Quantum sensing based on entangled photon pairs is gradually establishing itself as a cornerstone in modern communication networks. The unrivalled capability of quantum sensing techniques in distilling signals plagued by noise, renders them suitable for deployment in backscatter communication networks. Several attempts have been made recently to utilize pairs of entangled signal- idler photons, to enhance the sensitivity of photo- detection in backscatter networks. However, these efforts have always assumed the lossless retention of the idler mode, which is a challenging task from a practical perspective. In this study we examine the extent to which quantum correlations remain after retaining the idler mode in a lossy memory element, while the signal photon propagates through a lossy thermal channel as usual. We also examine briefly two different detection methods, and estimate the received signal-to-noise ratio for them both. This new proposed model is one step further towards realizing quantum backscatter communication.
Quantum sensing based on entangled photon pairs is gradually establishing itself as a cornerstone in modern communication networks. The unrivalled capability of quantum sensing techniques in distilling signals plagued by noise, renders them suitable for deployment in backscatter communication networks. Several attempts have been made recently to utilize pairs of entangled signal- idler photons, to enhance the sensitivity of photo- detection in backscatter networks. However, these efforts have always assumed the lossless retention of the idler mode, which is a challenging task from a practical perspective. In this study we examine the extent to which quantum correlations remain after retaining the idler mode in a lossy memory element, while the signal photon propagates through a lossy thermal channel as usual. We also examine briefly two different detection methods, and estimate the received signal-to-noise ratio for them both. This new proposed model is one step further towards realizing quantum backscatter communication.
We consider a joint communication and sensing problem over an optical link in which a low-power transmitter simultaneously communicates with a receiver and identifies the range of a defect producing a backscattered signal. We model the system as a lossy thermal-noise bosonic channel, in which the target location, modeled as a beamsplitter, affects the timing of the backscattered signal. Motivated by the envisioned deployment of entanglement-enabled quantum networks, we allow the transmitter to exploit shared entanglement to assist both sensing and communication. Since entanglement is known to enhance sensing, as demonstrated in Quantum Illumination (QI), and to increase communication rates through entanglement-assisted communication, the transmitter faces a trade-off in allocating its entanglement resources between the two tasks. Our main result is a characterization of these trade-offs in the form of an achievable rate/error-exponent region, which can outperform time-sharing and demonstrates a quantum advantage.
Superconducting circuits provide a suitable platform to build quantum computers with increasing number of quantum bits (qubits). As such the sheer number of qubits, i.e., the quantum mechanical computational units in a quantum computer, cannot be used to determine the computational power of the computer. On the other hand, a large number of qubits is required to execute well-known quantum algorithms with reliable accuracy. To even dream about solving complex tasks with the state-of-the-art quantum circuits, they have to be improved in many ways to achieve a high level of accuracy. An essential part of large quantum systems is the communication between distant computational units. Establishing efficient communication channels between different parts allows building a network of established components. However, the communication has to be resistant to significant levels of background noise. Quantum backscatter communication is particularly interesting communication paradigm that fulfills this requirement. Here, a single photon is generated and divided into an entangled pair of a signal and an idler photon. The idler photon is sent directly to a receiving quantum system whereas the signal photon is scattered from an auxiliary component. The backscattering event introduces a phase shift that carries the information being sent from the backscatterer to the receiving quantum system. The utilization of quantum entanglement improves the signal-to-noise ratio compared with the classical backscatter paradigm. In this thesis, we study a method to generate single signal photons conditioned on a quantum state of a superconducting qubit, which is a prerequisite for quantum backscattering communication. This process involves exciting the quantum bit to its second excited state and driving it with an appropriate pulse frequency such that the qubit evolves back to the ground state of the system while simultaneously generating a single photon. We find that with our current measurement setup, the signal photon is generated with a success rate of 70%. As our research group’s first trial for this type of measurement, the results show great promise for future work with the quantum backscatter project.
Two current hurdles of quantum RADAR/LiDAR technology are i.) The use of joint measurement techniques, whereby the idler remains in a delay line or a quantum memory to be measured later with the returning signal, and ii.) The difficulty in creating high photon flux signals for long range sensing. Our measurement and detection protocol using immediate-idler-detection (IID) helps to alleviate both of these issues. We present our recent experimental data from characterizing our proof-of-concept IID quantum LiDAR system and show that similar to delay line approaches, we achieve strong correlation even in extremely noisy channels where the noise level exceeds the signal strength by as much as one hundred times. We have found that even in very lossy channels, the integration time remains extremely short and roughly the same value even as the noise is increased. We also show preliminary results through foggy free space channels and found positive correlation SNR even when the visibility was as low as 15%. Our measurement and detection protocol was designed to align closely with classical RADAR and LiDAR signal processing to better align the quantum and classical sensor regimes and allows for the potential to scale upwards and produce higher photon-flux signals from multiple photon pair sources.
Backscattered signals are always obscured by the unavoidable channel noise. However, by exploiting quantum physics recent protocols had been developed to enhance the probability of detecting backscattered signals in a very noisy environment [1], [2]. In this paper we propose a new detection scheme that is simpler in nature than the sum frequency receiver that was proposed for the quantum illumination protocol [3]. Signals are generated using spontaneous parametric down conversion (SPDC) and are transmitted via the simple modulation technique of on-off keying (OOK), while the receiver design will rely upon the purely non-classical Hong-Ou-Mandel (HOM) effect.
In this paper, we propose a novel quantum backscatter communications (QBC) protocol, inspired by the quantum illumination (QI) concept. In the QBC paradigm, the transmitter generates entangled photon pairs. The signal photon is transmitted and the idler photon is kept at the receiver. The tag antenna communicates by performing the pulse amplitude modulation (PAM), binary phase shift keying (BPSK) or quadratic phase shift keying (QPSK) on the signal impinging at the antenna. Using the sum-frequency-generation receiver, our QBC protocol achieves a 6 dB error exponent gain for PAM and BPSK, and 3 dB gain for QPSK over its classical counterpart. Finally, we discuss the QI-enhanced secure backscatter communication.
Entanglement based (EB) communication, networking, and sensing represent excellent alternatives to corresponding classical counterparts in particular in either lowbrightness or high-attenuation regime and when the signal photons are buried in noise. In EB applications idler photons are stored in memory while the signal photons, carrying information, are transmitted over lossy, noisy, and for communication/networking over free-space optical (FSO) channels signal photons are also affected by the atmospheric turbulence and scattering effects. In optical phase conjugation (OPC)-based EB receiver, the OPC operation is performed on signal photons, which are severely affected by FSO channel impairments, with many signal photons being either absorbed or scattered by the FSO channel. Here we propose to perform the OPC operation on idler photons instead. The OPC operation is performed using low-cost, C-band components including tunable laser, periodically polled lithium niobate (PPLN) waveguides, tunable filter, and WDM demultiplexer; and this implementation is suitable for the photonic integrated circuit (PIC) realization. A similar approach is applied to implement the C-band devicesbased entanglement source. Given that the idler photons are brighter compared to the signal photons transmitted over either FSO channel or fiber-optics channel, the OPC operation is much more efficient. In the OPC-based receiver for FSO communications, we need to couple the signal photons to the single-mode fiber (SMF) before the OPC takes place, making the OPC process sensitive to the FSO-to-SMF coupling inefficiency. By performing the OPC on the idler photons instead, the OPC process is insensitive to poor coupling efficiency. In addition to being superior in terms of spectral efficiency compared to classical counterparts, in particular in highly lossy and noisy environment, the proposed EB scheme with the phaseconjugation on idler photons is superior to the corresponding scheme performing the phase-conjugation on signal photons in terms of receiver sensitivity making it an attractive option for radar applications. To validate the proposed concept at the University of Arizona campus we have developed a terrestrial FSO testbed. For detection probability of 0.99 the proposed EB radar outperforms the classical counterpart by 6.7 dB. We experimentally demonstrate that the target detection probability of the proposed entanglement based radar over turbulent FSO channel is significantly better than that of corresponding classical detection scheme.