We develop components and methodology in the rapidly growing field of quantum computing, especially in the framework of circuit quantum electrodynamics cQED. Our experiments with superconducting circuits are strongly supported by our quantum theory effort. Such computers are potentially useful in solving complex problems such as those related to nitrogen fixation, machine learning, or cryptology. In the cQED experiments, we focus particularly on the interplay of thermal and electric effects [ Nat. Especially, we study the coherence properties of quantum circuits coupled to tunable environments [ Nat.
In addition, we develop thermal microwave detectors [ Phys. Such components are important, for example, for quantum computing and communications based on propagating microwave photons.
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This project works in close collaboration with the Finnish Centre of Excellence in Quantum Technology QTF and the state-of-the-art cleanroom and low-noise measurement facilities of OtaNano. The goal of the project is to develop a multi-qubit quantum processor. We focus on transmon qubits which combine long coherence times and flexible coupling mechanisms to different types of quantum circuits. Based on our above-mentioned efforts on superconducting qubits, we study tunable environments for superconducting circuits. Our idea is to couple the qubits to the recently developed quantum-circuit refrigerator QCR [ Nat.
The operating principle of the QCR relies on voltage-controlled photon-assisted single electron tunneling. The tunable heat sink works with a dissipative element coupled to a quantum circuit, where the coupling strength is tunable through the relative detuning. By coupling a qubit to such devices, we aim to implement our theoretical ideas on a fast qubit reset [ NPJ Quant. B 96, ].
Our goal is to use such experiments for noise-enhanced quantum control experiments and to scale up the number of qubits to simulate dissipative open quantum circuits and complex Hamiltonians. Recently, we also implemented a thermal source of microwaves using a QCR coupled to a resonator [ Sci. By applying a large voltage on the tunnel junctions of the QCR, it starts to operate as a high-temperature environment as opposed to a low-temperature environment at low bias voltages.
Consequently, we were able to measure a large flux of photons emitted from the resonator. Applications to Bulk Superconductors. Thin Films. Applications to Thin Films.
Superconducting Quantum Electronics | Volkmar Kose | Springer
Technological Aspects. Tunnel Contacts. High Frequency Applications. The Biomagnetic Method. Current Dipole Model.
Detection Coil Configurations. Sensor Periphery.
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Current and Voltage Sensitivity. Noise Equivalent Power. Only the solid wave functions are used for computation.
Different wells correspond to a different number of flux quanta trapped in the superconducting loops. The bias current is adjusted to make the wells shallow enough to contain exactly two localized wave functions. The GHz energy gap between the energy levels of a superconducting qubit is intentionally designed to be compatible with available electronic equipment, due to the terahertz gap - lack of equipment in the higher frequency band.
The qubit energy level separation may often be adjusted by means of controlling a dedicated bias current line, providing a "knob" to fine tune the qubit parameters. An arbitrary single qubit gate is achieved by rotation in the Bloch sphere. The rotations between the different energy levels of a single qubit are induced by microwave pulses sent to an antenna or transmission line coupled to the qubit, with a frequency resonant with the energy separation between the levels.
Individual qubits may be addressed by a dedicated transmission line , or by a shared one if the other qubits are off resonance. The axis of rotation is set by quadrature amplitude modulation of the microwave pulse, while the pulse length determines the angle of rotation. More formally, following the notation of,  for a driving signal. The resulting transformation is. Coupling qubits is essential for implementing 2-qubit gates.
Superconducting Quantum Devices SQD 12222
Coupling two qubits may be achieved by connecting them to an intermediate electrical coupling circuit. In the first case, decoupling the qubits during the time the gate is off is achieved by tuning the qubits out of resonance one from another, i.
Notably, D-Wave Systems ' nearest-neighbor coupling achieves a highly connected unit cell of 8 qubits in the Chimera graph configuration. Generally, quantum algorithms require coupling between arbitrary qubits, therefore the connectivity limitation is likely to require multiple swap operations, limiting the length of the possible quantum computation before the processor decoherence.
Another method of coupling two or more qubits is by coupling them to an intermediate quantum bus. The quantum bus is often implemented as a microwave cavity , modeled by a quantum harmonic oscillator. Coupled qubits may be brought in and out of resonance with the bus and one with the other, hence eliminating the nearest-neighbor limitation.
The formalism used to describe this coupling is cavity quantum electrodynamics , where qubits are analogous to atoms interacting with optical photon cavity, with the difference of GHz rather than THz regime of the electromagnetic radiation. One popular gating mechanism includes two qubits and a bus, all tuned to different energy level separations.
The rotation direction depends on the state of the first qubit, allowing a controlled phase gate construction. More formally, following the notation of,  the drive Hamiltonian describing the system excited through the first qubit driving line is. Unwanted rotations due to the first and third terms of the Hamiltonian can be compensated with single qubit operations.
The remaining part is exactly the controlled-X gate. Architecture-specific readout measurement mechanisms exist.
Bottom-up superconducting and Josephson junction devices inside a group-IV semiconductor.
The readout of a phase qubit is explained in the qubit archetypes table above. A more general readout scheme includes a coupling to a microwave resonator, where the resonance frequency of the resonator is shifted by the qubit state. The list of DiVincenzo's criteria for a physical system to implement a logical qubit is satisfied by the superconducting implementation. The challenges currently faced by the superconducting approach are mostly in the field of microwave engineering.
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Quantum computing implementation. Bibcode : Natur. Ryan, Blake R. Retrieved Intel Newsroom. Gambetta, J.