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.
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.
Building a scalable quantum computer is one of the greatest challenges of this century. A quantum computer should be able to execute quantum algorithms which promise great speed up in solving certain computational problems. Microwave photons in superconducting circuits offer a promising platform for quantum computing due to their long coherence times, and the ease of their generation and control. Single-qubit quantum gates are implemented with conventional microwave components. However, implementing entangling two-qubit gates for photonic qubits is considerably difficult because photons do not interact with each other naturally. In this thesis, we investigate the effective interactions between photons mediated by superconducting qubits. Especially, the nonlinear effects in the photonic states generated by these interactions are studied. Based on the analysis of the scattering properties of the system, we present a design for a nonlinear phase shifter device, which yields a photon-number-dependent phase shift in the transmitted photons. Furthermore, we show how entangling two-qubit gates can be implemented using the presented nonlinear phase shifter.
Superconducting quantum circuits provide a versatile and powerful platform for quantum computing as well as for the study of fundamental physics and quantum technologies in general, such as radiation sensors. The development of novel detectors and measurement techniques is important for advancing quantum technologies by enabling detection schemes that are faster, more accurate, or less complex than the current state of the art. In this thesis, we improve upon the existing measurement techniques in superconducting quantum circuits by employing novel methods on a radiation sensor based on superconductor–normal-metal–superconductor (SNS) junctions, in addition to superconducting quantum bits (qubits). The presented methods are experimentally demonstrated in the five publications of this thesis. First, we demonstrate that an SNS sensor, essentially a power or a photon detector, has the capability to directly measure the variance of itinerant microwave photons. Experimentally, we show the measurement of the zero-delay second-order correlation function of interfering thermal and coherent photons displaying bunching with increasing proportion of thermal photons. We combine the signal field with a homodyne field and carry out Wigner function tomography by utilizing techniques from quantum optics. Second, we show a new readout technique for superconducting qubits by multichannel driving which utilizes effectively longitudinal coupling of the qubit and a readout cavity. We find that applying this technique reduces the readout error by 50%, while being roughly 20% faster than the conventional dispersive readout. We finish the thesis by displaying methods for making quantum circuits more compact. We demonstrate a rigorous process for the design of lumped-element resonators without utilizing any computer simulations. The fabricated resonators have a small physical footprint compared to that of coplanar waveguide resonators ubiquitous in superconducting circuits. We achieve an accuracy of 99% in the designed resonance frequency with a standard deviation of 1%, demonstrating the applicability of the model. The final publication demonstrates multiplexed readout of the SNS sensors, reducing the number of required control and measurement lines.
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.
Entangled photons are at the heart of experimental quantum physics. They were used for the first fundamental tests of quantum theory, and became a basic building block for many novel quantum protocols, such as quantum cryptography, dense coding or teleportation. Therefore, the efficient generation of entangled photons, as well as their distribution and accurate analysis are of paramount importance, particularly with regard to the practicability of many applications of quantum communication. This thesis deals largely with the problem of efficient generation of photonic entanglement with the principal aim of developing a bright source of polarization-entangled photon pairs, which meets the requirements for reliable and economic operation of quantum communication prototypes and demonstrators. Our approach uses a correlated photon-pair emission in nonlinear process of spontaneous parametric down-conversion pumped by light coming from a compact and cheap blue laser diode. Two alternative source configurations are examined within the thesis. The first makes use of a well established concept of degenerate non-collinear emission from a single type-II nonlinear crystal and the second relies on a novel method where the emissions from two adjacent type-I phase-matched nonlinear crystals operated in collinear non-degenerate regime are coherently overlapped. The latter approach showed to be more effective, yielding a total detected rate of almost 10^6 pairs/s at >98 % quantum interference visibility of polarization correlations. This performance, together with the almost free of alignment operation of the system, suggest that it is an especially promising candidate for many future practical applications, including quantum cryptography, detector calibration or use in undergraduate lab courses. The second issue addressed within the thesis is the simplification and practical implementation of quantum-assisted solutions to multiparty communication tasks. While the recent rapid progress in the development of bright entangled photon-pair sources has been followed with ample experimental reports on two-party quantum communication tasks, the practical implementations of tasks for more than two parties have been held back, so far. This is mainly due to the requirement of multiparty entangled states, which are very difficult to be produced with current methods and moreover suffer from a high noise. We show that entanglement is not the only non-classical resource endowing the quantum multiparty information processing its power. Instead, only the sequential communication and transformation of a single qubit can be sufficient to accomplish certain tasks. This we prove for two distinct communication tasks, secret sharing and communication complexity. Whereas the goal of the first is to split a cryptographic key among several parties in a way that its reconstruction requires their collaboration, the latter aims at reducing the amount of communication during distributed computational tasks. Importantly, our qubit-assisted solutions to the problems are feasible with state-of-the-art technology. This we clearly demonstrate in the laboratory implementation for 6 and 5 parties, respectively, which is to the best of our knowledge the highest number of actively performing parties in a quantum protocol ever implemented. Thus, by successfully solving and implementing a cryptographic task as well as a task originating in computer science, we clearly illustrate the potential to introduce multiparty communication problems into real life.
In a quantum computer, the information carriers, which are bits in ordinary computers, are implemented as devices that exhibit coherent superpositions of physical states and entanglement. Such components, known as quantum bits or qubits, can be realized with various different types of two-state quantum systems. Quantum computers will be built for computational speed, with hoped for applications especially in cryptography and in other tasks where classical computers remain inefficient. Circuit quantum electrodynamics (cQED) is a quantum-computer architecture which employs superconducting electronic components and microwave photon fields as building blocks. Compared to cavity quantum electrodynamics (CQED), where atoms are trapped in physical cavities, cQED is more attractive in that its qubits are tunable and conveniently integrable with the electronics already in use. This architecture has shown some of the most promising qubit designs, despite their coherence times reaching tens of microseconds, are still below the state of the art with spin qubits, which reach milliseconds. Coherence times are historically the most relevant parameters describing the fitness of a qubit, although these days not necessarily the limiting factor. This thesis presents a comprehensive set of theoretical and experimental methods for measuring the characteristic parameters of superconducting qubits. We especially study transmission-line-shunted plasma oscillation qubits, or transmons, and presents experimental results for a single sample. Transmon capacitively couples a superconducting quantum interference device (SQUID) with a coplanar waveguide (CPW) resonator, often with added frequency tunability utilizing an external magnet. The number of superconducting charge carriers tunnelled through a junction in the SQUID are used as qubit degrees of freedom. Readout of the qubit state is carried out by measuring transmission through the CPW. A cryogenic setup is employed with measurement and driving pulses delivered from microwave sources. Steady-state spectroscopy is employed to determine the resonance frequencies of the qubit and the resonator, qubit-resonator coupling constants, and the energy parameters of the qubit. Pulse-modulated measurements are employed to determine the coherence times of the qubit. The related analysis- and simulation programs and scripts are collected togithub.com/patomaki.
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.