November 23, 2020• Physics 13, 183

Classical computers can efficiently simulate the behavior of quantum computers if the quantum computer is imperfect enough.

With a few quantum bits, an ideal quantum computer can process vast amounts of information in a coordinated way, making it significantly more powerful than a classical counterpart. This predicted power increase will be great for users but is bad for physicists trying to simulate on a classical computer how an ideal quantum computer will behave. Now, a trio of researchers has shown that they can substantially reduce the resources needed to do these simulations if the quantum computer is imperfect [1]. The arXiv version of the trios paper is one of the most Scited papers of 2020 and the result generated quite a stir when it first appeared back in FebruaryI overheard it being enthusiastically discussed at the Quantum Optics Conference in Obergurgl, Austria, at the end of that month, back when we could still attend conferences in person.

In 2019, Google claimed to have achieved the quantum computing milestone known as quantum advantage, publishing results showing that their quantum computer Sycamore had performed a calculation that was essentially impossible for a classical one [2]. More specifically, Google claimed that they had completed a three-minute quantum computationwhich involved generating random numbers with Sycamores 53 qubitsthat would take thousands of years on a state-of-the-art classical supercomputer, such as IBMs Summit. IBM quickly countered the claim, arguing that more efficient memory storage would reduce the task time on a classical computer to a couple of days [3]. The claims and counterclaims sparked an industry clash and an intense debate among supporters in the two camps.

Resolving the disparity between these estimates is one of the goals of the new work by Yiqing Zhou, of the University of Illinois at UrbanaChampaign, and her two colleagues [1]. In their study, they focused on algorithms for classically replicating imperfect quantum computers, which are also known as NISQ (noisy intermediate-scale quantum) devices [4]. Todays state-of-the-art quantum computersincluding Sycamoreare NISQ devices. The algorithms the team used are based on so-called tensor network methods, specifically matrix product states (MPS), which are good for simulating noise and so are naturally suited for studying NISQ devices. MPS methods approximate low-entangled quantum states with simpler structures, so they provide a data-compression-like protocol that can make it less computationally expensive to classically simulate imperfect quantum computers (see Viewpoint: Pushing Tensor Networks to the Limit).

Zhou and colleagues first consider a random 1D quantum circuit made of neighboring, interleaved two-qubit gates and single-qubit random unitary operations. The two-qubit gates are either Controlled-NOT gates or Controlled-Z (CZ) gates, which create entanglement. They ran their algorithm for NISQ circuits containing different numbers of qubits, N, and different depths, Da parameter that relates to the number of gates the circuit executes (Fig. 1). They also varied a parameter in the MPS algorithm. is the so-called bond dimension of the MPS and essentially controls how well the MPS capture entanglement between qubits.

The trio demonstrate that they can exactly simulate any imperfect quantum circuit if D and N are small enough and is set to a value within reach of a classical computer. They can do that because shallow quantum circuits can only create a small amount of entanglement, which is fully captured by a moderate . However, as D increases, the team finds that cannot capture all the entanglement. That means that they cannot exactly simulate the system, and errors start to accumulate. The team describes this mismatch between the quantum circuit and their classical simulations using a parameter that they call the two-qubit gate fidelity fn. They find that the fidelity of their simulations slowly drops, bottoming out at an asymptotic value f as D increases. This qualitative behavior persists for different values of N and . Also, while their algorithm does not explicitly account for all the error and decoherence mechanisms in real quantum computers, they show that it does produce quantum states of the same quality (perfection) as the experimental ones.

In light of Googles quantum advantage claims, Zhou and colleagues also apply their algorithm to 2D quantum systemsSycamore is built on a 2D chip. MPS are specifically designed for use in 1D systems, but the team uses well-known techniques to extend their algorithm to small 2D ones. They use their algorithm to simulate an N=54, D=20 circuit, roughly matching the parameters of Sycamore (Sycamore has 54 qubits but one is unusable because of a defect). They replace Googles more entangling iSWAP gates with less entangling CZ gates, which allow them to classically simulate the system up to the same fidelity as reported in Ref. [2] with a single laptop. The simulation cost should increase quadratically for iSWAP-gate circuits, and although the team proposes a method for performing such simulations, they have not yet carried them out because of the large computational cost it entails.

How do these results relate to the quantum advantage claims by Google? As they stand, they do not weaken or refute claimswith just a few more qubits, and an increase in D or f, the next generation of NISQ devices will certainly be much harder to simulate. The results also indicate that the teams algorithm only works if the quantum computer is sufficiently imperfectif it is almost perfect, their algorithm provides no speed up advantage. Finally, the results provide numerical insight into the values of N, D, f, and for which random quantum circuits are confined to a tiny corner of the exponentially large Hilbert space. These values give insight into how to quantify the capabilities of a quantum computer to generate entanglement as a function of f, for example.

So, whats next? One natural question is, Can the approach here be transferred to efficiently simulate other aspects of quantum computing, such as quantum error correction? The circuits the trio considered are essentially random, whereas quantum error correction circuits are more ordered by design [5]. That means that updates to the new algorithm are needed to study such systems. Despite this limitation, the future looks promising for the efficient simulation of imperfect quantum devices [6, 7].

Jordi Tura is an assistant professor at the Lorentz Institute of the University of Leiden, Netherlands. He also leads the institutes Applied Quantum Algorithms group. Tura obtained his B.Sc. degrees in mathematics and telecommunications and his M.Sc. in applied mathematics from the Polytechnic University of Catalonia, Spain. His Ph.D. was awarded by the Institute of Photonic Sciences, Spain. During his postdoctoral stay at the Max Planck Institute of Quantum Optics in Germany, Tura started working in the field of quantum information processing for near-term quantum devices.

A nanopatterned magnetic structure features an unprecedently strong coupling between lattice vibrations and quantized spin waves, which could lead to novel ways of manipulating quantum information. Read More

Go here to see the original:

Imperfections Lower the Simulation Cost of Quantum Computers - Physics

- Quantum computing research helps IBM win top spot in patent race - CNET - January 14th, 2021
- Error Protected Quantum Bits Entangled: A Milestone in the Development of Fault-Tolerant Quantum Computers - SciTechDaily - January 14th, 2021
- You can find a $180K solar-powered car, qubit controls, and breathing tips at the NL Tech Pavilion at CES 2021 - TechRepublic - January 14th, 2021
- NSA Cites DoD Crypto Work in First-Ever Cybersecurity Year-End Report - MeriTalk - January 14th, 2021
- 'Magic' angle graphene and the creation of unexpected topological quantum states - Princeton University - December 15th, 2020
- Bringing Your Mainframe Into the Cloud Age - CMSWire - December 15th, 2020
- ASC20-21 Student Supercomputer Challenge Kickoff: Quantum Computing Simulations, AI Language Exam and Pulsar Searching with FAST - Business Wire - November 26th, 2020
- Virtual ICM Seminar with Hiroaki Kitano, 'Nobel Turing Challenge-Creating the Engine of Scientific Discovery' to Be Held Nov 26 - HPCwire - November 26th, 2020
- The Trillion-Transistor Chip That Just Left a Supercomputer in the Dust - Singularity Hub - November 26th, 2020
- Is the blockchain vulnerable to hacking by quantum computers? - Moneyweb.co.za - November 24th, 2020
- Can a Computer Devise a Theory of Everything? - The New York Times - November 24th, 2020
- Cracking the Secrets of an Emerging Branch of Physics: Exotic Properties to Power Real-World Applications - SciTechDaily - November 24th, 2020
- Inside the Competition That Will Save Bitcoin From Quantum Computers - Decrypt - November 22nd, 2020
- Foreign policy expert: China is 'outstripping us' in technologies of the future - Brainerd Dispatch - November 22nd, 2020
- Cracking the secrets of an emerging branch of physics - MIT News - November 22nd, 2020
- #SpaceWatchGL Opinion: Quantum Technology and Impact of the Global Space Security - SpaceWatch.Global - November 22nd, 2020
- A Scoville Heat Scale For Measuring The Progress Of Emerging Technologies In 2021 - Forbes - November 22nd, 2020
- Democracies must team up to take on China in the technosphere - The Economist - November 22nd, 2020
- Honeywell fires up the H1, its second-generation quantum computer - CNET - November 2nd, 2020
- Quantum computers could soon reveal all of our secrets. The race is on to stop that happening - ZDNet - November 2nd, 2020
- Quantum Computing Expert Warns Governments May Be First to Crack Algorithms Keeping Bitcoin and the Internet Secure - The Daily Hodl - November 2nd, 2020
- Australia's Archer and its plan for quantum world domination - ZDNet - November 2nd, 2020
- Quantum Computing Is Bigger Than Donald Trump - WIRED - November 2nd, 2020
- Will Quantum Mechanics Produce the True Thinking Computer? - Walter Bradley Center for Natural and Artificial Intelligence - November 2nd, 2020
- Strategic Partnership will aid smooth work in the event of regional crisis: Australia High Commissioner - The Hindu - November 2nd, 2020
- Valuation of quantum computer maker D-Wave slashed by more than half after company struggles to raise financing - The Globe and Mail - October 27th, 2020
- 60-year-old limit to lasers overturned by quantum researchers - Griffith News - October 27th, 2020
- A Measured Approach to Regulating Fast-Changing Tech - Harvard Business Review - October 27th, 2020
- The Importance of Funding Quantum Physics, Even in a Pandemic - Inside Philanthropy - October 23rd, 2020
- Material found in paint may hold the key to a technological revolution - Advanced Science News - October 23rd, 2020
- What is Quantum Computing, and How does it Help Us? - Analytics Insight - October 13th, 2020
- QCE20: Here's what you can expect from Intel's new quantum computing research this week - Neowin - October 13th, 2020
- Canadian quantum computing firms partner to spread the technology - IT World Canada - October 13th, 2020
- Ten-year Forecasts for Quantum Networking Opportunities and Deployments Over the Coming Decade - WFMZ Allentown - October 13th, 2020
- Berkeley Lab Technologies Honored With 7 R&D 100 Awards - Lawrence Berkeley National Laboratory - October 5th, 2020
- IBM Partners With HBCUs to Diversify Quantum Computing Workforce - Diverse: Issues in Higher Education - September 25th, 2020
- IBM, Alphabet and well-funded startups in the race for quantum supremacy - IT Brief Australia - September 25th, 2020
- How This Bangalore Based Startup Is Driving Innovation With Quantum Technology-Based Products - Analytics India Magazine - September 25th, 2020
- New faculty add to Yale's strength in applied mathematics - Yale News - September 25th, 2020
- NU receives $115 million federal grant to research and develop beyond state-of-the-art quantum computer - Daily Northwestern - September 24th, 2020
- IBM Just Committed to Having a Functioning 1,000 Qubit Quantum Computer by 2023 - ScienceAlert - September 24th, 2020
- IBM plans to build a 1121 qubit system. What does this technology mean? - The Hindu - September 24th, 2020
- Extending the life of the qubit | Temple Now - Temple University News - September 24th, 2020
- OSTP, NSF, DoE, and IBM make major push to strengthen research in AI and quantum - BlackEngineer.com - September 24th, 2020
- Heres why quantum computing is a cat among the pigeons - BusinessLine - September 12th, 2020
- The Hyperion-insideHPC Interviews: ORNL Distinguished Scientist Travis Humble on Coupling Classical and Quantum Computing - insideHPC - September 12th, 2020
- Oxford Instruments Partners With The 10 Million Consortium, To Launch The First Commercial Quantum Computer In UK - AZoNano - September 10th, 2020
- Combinations of new technologies will upend finance - The Australian Financial Review - September 10th, 2020
- Quantum Computing Market Analysis by Growth, segmentation, performance, Competitive Strategies and Forecast to 2026 - Galus Australis - September 10th, 2020
- The Quantum Dream: Are We There Yet? - Toolbox - September 7th, 2020
- 17 extremely useful productivity tips from this years 40 Under 40 - Yahoo Finance UK - September 7th, 2020
- How Amazon Quietly Powers The Internet - Forbes - September 7th, 2020
- Study Expands Types of Physics, Engineering Problems That Can Be Solved by Quantum Computers - HPCwire - September 4th, 2020
- New evidence that the quantum world is even stranger than we thought - Purdue News Service - September 4th, 2020
- How Andersen Cheng plans to defend against the quantum computer - The Independent - September 4th, 2020
- Quantum computer to be hosted in Abingdon - ClickLancashire - September 4th, 2020
- Assistant director of NSFs Computer and Information Science and Engineering to give virtual talk Sept. 11 - Vanderbilt University News - September 4th, 2020
- Fermilab to lead $115 million National Quantum Information Science Research Center to build revolutionary quantum computer with Rigetti Computing,... - August 29th, 2020
- I confess, I'm scared of the next generation of supercomputers - TechRadar - August 29th, 2020
- Q-NEXT collaboration awarded National Quantum Initiative funding - University of Wisconsin-Madison - August 29th, 2020
- UArizona Scientists to Build What Einstein Wrote off as Science Fiction - UANews - August 29th, 2020
- Quantum leap? US plans for unhackable internet may not fructify within a decade, but India is far behind - The Financial Express - August 4th, 2020
- Google distinguished scientist Hartmut Neven is one of Fast Company's - Fast Company - August 4th, 2020
- Quantum physicists say time travelers don't have to worry about the butterfly effect - The Next Web - August 2nd, 2020
- Week in review: BootHole, RCEs in industrial VPNs, the cybersecurity profession crisis - Help Net Security - August 2nd, 2020
- New UC-led institute awarded $25M to explore potential of quantum computing and train a future workforce - University of California - July 31st, 2020
- The future of encryption: Getting ready for the quantum computer attack - TechRepublic - July 31st, 2020
- IBM and University of Tokyo team up for Quantum Innovation Initiative Consortium - SmartPlanet.com - July 31st, 2020
- 'Butterfly effect' is wrong and reality can 'heal itself', quantum scientists find in time travel experiment - The Independent - July 31st, 2020
- Research: the butterfly effect does not exist in the quantum model - FREE NEWS - July 31st, 2020
- Solving problems by working together: Could quantum computing hold the key to Covid-19? - ITProPortal - July 2nd, 2020
- Spain Introduces the World's First Quantum Phase Battery - News - All About Circuits - July 2nd, 2020
- Professor tackles one more mystery about quantum mechanics and times flow - GeekWire - July 2nd, 2020
- This Week's Awesome Tech Stories From Around the Web (Through June 27) - Singularity Hub - June 29th, 2020
- Kudos: Read about faculty, staff and student awards, appointments and achievements - Vanderbilt University News - June 29th, 2020
- This Is the First Universal Language for Quantum Computers - Popular Mechanics - June 21st, 2020
- Universal Quantum raises $4.5 million to build a large-scale quantum computer - VentureBeat - June 17th, 2020
- Ethereum (ETH) Might Not have Quantum Resistance on its Roadmap, the QRL Team Reveals - Crowdfund Insider - June 17th, 2020
- Craig Knoblock Named Michael Keston Executive Director of the USC Information Sciences Institute - USC Viterbi School of Engineering - June 17th, 2020
- European quantum computing startup takes its funding to 32M with fresh raise - TechCrunch - June 11th, 2020

## Recent Comments