# New tool for characterizing quantum simulators

September 06, 2017Many phenomena in the quantum world cannot be investigated directly in the laboratory, and even supercomputers fail when trying to simulate them. However, scientists are now able to control various quantum systems in the laboratory very precisely and these systems can be used to simulate other quantum systems. Such Quantum Simulators are therefore considered to be one of the first concrete applications of the second quantum revolution.

However, the characterization of large quantum states, which is necessary to guide the development of large-scale quantum simulators, proves to be difficult. The current gold standard for quantum-state characterization in the laboratory - quantum-state tomography - is only suitable for small quantum systems composed of a handful of quantum particles. Researchers from the Institute of Experimental Physics at the University of Innsbruck and the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences have now established a new method in the laboratory that can be used to efficiently characterize large quantum states.

**A collaborative effort**

In ion traps, charged atoms (ions) are cooled to temperatures close to absolute zero and manipulated with the aid of lasers. Such systems represent a promising approach to performing quantum simulations that can go beyond the capabilities of modern supercomputers. The Innsbruck quantum physicists are amongst the world leaders in this field and can currently entangle 20 or more ions in their traps. In order to fully characterize such large quantum systems, they need new methods. For this, theorists around Martin Plenio from the University of Ulm, Germany, came to their aid. In 2010, the Plenio team proposed a new method for the characterization of complex quantum states called matrix-product-state tomography. Using this method, the state of a group of entangled quantum particles can be estimated precisely without the effort increasing dramatically as the number of particles in the group is increased. In collaboration with the teams around Martin Plenio from Ulm and Andrew Daley from the University of Strathclyde in Scotland, the Innsbruck experimental physicists around Christian Roos, Ben Lanyon and Christine Maier have now implemented this procedure in the laboratory.

**More efficient measurements**

As a test case, the physicists built a quantum simulator with up to 14 quantum bits (atoms), that was first prepared in a simple initial state without quantum correlations. Next, the researchers entangled the atoms with laser light and observed the dynamical propagation of entanglement in the system. "With the new method, we can determine the quantum state of the whole system by measuring only a small fraction of the system properties," says START prize winner Ben Lanyon. The theorists around Martin Plenio took the characterization of the global quantum state from the measured data: "The method is based on the fact that we can theoretically describe locally-distributed entanglement well and can now also measure it in the laboratory."

When the work group of Rainer Blatt realized the first quantum byte in 2005, more than 6,000 measurements were required for the characterization of the quantum state, taken over a period of ten hours. The new method requires only 27 measurements to characterise the same size system, taken over around 10 minutes. "We were able to show that this method can be used to identify large and complex quantum states efficiently," says Christine Maier, a team member from Innsbruck. Now the scientists want to further develop the algorithms so that they can also be used flexibly by other research groups.

**New gold standard**

The new method allows the complete characterization of systems containing large numbers of correlated quantum particles and thus provides a comparison option for quantum simulations. "We can use the new technique to calibrate quantum simulators, by comparing the states that we find in the lab with the ones expected from analytical calculations," explains Christian Roos. "Then we know if the simulator does what we want." The new method offers physicians a tool for many applications and could become a new standard for quantum simulations.

-end-

The work was financially supported by, among others, the Austrian Science Fund FWF and the European Union.Publication: Efficient tomography of a quantum many-body system. B. P. Lanyon, C. Maier, M. Holzäpfel, T. Baumgratz, C. Hempel, P. Jurcevic, I. Dhand, A. S. Buyskikh, A. J. Daley, M. Cramer, M. B. Plenio, R. Blatt, C. F. Roos.

*Nature Physics*2017 DOI: 10.1038/nphys4244.

University of Innsbruck

## Related Quantum Articles from Brightsurf:

Theoreticians show which quantum systems are suitable for quantum simulations

A joint research group led by Prof. Jens Eisert of Freie Universität Berlin and Helmholtz-Zentrum Berlin (HZB) has shown a way to simulate the quantum physical properties of complex solid state systems.

Quantum shake

There they were, in all their weird quantum glory: ultracold lithium atoms in the optical trap operated by UC Santa Barbara undergraduate student Alec Cao and his colleagues in David Weld's atomic physics group.

New evidence for quantum fluctuations near a quantum critical point in a superconductor

A study has found evidence for quantum fluctuations near a quantum critical point in a superconductor.

Quantum simulation of quantum crystals

International research team describes the new possibilities offered by the use of ultracold dipolar atoms

Quantum machines learn "quantum data"

Skoltech scientists have shown that quantum-enhanced machine learning can be used on quantum (as opposed to classical) data, overcoming a significant slowdown common to these applications and opening a ''fertile ground to develop computational insights into quantum systems''.

Simulating quantum 'time travel' disproves butterfly effect in quantum realm

Using a quantum computer to simulate time travel, researchers have demonstrated that, in the quantum realm, there is no 'butterfly effect.' In the research, information--qubits, or quantum bits--'time travel' into the simulated past.

Orbital engineering of quantum confinement in high-Al-content AlGaN quantum well

Recently, professor Kang's group focus on the limitation of quantum confine band offset model, the hole states delocalization in high-Al-content AlGaN quantum well are understood in terms of orbital intercoupling.

Quantum classifiers with tailored quantum kernel?

Quantum information scientists have introduced a new method for machine learning classifications in quantum computing.

A Metal-like Quantum Gas: A pathbreaking platform for quantum simulation

Coherent and ultrafast laser excitation creates an exotic matter phase with spatially overlapping electronic wave-functions under nanometric control in an artificial micro-crystal of ultracold atoms.

Quantum leap: Photon discovery is a major step toward at-scale quantum technologies

A team of physicists at the University of Bristol has developed the first integrated photon source with the potential to deliver large-scale quantum photonics.

Read More: Quantum News and Quantum Current Events

A joint research group led by Prof. Jens Eisert of Freie Universität Berlin and Helmholtz-Zentrum Berlin (HZB) has shown a way to simulate the quantum physical properties of complex solid state systems.

Quantum shake

There they were, in all their weird quantum glory: ultracold lithium atoms in the optical trap operated by UC Santa Barbara undergraduate student Alec Cao and his colleagues in David Weld's atomic physics group.

New evidence for quantum fluctuations near a quantum critical point in a superconductor

A study has found evidence for quantum fluctuations near a quantum critical point in a superconductor.

Quantum simulation of quantum crystals

International research team describes the new possibilities offered by the use of ultracold dipolar atoms

Quantum machines learn "quantum data"

Skoltech scientists have shown that quantum-enhanced machine learning can be used on quantum (as opposed to classical) data, overcoming a significant slowdown common to these applications and opening a ''fertile ground to develop computational insights into quantum systems''.

Simulating quantum 'time travel' disproves butterfly effect in quantum realm

Using a quantum computer to simulate time travel, researchers have demonstrated that, in the quantum realm, there is no 'butterfly effect.' In the research, information--qubits, or quantum bits--'time travel' into the simulated past.

Orbital engineering of quantum confinement in high-Al-content AlGaN quantum well

Recently, professor Kang's group focus on the limitation of quantum confine band offset model, the hole states delocalization in high-Al-content AlGaN quantum well are understood in terms of orbital intercoupling.

Quantum classifiers with tailored quantum kernel?

Quantum information scientists have introduced a new method for machine learning classifications in quantum computing.

A Metal-like Quantum Gas: A pathbreaking platform for quantum simulation

Coherent and ultrafast laser excitation creates an exotic matter phase with spatially overlapping electronic wave-functions under nanometric control in an artificial micro-crystal of ultracold atoms.

Quantum leap: Photon discovery is a major step toward at-scale quantum technologies

A team of physicists at the University of Bristol has developed the first integrated photon source with the potential to deliver large-scale quantum photonics.

Read More: Quantum News and Quantum Current Events

Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.