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Quantum mechanics is very hard to simulate on a classical computer. The challenge lies in capturing all possible quantum states allowed in a given system which could be populated at once. In other words, a system of 50 quantum bits already requires 250 classical bits of information to store all possible quantum states the system may visit in a given dynamical evolution. Computing such an evolution is already not possible with the largest supercomputer on Earth.
Quantum simulators are a special type of quantum computer that uses qubits to simulate complex interactions between particles. According to definitions used in the research community, quantum simulators are designed to model specific quantum processes, whereas quantum computers are universally applicable to any desired calculation.
Quantum simulators consist of small arrays of quantum bits (qubits) that can each represent multiple states of information simultaneously. Qubits are the informational medium of quantum computers, analogous to a bit in an ordinary computer. Yet rather than existing as a 1 or 0, as is the case in a conventional bit, a qubit can exist in some superposition of both of these states at the same time.
Qubits come in different forms, and atoms—the versatile building blocks of everything—are one of the leading choices for making qubits. In recent years, scientists have controlled 10 to 20 atomic qubits in small-scale quantum simulations. Lasers are used to manipulate the qubits in a vacuum chamber to simulate quantum interactions between the particles.
Quantum simulators are designed to model and mimic complex physics phenomena in a way that is impossible with conventional machines, even supercomputers. They could tackle very specific problems in scientific fields such as high-energy physics and chemistry techniques also help improve atomic clocks. Quantum simulators might also help study problems such as how the universe began, how to engineer novel technologies (for instance, room-temperature superconductors or atom-scale heat engines), or accelerate the development of quantum computers.
“Quantum simulations are widely believed to be one of the first useful applications of quantum computers. After perfecting these quantum simulators, we can then implement quantum circuits and eventually quantum-connect many such ion chains together to build a full-scale quantum computer with a much wider domain of applications,” said Alexey Gorshkov, a NIST theoretical physicist, JQI and QuICS fellow, and adjunct assistant professor in the UMD Department of Physics.
A smaller-sized version of a quantum computer can already simulate other quantum systems of significant relevance. Such a quantum simulator is suited to explore a wide range of physical systems e.g. exotic phase transitions in condensed matter physics, novel materials, fertilizers, drugs etc.
For instance, proof of concept experiments on optical lattices have already shown that cold atoms simulate the behavior of electrons in a real material. Those experiments allowed the exploration of critical properties of artificial matter. Many quantum practitioners hold the opinion that quantum simulation is the way to push technology in order to explore the computational power of quantum mechanics as the first true application of quantum processors.
There is fierce race among industry behemoths, startups and university researchers to build prototype that can entangle and control more and more qubits.
In recent work, 51 qubit quantum simulator has been designed by Harvard and MIT researchers that uses rubidium atoms confined by an array of laser beams. One of the group’s key achievements of Omran and his colleagues was by using 101 lasers to shine upon a “dilute vapor of rubidium atoms,” they could “create perfect atom arrays of any desired size and pattern with up to 51 particles.”Researchers then by focusing additional lasers on individual electrons provided energy to electrons tightly orbiting the atomic nucleus into a much larger orbit—a Rydberg state effectively turning the atoms into qubits and to effectively manipulate them as qubits.
“The distance between atoms determines the interaction strength, and since we can control the position of each atom individually, we can program various interaction patterns and study the evolution of this quantum many-body system,” Omran explains.
Another group at University of Maryland and National Institute of Standards and Technology team were able to create a quantum simulator with 53 qubits. For the Maryland experiment, each of the qubits was a laser cooled ytterbium ion. Each ion had the same electrical charge, so they repelled one another when placed in close proximity. The system created by Monroe and his colleagues used an electric field to force the repelled ions into neat rows.
In 2016, Physicists at the National Institute of Standards and Technology (NIST) have “entangled” or linked together the properties of up to 219 beryllium ions (charged atoms) to create a quantum simulator. The behavior of the entangled ions rotating in a flat crystal just 1 millimeter in diameter can also be tailored or controlled to a greater degree than before.
A group of physicists in China has also joined the race to couple together increasing numbers of superconducting qubits. The researchers have shown that they can entangle 10 qubits connected to one another via a central resonator and say that their result paves the way to quantum simulators that can calculate the behaviour of small molecules and other quantum-mechanical systems much more efficiently than even the most powerful conventional computers.
Quantum Computing Moves Forward with Record Setting UMD-NIST 53 Qubit Quantum Simulator
By deploying 53 individual ytterbium ions—charged atoms trapped in place by gold-coated and razor-sharp electrodes—the UMD-NIST quantum simulator is on the cusp of exploring physics that is unreachable by even the fastest modern supercomputers.
Lasers are used to manipulate all the ytterbium qubits into the same initial state. Then another set of lasers is used to manipulate the qubits so that they act like atomic magnets, where each ion has a north and south pole. The qubits either orient themselves with their neighboring ions to form a ferromagnet, where their magnetic fields are aligned, or at random. By changing the strength of the laser beams that are manipulating the qubits, the researchers are able to program them to a desired state (in terms of magnetic alignment).
“Each ion qubit is a stable atomic clock that can be perfectly replicated,” said UMD team lead Christopher Monroe, a Distinguished University Professor of Physics and Bice Sechi-Zorn Professor at UMD, and co-founder and chief scientist of IonQ Inc., a UMD-based quantum computing startup company. “They are effectively wired together with external laser beams. This means that the same device can be reprogrammed and reconfigured, from the outside, to adapt to any type of quantum simulation or future quantum computer application that comes up.”
“What makes this problem hard is that each magnet interacts with all the other magnets,” said UMD research scientist Zhexuan Gong, lead theorist and a co-author of the study. “With the 53 interacting quantum magnets in this experiment, there are over a quadrillion possible magnet configurations, and this number doubles with each additional magnet. Simulating this large-scale problem on a conventional computer is extremely challenging, if at all possible.”
The building of qubit simulators is a key step in efforts to build a full-fledged quantum computer capable of tackling any complex computational problem. And, according to the UMD-NIST team, adding even more qubits is just a matter of lassoing more atoms into the mix.
“We are continuing to refine our system, and we think that soon, we will be able to control 100 ion qubits, or more,” said Jiehang Zhang, a postdoctoral researcher in the UMD Department of Physics, and the lead author of a paper about the team’s 53 qubit quantum simulator that appears in this week’s issue of the journal Nature. “At that point, we can potentially explore difficult problems in quantum chemistry or materials design.”
NIST’s Super quantum simulator ‘entangles’ hundreds of ions
Described in the June 10, 2016, issue of Science, NIST’s latest simulator improves on the same research group’s 2012 version by removing most of the earlier system’s errors and instabilities, which can destroy fragile quantum effects.
“Here we get clear, indisputable proof the ions are entangled,” NIST postdoctoral researcher Justin Bohnet said. “What entanglement represents in this case is a useful resource for something else, like quantum simulation or to enhance a measurement in an atomic clock.”
In the NIST quantum simulator, ions act as quantum bits (qubits) to store information. Trapped ions are naturally suited to studies of quantum physics phenomena such as magnetism. Quantum simulators with hundreds of qubits have been made of other materials such as neutral atoms and molecules. But trapped ions offer unique advantages such as reliable preparation and detection of quantum states, long-lived states, and strong couplings among qubits at a variety of distances.
The ion crystals are held inside a Penning trap, which confines charged particles by use of magnetic and electric fields. The ions naturally form triangular patterns, useful for studying certain types of mag-netism. NIST is the only laboratory in the world generating two-dimensional arrays of more than 100 ions. Based on lessons learned in the 2012 experiment, NIST researchers designed and assembled a new trap to generate stronger and faster interactions among the ions. The interaction strength is the same for all ions in the crystal, regardless of the distances between them.
The researchers used lasers with improved position and intensity control, and more stable magnetic fields, to engineer certain dynamics in the “spin” of the ions’ electrons. Ions can be spin up (often envisioned as an arrow pointing up), spin down, or both at the same time, a quantum state called a super-position. In the experiments, all the ions are initially in independent superpositions but are not communicating with each other. As the ions interact, their spins collectively morph into an entangled state involving most, or all of the entire crystal.
Researchers detected the spin state based on how much the ions fluoresced, or scattered laser light. When measured, unentangled ions collapse from a superposition to a simple spin state, creating noise, or random fluctuations, in the measured results. Entangled ions collapse together when measured, reducing the detection noise.
Crucially, the researchers measured a sufficient level of noise reduction to verify entanglement, results that agreed with theoretical predictions. This type of entanglement is called spin squeezing because it squeezes out (removes) noise from a target measurement signal and moves it to another, less import-ant aspect of the system. The techniques used in the simulator might someday contribute to the development of atomic clocks based on large numbers of ions (current designs use one or two ions).
“The reduction in the quantum noise is what makes this form of entanglement useful for enhancing ion and atomic clocks,” Bohnet said. “Here, spin squeezing confirms the simulator is working correctly, because it produces the quantum fluctuations we are looking for.” The work was funded in part by the National Science Foundation, Army Research Office and Air Force Office of Scientific Research.
Ten superconducting qubits entangled by physicists in China
Superconducting circuits create qubits by superimposing two electrical currents, and hold the promise of being able to fabricate many qubits on a single chip through the exploitation of silicon-based manufacturing technology. In the latest work, a multi-institutional group led by Jian-Wei Pan of the University of Science and Technology of China in Hefei, built a circuit consisting of 10 qubits, each half a millimetre across and made from slivers of aluminium laid on to a sapphire substrate. The qubits, which act as non-linear LC oscillators, are arranged in a circle around a component known as a bus resonator.
Initially, the qubits are put into a superposition state of two oscillating currents with different amplitudes by supplying each of them with a very low-energy microwave pulse. To avoid interference at this stage, each qubit is set to a different oscillation frequency. However, for the qubits to interact with one another, they need to have the same frequency. This is where the bus comes in. It allows qubits to transfer energy from one another, but does not absorb any of that energy itself.
“Magical interaction”
The end result of this process, says team member Haohua Wang of Zhejiang University, is entanglement, or, as he puts it, “some kind of magical interaction”. To establish just how entangled their qubits were, the researchers used what is known as quantum tomography to find out the probability of detecting each of the thousands of possible states that this entanglement could generate. The outcome: their measured probability distribution yielded the correct state on average about two thirds of the time. The fact that this “fidelity” was above 50%, says Wang, meant that their qubits were “entangled for sure”.
According to Shibiao Zheng of Fuzhou University, who designed the entangling protocol, the key ingredient in this set-up is the bus. This, he says, allows them to generate entanglement “very quickly”.
The previous record of nine for the number of entangled qubits in a superconducting circuit was held by John Martinis and colleagues at the University of California, Santa Barbara and Google. That group uses a different architecture for their system; rather than linking qubits via a central hub they instead lay them out in a row and connect each to its nearest neighbour. Doing so allows them to use an error-correction scheme that they developed known as surface code.
High fidelity
Error correction will be vital for the functioning of any large-scale quantum computer in order to overcome decoherence – the destruction of delicate quantum states by outside interference. In 2015, Martinis and co-workers showed that superconducting quantum computers could in principle be scaled up, when they built two-qubit gates with a fidelity above that required by surface code – introducing errors less than 1% of the time.
Martinis praises Pan and colleagues for their “nicely done experiment”, in particular for their speedy entangling and “good single-qubit operation”. But it is hard to know how much of an advance they have really made, he argues, until they fully measure the fidelity of their single-qubit gates or their entangling gate. “The hard thing is to scale up with good gate fidelity,” he says.
Wang says that the Chinese collaboration is working on an error-correction scheme for their bus-centred architecture. But he argues that in addition to exceeding the error thresholds for individual gates, it is also important to demonstrate the precise operation of many highly entangled qubits. “We have a global coupling between qubits,” he says. “And that turns out to be very useful.”
Wang and his colleagues aim to develop a “quantum simulator” consisting of perhaps 50 qubits, which could outperform classical computers when it comes to simulating the behaviour of small molecules and other quantum systems.
Scientists have found a promising new way to build the next generation of quantum simulators combining light and silicon micro-chips.
Scientists from the University of Bristol and the Technical University of Denmark have found a promising new way to build the next generation of quantum simulators combining light and silicon micro-chips.
In the roadmap to develop quantum machines able to compete and overcome classical supercomputers in solving specific problems, the scientific community is facing two main technological challenges. The first is the capability of building large quantum circuits able to process the information on a massive scale, and the second is the ability to create a large number of single quantum particles that can encode and propagate the quantum information through such circuits.
Both these two requirements need to be satisfied in order to develop an advanced quantum technology able to overcome classical machines. A very promising platform to tackle such challenges is silicon quantum photonics. In this technology, the information carried by photons, single particle of lights, is generated and processed in silicon micro-chips. These devices guide and manipulate light at the nanoscale using integrated waveguides — the analogue of optical fibres at the nanometre-scale.
Crucially, the fabrication of photonic chips requires the same techniques used for fabricating electronic micro-chips in the semiconductor industry, making the fabrication of quantum circuits at a massive scale possible. In the University of Bristol’s Quantum Engineering Technology (QET) Labs, the team have recently demonstrated silicon photonic chips embedding quantum interferometres composed of almost a thousand optical components, orders of magnitude higher that what was possible just few years ago.
However, the big question that remained unanswered was if these devices were also able to produce a number of photons large enough to perform useful quantum computational tasks. The Bristol-led research, published today in the journal Nature Physics, demonstrates that this question has a positive answer. By exploring recent technological developments in silicon quantum photonics, the team have demonstrated that even small-scale silicon photonic circuits can generate and process a number of photons unprecedented in integrated photonics.
In fact, due to imperfections in the circuit such as photon losses, previous demonstrations in integrated photonics have been mostly limited to experiments with only two photons generated and processed on-chip, and only last year, four-photon experiments were reported using complex circuitry.
In the work, by improving the design of each integrated component, the team show that even simple circuits can produce experiments with up to eight photons, double than the previous record in integrated photonics. Moreover, their analysis shows that by scaling up the circuit complexity, which is a strong capability of the silicon platform, experiments with more than 20 photons are possible, a regime where photonic quantum machines are expected to surpass the best classical supercomputers. The study also investigates possible applications for such near-term photonics quantum processors entering a regime of quantum advantage.
In particular, by reconfiguring the type of optical non-linearity in the chip, they demonstrated that silicon chips can be used to perform a variety of quantum simulation tasks, known as boson sampling problems. For some of these protocols, for example the Gaussian Boson Sampling, this new demonstration is a world-first. The team also demonstrated that, using such protocols, silicon quantum devices will be able to solve industrially relevant problems. In particular, they show how the chemical problem of finding the vibrational transitions in molecules undergoing an electronic transformation can be simulated on our type of devices using Gaussian Boson Sampling.
Lead author Dr Stefano Paesani from the University of Bristol’s Centre for Nanoscience and Quantum Information, said: “Our findings show that photonic quantum simulators surpassing classical supercomputers are a realistic near-term prospect for the silicon quantum photonics platform. “The development of such quantum machines can have potentially ground-breaking impacts on industrially relevant fields such as chemistry, molecular designing, artificial intelligence, and big-data analysis.
“Applications include the design of better pharmaceutics and the engineering of molecular states able to generate energy more efficiently.” Co-author, Dr Raffaele Santagati, added: “The results obtained make us confident that the milestone of quantum machines faster than any current classical computers is within reach of the integrated quantum photonics platform.
“While it is true that also other technologies have the capability to reach such regime, for example trapped ions or superconducting systems, the photonics approach has the unique advantage of having the near-term applications we investigated. The photonic path, although perilous, is set, and is very much worth pursuing.”
Professor Anthony Laing, Associate Professor of Physics at Bristol supervised the project. He said: “In quadrupling the number of photons both generated and processed in the same chip, the team have set the scene for scaling up quantum simulators to tens of photons where performance comparisons with today’s standard computing hardware become meaningful.”
