We introduce a magnetic-flux-tunable phase shifter for propagating microwave photons, based on three equidistant superconducting quantum interference devices (SQUIDs) on a transmission line. We experimentally implement the phase shifter and demonstrate that it produces a broad range of phase shifts and full transmission within the experimental uncertainty. Together with previously demonstrated beam splitters, this phase shifter can be utilized to implement arbitrary single-qubit gates for qubits based on propagating microwave photons. These results complement previous demonstrations of on-demand single-photon sources and detectors, and hence assist in the pursuit of an all-microwave quantum computer based on propagating photons.
We introduce a magnetic-flux-tunable phase shifter for propagating microwave photons, based on three equidistant superconducting quantum interference devices (SQUIDs) on a transmission line. We experimentally implement the phase shifter and demonstrate that it produces a broad range of phase shifts and full transmission within the experimental uncertainty. Together with previously demonstrated beam splitters, this phase shifter can be utilized to implement arbitrary single-qubit gates for qubits based on propagating microwave photons. These results complement previous demonstrations of on-demand single-photon sources and detectors, and hence assist in the pursuit of an all-microwave quantum computer based on propagating photons.
We report a generic scheme to implement transmission-type quantum gates for propagating microwave photons, based on a sequence of lumped-element components on transmission lines. By choosing three equidistant superconducting quantum interference devices (SQUIDs) as the components on a single transmission line, we experimentally implement a magnetic-flux-tunable phase shifter and demonstrate that it produces a broad range of phase shifts and full transmission within the experimental uncertainty. Together with previously demonstrated beam splitters, these phase shifters can be utilized to implement arbitrary single-qubit gates. Furthermore, we theoretically show that replacing the SQUIDs by superconducting qubits, the phase shifter can be made strongly nonlinear, thus introducing deterministic photon--photon interactions. These results critically complement the previous demonstrations of on-demand single-photon sources and detectors, and hence pave the way for an all-microwave quantum computer based on propagating photons.
We implement a broadly tunable phase shifter for microwaves based on superconducting quantum interference devices (SQUIDs) and study it both experimentally and theoretically. At different frequencies, a unit transmission coefficient, |S21| = 1, can be theoretically achieved along a curve where the phase shift is controllable by the magnetic flux. The fabricated device consists of three equidistant SQUIDs interrupting a transmission line. We model each SQUID embedded at different positions along the transmission line with two parameters, capacitance and inductance, the values of which we extract from the experiments. In our experiments, the tunability of the phase shift varies from 0.07 × π to 0.14 × π radians along the full-transmission curve with the input frequency ranging from 6.00 GHz to 6.28 GHz. The reported measurements are in good agreement with simulations, which is promising for future design work of phase shifters for different applications.
Microwave photons in superconducting circuits is a promising approach for quantum computing due to long coherence times and the easy controllability of superconducting devices. However, as the search of the ultimate design of a quantum computer is still ongoing, various suggestions for the realization of a quantum bit, qubit, exist even in the area of superconducting circuits. In this thesis, we study theoretically quantum computing using itinerant microwave photons as qubits. Basic tools for treating the reflection of wave packets and the noise generated by resistive elements in a circuit are derived starting from a widely used model for superconducting transmission lines. We also present simple quantum devices that can act as quantum gates for microwave qubits, including a novel suggestion for a tunable phase shifter. A tunable phase shifter is a useful component in the study of microwave photons in general as well as in quantum computing. The tunable phase shifter consists of three superconducting quantum interference devices (SQUIDs) that can be treated as tunable inductors in the low-power regime. The results of this thesis can be used both in applications related to quantum computing and in the general study of quantum mechanics in circuits. In particular, we conclude that there exist feasible system parameters, for which it should be possible to demonstrate the tunable phase shifter utilizing available experimental techniques. We also show the principle how the possible nonlinearity of the phase shifter can be used in a two-qubit gate providing interactions between microwave photons.
A quantum computer is a promising addition to a classical computer due to increase in performance on certain computational problems. A classical computer computes by manipulating bits, which assume a value either 0 or 1, using logical gates. Similarly, a quantum computation is carried out by manipulating quantum bits, so-called qubits, using quantum gates. The main advantage of qubits is that they can be not only in the states representing 0 and 1, but also in any superposition of these two states. Many different physical realizations for qubits have been proposed. One of the most promising candidates for the hardware of quantum computing are superconducting circuits. Here, the qubit can be represented in number of ways. For example, the two different states can be the direction of current circulating in a superconducting loop, or presence and absence of a photon in a transmission line. In this thesis, we study a tunable phase gate for microwave photons. The gate is implemented by a transmission line interrupted by three superconducting quantum interference devices (SQUIDs), which we model as inductors. We theoretically show that this system fulfills the requirements of a phase gate and that the tunability of the phase shift is frequency dependent. In addition, we consider a non-ideal system by including the effects of the capacitance associated with the SQUIDs. We find that the capacitance has no adverse effects, and in the best case, it may even increase the range of tunability. We also measure the phase shift at frequency of 6.3 GHz and find that the system is well described by the theory. To our knowledge, similar phase gate has not been experimentally studied before.
The authors report on a novel microwave phase shifter featuring rapid electronic adjustment, continuous phase control true time delay operation, high device fault tolerance, and very broadband operation. By coupling a large number of superconducting quantum interference devices (SQUIDs) to a superconducting microstrip transmission line, a variable magnetic medium in which the wave velocity is controlled electronically is created. The authors have measured 60 degrees phase shift at 10 GHz, and wideband operation from 5 to 15 GHz for an 8-cm-long Nb transmission line coupled to 1600 SQUIDs, each containing a single Nb/AlO/sub x//Nb tunnel junction. The observed phase shift corresponds to a change in wave velocity of about 1 part in 60.>
We propose a frequency-tunable source to emit entangled microwave photons on the platform of a superconducting circuit, in which two superconducting transmission-line resonators are coupled via a capacitor and one resonator is inserted with a superconducting quantum interference device (SQUID) in the center. By pumping the circuit appropriately with an external coherent microwave signal through the SQUID, microwave photons are emitted in pairs out of the circuit. The entanglement between the two modes is demonstrated by numerically calculating the second-order coherence function and the logarithmic negativity of the output microwave signals. Due to the tunability of SQUID’s equivalent inductance, the frequencies of the entangled microwave photons can be tuned by an external flux bias in situ. Our proposal paves a new way for obtaining entangled frequency-tunable two-mode microwave photons.