# Presentation: 908494

## Quantum Computing with Superconducting Flux QubitsS. Han, University of Kansas, DMR-0325551

- SEM micrograph of a SQUID flux qubit. The qubit has a second order gradiometer configuration which makes it immune to fluctuations of homogeneous magnetic field. Experiments show the qubit has a relaxation time of ~7 s, corresponding to a maximum decoherence time of ~14 s, a figure that is very promising for realization of quantum computing..

- We have designed, fabricated, and characterized SQUID flux qubits aiming for scalable quantum computation. Energy relaxation time (T1) between the “0” and “1” states of the qubit has been measured using a high-precision time-domain technique. The result of T1~7 s is very encouraging. Temperature dependence of relaxation time agrees very well with the spin-boson model.

- Detector

- Qubit

- Bias control

- Supercurrent

- ‘0’

- ‘1’

**Notes:**
Quantum computers make essential use of the quantum mechanical states of the entire macroscopic computer. They are predicted to be able to solve a number of very important problems that are intractable for existing or projected classical computers. Examples on such problems are the factoring of very large numbers—a key to cryptography—and accurately modeling the true quantum nature of small systems, e.g. the many nano-scale devices being developed for the next-generation electronics. An enormous obstacle to the development of these advanced computers is the requirement that the computer be able to maintain the quantum mechanical coherence among all its elements throughout a calculation. Inevitable interactions of macroscopic objects with the external world, or environment which includes electronic circuits used for manipulating and readout the state of quantum computers, can rapidly destroy this coherence and are, in fact, the major reason quantum effects are not seen in our everyday experiences. We use the highly coherent state of superconducting material to form the basic element, or qubit, of the computer. This approach allows us to make the use of modern integrated circuit technology to scale the computer to a useful size. Here, we show a superconducting qubit, with its “0” and “1” states represented by persistent currents flowing in opposite directions (the red and green curved arrows), and its state control and readout circuits (the “qubit”, “bias control”, and “detector” in the top-right picture, respectively). The bottom-left picture shows a measurement of the qubit’s relaxation time which ultimately determines how long the qubit can maintain coherence. We found that for the qubit measured here the relaxation time is almost 7 microsecond, a figure that is very promising for realizing useful quantum computation.  

## Epitaxial Barrier NbN/AlN/NbN Tunnel Junctionsfor Quantum Information ApplicationsZ. Wang, Kobe Advanced ICT Research Center, NICT, JAPAN, DMR-0325551

- TEM micrograph of the junction cross section (center and left) and electron diffraction patterns (right) for an NbN/AlN/NbN junction with a *J**c* of 400 A/cm2.

- We fabricated high-quality NbN tunnel junctions with ***epitaxial*** AlN barrier by reactive dc-magnetron sputtering. The junctions show excellent properties in a wide range of *J**c,* from 100 A/cm2 to 20 kA/cm2, suitable for quantum information applications.
- TEM and electron diffraction show atomically smooth and sharp interfaces between the superconducting NbN electrodes and the insulating AlN tunnel barrier

** ****ASC2006-1EG01**

- The epitaxial tunnel barrier greatly reduce the number of defects (microscopic two-level fluctuators) which have been identified as the dominant decoherence mechanism in all types of solid-state Josephson superconducting qubits.

**Notes:**
Superconductor-insulator-superconductor Josephson tunnel junction is the elemental device for all types of quantum computer that use superconducting materials. To obtain junctions with coherence time sufficiently long for practical quantum computing It is extremely important to have an insulating tunnel barrier that is free of impurities and defects as much as possible. Recently, researchers found that amorphous aluminum oxide – the most commonly used material for tunnel barrier – has relatively high defect density which limited coherence time to less than 1 microsecond for Nb/AlO/Nb junctions. This problem can be almost completely eliminated by using epitaxial tunnel barrier which maintain nearly a perfect crystal structure.  We have chosen AlN (aluminum nitride) as the barrier material between NbN superconducting electrodes because its chemical compatibility and small lattice mismatch with NbN. Recently, we successfully made NbN/AlN/NbN tunnel junctions that have excellent electrical properties such as very low leakage current, high quality current-voltage characteristics, and large superconducting energy gap. Cross-section TEM (transmission electron microscopy) and electron diffraction analysis of the fabricated tunnel junction show that the crystal structure of AlN barrier is well maintained. The epitaxial growth of tunnel is a breakthrough in our battle against qubit decoherence caused by material defects.  

## Epitaxial Barrier NbN/AlN/NbN Tunnel Junctionsfor Quantum Information ApplicationsZ. Wang, Kobe Advanced ICT Research Center, NICT, JAPAN, DMR-0325551

- Is acting as a base for basic research that plays an important role in the National Institute of Information and Communications Technology, Incorporated Administrative Agency.
- We are conducting a variety of bio, brain, nanotechnology, superconductor, quantum and laser, etc. research, aiming at the creation of knowledge and technological breakthroughs, and dream of contributing to future, affluent info-communications.
**The Superconducting Electronics Group web site**
- http://www-karc.nict.go.jp/102/E_index.html
- contains our research subjects, activities, and recent research presentations.

**Equipments and facilities**
- We have a 600 m2 area clean room (class 100 and class 1,000) for fabrication of superconducting junctions and circuits. Various equipments are available both for device fabrication and analysis in the clean room. We have rf and dc reactive sputtering systems, RIE, ECR, laser ablation, ion-beam deposition, i-line stepper, e-beam writer, x-ray diffraction, SEM, AFM, etc.

**Epitaxial Barrier NbN/AlN/NbN Tunnel Junctions****for Quantum Information Applications****Z****. ****Wang****, ****Kobe Advanced ICT Research Center, NICT, JAPAN,**** DMR-0325551**

**Notes:**
This slide is self-explanatory.

## Quantum Computer: Need to Reduce Decoherence  Great Josephson Junctions Required

**J. E. Lukens, Stony Brook University****,**** DMR-0325551**

**Notes:**
Defects in tunnel barrier of Josephson junctions produce 1/f critical current (Ic) fluctuations which in turn cause decoherence in superconducting qubits. We have carefully tuned our niobium-aluminum-niobium (Nb/Al/Nb) trilayer junction fabrication process to obtain high quality junctions for our superconducting qubit circuits (top-right picture). The severity of 1/f critical current fluctuation is quantified by current power spectral density S1. The bottom-left figure shows that 1/f fluctuations in our Nb junctions is about 100 times lower than that observed in junctions made by other research groups around the world and thus are expected to have significantly longer coherence times.