Patent Grant
10620503
This application claims the benefit of Canadian patent application Serial No. 3,012,946, filed on Jul. 31, 2018, and incorporates such application by reference into this disclosure as if fully set out at this point.
There is provided a fiber-based broadband polarization-entangled photon source having a simple configuration. The present invention is compact, robust, low-power, low-cost, alignment-free and room-temperature operable, featuring a simple turn-key, plug-and-play operation.
Quantum technology is on the verge of becoming the next transformative discipline. Quantum entangled sources, i.e. sources that emit entangled particles, are a core quantum technology. Entangled sources have one property that no classical source has, that is quantum entanglement. Once described by Einstein as “spooky action at a distance”, entanglement is now an extremely powerful resource for quantum computing, quantum communication and quantum sensing. Entangled photons are pairs of photons exhibiting quantum correlations which can be harnessed to perform more efficient computation, higher resolution imaging, or secure cryptographic key distribution. Different types of entanglement exist, such as position-time entanglement, quadrature entanglement, frequency entanglement and modal entanglement. Polarization-entanglement is by far the most widely encountered and arguably the most promising for practical applications, because polarization correlation is easy to measure and verify, without requiring stable interferometers. Moreover, telecom-wavelength entangled photon sources are particularly useful, because their wavelengths correspond to the low-loss region of the existing telecom fiber, enabling easy distribution of entanglement over long distances, which is essential to distributed quantum computing and quantum cryptography.
While there are a number of technologies that can produce polarization-entangled photon pairs in the telecom wavelength region, they tend to be bulky and complex. The quantum industry cannot survive on fragile, bulky and expensive entangled sources that work only in highly controlled laboratory environments. The more robust, compact, efficient, and affordable the entangled sources become, the sooner quantum technology will flourish.
Polarization-entangled EPR (Einstein-Podolsky-Rosen) pairs can be generated through spontaneous parametric down conversion (SPDC) in nonlinear optical media, typically birefringent crystals or waveguides that have a large second-order nonlinearity, χ(2). Pump photons traveling in the nonlinear medium have a small probability of decaying into two correlated daughter photons (the signal and the idler). Momentum conservation (i.e., phase matching) is satisfied either through choosing a particular crystal orientation, or through quasi-phase matching (QPM), attainable with a periodic structure. Polarization entanglement of the two daughter photons is either inherent in the SPDC process or can be achieved through post selection or erasure of distinguishability.
While crystals and waveguides with large χ(2) can yield high-flux photon pairs, the coupling efficiency into a fibre is poor. They also require bulky optical elements and precision alignment. Fibre-based photon pair sources in the telecom wavelength region are desirable as generation and transport of photon pairs can then be done seamlessly in an all-fiber system, eliminating bulky optics and alignment.
However, conventional fibres do not exhibit 2nd-order nonlinearity (χ(2)=0) due to glass symmetry. Prior work on fiber-based polarization-entangled photon pairs had been limited to using third-order nonlinearity, χ(3), via the spontaneous four-wave-mixing (SFWM) process, involving two pump photons having nearly the same wavelength (near degeneracy) as those of the signal and the idler in the telecom band.
Major disadvantages of the SFWM approach are:
(1) Due to the weak χ(3) in fibre, typically very long fibres are required (e.g., 300 m of DSF); in some specialty fibers (such has photonics crystal fibers), high nonlinearity can be achieved, but the fiber fabrication process is complex and such photonic crystal fibers are expensive.
(2) Operating near degeneracy requires the suppression of Raman-scattered photons as they spectrally overlap with the signal and the idler. Raman suppression is achieved by cooling the fibre in either liquid nitrogen or liquid helium, but residual Raman noise is still a major limitation to the entanglement quality of the source;
(3) The χ(3) tensor of silica is such that the photon pairs have the same polarization as the pump, and therefore are not polarization-entangled. Additional steps are required to convert them into polarization-entangled pairs, adding more complexity and cost.
These disadvantages largely diminished the attractiveness of using fibre, as the overall system would still be bulky and costly.
A further observation is that in the majority of the experimentally demonstrated systems that produce polarization-entangled photon pairs, the birefringence of the nonlinear medium (be it a crystal, a waveguide, or a polarization-maintaining fiber) is very large, leading to distinguishability due to polarization-dependent differential group delay (DGD). This DGD must be compensated to obtain high quality polarization entanglement, introducing an additional degree of complexity.
Although progress has been made in the design of bulky nonlinear crystals or waveguides, challenges remain. Operation and delicate alignment in free-space and bulk optics is challenging for infield applications. In addition, due to the large group birefringence in nonlinear crystals, temporal compensation is always needed to achieve polarization entanglement. One has to use interferometer, or an additional birefringent crystal, or a pre-compensator for pump, to eliminate the temporal distinguishability in the birefringent biphoton source. Such requirements increase the complexity and reduce the robustness of an entangled photon source.
An initial proposal of using periodically poled fiber for polarization-entangled photon generation was published in L. G. Helt et al., Proposal for in-fiber generation of telecom-band polarization-entangled photon pairs using a periodically poled fiber, Optics Letter, 34, Issue 14, pp. 2138-2140, 2009.
In 2012, the direct entangle generation was experimentally demonstrated using a fibre without any compensation or erasure of distinguishability; see for example Zhu, Eric Y., et al. “Direct generation of polarization-entangled photon pairs in a poled fiber.” Physical review letters 108.21 (2012): 213902.
The following article discusses broadband emission of the source: E. Y. Zhu, et al, “Poled-fiber source of broadband polarization-entangled photon pairs,” Opt. Lett. 38, pp. 4397-4400, 2013.
The following article discusses compensation-free broadband polarization entanglement generation achieved in a periodically poled fiber: Chen, Changjia et al., Compensation free broadband entangled photon pair sources, Optics Express 25, pp. 22667-22678, Sep. 8, 2017. This reference is incorporated herein by reference in its entirety.
The present invention relates to a compensation-free, alignment-free, diode-pumped, broadband, polarization-entangled photon source that is based on a low-birefringence fiber. One aspect of the present invention uses a periodically poled silica fiber (PPSF) in a polarization entangled photon source for solving some of the problems associated with prior known solutions. The all-fiber-based optical structure grants strong robustness against environmental perturbation. Compared to the free space bulk optics, no spatial alignment is required in PPSF source. It can be steadily operating at room temperature. Taking advantage of a negligible group birefringence, PPSF sources are able to generate high quality and broadband entanglement over 80 nm bandwidth, which is desirable in applications such as broadband quantum sensing and multi-channel quantum communication. Moreover, it has been shown experimentally and theoretically that no additional temporal compensation is needed in PPSF-based sources. Thus, the structure of the PPSF-based source is extremely simple and compact.
The invention will be better understood with reference to the following drawings showing exemplary embodiments in which:
In one aspect of the present invention, there is provided a weakly birefringent periodically-poled fiber as a nonlinear medium, pumped by a narrow-band, linearly-polarized laser (which can optionally be a fiber-pigtailed diode laser), and followed by spectral filters that eliminate the pump photons as well as fluorescence noise photons.
In a further aspect, the nonlinear fiber can be heated with an optional heater 12, shown in
The nonlinear fiber is weakly birefringent and has two defined polarization axes which can be aligned during splicing to a polarization-maintaining fiber from the pump. In one embodiment, it can also be connectorized with the connector key 14, shown in
The low group birefringence of the nonlinear fiber allows the generation of polarization-entangled photon pairs directly, while the birefringence is sufficiently low that high-concurrence entanglement can be obtained over a large bandwidth without compensation. Details are discussed in Chen, Changjia et al., Compensation free broadband entangled photon pair sources, Optics Express 25, pp. 22667-22678, Sep. 18, 2017, and this reference is incorporated herein by reference.
The present invention features an extremely simple configuration capitalizing on the compensation-free and direct generation capability of a low-birefringence fiber. The present invention employs a broad bandwidth, high-quality polarization-entanglement. The source is also a hyper-entangled source in the sense that the photon pairs are entangled in both polarization- and in-spectral degree of freedom.
The present invention has several advantages such as being compact, robust, low-power, low-cost, alignment-free and room-temperature operable, featuring a simple turn-key, plug-and-play operation. These features are due to many aspects. For starters, the present invention is based on an induced second nonlinearity of the optical fiber that ensures much shorter nonlinear interaction length. In one aspect, the present invention relies on type II phase matching to generate polarization-entangled photons directly. In a further aspect the present invention relies on weak group birefringence such that no compensation or erasure of distinguishability is necessary. It is single-mode-fiber based so no beam alignment is necessary. The nonlinear fiber can be spliced and/or connectorized with polarization axis alignment such that no further polarization alignment is necessary once the nonlinear fiber is spliced or connectorized. It takes advantage of the mature telecom fiber technologies and uses mostly low-cost components widely available in the telecom industry.
The present invention has further advantages over the known prior solutions. Previous technologies that were based on crystals and waveguides (not fiber-based) rely on fiber-to-waveguide coupling. Elimination of the fiber coupling makes the present invention robust, alignment-free, reduces loss and lowers costs. Some previous fiber-based technologies rely on cryogenic cooling to reduce Raman noise. Conversely, the present invention does not need cryogenic cooling. In addition, most other technologies use either an interferometer configuration or compensator(s) to erase distinguishability, resulting in system complexity. Conversely, the present invention generates it directly (using type II phase matching) and without compensation (due to low group birefringence).
Variations to the present invention exist. For example, the present invention can use other types of nonlinear fibers, not necessarily poled fibers, provided the fiber (1) has sufficiently large second-order nonlinearity, (2) can be type II phase matched (or quasi-phase matched) for the desirable pump, signal and idler wavelengths; and (3) has low birefringence (specifically low group birefringence such that the polarization walk off is negligible compared to spectral walk-off over the desirable signal/idler bandwidth). In other examples, the pump can be a different type, such as continuous wave, Q-switched, mode-locked, or otherwise pulsed, provided the pump is linearly polarized and has sufficient power within the phase-matching bandwidth. The pump can optionally be replaced by a wavelength tunable laser to excite other types of phase matching (such as type 0, type I, etc.) This would allow correlated photon pair generation for better brightness.
In other variations, the nonlinear fiber can be placed in a resonant cavity 10, for example formed by fiber gratings 16, to obtain high spectral brightness or more desirable spectral properties. In other alternatives, various stabilization schemes may be employed, such as pump wavelength stabilization, temperature stabilization for the entire module, etc. Other variations include using filters inside the module to separate signal and idler photons or using spectral filtering/shaping of any kind inside the module on the signal and idler spectra.
The present invention preferably uses diode pumping. In order to enable an in-field robust quantum operation, a compact and stable system is desired. Due to the compactness and stability of a laser diode, it is more favorable for practical quantum application. For a less than 1 micron small size compact entangled photon source, laser diode is preferred. References respecting diode pumping can be found in Bedington, Robert, Juan Miguel Arrazola, and Alexander Ling. “Progress in satellite quantum key distribution.” npj Quantum Information 3.1 (2017): 30, and Yin, Juan, et al. “Satellite-based entanglement distribution over 1200 kilometers.” Science 356.6343 (2017): 1140-1144.
One embodiment of the packaged entanglement source setup is shown in
The exemplary components shown in
In a further variation, a fibre Bragg grating (FBG) 22 can be provided at the output of the pump. The power reflectivity profile of the FBG can be designed to reflect the desired pump wavelength in order to stabilize the pump wavelength. A fiber Bragg grating (FBG) can be written, spliced or connected to the output fiber of the pigtailed pump laser diode. As a result, an additional optical feedback forces the laser diode to emit light and operate at a wavelength within the reflection bandwidth of that FBG. Consequently, the wavelength of the pump laser will be stabilized and locked to the center of the reflection bandwidth curve of the FBG and hence, the operating wavelength becomes impermeable to the instabilities and thermal noise of the pump laser diode. The spectral bandwidth of the FBG can also be fixed by using an active thermoelectric cooler to maintain its center wavelength as well as the operating wavelength of the pump laser diode. Also, this may reduce the linewidth of the pump laser diode.
The direct generation of polarization entanglement via type-II SPDC process produces bipartite Bell states. The state in terms of frequency can be written as:
|Φ>∝∫∫[|HiVs+exp(i*ϕ(ωs,ωi))*|VsHi>]dωsdωi
The subscripts s,i denote signal and idler downconverted photons respectively. ϕ(ωs, ωi) is the phase that depends on frequency. The frequency dependence of phase might come from the birefringence and dispersion of the non-linear medium itself or the compensator, for example, the beamsplitter in interferometric compensator and the post compensation crystal. In the case of narrowband filtering or direct narrowband polarization entanglement generation, the frequency-dependent phase should not be a problem since it can be approximated by a constant in a small bandwidth region. However, when it comes to broadband generation over several THz, the phase term ϕ(ωs, ωi) variation becomes significant. In the presence of this phase, the ensemble average of state Eq. (1) over a relative broad spectrum will lead to significant degradation of entanglement and even turn out to be a mixed state. It has been demonstrated that the polarization entanglement in PPSF with narrowband filtering at different wavelength theoretically shows that the phase is nearly constant over the whole bandwidth. The present invention further demonstrates the genuine broadband polarization entanglement in a compact PPSF source using broad-band commercial L/C band WDMs as biphoton splitters.
When the bandwidth of generated entangled biphotons is narrow, the phase and amplitude difference between f−(ωs, ωi) and f+(ωs, ωi) in Eq. (1) is trivial, since it can be approximated as a constant in a small range. Even in the case of broadband generation over several THz, the ensemble average of state Eq. (1) over a broad spectrum for the PPSF source still maintains high polarization-entanglement quality due to its low group birefringence. To demonstrate the genuine broadband nature of the source, there can be performed quantum state tomography (QST) (
Moreover, the spectral property of the source can be characterized using a fiber spectrometer. Details of this type of spectral characterization can be found for example in Zhu, E. Y., et al. “Multi-party agile quantum key distribution network with a broadband fiber-based entangled source.” arXiv preprint arXiv:1506.03896 (2015). An exemplary spectrometer setup is shown in
To demonstrate the operation stability of the source, the coincidence-to-accidental (CAR) ratio can be continually measured over a 10 hour period without any additional control of environmental factors. The CAR measurement was performed at a pump power of ˜9 mW. At this pump power, the generation rate is so high that the detectors are saturated if L/C band splitter is used. The present invention preferably uses a set of narrower bandwidth spectral filters for demonstration, as shown in example
The performance of the source can be further improved. On one hand, because of the low-noise nature of the PPSF-based source, the generation rate can always be increased by higher power pumping, without losing entanglement quality. On the other hand, the in-source loss at present is large but can be greatly improved in the future. For example, the PMF-PPSF and PPSF-SMF splices contributes up to 1.5 dB loss each, while it can be further optimized to <0.5 dB. And the pump suppression filters bring in another 4 dB loss, which can be reduced by using better components. Last but not least, because of the weak group birefringence, it is possible and also convenient to cascade several PPSFs to increase spectral brightness of the source without losing its broadband spectral property. Connecting multiple PPSFs to the output of the pump can increase the brightness of the source.
It will be appreciated by one skilled in the art that variants can exist in the above-described arrangements and applications. For example, the device is not limited to a specific wavelength range, but rather can be designed to operate any many spectral bands. The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.
It is to be understood that the terms “including”, “comprising”, “consisting” and grammatical variants thereof do not preclude the addition of one or more components, features, steps, or integers or groups thereof and that the terms are to be construed as specifying components, features, steps or integers.
If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element.
It is to be understood that where the claims or specification refer to “a” or “an” element, such reference is not be construed that there is only one of that element.
It is to be understood that where the specification states that a component, feature, structure, or characteristic “may”, “might”, “can” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included.
Where applicable, although state diagrams, flow diagrams or both may be used to describe embodiments, the invention is not limited to those diagrams or to the corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described.
Methods of the present invention may be implemented by performing or completing manually, automatically, or a combination thereof, selected steps or tasks.
The term “method” may refer to manners, means, techniques and procedures for accomplishing a given task including, but not limited to, those manners, means, techniques and procedures either known to, or readily developed from known manners, means, techniques and procedures by practitioners of the art to which the invention belongs.
The term “at least” followed by a number is used herein to denote the start of a range beginning with that number (which may be a ranger having an upper limit or no upper limit, depending on the variable being defined). For example, “at least 1” means 1 or more than 1. The term “at most” followed by a number is used herein to denote the end of a range ending with that number (which may be a range having 1 or 0 as its lower limit, or a range having no lower limit, depending upon the variable being defined). For example, “at most 4” means 4 or less than 4, and “at most 40%” means 40% or less than 40%.
When, in this document, a range is given as “(a first number) to (a second number)” or “(a first number)-(a second number)”, this means a range whose lower limit is the first number and whose upper limit is the second number. For example, 25 to 100 should be interpreted to mean a range whose lower limit is 25 and whose upper limit is 100. Additionally, it should be noted that where a range is given, every possible subrange or interval within that range is also specifically intended unless the context indicates to the contrary. For example, if the specification indicates a range of 25 to 100 such range is also intended to include subranges such as 26-100, 27-100, etc., 25-99, 25-98, etc., as well as any other possible combination of lower and upper values within the stated range, e.g., 33-47, 60-97, 41-45, 28-96, etc. Note that integer range values have been used in this paragraph for purposes of illustration only and decimal and fractional values (e.g., 46.7-91.3) should also be understood to be intended as possible subrange endpoints unless specifically excluded.
It should be noted that where reference is made herein to a method comprising two or more defined steps, the defined steps can be carried out in any order or simultaneously (except where context excludes that possibility), and the method can also include one or more other steps which are carried out before any of the defined steps, between two of the defined steps, or after all of the defined steps (except where context excludes that possibility).
Further, it should be noted that terms of approximation (e.g., “about”, “substantially”, “approximately”, etc.) are to be interpreted according to their ordinary and customary meanings as used in the associated art unless indicated otherwise herein. Absent a specific definition within this disclosure, and absent ordinary and customary usage in the associated art, such terms should be interpreted to be plus or minus 10% of the base value.
Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While the inventive device has been described and illustrated herein by reference to certain preferred embodiments in relation to the drawings attached thereto, various changes and further modifications, apart from those shown or suggested herein, may be made therein by those of ordinary skill in the art, without departing from the spirit of the inventive concept the scope of which is to be determined by the following claims.
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| E. Y. Zhu, et al,, “Poled-fiber source of broadband polarization-entangled photon pairs”, Publisher: Opt. Lett. 38, pp. 4397-4400, 2013. |
| Zhu, et al., “Multi-party agile quantum key distribution network with a broadband fiber-based entangled source”, Publisher: arXiv preprint arXiv:1506.03896 (2015). |
| Zhu, Eric Y., et al., “Direct generation of polarization-entangled photon pairs in a poled fiber”, Publisher: Physical review letters 108.21 (2012): 213902. |
| Number | Date | Country | |
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| 20200041866 A1 | Feb 2020 | US |
