cleland lab research

superconducting quantum integrated circuits

In the Cleland lab, we are developing superconducting quantum integrated circuits for applications in quantum computation and quantum simulation. These circuits rely centrally on the physics of the Josephson junction, implemented primarily in a variant of the transmon qubit. We have also done extensive experimentation using the Josephson phase qubit.

Josephson qubits have typical ground-excited state energy splittings in the few GHz range, with qubit control provided by external tuning of the qubit splitting as well as using controlled classical microwave signals at the splitting frequency. Qubits are coupled together using capacitive or inductive coupling elements, or via superconducting resonators (see below). Circuits are designed and built using standard semiconductor processing technology, and then tested at mK temperature using a dilution refrigerator.

Superconducting qubits are appealing for engineering quantum integrated circuits because of their intrinsically low loss, their relatively simple fabrication and ease of operation. Superconducting qubits now can achieve quite long coherence times, with both T₁ and T₂ in the tens of microseconds range, corresponding to a few hundred gate operations. We are continuing to develop better materials, processing, and circuit engineering to improve these basic figures of merit. We have built circuits comprising of up to nine coupled qubits; demonstrated the factoring the number 15 using Shor's algorithm; generating a five-qubit GHZ state; and shown a bit-flip error detection circuit.

Presently, we are focusing our research efforts on developing these devices for testing concepts in quantum communication in addition to quantum computing.

photo e.lucero

superconducting electromagnetic resonators

We have developed very high quality factor, superconducting electromagnetic resonators, for use as computational and memory elements for quantum information. With resonance frequencies in the 5-10 GHz band, we can achieve quality factors as high as a few times 10⁶ at low temperatures and low excitation powers. The corresponding T₁ energy relaxation times as long as 100 μs, with T₂ phase coherence times just under twice this value.

In one experiment, we coupled an individual Josephson qubit to a resonator, and using a sequence of qubit preparation and tune-to-resonance protocols, demonstrated that we can pump photons one at a time, on demand, into the resonator. By subsequently measuring the resonator with the qubit, we have shown that the resulting Fock (photon number) states in the resonator have very high purity, with up to 20 photons stored in the resonator at one time.

By employing a protocol developed for ions interacting in a harmonic ion trap, we have extended our ability to generate Fock states in the resonator to being able to create arbitrary quantum superpositions of Fock states, creating states with e.g. zero, three and six photons simultaneously, with full quantum control over the relative amplitude and phase of each of the elements of the superposed state.

These resonators provide a robust quantum memory for quantum information processing, as well as allowing large on-off coupling when used as an intermediary between qubits.