Tunable phase gate for microwave photon qubits

Taewan Noh, Gwanyeol Park, Gahyun Choi +5 more

Tuomas Ollikainen

Roope Kokkoniemi

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.

Janne I. Kokkala

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.

Shwetank Kumar, Matthias Steffen, Mary-Beth Rothwell +3 more

Xue Dong, Re-Bing Wu

Towards distributed quantum computing, the logical operation of flying qubits leads to the flying-qubit gate control problem. This paper studies the design of single flying-qubit gates using a superconducting qubit with a tunable waveguide coupler. We show that, without microwave driving, only phase gates are realizable, and we derive an analytical expression of the tuning scheme for arbitrary phase gates. The resulting scheme is demonstrated by numerical examples.