Nav: Home

Digital quantum simulators can be astonishingly robust

May 14, 2019

In solving quantum-physical problems in many-body systems, such as predicting material properties, conventional computers rapidly reach the limits of their capacity. Digital quantum simulators might help, but until now they are drastically limited to small systems with few particles and only short simulation times. Now, Heidelberg University physicist Dr Philipp Hauke and colleagues from Dresden and Innsbruck (Austria) have demonstrated that such simulations can be more "robust" and hence much more stable than previously assumed. The results of their research were published in "Science Advances".

In quantum physics, many-body theory describes a large number of interacting particles. In the state of thermodynamic equilibrium, the many-body system can be described by only a handful of values such as temperature or pressure, which are largely homogeneous for the entire system. But what happens over time after a major perturbation, such as when energy is abruptly deposited in a material sample by short laser pulses? Precisely calculating the so-called nonequilibrium dynamics of interacting many-body systems is a high-profile problem in quantum physics.

Calculations using conventional computers require resources that increase exponentially with the number of constituent quantum particles. "So computationally exact methods fail with just a few dozen particles. That is far less than the number needed to predict material properties, for example. In such cases, scientists rely on approximation methods that are often uncontrolled, particularly when it comes to dynamic properties," explains Dr Hauke, a researcher at the Kirchhoff Institute for Physics and the Institute for Theoretical Physics of Heidelberg University. Digital quantum simulation provides one possible workaround. The nonequilibrium dynamics are studied with simulators that themselves are governed by quantum-mechanical laws.

Depicting the time evolution in a quantum computer requires discretising it into individual operations. But this approach - also known as Trotterization - unavoidably generates an error inherent in the simulation itself. This Trotter error can be mitigated by sufficiently fine discretisations. Extremely small discretisation steps must be chosen, however, to depict reliably a longer time evolution. Until now, research has maintained that the error quickly grows over long time periods and with a larger number of particles - which for all practical purposes drastically limits digital quantum simulation to small systems and short times.

Using numerical demonstrations and analytical arguments, the researchers have now shown that quantum simulation is much more "robust" and hence more stable than previously assumed, as long as only values that are relevant in practice - such as averages across the entire system - are considered and not the full state of each individual particle. For such values, there is a sharp threshold between a region with controllable errors and a simulation that can no longer deliver a usable result. Below this threshold, the Trotter error has only limited impact - in fact for all time periods that could be practically simulated and largely independent of the number of constituent particles.

At the same time, the research showed that digital quantum simulation can deliver astonishingly precise results using unexpectedly large Trotter steps. "A simulation that can predict the behaviour of many quantum particles over a longer time therefore becomes more and more likely. This further opens the door for practical applications, ranging from materials science and quantum chemistry to issues in fundamental physics," states Dr Hauke, who heads the "Quantum optics and quantum many-body theory" research group.
The research was conducted in collaboration with Dr Markus Heyl of the Max Planck Institute for the Physics of Complex Systems in Dresden and Prof. Dr Peter Zoller of the University of Innsbruck. At Heidelberg University, it was carried out as part of the Collaborative Research Centre "Isolated Quantum Systems and Universality in Extreme Conditions" (SFB 1225).

University of Heidelberg

Related Quantum Articles:

Quantum nanoscope
Researchers have studied how light can be used to 'see' the quantum nature of an electronic material.
'Quantum leap' for Liverpool
Physicists from the University of Liverpool have made a huge step forwards towards building a novel experiment to probe the 'dark contents' of the vacuum.
Testing quantum field theory in a quantum simulator
Quantum field theories are often hard to verify in experiments.
Quantum reservoir for microwaves
EPFL researchers use a mechanical micrometer-size drum cooled close to the quantum ground state to amplify microwaves in a superconducting circuit.
Looking for the quantum frontier
Researchers have developed a new theoretical framework to identify computations that occupy the 'quantum frontier' -- the boundary at which problems become impossible for today's computers and can only be solved by a quantum computer.
Quantum mechanics are complex enough, for now...
Physicists have searched for deviations from standard quantum mechanics, testing whether quantum mechanics requires a more complex set of mathematical rules.
Seeing the quantum future... literally
Sydney scientists have demonstrated the ability to 'see' the future of quantum systems and used that knowledge to preempt their demise, in a major achievement that could help bring the strange and powerful world of quantum technology closer to reality.
The sound of quantum vacuum
Quantum mechanics dictates sensitivity limits in the measurements of displacement, velocity and acceleration.
New quantum states for better quantum memories
How can quantum information be stored as long as possible?
Watching quantum jumps
When a quantum system changes its state, this is called a quantum jump.

Related Quantum Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Jumpstarting Creativity
Our greatest breakthroughs and triumphs have one thing in common: creativity. But how do you ignite it? And how do you rekindle it? This hour, TED speakers explore ideas on jumpstarting creativity. Guests include economist Tim Harford, producer Helen Marriage, artificial intelligence researcher Steve Engels, and behavioral scientist Marily Oppezzo.
Now Playing: Science for the People

#524 The Human Network
What does a network of humans look like and how does it work? How does information spread? How do decisions and opinions spread? What gets distorted as it moves through the network and why? This week we dig into the ins and outs of human networks with Matthew Jackson, Professor of Economics at Stanford University and author of the book "The Human Network: How Your Social Position Determines Your Power, Beliefs, and Behaviours".