# MAgnetic Gradient Induced Coupling (MAGIC)

Theoretical work on MAGIC that has been done by our group can be found here.

## Blueprint for a Microwave Ion Trap Quantum Computer

### References: [QIS46]

A universal quantum computer will have fundamental impact on a vast number of research fields and technologies. Therefore an increasingly large scientific and industrial community is working towards the realization of such a device. A large scale quantum computer is best constructed using a modular approach. We present the blueprint for an ion trap based scalable quantum computer module which makes it possible to create an arbitrarily large quantum computer architecture powered by long-wavelength radiation. This quantum computer module controls all operations as a stand-alone unit, is constructed using silicon microfabrication techniques and within reach of current technology. To perform the required quantum computations, the module makes use of long-wavelength-radiation quantum gate technology and relies only on a vacuum environment and global laser and microwave fields. To scale this microwave quantum computer architecture beyond one module we also present a new approach that makes use of ion transport between different modules, thereby allowing connections between arbitrarily many modules for a large scale architecture. A high-error-threshold surface error correction code making use of such module interactions can be implemented in the proposed architecture to execute fault-tolerant quantum logic operations. With only minor adjustments these modules are also suitable for alternative ion trap quantum computer architectures, such as schemes using photonic interconnects.

## Versatile microwave-driven trapped ion spin system for quantum information processing

### References: [QIS47]

Using trapped atomic ions, we demonstrate a tailored and versatile effective spin system suitable for quantum simulations and universal quantum computation. By simply appl
ying microwave pulses, selected spins can be decoupled from the remaining system and, thus, can serve as a quantum memory, while simultaneously, other coupled spins
perform conditional quantum dynamics.

Also, microwave pulses can change the sign of spin-spin couplings, as well as
their effective strength, even during the course of a quantum algorithm. Taking advantage of the simultaneous long-range coupling between three spins, a coherent quantum Fourier transform — an essential building block for many quantum algorithms — is efficiently realized.

This approach, which is based on microwave-driven trapped ions and is complementary to laser-based methods, opens a new route to overcoming technical and physical challenges in the quest for a quantum simulator and a quantum computer.

## A trapped-ion-based quantum byte with 10^{−5} next-neighbour cross-talk

### References: [QIS39] [Nature Communications]

The addressing of a particular qubit within a quantum register is a key prerequisite for scalable quantum computing.
We demonstrate addressing of individual qubits within a quantum byte (eight qubits) and measure the error induced in non-addressed qubits (cross-talk) associated with the application of single-qubit gates.
This cross-talk is on the order of 10^{−5} breaching the threshold for fault-tolerant quantum computing.
The quantum byte is implemented using ^{171}Yb^{+} ions confined in a Paul trap where a static magnetic gradient field is applied that lifts the hyperfine qubits’ degeneracy.
Hyperfine qubits are individually addressed using microwave radiation.
In addition, we demonstrate a method for addressing individual qubits where an appropriate choice of addressing frequency and microwave pulse duration allows for further lowering cross-talk.

Figure 1:

Addressing of a single qubit within a quantum byte.
(a) Spatially resolved resonance fluorescence (near 369 nm) of eight ions held in a linear Paul trap detected by an EMCCD camera.
(b) Microwave-optical double resonance spectrum for a fixed pulse length of 10 μs serves for determining the microwave addressing frequency of an individual ion.
Here, the state selective resonance fluorescence signal only in the region of ion 1 is considered.
(c) Same as in (b), however measuring the signal in the region of next-neighbor ion 2.
Non-nearest-neighbor ions (3 through 8) are not affected by manipulating qubit 1 either. Their signal is simultaneously measured but not shown for clarity.
(d) Rabi oscillations are only observed in the region of ion 1 when irradiating all ions at the microwave addressing frequency of ion 1.
(e) Qubit 2 is left virtually unaffected.
Solid lines represent fits of the data. Two points with error bars are displayed in each graph representing typical statistical standard deviations.
Each data point represents 50 repetitions.

## Designer Spin Pseudomolecule Implemented with Trapped Ions in a Magnetic Gradient

### References: [QIS33]

We report on the experimental investigation of an individual pseudomolecule using trapped ions with adjustable magnetically induced J-type coupling between spin states. Resonances of individual spins are well separated and are addressed with high fidelity. Quantum gates are carried out using microwave radiation in the presence of thermal excitation of the pseudomolecule’s vibrations. Demonstrating controlled-NOT gates between non-nearest neighbors serves as a proof-of-principle of a quantum bus employing a spin chain. Combining advantageous features of nuclear magnetic resonance experiments and trapped ions, respectively, opens up a new avenue toward scalable quantum information processing.

## Individual Addressing of Trapped Ions and Coupling of Motional and Spin States Using RF Radiation

### References: [QIS22]

Individual electrodynamically trapped and laser cooled ions are addressed in frequency space using radio-frequency radiation in the presence of a static magnetic ﬁeld gradient. In addition, an interaction between motional and spin states induced by an rf ﬁeld is demonstrated employing rf optical double resonance spectroscopy. These are two essential experimental steps towards realizing a novel concept for implementing quantum simulations and quantum computing with trapped ions.