**Quantum particles have the unique property of being able to exist in multiple states at the same time, but according to the principles of quantum mechanics they lose that property as soon as they are measured.**

Scientists at Delft University of Technology (TU Delft, The Netherlands) have found a way to circumvent this phenomenon and also to manipulate quantum states by measurement. This result is very important for the development of quantum computers, which could solve complex problems much faster than supercomputers. The Delft researchers' findings will be published this week in Nature. The research was funded mainly by the Netherlands Organisation for Scientific Research (NWO) and the Foundation for Fundamental Research on Matter (FOM).

## Schrödinger’s Cat

The fact that quantum particles lose their quantum-mechanical properties as soon as they are measured is known to us mainly through ‘Schrödinger’s Cat’. The famous founder of quantum mechanics performed the following thought experiment: put a cat in a box with a flask of poison and a switch based on a quantum particle. This imaginary switch thus has the property of being ‘on’ and ‘off' simultaneously; therefore the cat in the box is both dead and alive at the same time. This state persists until the box is opened, when nature is forced to make a decision as the cat will be either dead or alive; the outcome is random.

## Looking into the box

This week an article titled '

*Deterministic entanglement of superconducting qubits by parity measurement and feedback*' by scientists of the Kavli Institute of Nanoscience at TU Delft, led by Leonardo DiCarlo, will appear in Nature. The article presents a new way to look inside Schrödinger’s box and still maintain a quantum superposition. They are also able to measure - and, if required, correct - a quantum state so that the final superposition is no longer random.## Erwin and Niels

In their research, the Delft team used two quantum bits (qubits), the building blocks of the quantum computer. In Schrödinger's analogy, each qubit represents a cat, which means that they put not one but two cats in the sealed box: ‘We'll call them Erwin and Niels for convenience’, explains DiCarlo. ‘According to quantum theory, four possible states exist at the same time: Erwin and Niels are dead, Erwin and Niels are alive, Erwin is dead but Niels is alive, and vice versa. Normally, as soon as you look inside the box, the probability of each outcome is 25%. Now, however, we can look inside the box and determine whether the cats share the same fate: both dead or alive, or one dead and the other alive. In all cases, Erwin and Niels are still simultaneously dead and alive after the observation, and quantum superposition is therefore maintained. However, now the possible number of outcomes is no longer four, but two.'

## Feedback control

The outcome of the measurement is still random, entirely in accordance with laws of quantum mechanics, but the scientists have gone a step further. DiCarlo: 'To stay with the example of the cats, if the measurement shows that one of the cats is dead and the other one is alive, we can alter the state of one cat so that they are both dead or both alive. The quantum state is still maintained: they are still dead and alive at the same time, but we can influence the outcome so that their fate is the same.’

For this purpose the researchers developed a method that uses feedback control. Two years ago, this was almost unthinkable for qubits. Until recently, these circuits retained their quantum behaviour for barely a millionth of a second. ‘Advances in superconducting qubits have increased this time by a factor of 10-100, so that we were finally able to close the feedback loop quickly enough,’ explains the first author of the Nature article, Diego Ristè.

For this purpose the researchers developed a method that uses feedback control. Two years ago, this was almost unthinkable for qubits. Until recently, these circuits retained their quantum behaviour for barely a millionth of a second. ‘Advances in superconducting qubits have increased this time by a factor of 10-100, so that we were finally able to close the feedback loop quickly enough,’ explains the first author of the Nature article, Diego Ristè.

## Quantum computer

This method is important for the development of the quantum computer, pursued by a large team of TU Delft researchers. Theoretical physicist Yaroslav Blanter: 'The main problem with quantum bits is that they lose their quantum state after a time. There is a method that can maintain this state, using quantum error correction. Our experiment demonstrates the two steps that are crucial in carrying out quantum error correction and preserving quantum states longer. The next step for the team will be to develop within five years a self-correcting quantum memory using 20 qubits.

Other authors of the Nature article are M. Dukalski, C. A. Watson, G. de Lange, M. J. Tiggelman, R. N. Schouten, all from TU Delft, and K. W. Lehnert of the University of Colorado and NIST.

We acknowledge funding from the Dutch Organization for Fundamental Research on Matter (FOM), the Netherlands Organization for Scientific Research (NWO, VIDI scheme), the EU FP7 integrated projects SOLID and SCALEQIT, and partial support from the DARPA QuEST program.

We acknowledge funding from the Dutch Organization for Fundamental Research on Matter (FOM), the Netherlands Organization for Scientific Research (NWO, VIDI scheme), the EU FP7 integrated projects SOLID and SCALEQIT, and partial support from the DARPA QuEST program.

## More information

**'Deterministic entanglement of superconducting qubits by parity measurement and feedback'**Nature, 17 October 2013

Authors: D. Ristè,1 M. Dukalski,1 C. A. Watson,1 G. de Lange,1 M. J. Tiggelman,1 Ya. M. Blanter,1 K. W. Lehnert,2 R. N. Schouten,1 and L. DiCarlo1

1 Kavli Institute of Nanoscience, Delft University of Technology, The Netherlands

2 JILA, National Institute of Standards and Technology and Department of Physics, University of Colorado, Boulder, Colorado, USA

Contact: dr. Leonardo DiCarlo, Assistant Professor Quantum Transport, Kavli Institute of Nanoscience

l.dicarlo@tudelft.nl, +31 15 278 6097, www.dicarlolab.tudelft.nl

Science Information Officer TU Delft Roy Meijer, +31 15 278 1751, +31 6 14015008,r.e.t.meijer@tudelft.nl

l.dicarlo@tudelft.nl, +31 15 278 6097, www.dicarlolab.tudelft.nl

Science Information Officer TU Delft Roy Meijer, +31 15 278 1751, +31 6 14015008,r.e.t.meijer@tudelft.nl

Image: Artist's impression of two superconducting qubits inside a microwave-frequency cavity, also illustrating the creation of their entanglement by a parity measurement.

The starting state of the qubits, a superposition of the states 00, 01, 10, and 11, is represented by four colored beams impinging on the cavity. Only two of the beams cross the cavity and become intertwined, symbolizing the generation of entanglement in the form of a 01 and 10 superposition.

The entanglement is created by the measurement, here represented by the white beam traversing the cavity through two connectors.

Copyright Tremani /TU Delft