Quantum technology (QT) applies quantum mechanical properties such as quantum entanglement, quantum superposition, and No-cloning theorem to quantum systems such as atoms, ions, electrons, photons, or molecules.
Quantum bit is the basic unit of quantum information. Whereas in a classical system, a bit is either in one state or the another. However, quantum qubits can exist in large number of states simultaneously, property called Superposition.
Quantum entanglement is a phenomenon where entangled particles can stay connected in the sense that the actions performed on one of the particles affects the other no matter what’s the distance between them. No-cloning theorem tells us that quantum information (qubit) cannot be copied.
Quantum communication refers to a quantum information exchange that uses photons as quantum information carriers over optical fibre or free-space channels.
Today, quantum data transfer rates remain quite low, and so communicating entire messages is not yet practical. Instead, Quantum Cryptography or Quantum key distribution (QKD) is being used that employs single or entangled photons to generate shared secret key between the parties that is perfectly secure. The security is guaranteed by Heisenberg’s uncertainty principle. This ensures that any attempts to intercept and measure quantum transmissions, will introduce an anomalously high error rate in the transmissions and therefore will be detectable.
QKD technology requires single-photon sources (SPSs), single-photon detectors, modulating schemes, and protocols. Sensitive superconducting detectors also require cryogenic refrigerated devices. Currently point to point fiber optic links are commercially available with limited distance due to photon losses. QKD is also being tested on free space channels from ground to satellites and drones. They are now being expanded to quantum network that contains elements such as a quantum repeater and quantum switch.
QKD systems have been shown to be unconditionally secure, however, this is true only for an ideal system. Practical implementations have been shown vulnerable to many attacks such as photon number splitting (PNS) attacks that uses extra photons generated by photon sources, Trojan-horse attack that sends bright light from the quantum channel and analyzes the back-reflections, and high-power laser attack that may cause detector blinding.
Communication using QKD can be delivered through fiber-optic networks, over the air, and drones to satellites. Current limitations of QKD are high cost of dedicated hardware, limited transmission speed and distance, and the need for repeaters. Currently Most Quantum Communication links are direct point-to-point links through telecom optical fibers and, limited to about 300-600 kms due to losses in the fiber.
The next important milestone is to extend QKD from point-to-point configuration to National scale multi-user QKD networks. The quantum signals can’t be amplified like an electronic signal, therefor these networks require development of quantum repeaters, and quantum memories. However, no reliable and practical quantum memory is available yet.
There is ongoing Global Quantum communications race, China has taken early lead and US, Europe, Japan and others are trying to catch up. China has created a 2000 Km fiber based Quantum network linking four fiber-based quantum metropolitan area networks (QMANs) and a quantum satellite link spanning 2600 km between two observatories.
The quantum fiber network comprises more than 700 fibre links supported by 32 “trusted relay nodes” and serves about 150 users. China was also first country to launch a quantum communication satellite in 2016 known as Micius. They established a secure satellite link between two ground stations, separated by over 1,100km. using entangled particles for simultaneous transmission of keys. They have also conducted the world’s first quantum-secured video call.
Militaries are now testing the Quantum Communications for battlefield networks. Navies are planning to extend the range of quantum networks to even ships at sea, through quantum satellites. Secure communications with submarines are critical to maintain nuclear deterrence capability. Researchers are now developing secure QKD communications between submarines and its communication with surface vessels, or even satellites. Currently Submarine communications use ELF or VLF radio waves as only low frequencies can penetrate the water at those depths. However, adversaries can intercept these messages could determine their position and sink them.
NIST optical frequency conversion technology
Optical frequency conversion, in which the color of light is changed, is a process that has numerous applications in physics and technology. For example, green laser pointers typically involve the second harmonic generation, where a strong beam produced by a laser at 1064 nm is frequency-doubled in a nonlinear crystal to produce a visible beam at 532 nm.
From the perspective of quantum and classical information processing, the ability to manipulate the color of light, and in particular, quantum states of light, can be an extremely important resource. First, it can be used to connect quantum systems operating in different frequency regions. For example, high performance quantum memories often accept photons in the visible spectrum. However, stable single photon generation in the telecommunications band, which is in the near-infrared, might be preferable because low-loss, long-distance transmission of light through optical fibers is possible at those wavelengths.
Frequency conversion, if accomplished in a manner that preserves all other quantum properties of light, can enable interfacing between these different components of future quantum information processing systems. Such quantum frequency conversion may also be particularly relevant for solid-state quantum optical systems, such as those based on semiconductor quantum dots. This is because such systems invariably suffer from “inhomogeneous broadening”, in which small variations in geometry and/or composition cause the quantum dots to emit at slightly different wavelengths, which can be limiting in applications where identical photons are required. Also, detector technology can also benefit from frequency conversion.
Commercially available silicon single photon counters operate in the visible wavelength region with high quantum efficiency, low dark count noise, and the ability to detect signals with low timing jitter and high repetition rate. On the other hand, single photon counters operating in the telecommunications-band typically do not have the same level of performance. Frequency conversion can be used to detect low levels of near-infrared light with visible wavelength detectors, which, in addition to quantum information processing applications, may benefit basic research in optical spectroscopy of nanoscale systems and applications such as optical remote sensing.
Finally, not only is the color of the photons important, but so is the shape – that is, the temporal profile of the photon wavepackets. This is because different physical systems optimally produce, or coherently absorb, photons of different shapes. We are therefore also investigating approaches to manipulate the shape of single photon wavepackets.
“Our research on quantum frequency conversion follows two main tracks. First, we combine relatively mature frequency conversion technology based on periodically-poled lithium niobate waveguides with quantum light generated by single semiconductor quantum dots in proof-of-principle experiments that demonstrate some of the potential functionality that can be achieved. Second, we develop novel frequency conversion technology in nanophotonic systems compatible with scalable fabrication processes, as a route to creating new technology that may be more relevant to future technologies.”
Telecom-to-visible wavelength conversion
Working with colleagues at NIST’s Information Technology Laboratory (ITL), we are performing experiments on quantum frequency conversion of single photons for advanced detector technology and hybrid quantum systems. To start, single photons at the telecommunications wavelength of 1300 nm are generated from a single semiconductor quantum dot and extracted into an optical fiber.
They are then combined with a strong 1550 nm pump laser in a periodically-poled lithium niobate waveguide (PPLN WG). This PPLN WG ensures momentum conservation between the three optical waves involved in this process – the 1300 nm single photons, the 1550 nm pump beam, and the newly generated photons at 710 nm. The photon stream is split into two paths, with each path sent to a single photon counter that provides a record of the arrival times of the photons. The newly generated photons are then detected by visible wavelength single photon counters, for example, in a photon correlation setup used to measure the quantum properties of the optical field
A breakthrough in frequency up-conversion of single photons, reported in May 2022
Preserving photon statistics of quantum optical states during frequency conversion is critical in modern quantum technologies, as quantum networks sometimes demand the interfacing of many subsystems operating in significantly different spectral regions. However, current approaches offer only very small frequency shifts and limited tunability. They also suffer from high insertion loss and Raman noise originating in the materials used.
In a new study, scientists reported a breakthrough in frequency up-conversion of single photons based on a hollow-core photonic crystal fiber (PCF) filled with hydrogen gas. Scientists created a spatio-temporal hologram of molecular vibrations in the gas by stimulating Raman scattering. They then used this hologram for highly efficient, correlation-preserving frequency conversion of single photons.
The system operates at a pressure-tunable wavelength, which could be helpful in quantum communications where efficient sources of indistinguishable single-photons are unavailable at wavelengths compatible with existing fiber networks.
The approach combines quantum optics, gas-based nonlinear optics, hollow-core PCF, and the physics of molecular vibrations to form an efficient tool that can operate in any spectral band from the ultraviolet to the mid-infrared – an ultra-broad working range inaccessible to the existing technologies. The findings may be used to develop fiber-based tools in quantum communications and quantum-enhanced imaging technologies.
SBIR Quantum frequency conversion for quantum communication
The Department of Defense and the Army has a vested interest in secure communications. Quantum communication has been shown to be secure against eavesdropping due to the nature of entanglement.
Potential next-generation quantum communication systems include the transmission of quantum information entangled with quantum memories. Quantum memories allow for extended distance quantum key distribution (QKD) and for storage and later retrieval of quantum information in a network composed of many nodes. In these cases, frequency conversion of single photons is needed.
Such quantum frequency conversion has been demonstrated for certain wavelengths with periodically poled lithium niobate (and similar periodically poled ferroelectric waveguides). The goal of SBIR is to develop a plug-and-play nonlinear device based on periodically poled lithium niobate waveguide (or similar) having high difference, and sum, conversion efficiency with at least 10 dB signal-to-noise.
The objective here is to develop a complete package that supports quantum frequency conversion between the specified wavelengths. This quantum frequency conversion package would allow for long-haul quantum communication because the output/input is a photon in the telecommunications band. Successful demonstration of the packaged outlined below will directly and significantly impact quantum communication, long-haul quantum communication, and hybrid quantum technologies.
Periodically poled lithium niobate (PPLN), a ferroelectric crystal, is a versatile nonlinear medium. It is well established as a material of choice for optical amplifiers, second harmonic generation and nonlinear waveguides. For efficient use of PPLN in frequency conversion, a waveguide is often fabricated into the PPLN. Obtaining efficient input coupling of two nondegenerate optical frequencies into a PPLN waveguide is challenging. Moreover, obtaining an efficient waveguide for both the nondegenerate frequencies in order to achieve high efficiency frequency conversion is another difficulty. This call is for two devices, one capable of difference frequency generation (DFG) and a second capable of sum frequency generation (SFG). The aim of this program is to design, fabricate and successfully demonstrate a complete packaged PPLN (or other nonlinear compact medium packaged<30 cm3) having high efficiency, fiber or other waveguide coupling at the input and have an output signal-to-noise of at least 10 dB. There is appreciable overlap in the design so simultaneous work on DFG and SFG is very reasonable. Any needed power supplies or oven controllers can be separate to the packaged PPLN and their size is not critical.
PHASE I: The design of the periodically poled lithium niobate (or other nonlinear medium) waveguide must be demonstrated. The design and simulations must show (i) high efficiency DFG (of inputs 795 nm and 1989 nm) and high efficiency SFG (of inputs 1324 nm and 1989 nm), where both DFG and SFG are at the level>30%/W/cm2 (where W/cm2 is the product of the input powers divided by the squared length with<300 mW total input power) (ii) a signal-to-noise of the output of at least 10 dB and (iii) the input coupling efficiency should be>40% for all DFG and SFG inputs, (iv) provide a design of the coupling method of the inputs to the PPLN waveguide for both DFG and SFG. The designed holder for the PPLN should be<30 cm3.
PHASE II: Difference Frequency Generation: The fabrication of the periodically poled lithium niobate waveguide (or other nonlinear medium) must be completed and the following demonstrated: (i) high efficiency DFG (of 795 nm and 1989 nm) and SFG (of 1324 nm and 1989 nm), both>30%/W/cm2 and total input power of<300 mW (ii) the signal-to-noise of the output of at least 10 dB, (iii) the input coupling efficiency should be>40% for all SFG and DFG inputs. The efficiency and signal-to-noise can be experimentally demonstrated with ample input powers that is well above the single photon level. The package (<30cm3) should be designed for optimal coupling of the inputs into the waveguide, where for DFG the inputs are either fiber or waveguide coupling into the PPLN and the inputs are fiber coupled for SFG.
PHASE III: The complete packaged device (<30cm3) must be delivered, one for sum frequency generation and one for difference frequency generation. That is, a periodically poled lithium niobate waveguide (or other nonlinear medium) and (i) be a complete plug-and-play package, specifically, it should include the waveguide, its housing (ii) the inputs should be either fiber coupled or waveguide coupled into the PPLN waveguide for DFG and be fiber coupled into the PPLN waveguide for SFG (iii) the output should be fiber coupled for both DFG and SFG and (iv) the signal-to-noise of the output should be at least 10 dB (measured for DFG at 795 nm vs 1324 nm and for SFG 1324 nm and 795 nm) (v) the product should be tunable in the input of at least 5 nm with the output having at least 10 dB signal-to-noise. The output does not need to be measured at the single photon level. Note that any needed power supplies or oven controllers can be separate to the packaged PPLN and their size is not critical.
The compact, portable and robust nature of the device is an important feature. The product”s commercialization would serve as a device to bridge two neutral atom-based quantum systems that are remotely situated but connected by telecommunications fibers. This device could be integrated into a more secure quantum communication network for the DoD. Beyond a research tool, this device would operate as bridge hybrid quantum systems which require frequency conversion for spectral overlap. Furthermore, commercialization of the methodology for optimal coupling and conversion would allow for consumer access to these specialized devices for specific use in extending the technique into other wavelength regimes where the inputs are highly nondegenerate.