second fiber portion106bof the optical fiber link106and configured for imparting a phase changing contribution to the second fiber portion106bvia its coupling thereto. In some embodiments, the first fiber portion106ais spaced apart from but serially connected to the second fiber portion106b. In some embodiments, the first fiber portion106aand the second fiber portion106bcan partially or wholly overlap with one another. The first and second phase change devices112and114can be interchanged along the optical fiber link106, meaning that the first fiber portion106acan be upstream from the second fiber portion106b, or vice versa. The optical fiber link106can have a length extending between 1 m and 500 m, and more preferably between 1 m and 250 m.

It is noted that the coupling between the first and second phase change devices112and114and the corresponding first and second fiber portions106aand106bcan be a mechanical coupling, a thermal coupling, or an optical coupling, depending on the type of phase change device used. The phase changing contribution can thus be a mechanical contribution, a thermal contribution, an optical contribution, or a combination thereof. For instance, in some embodiments, the phase change device is configured to impart a mechanical force to the corresponding fiber portion. The mechanical force can be a compressive force squeezing the corresponding fiber portion across its diameter, a tension force pulling the fiber portion at two opposite locations thereof, and the like. Examples of such phase change devices can include, but are not limited to, piezoelectric-based devices, servomotor devices, electro-mechanical actuators, and the like. Alternately or additionally, the phase change device can be configured to impart a thermal stress to the fiber portion. In these embodiments, the thermal stress can be localized or distributed along a given length of the corresponding fiber portion. In these embodiments, the thermal stress can cause the fiber portion to increase or decrease in temperature. Examples of such phase change devices can include, but are not limited to, thermoelectric-based or Peltier devices. In some embodiments, the phase change device has an optical element configured to heat the fiber portion by directing optical light pulses repeatedly towards the fiber portion. Examples of such phase change devices can include, but are not limited to, devices based on ultra-fast laser pulses and acousto-optic modulated lasers. Alternately or additionally, the phase change device can be configured to exploit the electro-optic effect. In these embodiments, the refractive index of the fiber portion is adjusted via the application of an electromagnetic field. Examples of such phase change devices can include, but are not limited to, free-space and fiber-coupled electro-optic modulators such as lithium niobate phase modulators. It is intended that any type of matter interactions which can cause a phase changing contribution, either resulting in fast and slow phase changes, can be used in the optical phase change apparatus110.

Regardless of the type of phase change device used, the first and second phase change devices112and114are configured for imparting their own phase changing contributions to the first and second fiber portions106aand106b, respectively. As best shown inFIG.2A, the first phase change device112is selected to operate within a first response frequency range116whereas the second phase change device114is selected to operate within a second response frequency range118. More specifically, the second response frequency range118has a maximum response frequency value FR,2,MAXgreater than a maximum response frequency value FR,1,MAXof the first response frequency range116. In other words, the second phase change device114operates within a faster response frequency range while the first phase change device112operates within a slower response frequency range. For instance, the maximum response frequency FR,2,MAXof the second response frequency range118can be at most about 40 GHZ, preferably at most about 100 MHz and most preferably at most about 100 KHz. The maximum response frequency FR,1,MAXof the first response frequency range116can be of at most about 160 Hz, preferably at most about 50 Hz and most preferably at most about 10 Hz. In some embodiments, the first and second response frequency ranges116and118have partially overlapping portions or tails such as illustrated inFIG.2A. In some embodiments, the first and second response frequency ranges116and118are fully separate from one another.

The optical phase change apparatus110is operated such that, upon determining that the optical signal104propagating along the optical fiber link106experiences a given phase change including one or more given frequency values greater than the first response frequency range116, the first phase change device112is operated to impart, along the first fiber portion106a, a first phase change at a first response frequency value within the first response frequency range116, and the second phase change device114is operated to impart, along the second fiber portion106b, a second phase change at a second response frequency value within the second response frequency range118and greater than the first response frequency range116. The first and second phase changes added to one another corresponds to the given phase change in the sense that they cancel out the given phase change or re-center the phase with respect to a reference phase value.

As best shown inFIG.2B, the first and second phase change devices112and114have respective dynamic phase ranges120and122. More specifically, the first phase change device112operates within a first phase range120and the second phase change device114operates within a second phase range122, with the first phase range120being greater than the second phase range122. Correspondingly, the first phase change lies within the first phase range and the second phase change lies within the second phase range. It was found that the limited phase range of the second phase change device114can often times result in a phase slip, i.e., when the phase adjustment reaches the limit of the second phase change device's phase range and it jumps or “slips” to a different period of oscillation, causing unwanted disturbances to the overall system. As such, combining the use of the first and second phase change devices112and114can help prevent the second phase change device114from working beyond the second phase range122and thus can limit phase slipping.

As such, the first and second phase change devices112and114are complementary to one another in the sense that the first phase change device112is configured to be more efficient in addressing slower but larger phase changes whereas the second phase change device114is configured to be more efficient in addressing smaller but faster phase changes. Put otherwise, the first phase change device112operates within a larger phase range while the second phase change device operates within a smaller phase range. For instance, the first phase range120is of at most fifty thousand π, preferably at most five hundred π and most preferably at most forty π. In some embodiments, the second phase range122is of at most one hundred π, preferably at most ten π and most preferably at most two π. In view of the above, the first phase change device112can be used for coarse tuning the optical phase of the optical signal while the second phase change device114can be used for fine tuning the optical phase of the optical signal.

FIGS.3and4show how typical environmentally induced phase changes are addressed using the optical phase change apparatus110ofFIG.1. Referring specifically toFIG.3A, environmental conditions perturb the optical fiber link over time. These environmental perturbations cause a given optical phase change124such as the one shown inFIG.3B. In some embodiments, the given optical phase change124is measured using a detector which generates a time-varying signal. When a Fourier transform is performed on the time-varying signal, a frequency spectrum124′ of the given optical phase change124can be obtained. An example of the frequency spectrum124′ of the given optical phase change124is shown inFIG.3C. As illustrated, the frequency spectrum124′ of the given optical phase change124includes one or more frequency values which are greater than the maximum response frequency FR,1,MAXof the first response frequency range116and which are part of the second response frequency range118. As depicted, the given optical phase change124includes a first portion124ain which the optical phase increases slightly at a slow rate of change, a second portion124bin which the optical phase increases abruptly at a faster rate of change and ultimately ends with a third portion124cin which the optical phase plateaus. In response to such optical phase increase, the first optical phase change device112is operated to impart a first phase change126as shown inFIG.3Dwhile the second optical phase change device114is operated to impart a second phase change128as shown inFIG.3E. As illustrated, during the first portion124aof the given phase change124, the first phase change device112is used to compensate for the slow increase of the phase. However, when the phase increases at a steeper rate of change, during the second portion124bof the given phase change124, contributions from the second phase change device114are relatively more important to ensure that the faster rate of change of the given phase change124is met with a corresponding phase compensation. When combined with one another, the first and second phase changes126and128imparted by the first and second phase change devices112and114, respectively, cancel out the perturbation of the optical signal's phase, as shown inFIG.3F.

Referring specifically toFIG.4A, environmental conditions perturb the optical fiber link over time. These environmental perturbations cause a given optical phase change124such as the one shown inFIG.4B. As depicted, the given optical phase change124includes a first portion124ain which the optical phase decreases slightly at a slow rate of change, a second portion124bin which the optical phase falls abruptly at a faster rate of change and ultimately ends with a third portion124cin which the optical phase plateaus. In response to such optical phase decrease, the first optical phase change device112is operated to impart a first phase change126as shown inFIG.4Cwhile the second optical phase change device114is operated to impart a second phase change128as shown inFIG.4D. As illustrated, during the first portion124aof the given phase change124, the first phase change device112is used to compensate for the slow decrease of the phase. When the phase decreases at a steeper rate, during the second portion124bof the given phase change124, contributions from the second phase change device114are relatively more important to ensure that the faster rate of change of the given phase change is met with a corresponding phase compensation. When combined with one another, the first and second phase changes126and128imparted by the first and second phase change devices112and114, respectively, cancel out the perturbation of the optical signal's phase, as shown inFIG.4E. In view of these examples, it is understood that the optical phase change apparatus110described herein can be used to compensate for both small and large phase changes occurring at both faster and slower rates of change.

FIG.5shows an example of an optical phase change apparatus210, in accordance with an embodiment. As depicted, the optical phase change apparatus210has an optical fiber link206along which an optical signal204propagates. In this embodiment, the optical phase change apparatus210has an enclosure230inside which the optical fiber link206extends. As shown, the enclosure230is provided with an entry port230aand an exit port230bbetween which the optical fiber link106extends. In some embodiments, the enclosure230includes a thermally insulating material (e.g., plastic, foam, rubber, vacuum flask) for thermal and/or mechanical isolation from the surrounding environment205.

As illustrated, the optical phase change apparatus210has a first phase change device212and a second phase change device214which are both enclosed inside the enclosure230. More specifically, the first phase change device212is coupled with a first fiber portion206aof the optical fiber link206and configured for imparting a phase changing contribution to the first fiber portion206avia the coupling. A second phase change device214is also coupled with a second fiber portion206bof the optical fiber link206. The second phase change device214is configured for imparting a phase changing contribution to the second fiber portion206bvia the coupling. Although the first and second fiber portions206aand206bare shown to be spaced apart from one another along a length of the optical fiber link206in this example, the first and second fiber portions can partially or wholly overlap with one another in some other embodiments. Typically, as the first phase change imparted by the first phase change device212on the first fiber portion206ais slower and of larger amplitude, the first fiber portion206acan have a relatively longer length. On the contrary, the second fiber portion206bcan have a relatively shorter length as it is used to impart rather faster phase changes of relatively small amplitudes.

The optical phase change apparatus210has a controller240which is communicatively coupled at least to the first phase change device212and to the second phase change device214. The controller240has a processor and a non-transitory memory having stored thereon instructions which when executed by the processor perform steps. The controller240can thus be used to receive phase change data indicative of a given phase change experienced by the optical signal204propagating along the optical fiber link206, and to operate the first and second phase change devices212and214to compensate the given phase change with complementary first and second phase changes, respectively. In some embodiments, the controller240monitors a current phase signal indicative of a current phase of the optical signal204and operates or otherwise sends operation instructions to the first and second phase change devices212and214accordingly, in a real time or quasi-real time manner. For instance, the controller240can instruct the first phase change device212to impart the first phase change. Similarly, the controller240can instruct the second phase change device214to impart the second phase change. The current phase of the optical signal204can be measured or otherwise monitored either downstream or upstream from the optical phase change apparatus210, depending on the embodiment.

As shown, in this embodiment, the first phase change device212is provided in the form of a thermoelectric element242operating within a first response frequency range. The thermoelectric element242is thermally coupled to the first fiber portion206a. The thermoelectric element242can be driven to heat or cool the first fiber portion206a, depending on what phase change is required at any given moment. The thermoelectric element242can be driven by a thermoelectric driver244in communication with the controller240. In this specific embodiment, the first fiber portion206ais looped about a first cylindrical surface246made of a thermally conductive material. As such, by thermally coupling the thermoelectric element242to the first cylindrical surface246, temperature variations imparted by the thermoelectric element242can be conveyed to the first fiber portion206avia the first cylindrical surface246. It is noted that the first phase change is mostly caused by a change of refractive index of the propagating medium of the first fiber portion206aupon cooling and heating, and that only a small portion of the first phase change is due to the expansion or contraction of the first fiber portion206aalong its length. In some embodiments, the first cylindrical surface246is part of a first mandrel248having a base248afixed within the enclosure230. The thermoelectric element242can be thermally coupled to the base248aof the first mandrel248or to a portion of the enclosure230receiving the first mandrel248, depending on the embodiment. The first mandrel248has a diameter which is large enough not to incur losses to the optical signal104propagating around the first cylindrical surface246. In some embodiments, the first fiber portion206aacts as a first delay line where the optical signal can be delayed by a given amount of time corresponding to a length of the first fiber portion206a. For instance, the first fiber portion206acan extend along at least 1 m, preferably at least 200 m and most preferably at least 250 m, depending on the embodiments. Additionally, a temperature sensor250can be positioned proximate to the first fiber portion206aand/or to the thermoelectric element242to monitor a current temperature of the first fiber portion206aand/or of the thermoelectric element242. A temperature reader252can be communicatively coupled to the temperature sensor250and/or to the controller240, such as shown inFIG.5. It is understood that since the phase changing contribution imparted by the thermoelectric element242requires heating and cooling, which can only be performed at relatively slow rates of change, the first response frequency can range between 0 Hz and 100 Hz, for instance.

In some alternate embodiments, the first phase change device212does not necessarily have a thermoelectric element, but rather has an electro-mechanical element. In these embodiments, the phase change device is configured to impart a mechanical force to the first fiber portion. The mechanical force can be a tension force stretching the first fiber portion, for example. In such embodiments where the first phase change device has an electro-mechanical element, the first response frequency can range between 0 Hz and 160 Hz, for instance. In some embodiments, the electro-mechanical element may be a solenoid element.

In this example, the second phase change device214is provided in the form of a piezoelectric element254mechanically coupled to the second fiber portion206b. The piezoelectric element254can be driven to apply a mechanical stress to the second fiber portion206b. More specifically, in this example, the second fiber portion206bis looped around a second cylindrical surface256made of a rigid material. The piezoelectric element254can be driven by a piezoelectric driver256in communication with the controller240. As such, as the piezoelectric element254is mechanically coupled to the second cylindrical surface256, the second cylindrical surface256can expand and contract repeatedly. Such expansions and contractions imparted by the piezoelectric element254can be conveyed to the second fiber portion206bvia the second cylindrical surface256. In this specific embodiment, the second cylindrical surface256is part of a second mandrel258having a base258afixed within the enclosure230. The second mandrel258has a diameter which is large enough not to incur losses to the optical signal propagating around the second cylindrical surface256. It is intended that the second cylindrical surface256can be omitted, as the piezoelectric element254can be in direct coupling with the second fiber portion206bin some other embodiments. In some embodiments, the second fiber portion206balso acts as a second delay line where the optical signal can be delayed by a given amount of time corresponding to a length of the second fiber portion206b. In some embodiments, the second delay line is shorter than the first delay line, or vice versa, depending on the embodiment. Although the second fiber portion206bis shown to be looped around the cylindrical surface256in this embodiment, it is intended that the second fiber portion206bis not necessarily coiled in some other embodiments. Indeed, in some embodiments, the second fiber portion206bcan extend linearly when put under tension at both opposite ends of the second fiber portion206b, with the phase changing contribution being imparted at either one or both of the ends of the second fiber portion206b. For instance, the linearly extending fiber portion can be so affixed to a metal plate, rod, or anvil. In these embodiments, the phase changing contribution can be locally imparted at a point along the tensioned fiber or along a range thereof. It is understood that since the phase changing contribution imparted by the piezoelectric element254is driven with an electrical signal, which can bear quite high frequencies, the second response frequency range can range between 0 Hz and 100 kHz, for instance. In embodiments where the phase changing contribution is imparted using a lithium niobate phase modulator, the second response frequency range can range between 0 Hz and 40 GHz. Although the lithium niobate phase modulators may be more lossy compared to piezoelectric elements and thermoelectric elements, they can be satisfactory in applications where faster response frequencies are desired. As such, the second response frequency range of the second phase change device214has a maximum response frequency value greater than a maximum response frequency value of the first response frequency range.

In some embodiments, the second phase change device214does not necessarily have a piezoelectric element or an electro-optic element, but rather has a thermoelectric element similarly to the first phase change device212. In these embodiments, the second fiber portion can be wound about the second mandrel, but since the second fiber portion is shorter than the first fiber portion (due in part to the mandrel having a smaller diameter than a diameter of the first mandrel), the second phase change device214provides a second response frequency range which is faster than the first response frequency range. In embodiments where thermoelectric elements are used, it is encompassed that any positioning of the thermoelectric element is satisfactory as long as it can impart a phase changing contribution to the corresponding fiber portion. For instance, the thermoelectric element can be mounted inside the cylindrical surface, at a top or base thereof, or to a plate on which the cylindrical surface is seated. In some embodiments, the second phase change device214can include a combination of a piezoelectric element and a thermoelectric element. Depending on the embodiment, the second fiber portion can be i) wound around a piezoelectric mandrel which expands or contracts radially upon application of a driving voltage; ii) mounted over a piezoelectric anvil which pushes or pulls laterally on the second fiber portion upon application of a driving voltage; iii) mounted on a piezoelectric rod which lengthens or shrinks the second fiber portion upon application of a driving voltage; and iv) held between two endpoints, at least one of which moves due to a piezoelectric actuator upon application of a driving voltage.

When driven by a DC signal rather than an AC signal, a phase change device (such as each of the phase change devices212and214) can have a response frequency range with a minimum response frequency value of 0 Hz. Therefore, the shape of the first and second response frequency ranges can depend on how the phase change devices212and214are driven. For instance, the thermoelectric element242can be driven using a DC signal260such as shown inFIGS.6A and7A. The resulting response frequency range216can be such as shown in the solid lines ofFIGS.6B and7B. More specifically, such a first response frequency range216can extend from 0 Hz to a given maximum response frequency value FR,1,MAX. In some other embodiments, the thermoelectric element242can be driven using an AC signal as well. Referring now toFIG.6A, the piezoelectric element254can be driven using an AC signal262. In these embodiments, the second response frequency range218can extend from a given minimum response frequency value FR,2,MINto a given maximum response frequency value FR,2,MAX. In some embodiments, the minimum response frequency value FR,2,MINof the second response frequency range218is greater than the maximum response frequency value FR,1,MAXof the first response frequency range216. In contrast, such as shown inFIG.7A, when the piezoelectric element254is driven by a DC signal264, e.g., a fast DC signal, the minimum response frequency value FR,2,MINcan be 0 Hz. In these embodiments, the second phase change device can be used to impart either fast or slow phase changes to the second fiber portion.

FIG.8shows a flow chart of a method800of changing a phase of an optical signal, in accordance with an embodiment. The method800is described with reference to the optical phase change device ofFIG.1. Accordingly, alternatives and options described with reference toFIG.1also apply to the method800.

At step802, an optical signal is propagated along an optical fiber link. The optical fiber link can include one or more sections of optical fiber serially connected to one another. In an exemplary photonic FTQC system, the optical fiber link can optically connect between and/or within one or more sub-systems of the photonic FTQC system. In a given photonic FTQC system, the method800can be performed along one or more optical fiber links sequentially or simultaneously, in order to control the timing and phase of the optical signals propagated along the optical fiber links. However, for ease of reading, the method800is described with respect to only one optical fiber link.

At step804, a first fiber portion of the optical fiber link is coupled to a first phase change device. As discussed above, this coupling can be a thermal coupling in embodiments where the first phase change device includes a thermoelectric element. The first phase change device is configured for imparting a phase changing contribution to the first fiber portion via the coupling.

At step806, a second fiber portion of the optical fiber link is coupled to a second phase change device. As discussed above, this coupling can be a mechanical coupling in embodiments where the second phase change device includes a piezoelectric element. Otherwise, the coupling may be an optical coupling in embodiments wherein the second phase change device includes a pulse emitting device directed at the second fiber portion. It is intended that the second phase change device is configured for imparting a phase changing contribution to the second fiber portion via that coupling.

As discussed in detail above, the first phase change device operates within a first response frequency range while the second phase change device operates within a second response frequency range, and the second response frequency range has a maximum response frequency value greater than a maximum response frequency value of the first response frequency range. As such, the second phase change device can impart a phase change at a greater response rate or frequency than the first phase change device.

At step808, it is determined that the optical signal propagated along the optical fiber link has experienced a given phase change including a frequency greater than a first response frequency range of the first phase change device. The given phase change can be detected in real time thanks to an optical phase change feedback sub-system, examples of which are described below with reference toFIGS.9-13. In embodiments where the given optical phase change is not detected, it can be estimated based on known temperature variations occurring around the optical fiber link or based on known mechanical stress variations imparted along the optical fiber link. Such known temperature variations, known mechanical stress variations, and associated phase changes, can be stored on a computer memory accessible by the controller, for instance.

At step810, the first phase change device imparts, along the first fiber portion, a first phase change at a first response frequency value within the first response frequency range. The first phase change is meant to be a slower variation of the phase of the optical signal, thanks to the slower response frequency range of the first phase change device.

At step812, the second phase change device imparts, along the second fiber portion, a second phase change at a second response frequency value within the second response frequency range and greater than the first response frequency range. The second phase change is meant to be a faster variation of the phase of the optical signal, due to the faster response frequency range of the second phase change device.

By performing steps810and812, either simultaneously or sequentially, and in any order, the first and second phase changes can be made to cancel out the given phase change, thereby canceling out the environmentally induced perturbations. The method800can include a step of monitoring a current phase signal indicative of a current phase of the optical signal. In these embodiments, the steps810and812can be based on the current phase signal and on how it compares to a reference phase value, for instance. In some embodiments, the current phase value carried by the current phase signal is compared (e.g., subtracted) to a reference phase value, and the steps810and812are based on that comparison (e.g., on that difference).

In some embodiments, the construction of the first and second phase change devices enables the first phase change device to operate within a first phase range and the second phase change device to operate within a second phase range, with the first phase range greater than the second phase range. In these embodiments, the first phase change imparted by the first phase change device can have a slower rate of change but a larger phase change amplitude-wise while the second phase change imparted by the second phase change device can have a faster rate of change but smaller phase change amplitude-wise. In some embodiments, upon determining that the optical signal experiences a steeper phase change, the second phase change rapidly imparted by the second phase change device can be greater at first but then can decrease once the first phase change imparted by the first phase change gradually catches up over time due to its slower response frequency range. In some embodiments, the steps810and812are performed in a way that forces the second phase change device to work as far as possible from the maximum value of its second phase range. In other words, the steps810and812are performed to keep the amplitude of the second phase change as close as possible to a minimum phase range value of the second phase range. For long-term operation, phase slip is avoided by adjusting the first phase change imparted by the first phase change device to keep the second phase change device within its dynamic phase range.

FIGS.9-13show examples of optical phase change feedback sub-systems that can be used with the optical phase change apparatus110described herein. Such feedback sub-systems can include programmable logic devices (PLD) acting as a feedback loop to correct the phase of the optical signal in real time. More specifically,FIG.9shows an example of an optical phase change feedback sub-system900incorporating a reference timer980generating a reference signal982which is communicated either directly or indirectly to the controller of the optical phase change apparatus110. In the illustrated embodiment, the reference signal982is sent to a destination apparatus984receiving both the optical signal104from the optical fiber link106and the reference signal982. By interacting the optical signal104with the reference signal982at the destination apparatus984, a feedback signal986can be generated to stabilize the phase change correction that is performed in real time or quasi-real time. In this embodiment, the feedback signal986can be used to monitor a given phase change experienced by the optical signal104over time. As such, the feedback signal986can be used as a basis for the operation of the first and second phase change devices and associated first and second phase changes.

An example of a destination apparatus1084is shown in greater detail inFIG.10. As shown, the destination apparatus1084has a coherent optical detector1088receiving the optical signal104from the optical fiber link106but also a reference light signal1082generated by a reference light source1083. Both the optical signal104and the reference light signal1082impinging simultaneously onto the coherent optical detector1088can create an electric beat output1090which can be monitored in real time using an electronic signal analyzer1092. In this example, the feedback signal1086is generated by the electronic signal analyzer1092based on the electric beat output1090. The large phase adjustments provided by the first phase change device are typically used to control the relative timing and account for timing errors, and the small adjustments provided by the second phase change device are generally used to control the relative optical phase and account for phase errors. It should be noted that the term “timing errors” here refers to delays of much more than one optical wave period of the light, whereas the term “optical phase errors” are indistinguishable every 2π radians. For example, a timing error may be 10 picoseconds, which corresponds to2000optical wave periods at a wavelength of 1550 nm (frequency of 193 THz), yet the optical phase repeats every 5.2 femtoseconds (1/193 THz). Thus, the feedback can first indicate the timing error as a large change and then make small adjustments to achieve the desired alignment of optical phase.

Another example of a destination apparatus1184is shown in greater detail inFIG.11. As shown, the destination apparatus1184has a first photodetector1194areceiving the optical signal104from the optical fiber link106. Upon reception, the first photodetector1194agenerates a first electrical signal output1196a. In parallel, a reference light source1198propagates a reference light signal1199onto a second photodetector1194bwhich generates, upon reception of the reference light signal1199, a second electrical output1196b. Both the first and second electrical outputs1196aand1196bare sent to an electronic signal analyzer1192which can produce a feedback signal1186based on a beat occurring between the first and second electrical outputs1196aand1196b.

Another example of a destination apparatus1284is shown in greater detail inFIG.12. As depicted in this example, the reference light source and associated photodetector are omitted. More specifically, the destination apparatus1284has a photodetector1294receiving the optical signal104from the optical fiber link106. Upon reception, the photodetector1294generates an electrical signal output1296which is transmitted to an electronic signal analyzer1292. In this specific embodiment, a reference timer1280is provided which generates a reference signal1282sent to the electronic signal analyzer1292as well. As shown, the feedback signal1286in this example results from a beat occurring between the reference signal1282and the electrical signal output1296generated by the photodetector1294.

FIG.13shows an example of an optical phase change feedback sub-system1300in which feedback signal1386is sent to one or more controllers pertaining to two distinct optical phase change apparatuses110aand110b. As shown, the optical phase change apparatus110ahas an optical fiber link106along which a first optical signal104apropagates. Similarly, the optical phase change apparatus110bhas an optical fiber link106′ along which a second optical signal104bpropagates. In this embodiment, the two optical phase change apparatuses110aand110bare independent from one another, meaning that the optical fiber links106and106′ can experience different environmentally induced phase changes, but are both using the same feedback signal1386. It is intended that the same feedback signal1386can be used by more than two optical phase change apparatuses, depending on the embodiment. It was found particularly valuable to provide a single optical phase change feedback sub-system1300to control multiple optical signals at a destination, as it reduces the overall footprint and cost of the corresponding system.

With respect to controllers, computers, processors, or processing units, the expression “configured to” relates to the presence of hardware or a combination of hardware and software which is operable to perform the associated functions. The controller240shown inFIG.5can be provided as a combination of hardware and software components. The hardware components can be implemented in the form of a computer1400, an example of which is described with reference toFIG.14. Moreover, the software components of the controller240can be implemented in the form of one or more software applications.

Referring toFIG.14, the computing device1400can have a processor1402, a memory1404, and I/O interface1406. Instructions1408for running the one or more software applications can be stored on the memory1404and accessible by the processor1402.

The processor1402can be, for example, a general-purpose microprocessor or microcontroller, a digital signal processing (DSP) processor, an integrated circuit, a field-programmable gate array (FPGA), a reconfigurable processor, a programmable read-only memory (PROM), or any combination thereof.

The memory1404can include a suitable combination of any type of computer-readable memory that is located either internally or externally such as, for example, random-access memory (RAM), read-only memory (ROM), compact disc read-only memory (CDROM), electro-optical memory, magneto-optical memory, erasable programmable read-only memory (EPROM), and electrically erasable programmable read-only memory (EEPROM), Ferroelectric RAM (FRAM), or the like.

Each I/O interface1406enables the computing device1400to interconnect with one or more input devices, such as optical phase change feedback sub-systems, or with one or more output devices such as the first and second optical phase change devices, a memory system, and/or a telecommunications network.

The computing device1400and the software layers described above are meant to be examples only. Other suitable embodiments of the controller240can also be provided, as will be apparent to the skilled reader.

As can be understood, the examples described above and illustrated are intended to be exemplary only. For instance, third, fourth, and other phase change devices can be coupled along different fiber portions of the optical fiber link. In these embodiments, each of the phase change devices has a respective response frequency range, and the response frequency ranges are complementary to one another so as to avoid unwanted phase slips. The optical phase change apparatus can be separate from the photonic FTQC system. In some other embodiments, an optical apparatus can be provided with the optical fiber link, the first phase change device, and the second phase change device. Accordingly, the controller may be omitted as this optical apparatus may be sold without the controller in some circumstances. The scope is indicated by the appended claims. cm What is claimed is: