A working quantum computer has the potential to transform the information economy and create the industries of the future, solving in hours or minutes problems that would take conventional computers – even supercomputers – centuries, and tackling otherwise intractable problems that even supercomputers could not solve. Applications include for software design, machine learning, scheduling and logistical planning, financial analysis, stock market modelling, software and hardware verification, climate modelling, rapid drug design and testing, and early disease detection and prevention.
Quantum bits, or qubits, are the basic building blocks of quantum computers, just as bits are that of modern computers. To give an example, Google’s quantum computer has nine qubits, while IBM’s quantum computer has 16 qubits. IBM plans to scale their quantum computer between 50 and 100 qubits within the next decade.
Researchers around the world have been exploring a range of different physical systems to act as qubits, including silicon-based nuclear spins, trapping and isolating ions by using electromagnetic fields, photons trapped in microwave cavities, nuclear spins, electron spins in quantum dots, superconducting loops and Josephson junctions, among others. Google’s Quantum AI Lab, have experimented with qubits based on superconducting metal circuits. In 2015, Google researchers became the first to demonstrate surface code error correction on a linear array of nine superconducting qubits.
Researchers at the University of New South Wales in Australia have designed a new type of quantum bit (qubit), which they say will enable large-scale quantum computing at a lower cost. The new ‘flip-flop qubits’ are able to communicate over distances of more than 150nm, which researcher leader Andrea Morello said might actually leave room to “cram other things between qubits.” What the team have invented is a new way to define a ‘spin qubit’ that uses both the electron and the nucleus of the atom. Crucially, this new qubit can be controlled using electric signals, instead of magnetic ones,” said Prof Morello.
Morello and his team proposed a method of using both the electron and nucleus of a single phosphorous atom, to create a qubit inside a layer of silicon. ‘Pulling’ the electron away from the nucleus would extend the electric field that qubits use for entanglement. As well as leaving more space, the new chip designs would also overcome the need for atoms to be very precisely placed.
Even more important, though, is the fact that the new chips could be produced using existing manufacturing technology, which opens up the possibility of mass production. Morello said that this “makes the building of a quantum computer much more feasible.” “This new idea allows us to fabricate multi-qubit processes with current technology,” says Guilherme Tosi, the lead scientist.
Australia’s first quantum computing company, Silicon Quantum Computing Pty Ltd, has been launched to advance the development and commercialisation of the University of New South Wales (UNSW Sydney)’s world-leading quantum computing technology. Working alongside the Australian Research Council (ARC) Centre of Excellence for Quantum Computation and Communication Technology (CQC2T), Silicon Quantum Computing Pty Ltd will operate from new laboratories within CQC2T’s UNSW headquarters.
It will drive the development and commercialisation of a 10-qubit quantum integrated circuit prototype in silicon by 2022 as the forerunner to a silicon based quantum computer.
CQC2T is home to an incredibly strong team of silicon quantum computing researchers being the only group in the world that can make atomically precise devices in silicon. Led by UNSW Scientia Professor Michelle Simmons, the Centre’s teams have produced the longest coherence time qubits in the solid state, the ability to optically address single dopant atoms in silicon, the lowest noise silicon devices and the first two qubit gate in silicon.
Intel Bets It Can Turn Everyday Silicon into Quantum Computing’s Wonder Material
Intel’s group has reported that they can now layer the ultra-pure silicon needed for a quantum computer onto the standard wafers used in chip factories. A quantum computer would need to have thousands or millions of qubits to be broadly useful, and to get to hundreds of thousands of qubits, we will need incredible engineering reliability, and that is the hallmark of the semiconductor industry, according to Andrew Dzurak, who works on silicon qubits at the University of New South Wales in Australia says. Another reason to work on silicon qubits is that they should be more reliable than the superconducting equivalents.
Intel’s silicon qubits represent data in a quantum property called the “spin” of a single electron trapped inside a modified version of the transistors in its existing commercial chips. “The hope is that if we make the best transistors, then with a few material and design changes we can make the best qubits,” says Clarke.
The new process that helps Intel experiment with silicon qubits on standard chip wafers, developed with the materials companies Urenco and Air Liquide, should help speed up its research, says Andrew Dzurak, who works on silicon qubits at the University of New South Wales in Australia.
Companies developing superconducting qubits also make them using existing chip fabrication methods. But the resulting devices are larger than transistors, and there is no template for how to manufacture and package them up in large numbers, says Dzurak.
Two teams from Australia are experimenting with two different types of Qbits, one team is using natural atom made of Phosphorous which contains two quantum bits electron and nuclear spin and on which they have achiever 99.99% accuracy that leads to 1 error in every 10,000 operations.
The second team is working with artificial atom that harness silicon to build a quantum processor with advantage of it being compatible with the microelectronics of existing computers. The spin of an electron or a nucleus in a semiconductor naturally implements the unit of quantum information – the qubit – while providing a technological link to the established electronics industry. However naturally occurring silicon contains about 92% 28Si, and other isotopes including 29Si at about 4.7%, which is a dominant factor for decoherence. For silicon then, coherence time can be drastically improved through the isotopic enrichment of the spin-zero nuclear species 28Si.
So far, the UNSW team has demonstrated a system with quantum bits, or qubits, only in a single atom. Useful computations will require linking qubits in multiple atoms. But the team’s silicon qubits hold their quantum state nearly a million times longer than do systems made from superconducting circuits, a leading alternative, UNSW physicist Guilherme Tosi told participants at the event. This helps the silicon qubits to perform operations with one-sixth of the errors of superconducting circuits. A second group from the UNSW has a less robust silicon design that has already demonstrated calculations that link up two qubits, a building block that paves the way for creating more-complex devices.
Systems will need to be scaled up to a large number of qubits to execute nontrivial quantum algorithms so that these quantum devices can simulate quantum systems efficiently, crack modern encryption codes, search through huge databases, as well as solve a wide range of optimization problems.
Australian engineers have created a new ultra stable quantum bit
Australian engineers team from Australia’s University of New South Wales (UNSW) had earlier reported to have created a new quantum bit which remains in a stable superposition for 10 times longer than previously achieved, dramatically expanding the time during which calculations could be performed in a future silicon quantum computer. “We have created a new quantum bit where the spin of a single electron is merged together with a strong electromagnetic field,” said Arne Laucht, a Research Fellow at the School of Electrical Engineering & Telecommunications at UNSW, and lead author of the paper. “This quantum bit is more versatile and more long-lived than the electron alone, and will allow us to build more reliable quantum computers.”
“The greatest hurdle in using quantum objects for computing is to preserve their delicate superpositions long enough to allow us to perform useful calculations,” said Andrea Morello, leader of the research team and a Program Manager in the Centre for Quantum Computation & Communication Technology (CQC2T) at UNSW.
“Our decade-long research program had already established the most long-lived quantum bit in the solid state, by encoding quantum information in the spin of a single phosphorus atom inside a silicon chip, placed in a static magnetic field,” said Andrea Morello. The results are striking: since the electromagnetic field steadily oscillates at a very high frequency, any noise or disturbance at a different frequency results in a zero net effect. The researchers achieved an improvement by a factor of 10 in the time span during which a quantum superposition can be preserved.
Specifically, they measured a dephasing time of T2*=2.4 milliseconds – a result that is 10-fold better than the standard qubit, allowing many more operations to be performed within the time span during which the delicate quantum information is safely preserved. “This new ‘dressed qubit’ can be controlled in a variety of ways that would be impractical with an ‘undressed qubit’,”, added Morello. “For example, it can be controlled by simply modulating the frequency of the microwave field, just like in an FM radio. The ‘undressed qubit’ instead requires turning the amplitude of the control fields on and off, like an AM radio.
“In some sense, this is why the dressed qubit is more immune to noise: the quantum information is controlled by the frequency, which is rock-solid, whereas the amplitude can be more easily affected by external noise”.
Since the device is built upon standard silicon technology, this result paves the way to the construction of powerful and reliable quantum processors based upon the same fabrication process already used for today’s computers. What Laucht and colleagues did was push this further: “We have now implemented a new way to encode the information: we have subjected the atom to a very strong, continuously oscillating electromagnetic field at microwave frequencies, and thus we have ‘redefined’ the quantum bit as the orientation of the spin with respect to the microwave field.”
Quantum computer in Silicon
Scientists and engineers from the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology (CQC2T), are developing a scalable quantum computer in silicon. They found that a single atom of phosphorus could be used to tightly hold an electron, which also carries a “spin” (like a tiny magnet) that could be used as a quantum bit.
The idea of silicon quantum computing was first proposed in 1998 by Bruce Kane, a physicist at the University of Maryland, in College Park. Quantum computers based on familiar silicon could theoretically be manufactured in conjunction with the conventional semiconductor techniques found in today’s computer industry. A silicon approach to quantum computing also offers the advantage of strong stability and high coherence times for qubits. (High coherence times mean the qubits can continue holding their information for long enough to complete calculations.) Kane proposed using the quantum characteristic of spin in the nucleus of the phosphorus donor atom as the qubit.
Morello and Dzurak were among the physicists impressed by Kane’s proposal, but they chose to investigate electron spins instead, because electron spins in silicon have very long coherence times—that is, it takes a relatively long time for such a qubit to lose its information.
So far, the UNSW team has demonstrated a system with quantum bits, or qubits, only in a single atom. Australian teams demonstrated mastery of single qubits based on electron spin in 2012 and control of nuclear spin qubits in 2013. Useful computations will require linking qubits in multiple atoms.
But the team’s silicon qubits hold their quantum state nearly a million times longer than do systems made from superconducting circuits, a leading alternative, said UNSW physicist Guilherme Tosi reported by Nature. This helps the silicon qubits to perform operations with one-sixth of the errors of superconducting circuits.
The scheme showcased at the innovation forum by Tosi and fellow physicist Vivien Schmitt uses qubits that are the spins of the electrons and nuclei in phosphorus atoms embedded in a silicon lattice, and are controlled using a special system of electric fields. Because the spins respond only to very specific, tuneable frequencies, they are robust to electrical noise. That allows the qubits to keep their quantum states for one minute and to operate perfectly 99.9% of the time, said Tosi.
Moreover, the electrically controlled qubits can communicate with each other at larger distances than can the qubits in other silicon designs. That bodes well for scaling up because the qubits can be far enough apart to allow room for control and read-out instruments to be placed between them. The atoms also do not need to be placed precisely, so they would fit with existing microprocessor-fabrication techniques, added Tosi.
Researchers at the University of New South Wales (UNSW) built a two-qubit logic gate in silicon
A second group from the UNSW led by physicist Andrew Dzurak, uses as its qubits the spins of electrons in a set-up that is based on modified electrical transistors. Although the qubits are less robust than those in the Morello design, Dzurak’s team demonstrated two-qubit calculations last October.
Researchers reported in the journal Nature that they have built a two-qubit logic gate containing two entangled qubits, based on spins of trapped electrons in silicon for the first time and thereby clearing the hurdle to making silicon-based quantum computer processors a reality.
Such trapped electrons can be integrated with existing CMOS technology, to create quantum computer chips that could store thousands, even millions of qubits on a single silicon processor chip. UNSW scientists have patented a design for a full-scale quantum chip that would hold millions of silicon qubits.
The UNSW researchers then simulated fundamental gate: the controlled NOT or CNOT operation through their two-qubit logic gate. Depending on the state of the control qubit, the CNOT gate changes an “up” spin into a “down” spin, and the other way around. The CNOT is the fundamental gate that can be combined to form complex quantum computations just as NAND gate is fundamental gate in conventional computers.
“For the creation of the two-qubit gate the researchers modified the design of a CMOS transistor. Two gates are placed next to each other on an insulating layer of silicon dioxide that separates them from a layer of almost pure silicon-28 isotope, writes Alexander Hellemans,” in IEEE spectrum.
Controlling the voltage of the gates allows the trapping of a single electron in the region under the gate. The quantum states of both electrons can be controlled by gigahertz-frequency pulses transmitted by the “electron spin resonance” (ESR) line, in combination with a 1.4 Tesla magnetic field.
The ESR line allows the spin state for each electron to be set independently for “one-qubit” operations. Voltage pulses entangle the two qubits, allowing them to operate as a CNOT gate; changing the spin of one electron results in changing the spin of the other electron.
The authors write in Nature, “Here we present a two-qubit logic gate, which uses single spins in isotopically enriched silicon and is realized by performing single- and two-qubit operations in a quantum dot system using the exchange interaction, as envisaged in the Loss–DiVincenzo proposal.
“We realize CNOT gates via controlled-phase operations combined with single-qubit operations. Direct gate-voltage control provides single-qubit addressability, together with a switchable exchange interaction that is used in the two-qubit controlled-phase gate. By independently reading out both qubits, we measure clear anticorrelations in the two-spin probabilities of the CNOT gate.”
Australian scientists design a full-scale architecture for a quantum computer in silicon
“Our Australian team has developed the world’s best qubits in silicon,” says University of Melbourne Professor Lloyd Hollenberg, Deputy Director of the CQC2T who led the work with colleague Dr Charles Hill. “However, to scale up to a full operational quantum computer we need more than just many of these qubits – we need to be able to control and arrange them in such a way that we can correct errors quantum mechanically.”
Australian scientists have designed a 3D silicon chip architecture based on single atom quantum bits, one of the final hurdles to scaling up to an operational quantum computer many thousands of qubits. Researchers detailed an architecture that sandwiches a 2-D layer of nuclear spin qubits between an upper and lower layer of control lines. Such triple-layer architecture enables a smaller number of control lines to activate and control many qubits all at the the same time.
By applying voltages to a sub-set of these wires, multiple qubits can be controlled in parallel, performing a series of operations using far fewer controls. Importantly, with their design, they can perform the 2D surface code error correction protocols in which any computational errors that creep into the calculation can be corrected faster than they occur.
“This architecture gives us the dense packing and parallel operation essential for scaling up the size of the quantum processor,” says Scientia Professor Sven Rogge, Head of the UNSW School of Physics. “Ultimately, the structure is scalable to millions of qubits, required for a full-scale quantum processor.”
In theory, the new architecture could pack about 25 million physical qubits within an array that’s 150 micrometers by 150 µm. But those millions of qubits would require just 10,000 control lines. By comparison, an architecture that tried to control each individual qubit would have required over 1000 times more control lines.
“We have demonstrated we can build devices in silicon at the atomic-scale and have been working towards a full-scale architecture where we can perform error correction protocols – providing a practical system that can be scaled up to larger numbers of qubits,” says UNSW Scientia Professor Michelle Simmons, study co-author and Director of the CQC2T.
If the team can pull off this low error rate in a larger system, it would be “quite amazing”, said Hartmut Neven, director of engineering at Google and a member of the panel. But he cautioned that in terms of performance, the system is far behind others. The team is aiming for ten qubits in five years, but both Google and IBM are already approaching this with superconducting systems. And in five years, Google plans to have ramped up to hundreds of qubits.
Solid state scheme of Quantum Computers
In 2014 two research groups, one from the UNSW and one from Dutch-US collaboration, showed that the accuracy and lifetime of silicon qubits are now in a realm that makes them suitable for the manufacture of large-scale quantum computers.
Teams in Australia have used a specially purified type of silicon that contains only one isotope, called Si-28. This isotope is completely non-magnetic, because its nucleus has no spin. The electrical properties of a chip of purified Si-28 are identical to those of natural silicon, and so it works equally well for any electronic device. But when an electron or nuclear spin qubit are configured inside pure Si-28, the absence of magnetic noise allows to store and manipulate the quantum state with unprecedented accuracy.
In order to execute a quantum algorithm, we need to manipulate individual qubits and couple two qubits together. Both of these operations are accomplished with an array of gate electrodes that lies on top of the wafer, but is isolated from the pure silicon by a thin insulating layer of silicon dioxide. Metal A gates are deposited on the oxide above each donor, and J gates between adjacent donors.
The process adopted by UNSW researchers was to deposit a layer of hydrogen on a silicon wafer and used a scanning tunnelling microscope to create a pattern on the surface in an ultra-high vacuum. This was then exposed to phosphine gas and annealed at 350 degrees so phosphorus atoms became incorporated precisely into the silicon.
Using this process, they were able to remove all the silicon 29 isotopes that have magnetic spin and grow, a specially purified type of silicon that contains only one isotope, called Si-28. This isotope is completely non-magnetic, because its nucleus has no spin. When qubits or phosphorus donors are configured inside pure Si-28 substrate, they experience a spin free environment or the absence of magnetic noise. This allowed them to reach an accuracy of quantum operations up to 99.99 percent, much above the minimum requirement to ensure that the quantum error correction can be performed
Silicon-29 has a non-zero nuclear spin, which interferes with the qubit spins and leads to the breakdown of coherence, explains Dzurak. The Australian team led by Dzurak has discovered a way to create an “artificial atom” qubit with a device remarkably similar to the silicon transistors used in consumer electronics, known as MOSFETs.
Meanwhile, Morello’s team has used purified Si-28, whose electrical properties are identical to those of natural silicon and the phosphorous the standard dopant for conventional silicon-based semiconductor devices. Therefore the technology, techniques, and collective experience of huge silicon semiconductor industry can now potentially be utilized for building quantum computers.
The UNSW team, manipulated the electron spins by applying a magnetic field, created by running microwave power down a transmission line with a short circuit at the end. This allowed them to be able to address and manipulate qubits individually, The researchers also created a new method for addressing individual qubits or spins of individual electrons. They were able to place a variable cluster of phosphorous atoms in silicon, so that each qubit or electron is hosted by different number of phosphorous atoms, hence responds differently to the electromagnetic fields. The individual addressing of individual qubits, allows their quantum computers to in principle scalable to an arbitrary number of qubits.
Like superconducting qubits, silicon qubits must be kept at a fraction of a degree above absolute zero. Morton said that the Morello design’s big advantage is that the qubits are atomic scale. In principle, that would allow many more to be placed on each chip than is possible with superconducting qubits, which are around 100 micrometres across. To seize this opportunity, the design needs to shed the bulky microwave devices that it proposes as its means to operate between distant qubits, said Neven.