Ultra-high brightness narrow-linewidth photon source for generating entangled photon pairs or heralding single photons

Abstract

A system and method are provided for generating entangled photon pairs containing two photons having different wavelengths. The technique involves pumping an atomic vapor of an alkali species using a pump laser and a coupled laser configured to drive a spontaneous four-wave mixing process within the atomic vapor. The pump laser and the coupled laser are detuned with respect to atomic transitions of the alkali species, thereby increasing the brightness of the entangled photon pair source.

Description

Background Art

[0001] (Research funded by the federal government) This invention was made with government support under DE-SC0021556 awarded by the United States Department of Energy. The government has certain rights in this invention.

[0002] A quantum network enables the transmission of information in the form of qubits ( "qubits") between physically separated quantum processors or other quantum devices (e.g., quantum sensors). Quantum networks can be used to enable optical quantum communication over long distances and can be implemented via standard telecommunications optical fibers through the transmission of single photons in which information is encoded (e.g., in polarization).

Summary of the Invention

[0003] The following is a non-limiting summary of some embodiments of the present application. Some aspects of the present application relate to a photon source comprising an atomic vapor cell containing atoms of an alkali atomic species. The atomic vapor cell is disposed in the beam paths of a first laser beam and a second laser beam generated during operation of the photon source, and the alkali atomic species includes a first energy level, a second energy level, and a third energy level, the second energy level being at a higher energy than the first energy level, and the third energy level being at a higher energy than the second energy level. During operation of the photon source, the first laser beam has a first wavelength set with a first detuning for a first atomic transition in the alkali atomic species, the first atomic transition occurring between the first energy level and the second energy level, the second laser beam having a second wavelength set with a second detuning for a second atomic transition in the alkali atomic species, the second atomic transition occurring between the second energy level and the third energy level, and the second detuning being at least partially based on the first detuning.

[0004] In some embodiments, during the operation of the photon source, the first laser beam is arranged to copropagate through the atomic vapor cell together with the second laser beam. In some embodiments, the first and second wavelengths have values ​​set to satisfy the conditions for a four-wave mixing process in alkali atomic species.

[0005] In some embodiments, the first detuning is greater than the one-photon resonance Doppler spreading of the alkali atomic species and less than or equal to 2π × 10 GHz. In some embodiments, the first detuning is approximately 2π × 1 GHz.

[0006] In some embodiments, the first detuning is approximately 2π × 1.1 GHz. In some embodiments, the first detuning is approximately 2π × 1.15 GHz. In some embodiments, the first energy level is |5S 1 / 2 >This is the state, and the second energy level is |5P 3 / 2 >This is the state, and the third energy level is |6S 1 / 2 >This is the state.

[0007] In some embodiments, the first wavelength is approximately 780 nm. In some embodiments, the second detuning is a two-photon detuning that is greater than the double-resonance Doppler spreading of the alkali atomic species and less than or equal to 2π × 10 GHz.

[0008] In some embodiments, the second detuning is approximately 2π × 2GHz. In some embodiments, the second detuning is approximately 2π × 2.4 GHz. In some embodiments, the second wavelength is approximately 1367 nm.

[0009] In some embodiments, the first laser beam and / or the second laser beam operate at a power of approximately 5 mW. In some embodiments, the first laser beam and / or the second laser beam operate with a beam diameter of approximately 50 μm.

[0010] In some embodiments, during the operation of the photon source, the atomic vapor cell is configured to output dichromatic entangled photon pairs. In some embodiments, each dichromatic entangled photon pair comprises a first photon and a second photon, the first and second photons having different wavelengths.

[0011] In some embodiments, the first photon has a wavelength suitable for long-distance communication technology. In some embodiments, the first photon has a wavelength in the range of 1300 nm to 1600 nm.

[0012] In some embodiments, the first photon has a wavelength of approximately 1324 nm. In some embodiments, the first photon has a wavelength of approximately 1324 nm, 1367 nm, 1476 nm, or 1529 nm.

[0013] In some embodiments, the second photon has a near-infrared (NIR) wavelength. In some embodiments, the second photon has a wavelength in the range of 700 nm to 925 nm.

[0014] In some embodiments, the second photon has a wavelength of approximately 795 nm. In some embodiments, the second photon has a wavelength of approximately 795 nm or 780 nm.

[0015] In some embodiments, the photon source further comprises a first output and a second output, which are optically coupled to the output of an atomic vapor cell, respectively, such that first photons exit the photon source from the first output and second photons exit the photon source from the second output.

[0016] In some embodiments, the photon source further comprises a single-photon detector optically coupled to either the first or second output. In some embodiments, the photon source further comprises a Fabry-Perot etalon optically coupled between the output of the atomic vapor cell and a first output, and a single-photon detector optically coupled to a second output.

[0017] In some embodiments, the photon source further comprises a bandpass filter optically coupled between the output of the atomic vapor cell and a first output and / or a second output. In some embodiments, the photon source further comprises a liquid crystal phase difference plate optically coupled between the output of the atomic vapor cell and a second output.

[0018] In some embodiments, the photon source further comprises a first laser light source configured to generate a first laser beam and a second laser light source configured to generate a second laser beam.

[0019] In some embodiments, the alkali species includes rubidium. Some aspects of this application relate to a method for generating entangled photon pairs from an atomic vapor cell containing atoms of an alkali atomic species, wherein the alkali atomic species includes a first energy level, a second energy level having a higher energy than the first energy level, and a third energy level having a higher energy than the second energy level. The method includes the steps of generating a first laser beam using a first laser, wherein the first laser beam has a first wavelength set to a first detuning for a first atomic transition in an alkali species, the first atomic transition occurring between a first energy level and a second energy level; generating a second laser beam using a second laser, wherein the second laser beam has a second wavelength set to a second atomic transition in an alkali species, the second atomic transition occurring between a second energy level and a third energy level, the second detuning being at least partially based on the first detuning; causing a four-wave mixing process in atoms by guiding the first and second laser beams to pass through the same region of an atomic vapor cell; and generating entangled photon pairs as a result of the four-wave mixing process.

[0020] In some embodiments, guiding the first and second laser beams to pass through the same region of the atomic vapor cell includes co-propagating the first and second laser beams through the atomic vapor cell.

[0021] In some embodiments, the step of generating a first laser beam includes generating a first laser beam having a first wavelength set to a first detuning greater than the one-photon resonant Doppler spreading of an alkali atomic species and less than or equal to 2π × 10 GHz.

[0022] In some embodiments, generating a first laser beam includes generating a first laser beam having a first wavelength with a first detuning of about 2π × 1 GHz. In some embodiments, generating a first laser beam includes generating a first laser beam having a first wavelength with a first detuning of about 2π × 1.1 GHz.

[0023] In some embodiments, generating a first laser beam includes generating a first laser beam having a first wavelength with a first detuning of about 2π × 1.15 GHz.

[0024] In some embodiments, the first energy level is |5S 1 / 2 >This is the state, and the second energy level is |5P 3 / 2 >This is the state, and the third energy level is |6S 1 / 2 >This is the state.

[0025] In some embodiments, the step of generating a first laser beam having a first wavelength includes generating a first laser beam having a first wavelength of about 780 nm. In some embodiments, the step of generating a second laser beam includes generating a second laser beam having a second wavelength set to be two-photon detuning and greater than the double-resonance Doppler spreading of alkali atomic species and less than or equal to 2π × 10 GHz.

[0026] In some embodiments, the step of generating a second laser beam includes generating a second laser beam having a second wavelength set to a second detuning of about 2π × 2 GHz.

[0027] In some embodiments, the step of generating a second laser beam includes generating a second laser beam having a second wavelength set to a second detuning of about 2π × 2.4 GHz.

[0028] In some embodiments, the step of generating a second laser beam includes generating a second laser beam having a second wavelength of about 1367 nm. In some embodiments, the step of generating a first laser beam and / or a second laser beam includes generating a laser beam having a power of about 5 mW.

[0029] In some embodiments, the step of generating a first laser beam and / or a second laser beam includes generating a laser beam having a beam diameter of about 50 μm.

[0030] In some embodiments, the step of inducing a four-wave mixing process in an atom causes the atomic vapor cell to output dichromatic entangled photon pairs. In some embodiments, each dichromatic entangled photon pair comprises a first photon and a second photon, the first and second photons having different wavelengths.

[0031] In some embodiments, the first photon has a wavelength suitable for long-distance communication technology. In some embodiments, the first photon has a wavelength in the range of 1300 nm to 1600 nm.

[0032] In some embodiments, the first photon has a wavelength of approximately 1324 nm. In some embodiments, the first photon has a wavelength of approximately 1324 nm, 1367 nm, 1479 nm, or 1529 nm.

[0033] In some embodiments, the second photon has a near-infrared (NIR) wavelength. In some embodiments, the second photon has a wavelength in the range of 700 nm to 925 nm.

[0034] In some embodiments, the second photon has a wavelength of approximately 795 nm. In some embodiments, the second photon has a wavelength of approximately 795 nm or 780 nm.

[0035] In some embodiments, the method further includes the step of detecting a second photon using a single-photon detector such that the first photon is output as a heralded single photon.

[0036] In some embodiments, the method further includes the step of filtering the first photon using a Fabry-Perot etalon. [Brief explanation of the drawing]

[0037] The attached drawings are not intended to be drawn to a fixed scale. In the drawings, identical or nearly identical components shown in various figures are represented by similar numbers. For clarity, not all components are labeled in all drawings. The drawings are as follows: [Figure 1] This is a schematic diagram of a photon source 100 according to some embodiments of the technology described herein. [Figure 2] This is an exemplary energy level diagram for rubidium, according to some embodiments of the technology described herein. [Figure 3] This is a schematic diagram of a photon source 300 configured for rack mounting according to some embodiments of the technology described herein. [Figure 4] This figure shows a heatmap of heralding efficiency as a function of vapor cell temperature and two-photon detuning for some embodiments of the technology described herein. [Figure 5A] This plot shows the unweighted scattering probabilities simulated as a function of atomic velocities when the pump detuning is near and far from resonance, according to some embodiments of the technique described herein. [Figure 5B] This plot shows the weighted scattering probabilities simulated as a function of atomic velocity when the pump detuning is near and far from resonance, according to some embodiments of the technique described herein. [Figure 6A]This is a plot showing the signal-idler coincidence rate measured as a function of coupled power for various pump powers, according to some embodiments of the technology described herein. [Figure 6B] This is a plot showing the measured maximum value of the signal-idler cross-correlation as a function of the coincidence rate, according to some embodiments of the technique described herein. [Figure 7A] This figure shows the real part of the density matrix reconstruction of the maximum likelihood of a signal idler state according to some embodiments of the technique described herein. [Figure 7B] This figure shows the imaginary part of the density matrix reconstruction of the maximum likelihood of a signal idler state according to some embodiments of the technique described herein. [Figure 8A] This is a schematic diagram of a heralding single-photon source 800a according to some embodiments of the technology described herein. [Figure 8B] This is a schematic diagram of a heralding single-photon source 800b according to some embodiments of the technology described herein. [Figure 9A] This is a schematic diagram of a heralding single-photon source 900 according to some embodiments of the technology described herein. [Figure 9B] This is a plot showing the heralding efficiency, measured as a function of etalon frequency, according to some embodiments of the technique described herein. [Figure 10] This is a flowchart illustrating a process 1000 for generating entangled photon pairs according to several embodiments of the technique described herein. [Modes for carrying out the invention]

[0038] Techniques for generating high-luminosity entangled photon pairs and / or heralding single photons using a photon source are described herein. These techniques involve the use of two pump laser beams having large detuning for two atomic transitions of atomic species used to form an atomic vapor. The two pump laser beams interact with atoms in the atomic vapor and, via a four-wave mixing process, induce the generation of entangled photon pairs having two wavelengths emitted from the atomic vapor cell. The entangled photon pairs can then be used for a variety of applications, including the transmission of quantum information. Alternatively, the photon source may function as a heralding single photon source by including a detector configured to detect one of the photons of the generated entangled photon pair.

[0039] Quantum communication leverages the special properties of quantum mechanics to exponentially improve the encoding, processing, and transmission of information. Whether the ultimate goal is to connect quantum computers, perform ultra-precise sensing measurements, or form quantum-protected communication networks, all will depend on connecting heterogeneous quantum devices. Such devices are often inherently communicationally incompatible, for example, they may operate using different frequencies or spatial modes (e.g., transmitting in free space or over optical fibers). For instance, an atomic magnetometer (AM) uses rubidium (Rb) atoms to measure magnetic fields with better sensitivity than 10 fT / √Hz by detecting changes in the polarization state of the magnetic field interacting with the Rb atoms. Networking arrays of AMs with polarization-entangled photon sources is desirable to improve measurement sensitivity, as has been proposed for long-baseline telescopes based on quantum repeaters. Many applications require multiple sensors working together to address the problems of distributed sensing.

[0040] However, this remains difficult to achieve because AM and other atomic-based sensors typically operate at wavelengths of around 780–795 nm, instead of the 1300 nm and / or 1550 nm commonly used for optical fiber communications. The same challenges are faced by room-temperature quantum memories using rubidium atoms, as well as many other atomic technologies such as quantum simulators and photonic phase modulators. Communication between quantum devices becomes even more complex as quantum technologies move from static quantum devices to mobile quantum devices (e.g., airborne or seaborne quantum nodes). To efficiently connect these mobile nodes to each other, photons with near-infrared (NIR) wavelengths (780–795 nm) have been shown to have reduced atmospheric losses and disturbances compared to photons with wavelengths suitable for optical fibers (e.g., infrared wavelengths). There is a significant and unmet need for devices that enable cross-device frequency compatibility in quantum-secure networks.

[0041] The inventors have recognized that by using multiple pairs of atomic transitions in a specific set of atomic species, it is possible to generate multiple entangled photon pairs corresponding to multiple far-field communication wavelengths and multiple free-space communication wavelengths. Accordingly, the inventors have developed a photon source using warm atomic vapors of alkali atomic species (e.g., rubidium, cesium, or other suitable alkali atomic species). The generated entangled photon pairs contain entangled photons in polarization space, but each photon in the entangled photon pair has a different wavelength. For example, one photon in the entangled photon pair may have a near-infrared (NIR) wavelength, and the other photon may have an infrared wavelength (e.g., suitable for far-field communication applications). The photon source utilizes the process of spontaneous four-wave mixing (SFWM) in a room-temperature atomic vapor cell to convert photons from two classical pump fields (e.g., from first and second pump lasers) into pairs of single photons at two different wavelengths entangled with each other. The photon source may use well-characterized rubidium vapor atomic transitions to access a wide range of wavelengths, including, for example, the O, S, and C bands for long-distance communication for fiber transmission, as well as NIR for quantum buffering, processing, and sensing.

[0042] The inventors further recognized that by detuning the first and second pump lasers from the atomic transitions of the alkali atomic species by a large detuning value, the brightness of the photon source can be increased (e.g., by 1000 times). In some embodiments, the first pump laser that provides the pump field is detuned from the first atomic transition of the alkali atomic species (e.g., between the first excited state and the second excited state) by a large one-photon detuning. For example, the detuning of the first pump laser is greater than the resonant Doppler broadening of the alkali atomic species (e.g., approximately equal to or greater than 2π×1 GHz, 2π×1.1 GHz, or 2π×1.15 GHz, and less than or equal to 2π×10 GHz). The second pump laser that provides the coupling field is detuned from the second atomic transition (e.g., between the second excited state and the third excited state) by a large two-photon detuning. For example, the detuning of the second pump laser is greater than the double-resonant Doppler broadening of the alkali atomic species (e.g., approximately equal to or greater than 2π×2 GHz or 2π×2.4 GHz, and less than or equal to 2π×10 GHz).

[0043] Normally, when such a large wavelength difference exists between the pump field and the coupling field, the supplied laser beam is limited to atoms in a relatively small velocity class in the atomic vapor. As a result, few entangled photon pairs are generated, and the brightness is in the range of about 5×10 3 s -1 ~about 8×10 5 s -1 . However, the inventors further recognized and understood that by reducing the interaction region (e.g., by reducing the beam diameter of the pump laser to, for example, 50 μm), the photon pair generation rate can be increased, thereby increasing the intensity of the pump laser without increasing the input power of the pump laser beam, and the pump laser beam can be maintained at a relatively low power (e.g., 5 mW). Therefore, the inventors developed a 1-to-1 optical system design in which the atomic vapor cell is placed at the center of a telescope to reduce the beam diameter.

[0044] In addition, the inventors have found that by aligning multiple pump laser beams so that they copropage through the atomic vapor cell, the effective interaction volume between the atoms of the atomic vapor and the pump lasers is further increased, and the photon pair production rate is increased by 10 7 s -1 It can be further improved to this extent, |Φ + >It was recognized that the lower limit of Bell state fidelity exceeds 95%. Finally, the inventors recognized and understood that the photons generated by a photon source such as those described herein are output at a wavelength significantly different from that of the pump laser, and as a result, it is possible to filter the generated photon pairs from the pump photons using off-the-shelf optical components, thereby reducing manufacturing costs and manufacturing time caused by the long lead time required for the manufacture of dedicated optical equipment.

[0045] I. Entangled photon source Figure 1 is a schematic diagram of a photon source 100 according to several embodiments of the technology described herein. The photon source 100 is configured to receive a first laser beam 102 and a second laser beam 104 (for example, via a suitable input port). The first laser beam 102 and the second laser beam 104 may each have different wavelengths. For example, the first laser beam 102 and the second laser beam 104 may have wavelengths that are set to detune the first and second atomic transitions of atoms in atomic vapor stored in an atomic vapor cell 114 located in the beam paths of the first and second laser beams 102 and 104.

[0046] In some embodiments, the first and second laser beams 102 and 104 can be optically coupled to the atomic vapor cell 114 by one or more optical components. As shown in the example in Figure 1, the first laser beam 102 and the second laser beam 104 can pass through bandpass filters 106a and 106b, respectively. The first laser beam 102 and the second laser beam 104 can then be coupled by a dichroic mirror 108 so that they co-propagate along the same beam path. The co-propagating laser beams 102 and 104 can then pass through a polarizing beam splitter 110 and lens 112. The polarizing beam splitter 110 may be configured to horizontally polarize both the first laser beam 102 and the second laser beam 104.

[0047] In some embodiments, the lens 112 may be configured to reduce the beam diameter of the copropagating laser beams 102 and 104. For example, the lens 112 reduces the beam diameter of the copropagating laser beams 102 and 104 to approximately 20-50 μm (1 / e). 2 This could be an achromatic lens with a focal length f ≈ 50 mm configured to focus the beam to the beam diameter. In some alternative embodiments, the optical components between the input port of the photon source 100 and the atomic vapor cell 114 may be arranged in an alternative configuration (for example, the polarizing beam splitter 110 may be removed and an alternative polarizing beam splitter may be placed between the bandpass filters 106a, 106b and the dichroic mirror 108), and it should be understood that the embodiments of this technology are not limited in this respect.

[0048] In some embodiments, the atomic vapor cell 114 includes a housing that supports a magneto-optical trap configured to confine atoms of the atomic vapor within the atomic vapor cell 114. The housing of the atomic vapor cell 114 may be hermetically sealed to prevent contamination and may include one or more optically transparent windows to allow light to enter and / or exit the atomic vapor cell 114. The atomic vapor cell 114 may have a length of several millimeters (e.g., 5 mm). In some embodiments, the atomic vapor cell 114 may be configured to confine room-temperature atomic vapor or a cold atomic atmosphere. In embodiments where the atomic vapor cell 114 is configured to confine room-temperature atomic vapor, the atomic vapor cell 114 may be heated by one or more ceramic heaters thermally coupled to the atomic vapor cell 114. In some embodiments, the atomic vapor cell 114 may be surrounded by shielding (e.g., including mu-metal) to prevent magnetic and / or electric fields from affecting the atomic vapor within the atomic vapor cell 114.

[0049] In some embodiments, the atomic vapor cell 114 may contain atomic vapor containing atoms that, upon receiving a pumping field (e.g., a laser beam), absorb received photons of a certain frequency and, after a two-step excitation and decay process, re-emit photons having an entangled polarization state. For example, the atomic vapor cell 114 may contain rubidium (e.g., 87 Rb, 85 It may contain atomic vapor of Rb (or any other suitable isotope). Alternatively, in some embodiments, the atomic vapor cell 114 may contain atomic vapor of another alkali metal. For example, the alkali metal may be an isotope of cesium (e.g., 133 It may contain Cs or any other suitable isotope.

[0050] In some embodiments, atomic vapor may exhibit two-photon resonance, enabling the generation of photon pairs entangled at two desired wavelengths. For example, as shown in Figure 2, 87 Rb is |5S 1 / 2 >→|5P 3 / 2 >→|6S 1 / 2> exhibits a two-photon resonance (or four-wave mixing process) along the transition, which can generate entangled photons with wavelengths of approximately 795 nm and 1324 nm in response to excitation by receiving light with wavelengths of 780 nm and approximately 1367 nm. Alternatively, or additionally, 87 Two-photon resonance in Rb vapor is used to generate photon pairs with wavelengths of approximately 1367 nm and 780 nm, 1476 nm and 795 nm, and / or 1529 nm and 780 nm, thereby providing a flexible photon source that can be used to generate wavelength spectra in the NIR, O, C, and / or S bands. These particular bands have a wide range of applications across quantum communications and quantum computing. For example, wavelengths of 1324 nm, 1476 nm, and 1529 nm correspond to the O, S, and C long-range communication bands, respectively, and are suitable wavelengths for long-distance optical fiber communications. Furthermore, wavelengths of 795 nm and 780 nm are commonly used in quantum buffers and sensors.

[0051] This exemplary transition cycle provides little pathway for the photon to spontaneously decay to the ground state, resulting in a higher entanglement rate and less output of uncorrelated photons. It should be understood that other similar two-photon resonances may exist in other isotopes of rubidium or other atomic systems. As a further example, 87 Rb system and 133 Potential wavelengths of photon pairs that can be generated in Cs systems are provided in Tables 1 and 2, respectively. These additional exemplary wavelengths can be used to interface with several Rydberg and ion technologies, such as neutral quantum computers and sensors.

[0052] [Table 1]

[0053] [Table 2]

[0054] The specific examples of laser wavelength pairs and input laser wavelength pairs provided herein are not the only wavelengths that can be used or generated, as the embodiments of the technology described herein are not limited in this respect. For example, in some embodiments, the laser wavelength pairs and / or entangled photon wavelength pairs may be in the ranges of 700 nm to 925 nm and 1300 nm to 1600 nm. Any suitable wavelength corresponding to the desired atomic transition energy of an atomic species can be selected from these ranges.

[0055] Large one-photon and two-photon detuning of the first and second laser beams 102 and 104 are performed so that the photon source exhibits a desired brightness value. The lower limit of two-photon detuning is determined by the velocity of atoms in the atomic vapor cell 114 at different temperatures. The atomic velocity is,

[0056]

number

[0057] It is excited resonantly. Here, c is the speed of light, δ is the two-photon detuning, and ω is the double excited state (for example, |6S in the example in Figure 2). 1 / 2 >This is the energy of the state. Ideally, this velocity is equal to the intrinsic atomic velocity v p

[0058]

number

[0059] It is sufficiently larger than , where m is the atomic mass of the atom in the atomic vapor. Alternatively, δ is

[0060]

number

[0061] It is desirable to have a value of [value]. In the example shown in Figure 2, δ >> 2π × 2GHz, as explained below. Although this technique is presented for a specific diamond-type two-photon scheme shown in Figure 2, it should be understood that it is generally applicable to atomic species that exhibit diamond-type two-photon excitations.

[0062] In some embodiments, the first and second laser beams 102 and 104 can be set to have a large detuning with respect to atomic transitions of atomic species in atomic vapor. In some embodiments, the detuning of the first laser beam 102 is greater than the resonant Doppler spreading of the atomic species. For example, the detuning of the first laser beam 102 may be approximately equal to or greater than 2π × 1 GHz, 2π × 1.1 GHz, or 2π × 1.15 GHz, and less than or equal to 2π × 10 GHz. As shown in the example in Figure 2, the first laser beam 102 is set such that the detuning amount Δt is approximately 2π × 1.1 GHz. 85 Rb | 5S 1 / 2 The first energy levels 202 and |5P in the F=3 state 3 / 2 The transition between the second energy level 204 in the state F'=4> can be frequency stabilized. That is, the first laser beam 102, 85 Rb | 5S 1 / 2 ,F=2>→|5P 3 / 2 From the transition F'=3>, detuning to the blue side is possible.

[0063] In some embodiments, the second laser beam 104 passes through a second energy level 204 and a third energy level 206 (e.g., |6S). 1 / 2 Atomic transitions between states and the first detuning Δ can be set. The detuning of the second laser beam 104 can be controlled, for example, using an electro-optic modulator (EOM, not shown). In some embodiments, the detuning of the second laser beam 104 is greater than the double-resonance Doppler spreading of the atomic species. For example, the detuning of the second laser beam 104 may be approximately equal to or greater than 2π × 2 GHz or 2π × 2.4 GHz, and less than or equal to 2π × 10 GHz.

[0064] By setting the first laser beam 102 and the second laser beam 104 to have detuning with respect to atomic transitions of atomic species in the atomic vapor cell 114, corresponding two-photon resonances can be used to generate entangled photon pairs during the decay of excited atomic states. As shown in the example in Figure 2, in some embodiments, the decay of excited atomic states is |5P 1 / 2 >Through state 208, an entangled photon pair can be obtained having a signal photon with a wavelength of approximately 1324 nm and an idler photon with a wavelength of approximately 795 nm.

[0065] Returning to Figure 1, the generated entangled photon pair can be coupled outside the atomic vapor cell 114 by another lens 116 positioned mirror-image inverted relative to lens 112 (for example, lens 116 may be an achromatic lens with a focal length f ≈ 50 mm). Both photons of the generated photon pair are coupled to an equivalent 1 / e² of approximately 20 μm. 2 The beams may then be coupled to a collimator (not shown) having a mode radius. In some embodiments, the generated photon pairs and the first and second laser beams 102, 104 may then pass through a dichroic mirror 118 configured to separate the two photons of the generated photon pairs (for example, based on wavelength).

[0066] In some embodiments, the first photon of the generated photon pair may be deflected in the direction of the output 122 by a dichroic mirror 118. The first photon may be a photon of a photon pair having a wavelength suitable for long-distance communication applications (e.g., in the range of 1300-1600 nm). Before being emitted from the photon source 100, the first photon may further pass through a bandpass filter 120a configured to filter out photons from the first and second laser beams 102, 104 so that only the first photon reaches the output 122.

[0067] In some embodiments, the second photon of a generated photon pair may pass through a liquid retardation plate 124 after passing through a dichroic mirror 118, before being guided towards the output 128 by a mirror 126. The second photon may be, for example, a photon of a photon pair having a near-infrared wavelength (e.g., in the range of 700-925 nm). The liquid retardation plate 124 may be positioned such that its retardation axis is perpendicular (e.g., out of the plane of the paper in Figure 1). By adjusting the retardation amount of the liquid retardation plate 124, any phase shift between the |H> and |V> polarization modes can be applied to the second photon. In some embodiments, a bandpass filter 120b may be optically coupled between the liquid retardation plate 124 and the output 128. The bandpass filter 120b may be configured to filter photons from the first and second laser beams 102, 104 so that only the second photon reaches the output 128.

[0068] In some embodiments, additional optical components not shown in the example of Figure 1 may be placed between the dichroic mirror 118 and the output 122, or between the liquid crystal phase difference plate 124 and the output 128. For example, one or more of a quarter-wave plate, a half-wave plate, and / or a polarizing beam splitter may be optically coupled along the beam path between the dichroic mirror 118 and the output 122, or along the beam path between the liquid crystal phase difference plate 124 and the output 128. These additional optical components may be used, for example, to select the generated photons of the polarization mode to be detected.

[0069] Although the example in Figure 1 shows an unmultiplexed photon source 100, it should be understood that the multiplexing of the photon source 100 can be achieved by splitting the first laser beam 102 and the second laser beam 104 into multiple paths (e.g., on a two-dimensional or three-dimensional grid), and the embodiments of this technology are not limited in this respect. This splitting can be achieved, for example, to simultaneously excite many small-sized atomic vapor cells or within a hollow core fiber. The multiplexed photon output can then be collected into different optical fiber outputs.

[0070] The inventors have further recognized and understood that miniaturization of the photon source (e.g., photon source 100) is advantageous for improved mass production, reduced form factor, and ease of compatibility and integration with existing long-distance communication infrastructure. As an example, Figure 3 shows an exemplary diagram of a photon source 300 arranged in a rack-mount housing according to several embodiments described herein. The rack-mount housing of Figure 3 has a footprint size of 6 inches (15.24 cm) × 15 inches (38.1 cm) × 2 inches (5.08 cm) or less, ensuring that the module can be used in a variety of locations and experiments, and can be integrated into a deployable rack-mount unit.

[0071] In some embodiments, the photon source 300 may include inputs 302 that can be optically coupled to first and second lasers, which may be located outside the photon source 300. For example, the first and second lasers may be located in a separate part of a rack-mount unit and optically coupled to the photon source 300 by optical fibers. Alternatively, the first and second lasers may be part of the photon source 300 and located within a rack-mount housing.

[0072] In some embodiments, the photon source 300 can receive first and second laser beams from the first and second lasers. The first and second laser beams can pass through the optical fiber spool 303 and be coupled to a free-space optical system by the fiber collimator 325. The first and second laser beams can be output from the fiber collimator 325 to a bandpass filter 306 (e.g., equivalent to the bandpass filters 106a and 106b described in relation to Figure 1).

[0073] In some embodiments, the first and second laser beams may be guided toward a dichroic mirror 308 by one or more mirrors 305. The first and second laser beams may also be arranged to copropagate along the same beam path by one or more mirrors 305 and / or the dichroic mirror 308. After passing through the dichroic mirror 308, the first and second laser beams may pass through a polarizer 310 before being incident on a telescope formed by lenses 312 and 316, with the atomic vapor cell 314 positioned at the center of the telescopic optical system. Lens 312 may be configured to reduce the beam diameter of the first and second laser beams before they are incident on the atomic vapor cell 314 (for example, as described in relation to lens 112 in the example of Figure 1).

[0074] In some embodiments, the first and second laser beams may induce a four-wave mixing process within the atomic vapor cell 314, as described above. The four-wave mixing process allows entangled photon pairs to be emitted from the atomic vapor cell 314 along with the photons of the first and second laser beams. The photons emitted from the atomic vapor cell 314 may then pass through a lens 316, which may be configured to symmetrically widen the beam diameter.

[0075] In some embodiments, photons emitted from the atomic vapor cell 314 may first pass through a dichroic mirror 318 configured to split entangled photon pairs having different wavelengths. The first photon of the entangled photon pair may then pass through a half-wave plate 319 and a bandpass filter 320 configured to filter out the remaining photons from the first and / or second laser beams. The second photon of the entangled photon pair may, guided (e.g. by another mirror), pass through a liquid crystal phase difference plate 324, and then through a separate half-wave plate 319 and bandpass filter 320. After exiting the bandpass filter 320, the first and second photons may be recombined into an optical fiber by a fiber collimator 325. The first and second photons may then pass through a separate fiber spool 303 and be emitted from the photon source 300 via the optical fiber output.

[0076] The inventors have recognized and understood that detuning of the pump laser beam has a significant effect on the operating parameters of the photon source (e.g., photon sources 100 and / or 300 as described herein). The heralding efficiency of the photon sources described herein is the probability of detecting one photon of a generated photon pair when detecting the other photon of the generated photon pair. Figure 4 is a heatmap of the heralding efficiency as a function of vapor cell temperature and two-photon detuning for several embodiments of the technology described herein. In Figure 4, the heralding efficiency is the probability of detecting a 795 nm photon when detecting a 1324 nm photon.

[0077] To obtain the data in Figure 4, the pump power was fixed at approximately 250 μW, and the coupling power was varied to maintain a detection signal rate of approximately 100 kcps. The two-photon detuning δ was varied using EOM in an offset dual resonance optical pumping (DROP) lock. Varying the vapor cell temperature changed the optical thickness of the atomic vapor in the vapor cell. Only the |VV> mode of the photon source was measured. Figure 4 shows that, for a fixed atomic temperature, the heralding efficiency can be increased by increasing the two-photon detuning.

[0078] This behavior can be understood by analyzing the three-level Hamiltonian related to the excitation process under the rotational wave approximation when the rubidium atom has velocity v. 1 / 2 >,|5P 3 / 2 >,|6S 1 / 2 The Hamiltonian in >} is as follows:

[0079]

number

[0080] Here, Ω p and Ω c These are the Rabi frequencies for the pump and coupling, respectively. |6S 1 / 2 >→|5P 1 / 2 >→|5S 1 / 2 The transition is |6S 1 / 2 >→|5P 3 / 2 >and|5P 3 / 2 >→|5S 1 / 2 In addition to the collapse channel, effectively |6S 1 / 2 >→|5S 1 / 2 It is being treated as a collapse.

[0081] Atomic motion can be treated as one-dimensional along the propagation of the pump and coupled laser beam. Due to large single-photon detuning, |5P 3 / 2>The state is avoided from being significantly occupied. Next, the stationary state of the Liouvilian associated with the reduced three-level system can be solved numerically. |6S 1 / 2 >→|5P 1 / 2 The decay rate of > is |6S 1 / 2 Since this decay rate is proportional to the steady-state occupancy rate, it can be used as a substitute for the signal photon scattering probability.

[0082] Figure 5A shows two examples of unweighted scattering probabilities numerically simulated as a function of atomic velocity according to several embodiments of the technique described herein. Curve 502 is the unweighted scattering probability for a small detuning of 2π × 500 MHz, and curve 504 is the unweighted scattering probability for a large detuning of 2π × 2000 MHz. Both curves 502 and 504 show sharp resonance peaks at v ≈ 250 m / s and v ≈ 1000 m / s, respectively, which are due to the two-photon Doppler shift.

[0083]

number

[0084] It appears in the scattering probability expressed as ω. |6S1 / 2> is, |6S 1 / 2 >This is the energy of the state. Curves 502 and 504 show |5P at v≈-1000m / s. 3 / 2 Two broader, less pronounced peaks are also observed, resulting from the resonant excitation of the state.

[0085] To determine the scattering probability in atomic vapor, as shown in Figure 5B, |6S 1 / 2> is weighted according to a Maxwell-Boltzmann distribution. Curve 506 is the weighted scattering probability for small detuning of 2π × 500 MHz, and curve 508 is the weighted scattering probability for large detuning of 2π × 2000 MHz. For small detuning, a significant proportion of the population is in the resonance velocity class, resulting in sharp features in the figure. However, for large detuning, the proportion of the population in the resonance velocity is almost negligibly small, and the unweighted scattering probability for this process is also significantly smaller than that for resonant scattering. Nevertheless, the majority of scattering occurs outside the resonance. Furthermore, as shown in Figure 5B, curve 508 follows a Maxwell-Boltzmann distribution. In the simulations in both Figures 5A and 5B, the pump detuning is fixed at a position far outside the resonance (Δ / 2π = 1150 MHz), and the Rabi frequencies of the pump and coupling are set to experimentally appropriate values ​​(Ω p / 2π=Ω c The frequency ( / 2π = 350 MHz) was maintained, and the temperature was fixed at T = 80°C.

[0086] The behavior shown in Figures 5A and 5B can be understood by considering the collective excitation projected onto the atomic system when the signal photon is detected. The likelihood of phase-matched emission of idlers is proportional to the number of atoms involved in the collective excitation. Assuming that the collective excitation has a distribution similar to that of the weighted scattering probability distribution, for a fixed atomic density, phase-matched emission of idlers is expected to occur more frequently in out-of-resonance excitations than in near-resonance excitations. Therefore, as shown in Figure 4, for a given vapor temperature, a higher heralding efficiency is expected when the photon source operates in the far out-of-resonance region.

[0087] From the theoretical model above, it is expected that the photon source behavior will be symmetric with respect to zero-two-photon detuning. However, Figure 4 shows that this is not the case. This unexpected behavior is due to the pump laser in positive two-photon detuning and approximately 99% 87 In atomic vapor having Rb purity 85This may be due to unwanted interactions with Rb impurities. Furthermore, the heralding efficiency is expected to saturate at a constant value in large two-photon detuning. However, Figure 4 shows a clear peak in the heralding efficiency as the two-photon detuning increases, which is related to the increase in coupling power used to perform large two-photon detuning.

[0088] In addition to the aforementioned trends in heralding efficiency when two-photon detuning is changed, as shown in Figure 4, when two-photon detuning is fixed, a peak in heralding efficiency is observed when the vapor cell temperature is changed. This phenomenon is due to competing processes: an increase in directional aggregate emission and a decrease in idler photon transmittance as the vapor temperature, and consequently the optical depth (OD), increases.

[0089] Next, the scaling characteristics of the photon source with coupled power and pump power are investigated. In the measurements used to generate Figures 6A to 7B, the photon source was operated with two-photon detuning at δ / 2π ≈ -2400 MHz and a spectroscopically measured atomic vapor cell temperature of approximately 93°C, and the measured maximum heralding efficiency was approximately 16% (or approximately 24% when corrected for idler detection efficiency). This value of maximum heralding efficiency is comparable to the detector-corrected heralding efficiency observed for a ladder pair photon source that is almost unaffected by the Doppler effect. The OD at resonance of the atomic vapor cell for idler photons is approximately 9. Measurements were acquired only for the |VV> mode of the photon source.

[0090] Figure 6A shows the measured signal-idler coincidence rate as a function of coupled power for various pump powers, according to several embodiments of the technology described herein. Figure 6A includes curves 602, 604, 606, and 608, which are curves fitted for measured coincidence rates as a function of coupled power for pump powers of 0.25 mW, 0.50 mW, 0.75 mW, and 1.00 mW, respectively. For low pump power and low coupled power, Figure 6A shows that the coincidence rate scales almost linearly as a function of power, and the measured scaling constant is approximately 3 × 10⁻⁶. 5 / s / mW 2 Either, or, considering detection efficiency, approximately 6 × 10 5 / s / mW 2 At high power, saturation of the coincidence rate is observed with increasing power. This saturation is partly due to a finite dead time for both the signal detector and the idler detector, which is approximately 20 ns. This dead time is taken into account in the fitting used to generate curves 602–608. However, even when these measurements are repeated using neutral density filters on both signal and idler paths, deviations from linearity at higher coupling powers are still observed. This deviation is due to saturation of the atomic medium.

[0091] As with other photon sources, as the pair generation rate increases, the signal-idler cross-correlation function g si It is expected to decrease. g relative to the measured clock values si The scaling of the maximum value is shown in Figure 6B, which includes data point 610 and fitting curve 612. Furthermore, a typical g si The value 614 is shown as an inset, and the value 614 is plotted as a function of the delay time τ between detections. Theoretically, g si ∝ 1 / concurrent count, and this inverse proportional scaling is observed for low concurrent count rates. However, for high concurrent count rates, g siThe behavior deviates from the expected inverse scaling. This deviation is due to a finite dead time in the detector, because this deviation disappears when measurements are repeated using attenuation filters on the signal arm and idler arm. Detector saturation is taken into account in the fitting used when generating curve 612.

[0092] When the maximum power for the pump beam and the coupled beam is 1 mW and 20 mW, respectively, the measured |VV>concurrency rate is approximately 1.7 × 10⁻⁶. 6 s -1 g si It is approximately 40. After correcting for the detector's detection efficiency and dead time saturation, this coincidence count rate is approximately 5 × 10⁻⁶. 6 s -1 This corresponds to the |VV> mode rate.

[0093] g si From the value 614, the two-photon linewidth after deconvolving the detector's finite response time is estimated to be less than 2π × 1 GHz. This estimated two-photon linewidth is similar to that observed in a ladder-type four-wave mixed system in which all velocity classes of atoms in the atomic vapor participate in collective excitation. The two-photon bandwidth is due to the convolution of Doppler broadening radiation and idler absorption as the idler propagates through the atomic vapor cell. Bandwidth variability due to OD is similarly expected. This observed bandwidth is equivalent to the bandwidth demonstrated by room-temperature atomic quantum memory, indicating that the photon sources described herein are suitable for all room-temperature atomic-based long-range compatible quantum repeaters.

[0094] According to the pumping method and rubidium Zeeman structure described herein, the photon source is |Φ +It is expected that entangled pairs of >= 1 / √2·(|HH>+|VV>) will be generated. Dichroic mirrors used to separate the signal photon from the idler photon (e.g., dichroic mirrors 118 and / or 318 as described herein), and mirrors used to couple the signal photon and idler photon to their respective optical fibers, introduce an arbitrary but stable phase shift between the |HH> and |VV> modes. The lagging amount of the liquid crystal phase difference plates (e.g., liquid crystal phase difference plates 124 and / or 324 as described herein) can be adjusted to compensate for these phase shifts, thereby |Φ + >The state can be restored.

[0095] To verify the entangled states generated after this operation, two-photon tomography is performed. To ensure that detector saturation does not affect the tomography, an attenuation filter with an OD of approximately 1 is used in both the signal path and the idler path. Using the maximum likelihood method, the density matrix of the two-photon states is reconstructed and shown in Figures 7A and 7B. Figure 7A shows the real part of the density matrix, and Figure 7B shows the imaginary part of the density matrix. From the reconstructed density matrix, 10 7 s -1 For entangled photon pair rates exceeding |Φ + The lower limit of fidelity to the Bell state is set to 95%.

[0096] II. Heralding single-photon source The inventors have further recognized and understood that the photon sources described herein (e.g., photon source 100 and / or 300) can be converted into heralding single-photon sources by placing a single-photon detector at one of the two outputs (e.g., output 122 or 128 as described in relation to Figure 1). Figures 8A and 8B are schematic diagrams of heralding single-photon sources 800a and 800b according to several embodiments described herein, respectively. Heralding single-photon sources 800a and 800b are similar to photon source 100 and operate with the same large detuning as described in relation to Figure 1. However, heralding single-photon sources 800a and 800b include a single-photon detector 802 or 804 instead of output 122 or 128, respectively. For example, the heralding single-photon source 800a in Figure 8A includes a single-photon detector 802 instead of output 122, and the heralding single-photon source 800a functions as a heralding single-photon source at NIR wavelengths (e.g., in the range of 700 nm to 925 nm, or in some embodiments, about 780 nm or 795 nm). Alternatively, the heralding single-photon source 800b in Figure 8B includes a single-photon detector 804 instead of output 128, and the heralding single-photon source 800b functions as a heralding single-photon source at far-field communication wavelengths (e.g., in the range of 1300 nm to 1600 nm, or in some embodiments, about 1324 nm, 1367 nm, 1479 nm or 1529 nm). It should be understood that the heralding single-photon sources 800a and 800b may, in some embodiments, be configured to generate a single heralding photon at any wavelength that may be relevant to alkali atomic species.

[0097] Probabilistic heralding photon sources are generally

[0098]

number

[0099] Heralding g that scales like this (2) It has g (2)θ is the degree of second-order coherence, η is the detection efficiency of the heralding photon, and n is the count as a function of time t. In order to be classified as a single photon,

[0100]

number

[0101] g (2) This must be less than 0.5. This corresponds to a rate of approximately 200 MHz for a bandwidth of approximately 1 GHz. Therefore, the photon source described herein, when operating as a heralding single-photon source, will have a rate of 2 × 10⁻¹⁶ per second. 8 It can generate individual single photons.

[0102] The inventors have recognized that the heralding efficiency of the heralding single-photon source described herein can be improved by adding filtering before the output of the single photon. Due to absorption in lower atomic transitions, there are single photons from higher atomic transitions that are not paired with photons produced by the lower atomic transitions. Therefore, the heralding efficiency of the heralding single-photon source can be improved by placing a filter before the output of the photon derived from the higher atomic transition.

[0103] Figure 9A is a schematic diagram of an example of a filtered heralding single-photon source 900 according to several embodiments of the technology described herein. The filtered heralding single-photon source 900 is similar to the heralding single-photon source 800b in Figure 8B, but further includes a filter 902 positioned along the optical path between the dichroic mirror 118 and the output 122. The filter 902 may be, for example, an etalon (e.g., a Fabry-Perot etalon).

[0104] Figure 9B is a plot showing the measured heralding efficiency values ​​904 for photons on lower atomic transitions of an atomic species (e.g., idler photons) as a function of the frequency of a filter (e.g., filter 902 in Figure 9A) placed on photons on higher atomic transitions of an atomic species (e.g., signal photons). Line 906 shows the heralding efficiency when no filter is present. As shown in Figure 9B, the heralding efficiency can be improved by filtering out unpaired single photons from the single-photon output.

[0105] The inventors have further recognized that, when a photon source operates as a heralding single-photon source, the photon source may be implemented using multiplexing so that the heralding single-photon source operates as multiple separate photon sources. Multiplexing may be implemented in one or more ways. A simple multiplexing scheme is based on entanglement with the intrinsic polarization of entangled photon pairs. That is, heralding is performed separately for |H> polarization and |V> polarization photon pairs to provide two multiplexed channels.

[0106] Alternatively or additionally, in some embodiments, time multiplexing may be implemented. To implement time multiplexing, the photon source may be repeatedly pumped and photons generated from higher atomic transitions (e.g., far-field photons) may be delayed. For example, photons generated from higher atomic transitions may be delayed using optical fibers of additional length. While the far-field photons are being delayed, a heralding click on the heralding in a given time mode is used to indicate that an NIR photon has been detected, and an optical switch may be used to select and transmit the appropriate far-field photon.

[0107] Alternatively or additionally, in some embodiments, spatial multiplexing may be implemented. Spatial multiplexing may be implemented to simultaneously generate entangled photon pairs using multiple photon sources (e.g., within the same atomic vapor cell or multiple atomic vapor cells). The long-distance communication photons can then be transmitted out of the photon source from the spatial mode associated with the received heralding click.

[0108] In some embodiments, multiple multiplexing techniques can be combined. For example, 10 multiplexing channels may be realized using polarization multiplexing and five time modes. In another example, time, frequency, polarization, and spatial multiplexing techniques may be performed simultaneously to multiplex in a four-dimensional state space, resulting in the generation of more than 1000 multiplexed states using the same photon source.

[0109] In some embodiments, time, frequency, polarization, and spatial multiplexing techniques may be performed simultaneously to generate multiple photon pairs from the same atom, creating a deterministic heralding single-photon source for applications in quantum computing and secure communications.

[0110] Alternatively or additionally, to reduce multiplexing, photon-resolved detectors may be used in place of single-photon detectors in Heralding single-photon sources (e.g., Heralding single-photon sources 800a, 800b, and / or 900). Photon-resolved detectors can reduce the number of multiplexed states required for the application by allowing the passage of p1 events and the filtering of p2 events.

[0111] Figure 10 is a flowchart illustrating a process 1000 for generating entangled photon pairs according to several embodiments of the technique described herein. Process 1000 may be performed using any one of the photon sources 100 and / or 300 described herein, and / or optionally any one of the heralding single-photon sources 800a, 800b, or 900. Process 1000 may begin with operation 1010, in which a first laser beam is generated using a first laser. The first laser beam has a first wavelength (for example, 780 nm or 795 nm in some embodiments).

[0112] In some embodiments, the first laser beam is set to a first detuning for the first atomic transition of an alkali atomic species contained in the atomic vapor cell. The alkali atomic species are, for example,87 Rb, 133 Cs, or any suitable alkali atomic isotope. The first atomic transition can occur between a first energy level and a second energy level of an atomic species, and the second energy level is higher than the first energy level. As one non-limiting example, in some embodiments, 87 the |5P 1 / 2 > state and the |5P 3 / 2 > state of Rb can occur.

[0113] In some embodiments, the first detuning for the first atomic transition can be a one-photon detuning Δ as described herein. The first laser beam can be generated at a first detuning that is greater than the one-photon resonance Doppler broadening of the alkali atomic species and less than or equal to 2π×10 GHz. As some non-limiting examples, in some embodiments, the first detuning can be about 2π×1 GHz, about 2π×1.1 GHz, or about 2π×1.15 GHz.

[0114] In some embodiments, after operation 1010, process 1000 proceeds to operation 1020, and a second laser beam is generated using a second laser. The second laser beam has a second wavelength (e.g., 1324 nm or 1367 nm in some embodiments). The second laser beam can be set at a second detuning with respect to a second atomic transition and the first detuning of the alkali atomic species contained within the atomic vapor cell. In some embodiments, the second atomic transition occurs between a second energy level and a third energy level of the alkali atomic species. As one non-limiting example, the second atomic transition can be 87 between the |5P 3 / 2 > state and the |6S 1 / 2 > state of Rb.

[0115] In some embodiments, the second detuning may be two-photon detuning. The second detuning may also be greater than the double-resonance Doppler spreading of the alkali species and less than or equal to 2π × 10 GHz. In some non-limiting examples, the second detuning may be about 2π × 2 GHz or about 2π × 2.4 GHz.

[0116] In some embodiments, generating a first laser beam in operation 1010 and / or generating a second laser beam in operation 1020 may include generating a first laser beam and / or a second laser beam having a power of about 5 mW. Alternatively or additionally, generating a first laser beam in operation 1010 and / or generating a second laser beam in operation 1020 may include generating a first laser beam and / or a second laser beam having a beam diameter of about 20 to 50 μm (for example, in some embodiments, the beam diameter is reduced within the volume of the atomic vapor cell).

[0117] In some embodiments, after operation 1020, process 1000 proceeds to operation 1030, in which a four-wave mixing process occurs in the atoms of the atomic vapor cell by guiding the first and second laser beams to pass through the same region of the atomic vapor cell. The first and second laser beams may be guided by, for example, one or more mirrors, dichroic mirrors, bandpass filters, and / or lenses. In some embodiments, guiding the first and second laser beams to pass through the same region of the atomic vapor cell includes co-propagating the first and second laser beams through the atomic vapor cell (for example, so that the first and second laser beams overlap within the same volume in the atomic vapor cell).

[0118] In some embodiments, after operation 1030, process 1000 may proceed to operation 1040, which may generate entangled photon pairs as a result of a four-wave mixing process. The entangled photon pair may be a dichromatic entangled photon pair such that the photon pair includes a first photon having a first wavelength and a second photon having a second wavelength. For example, the first photon may have a wavelength suitable for long-distance communication technology (e.g., in the range of 1300 nm to 1600 nm, such as about 1324 nm, 1367 nm, 1479 nm, and / or 1529 nm). The second photon may have a wavelength that is an NIR wavelength (e.g., in the range of 700 nm to 925 nm, or about 795 nm or about 780 nm).

[0119] In some embodiments, after operation 1040, process 1000 optionally proceeds to operation 1050, in which one of the entangled photon pairs is detected to generate a heralding single photon (for example, as described in relation to the heralding single-photon sources 800a, 800b, and / or 900 described herein). The detected photon may, in some embodiments, be detected using, for example, a single-photon detector. In some embodiments, the other photon of the entangled photon pair (e.g., the undetected photon) may be filtered using, for example, an etalon (e.g., a Fabry-Perot etalon) before being output from the heralding single-photon source.

[0120] The various embodiments described above can be used individually, in combination, or in various configurations not specifically described in the embodiments above, and therefore, in their application, are not limited to the details and configurations of the components described or shown in the drawings above. For example, an embodiment described in one embodiment can be combined in any way with an embodiment described in another embodiment.

[0121] Although some aspects and embodiments of the technology described in this disclosure have been thus described, it should be understood that various changes, modifications, and improvements will readily occur to those skilled in the art. Such changes, modifications, and improvements are intended to be within the spirit and scope of the technology described herein. For example, those skilled in the art will readily envision various other means and / or structures for performing the functions described herein and / or for obtaining one or more of the results and / or advantages, and each such variation and / or modification is considered to be within the scope of the embodiments described herein. Those skilled in the art will be able to recognize or confirm many equivalents to the specific embodiments described herein using only routine experimentation. Accordingly, the foregoing embodiments are presented by way of example only, and it should be understood that within the scope of the appended claims and their equivalents, embodiments of the present invention may be implemented in a manner different from that specifically described. Additionally, any combination of two or more of the features, systems, articles, materials, kits, and / or methods described herein is included within the scope of this disclosure if such features, systems, articles, materials, kits, and / or methods are not mutually inconsistent.

[0122] All definitions defined and used herein are to be understood as taking precedence over dictionary definitions, definitions in incorporated documents by reference, and / or ordinary meanings of defined terms.

[0123] The indefinite articles "a" and "an" used in this specification and the claims are to be understood to mean "at least one" unless explicitly indicated to the contrary. The use of "coupled" or "connected" means referring to elements or signals that are directly linked to each other or linked through intermediate components. Elements that are not "coupled" or "connected" are "separated" or "disconnected".

[0124] The use of "between" in a coupled signal chain does not imply a specific direction of signal flow in the signal chain unless otherwise stated. For example, if element B is described as coupled between element A and element C in a signal chain, then, unless otherwise stated, the signal may flow from element A to element C through element B, and / or from element C to element A through element B.

[0125] As used herein and in the claims, the phrase “and / or” should be understood to mean “either or both” of the elements thus combined, that is, elements that exist associatively in some cases and disjunctively in others. Multiple elements enumerated by “and / or” should be interpreted similarly, that is, “one or more” of the elements thus combined. Other elements other than those specifically identified by the “and / or” clause may exist at will, whether related to or unrelated to those specifically identified elements. Thus, as a non-restrictive example, a reference to “A and / or B” when used in conjunction with open-ended language such as “comprising” may, in one embodiment, refer to A only (optionally including elements other than B), in another embodiment, refer to B only (optionally including elements other than A), in yet another embodiment, refer to both A and B (optionally including other elements), and so on.

[0126] As used herein and in the claims, the phrase “at least one” with respect to a list of one or more elements means at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and all elements specifically enumerated in the list of elements, nor excluding any combination of elements in the list of elements. This definition also allows for the optional presence of elements other than those specifically identified in the list of elements to which the phrase “at least one” refers, whether related to or unrelated to those specifically identified elements. Therefore, as a non-limiting example, "at least one of A and B" (or equivalently, "at least one of A or B," or equivalently, "at least one of A and / or B") may mean, in one embodiment, at least one optionally comprising two or more A's and no B (and optionally comprising elements other than B); in another embodiment, at least one optionally comprising two or more B's and no A (and optionally comprising elements other than A); and in yet another embodiment, at least one optionally comprising two or more A's and at least one optionally comprising two or more B's (and optionally comprising other elements).

[0127] In the claims and the above specification, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” and “composed of” should be understood to be open-ended, meaning that they include but are not limited to. Only the transitional phrases “consist of” and “essentially consist of” are closed or semi-closed transitional phrases, respectively.

[0128] The terms “approximately” and “about” may be used in some embodiments to mean within ±20% of the target value, within ±10% of the target value, within ±5% of the target value, and within ±2% of the target value. The terms “approximately” and “about” may include the target value.

[0129] The use of ordinal terms such as “first,” “second,” and “third” in a claim to modify elements of a claim in the claims does not, by itself, imply any priority, order, or sequence of one claim element relative to another claim element, or the temporal order in which the actions of the method are performed, but is used simply as a label to distinguish one claim element having a particular name from another claim element having the same name (except for the use of ordinal terms) in order to distinguish the claim elements.

Claims

1. It is a photon source, An atomic vapor cell comprising an alkali atomic species, wherein the atomic vapor cell is positioned within the beampaths of a first laser beam and a second laser beam generated during the operation of the photon source, The alkali atom species includes a first energy level, a second energy level, and a third energy level, wherein the second energy level is at a higher energy than the first energy level, and the third energy level is at a higher energy than the second energy level, during the operation of the photon source, The first laser beam has a first wavelength to which a first detuning is set for a first atomic transition in the alkali atomic species, and the first atomic transition occurs between the first energy level and the second energy level. A photon source wherein the second laser beam has a second wavelength to which a second detuning is set for a second atomic transition in the alkali atomic species, the second atomic transition occurs between the second energy level and the third energy level, and the second detuning is at least partially based on the first detuning.

2. The photon source according to claim 1, wherein during the operation of the photon source, the first laser beam is arranged to copropagate together with the second laser beam through the atomic vapor cell.

3. The photon source according to claim 1 or 2, wherein the first wavelength and the second wavelength have values ​​set to satisfy the conditions for a four-wave mixing process in the alkali atomic species.

4. The photon source according to any one of claims 1 to 3, wherein the first detuning is greater than the one-photon resonance Doppler spreading of the alkali atomic species and is 2π × 10 GHz or less.

5. The photon source according to any one of claims 1 to 3, wherein the first detuning is approximately 2π × 1 GHz.

6. The photon source according to any one of claims 1 to 3, wherein the first detuning is approximately 2π × 1.1 GHz.

7. The photon source according to any one of claims 1 to 3, wherein the first detuning is approximately 2π × 1.15 GHz.

8. The first energy level is |5S １／２ >This is the state, and the second energy level is |5P ３／２ >This is the state, and the third energy level is |6S １／２ > The photon source according to any one of claims 1 to 7, which is in the state of >.

9. The photon source according to any one of claims 1 to 8, wherein the first wavelength is approximately 780 nm.

10. The photon source according to any one of claims 1 to 9, wherein the second detuning is a two-photon detuning that is greater than the double-resonance Doppler spreading of the alkali atomic species and is 2π × 10 GHz or less.

11. The photon source according to any one of claims 1 to 10, wherein the second detuning is approximately 2π × 2 GHz.

12. The photon source according to any one of claims 1 to 10, wherein the second detuning is approximately 2π × 2.4 GHz.

13. The photon source according to any one of claims 1 to 12, wherein the second wavelength is approximately 1367 nm.

14. The photon source according to any one of claims 1 to 13, wherein the first laser beam and / or the second laser beam operates with a power of about 5 mW.

15. The photon source according to any one of claims 1 to 14, wherein the first laser beam and / or the second laser beam operates with a beam diameter of about 50 μm.

16. The photon source according to any one of claims 1 to 15, wherein the atomic vapor cell is configured to output a pair of two-color entangled photons during the operation of the photon source.

17. The photon source according to claim 16, wherein each of the two-color entangled photon pairs comprises a first photon and a second photon, the first photon and the second photon having different wavelengths.

18. The photon source according to claim 17, wherein the first photon has a wavelength suitable for long-distance communication technology.

19. The photon source according to claim 17 or 18, wherein the first photon has a wavelength in the range of 1300 nm to 1600 nm.

20. The photon source according to any one of claims 17 to 19, wherein the first photon has a wavelength of about 1324 nm.

21. The photon source according to any one of claims 17 to 20, wherein the first photon has a wavelength of approximately 1324 nm, 1367 nm, 1476 nm, or 1529 nm.

22. The photon source according to any one of claims 17 to 21, wherein the second photon has a near-infrared (NIR) wavelength.

23. The photon source according to any one of claims 17 to 22, wherein the second photon has a wavelength in the range of 700 nm to 925 nm.

24. The photon source according to any one of claims 17 to 23, wherein the second photon has a wavelength of about 795 nm.

25. The photon source according to any one of claims 17 to 23, wherein the second photon has a wavelength of about 795 nm or 780 nm.

26. The photon source according to any one of claims 17 to 25, further comprising a first output and a second output, the first output and the second output being optically coupled to the output of the atomic vapor cell such that the first photons are emitted from the first output out of the photon source and the second photons are emitted from the second output out of the photon source.

27. The photon source according to claim 26, further comprising a single-photon detector optically coupled to either the first output or the second output.

28. A Fabry-Perot etalone optically coupled between the output of the atomic vapor cell and the first output, The photon source according to claim 26, further comprising a single-photon detector optically coupled to the second output.

29. The photon source according to any one of claims 26 to 28, further comprising a bandpass filter optically coupled between the output of the atomic vapor cell and the first output and / or the second output.

30. The photon source according to any one of claims 26 to 28, further comprising a liquid crystal phase difference plate optically coupled between the output of the atomic vapor cell and the second output.

31. A first laser light source configured to generate the first laser beam, The photon source according to any one of claims 1 to 30, further comprising a second laser light source configured to generate the second laser beam.

32. The photon source according to any one of claims 1 to 31, wherein the alkali atom species includes rubidium.

33. A method for generating entangled photon pairs from an atomic vapor cell containing an alkali atomic species, wherein the alkali atomic species includes a first energy level, a second energy level having a higher energy than the first energy level, and a third energy level having a higher energy than the second energy level, and the method is A step of generating a first laser beam using a first laser, wherein the first laser beam has a first wavelength to which a first detuning is set for a first atomic transition in the alkali atomic species, and the first atomic transition occurs between a first energy level and a second energy level. A step of generating a second laser beam using a second laser, wherein the second laser beam has a second wavelength to which a second detuning for a second atomic transition in the alkali atomic species is set, the second atomic transition occurs between the second energy level and the third energy level, and the second detuning is at least partially based on the first detuning, The steps include: guiding the first and second laser beams to pass through the same region of the atomic vapor cell to induce a four-wave mixing process in the atoms; A method comprising the step of generating entangled photon pairs as a result of the four-wave mixing process.

34. The method according to claim 33, wherein guiding the first and second laser beams to pass through the same region of the atomic vapor cell includes copropagating the first and second laser beams through the atomic vapor cell.

35. The method according to claim 33 or 34, wherein the step of generating the first laser beam comprises generating the first laser beam having a first wavelength set to a first detuning greater than the one-photon resonant Doppler spreading of the alkali atomic species and less than or equal to 2π × 10 GHz.

36. The method according to any one of claims 33 to 35, wherein the step of generating the first laser beam comprises generating the first laser beam having a first wavelength set to a first detuning of about 2π × 1 GHz.

37. The method according to any one of claims 33 to 35, wherein the step of generating the first laser beam comprises generating the first laser beam having a first wavelength set to a first detuning of about 2π × 1.1 GHz.

38. The method according to any one of claims 33 to 35, wherein the step of generating the first laser beam comprises generating the first laser beam having a first wavelength set to a first detuning of about 2π × 1.15 GHz.

39. The first energy level is |5S １／２ >This is the state, and the second energy level is |5P ３／２ >This is the state, and the third energy level is |6S １／２ The method according to any one of claims 33 to 35, which is in the state of >

40. The method according to any one of claims 33 to 39, wherein the step of generating the first laser beam having the first wavelength comprises generating the first laser beam having a first wavelength of about 780 nm.

41. The method according to any one of claims 33 to 40, wherein the step of generating the second laser beam includes generating the second laser beam having a second wavelength set to be two-photon detuning and having a second detuning greater than the double-resonance Doppler spreading of the alkali atomic species and less than or equal to 2π × 10 GHz.

42. The method according to any one of claims 33 to 41, wherein the step of generating the second laser beam comprises generating the second laser beam having a second wavelength set to a second detuning of about 2π × 2 GHz.

43. The method according to any one of claims 33 to 41, wherein the step of generating the second laser beam comprises generating the second laser beam having a second wavelength set to a second detuning of about 2π × 2.4 GHz.

44. The method according to any one of claims 33 to 43, wherein the step of generating the second laser beam comprises generating the second laser beam having a second wavelength of about 1367 nm.

45. The method according to any one of claims 33 to 44, wherein the step of generating the first laser beam and / or the step of generating the second laser beam includes generating a laser beam having a power of about 5 mW.

46. The method according to any one of claims 33 to 45, wherein the step of generating the first laser beam and / or the step of generating the second laser beam includes generating a laser beam having a beam diameter of about 50 μm.

47. The method according to any one of claims 33 to 46, wherein the step of causing the four-wave mixing process in an atom is to output a pair of two-color entangled photons to the atomic vapor cell.

48. The method according to claim 47, wherein each of the two-color entangled photon pairs comprises a first photon and a second photon, the first photon and the second photon having different wavelengths.

49. The method according to claim 48, wherein the first photon has a wavelength suitable for long-distance communication technology.

50. The method according to claim 48 or 49, wherein the first photon has a wavelength in the range of 1300 nm to 1600 nm.

51. The method according to any one of claims 48 to 50, wherein the first photon has a wavelength of about 1324 nm.

52. The method according to any one of claims 48 to 51, wherein the first photon has a wavelength of about 1324 nm, 1367 nm, 1479 nm, or 1529 nm.

53. The method according to any one of claims 48 to 52, wherein the second photon has a near-infrared (NIR) wavelength.

54. The method according to any one of claims 48 to 53, wherein the second photon has a wavelength in the range of 700 nm to 925 nm.

55. The method according to any one of claims 48 to 54, wherein the second photon has a wavelength of about 795 nm.

56. The method according to any one of claims 48 to 54, wherein the second photon has a wavelength of about 795 nm or 780 nm.

57. The method according to any one of claims 48 to 56, further comprising the step of detecting the second photon using a single-photon detector such that the first photon is output as a heralding single-photon.

58. The method according to claim 57, further comprising the step of filtering the first photon using a Fabry-Perot etalon.

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