Department of Physics

Research

  • Quantum computing with superconducting devices
  • Electronic transport in nanostructures
  • Noise in mesoscopic devices

Our newest research projects:

Check out here to see what we are interested in doing next.

On-going projects:

Transport phenomena in SET’s

In continuation of our work on Nb SET’s the QE team has investigated in more detail the potential of these structures for reducing the dephasing effects associated with the presence of nonequilibrium quasiparticles on the island. In the case of AlNbAl SET structures, it is expected that these processes will be suppressed due to the relatively large energy required to break the gap of Nb. However, in the samples that we have fabricated and measured, it turned out that several tunnelling effects coexist below the quasiparticle threshold voltage, where a rich structure of peaks and steps appears in the IV (see Figure 2). To identify what processes are responsible for these features we have fabricated several AlNb and AlAl single junctions and measured them at various magnetic fields and temperatures. This allowed us to characterize or eliminate effects such as resonances due to the electromagnetic environment, MAR, MPT, 3e-tunneling, etc.

We found that resonant tunnelling of Coper pairs is responsible for the series of equally-spaced peaks in the IV, and a higher-order tunnelling processes can explain the changes seen in the background current when a external magnetic field is applied.


Figure 2. IV features of an Al-Nb-Al SET, displaying gate-modulated Cooper pair resonances. The image on the right represents the current in the V-Vg plane with the background substracted. The period of the structure is set by the charging energy of the island (which was measured independently).

Suspended structures

The QE team has fabricated and measured a new type of SET structure. It is known that a major limitation for many superconducting devices is the 1/f noise that originates from the substrate. Apparently, the only way to avoid this is to use substrates with lower noise. But a more radical idea is to remove the substrate altogether! We have succeeded in fabricating a "suspended" Al single-electron transistor (SUSET) in which the island "floats" above the substrate (see Figure 3). The structure was remarkably resilient even after repeated cool-downs and mechanical manipulations, displaying the well-known SET features in the IV and conductance.



Figure 3. The SEM picture of a SUSET. The gate electrode points to the island, which is held above the substrate by the junction themselves. The two lines pointing downward are the usual by-products of the two-angle shadow evaporation. The inset shows the IV and dI/dV of this structure, with perfectly defined Josephson, JQP, and quasiparticle currents.

Rabi oscillations in coupled qubits

We have also started to investigate theoretically the quantum evolution of two-qubit systems when a microwave radiation is applied at the gate. The systems studied were a Cooper pair box capacitively coupled to a large Josephson junction (a system proposed by the Grenoble group to study entanglement) and two capacitively coupledCooper pair boxes (the NEC two-qubit). In both cases, we have developed a theory of Rabi oscillations that takes into account the coupling of the two qubits. In the first case, the system is analog to a laser-driven two-level atom in a QED cavity: there is a permanent exchange of quanta between the m.w. field, the Cooper pair box, and the junction, which leads to Rabi oscillations of various amplitudes. In the case of the NEC two-qubit, the quantum state can be manipulated by applying simultaneously two m.w. fields, of different frequency, at the gates of the qubits.

Previous projects:

Cooper pair pumps

We took part in the experiments done in the PICO group (J. Pekola) at HUT on transport in a three junction Cooper pair pump operated as a turnstile. The samples fabricated in Jyväskylä by A. Halvari were measured by J. Toppari in a 7 mK base-temperature refrigerator at the Low Temperature Lab (HUT) in Helsinki. The turnstile operation in this system is possible due to the hysteretic behaviour within the triangle opened around each triple node in the honeycomb stability diagram. A simple way to describe this process is to consider a path in the (q1,q2) plane with the constraint q1 = q2 + const., exiting the triangle at both extremes, and to assume that at every degeneracy point of the charging Hamiltonian, Hch, the system is driven into the state with lower energy. The argument is sufficient to explain the turnstile behaviour: within every traversal of the path one Cooper pair is transferred through the array in the direction of the bias voltage. This principle of operation involves coherent tunnelling, cotunnelling and relaxation of Cooper pairs.

In Figure 1 below we present the results from one of the experiments, where the measured current I = ΔI + I0, is shown as a function of the operating frequency f. The `pumped´ charge per cycle 2eQP , is found by fitting the current with the expression

I = 2efQP[1-exp(-fLZ/f )] + 2efQL + I0,

where the reduction of the current due to Landau-Zener interband transitions at frequencies above fLZ and the leak current, QL, due to inelastic tunnellings, have been taken into account. Values obtained from the fit are QP = (0.985 ± 0.068), fLZ = (26.2 ± 4.4) MHz and QL = (0.127 ± 0.046).

Figure 1: Current in a cycle through the degeneracy node of the stability diagram of the three junction CPP as a function of the frequency of the RF-signals applied in-phase to the two gates. The bias voltage was V = 50 μV and the AC-amplitude 1/6 times 2e-period in VgCg. The solid line is the fit by the equation above and the dashed line shows the ideal 2ef-dependence predicted by the theory. Other sample parameters were EC ≈ 129 μeV and EJ ≈ 19 μeV, yielding EC/EJ ≈ 0.15.