Quantum computer


Quantum Computation with Ions

The development of Quantum technologies, i.e. the attempts to make practical use of quantum effects such as superposition and entanglement, currently represents one of the fastest growing scientific areas. Possible applications include quantum communication, quantum simulation and quantum computing. Impressive demonstration experiments have been successfully carried out in the last two decades, and the first technologies have already reached a sufficient maturity to hit the market. However, attaining scalability, which means the ability to handle a large number of quantum bits (qubits) still represents a challenge. Atomic ions stored in Paul traps are currently the qubit which offers the best level of control. We attempt to demonstrate scalability by using micro structured segmented Paul traps. In this approach, a small number of qubits is stored in a processor region of the trap, manipulated by laser radiation, and then shuttled between processor and memory regions by means of electrostatic transport.

Our activities span a large bandwidth of physical and technological fields of interest. We make use of methods from solid state physics and semiconductor industries to manufacture our traps. Moreover, for control of multi-qubit systems, the development of custom electronics and software solutions is necessary.

The linear segmented microtrap is operated since 2006, with the long-term goal of realizing scalable quantum simulation and quantum computing. We are member of the EU Flagship project aqtion

Research Highlights:


Low-heating rate microchip trap

Our new segmented microchip trap is in operation and displays heating rates of below 20 phonons/s at room temperature, 1.5MHz axial trap frequency and about 250um ion-electrode distance. This is an about 20-fold improvement as compared to the best trap operated before. It enables sideband cooling down to 0.02 phonons on the axial mode, the lowest ion temperature achieved so far in our group. Strings of up to 6 ions can be stably stored at one segment pair. Improvements with respect to previous trap designs include galvoplating an additional 8um gold layer onto the trap electrodes, and avoiding glue for trap assembly. The latter measure significantly improved the pressure and leads to trapping times of >1day!


Bell states at a distance

Combining the entangling gate, ion separation and quantum state tomography, we have realized the distribution of entangles spin qubits in our segmented microtrap. The constituent ions of an odd Bell state are separated and shuttled to trap zones at a distance of 5.5mm. It was shown that the Bell state coherence can be retained for hold times of up to 20ms. This demonstrates how low-decoherence qubits can be distributed in a platform for scalable quantum information processing.



Ion separation

Separating two ion from a common potential well to two different ones is a fundamental building block for scalable quantum information with trapped ions. Throughout this provess, the ions are driven through a low-confinement situation. Thus, in order to suppress motional excitation, this process requires extremely careful control and precise calibration of the trap potentials.
Experimental Publication in Phys. Rev. A
Theory Publication in New J. Phys.




Ultrafast Coherent Ion Transport

We have realized fast, nonadiabatic shuttling of a single trapped ion in the microtrap, where a transport of 300 μm has been accomplished within only 4 μs. This corresponds to only a few trap oscillation cycles, and we could show that it is possible to control the shuttling such that the energy transfer is as low as 0.1 phonons on average. Furthermore, we managed to shuttle motional state superpositions without significant loss of coherence. ( Publication in Physical Review Letters, Accompanying Physics Viewpoint, QUANTUM press release)



Geometric Phasegate

The first geometric phase gate using spin dependent light forces was realized in 2011. The light force creates a geometric phase which depends on the total spin state of the qubits interacting with the light field, which can be exploited for creating entangled states. Improving the gate fidelity is part of the current activities. The gate was realized using axial vibrational modes, although, in the future we might also employ radial modes for this purpose.



Near-resonant Continuous Sideband Cooling

This cooling method combines high cooling rates with sub-Doppler cooling temperatures. For two ions, both normal modes of vibration can be cooled simultaneously. Stimulated Raman transitions are driven near the S1/2- P1/2- resonance, such that one- and two-photon transitions are taking place at similar rates, which provides fast cooling of a two ion crystal.



Development of Fast Multichannel Signal Generators

The operation of a segmented ion trap and the execution of transport operations require a device which is able to generate a set of arbitrary voltage waveforms. The required specifications on memory depth, update rate and signal integrity exclude commercial off-the-shelf solutions. We develop and test devices fulfilling these specifications, where modern cutting-edge FPGA technology is applied.


Robust Readout of the Spin Qubit

The evolution of a laser-driven two-level system with thermal fluctuations of the Rabi frequency due to the interaction with multiple thermally occupied vibrational modes is investigated theoretically. We find a simple model which correctly reproduces experimental data for square pulses and rapid adiabatic passage (RAP) pulses. This is of crucial importance for the realization of robust readout of the spin qubit at high fidelities. (further reading)


Quantum limited measurement of magnetic field gradients

The magnetic field gradient present within a microtrap was measured with an accuracy limited by projection noise. This measurement is of considerable importance for the realization of scalable quantum logic, as shuttling operations along a magnetic field gradient leads to the accumulation of additional quantum mechanical phases. (further reading)


Measurement of ion localization with standing waves

By means of a slowly moving resonant standing wave, we precisely measure the distance of two ions in units of the wavelength. From this, the Lamb-Dicke factor, the crystal temperature and the interferometer stability can be inferred. (further reading)


Quantum mechanical precision measurements with entangled states

We prepare Schrödinger cat states by separation of wave packet components in phase space by means of spin dependent light forces. Based on this scheme, we realize a quantum state tomography procedure, which enables us to track the ion's trajectory with sub-Heisenberg precision. (further reading)


A single ion as a precision probe for electric fields

A single ion provides the possibility for measuring electric fields by shuttling the ion through the trap structure and performing local spectroscopy. We find an impressive agreement between measured secular frequencies and values predicted from electrostatic simulations. (further reading)


Qubits and ground state cooling

Qubits are encoded in the spin states of the S1/2 ground state. We can realize qubit preparation, qubit readout and single qubit rotations with high fidelities. Single ions are prepared close to the vibrational ground state, which allows for measuring the heating rate by means of Jaynes Cummings dynamics on a blue sideband. (further reading)


Coherent dynamics in the microtrap

40Ca+ ions are trapped in our microtrap. The temperature can be reduced by sideband cooling, which allows for the observation of laser-driven coherent dynamics. (further reading)



The linear segmented microtrap and shuttling operations

The linear segmented microtrap consists of 31 DC electrode pairs and one RF electrode pair, where each DC may form its own, smaller Paul trap. Ions can be shuttled within the structure by variation of the dc voltages. (further reading)