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Quantum bits, or qubits, are the basic building blocks of quantum computers, just as bits are that of modern computers. Researchers around the world have been exploring a range of different physical systems to act as qubits, including 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.
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.
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. 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. Google’s Quantum AI Lab, have experimented with qubits based on superconducting metal circuits.
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.
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.
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.
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.
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.
Ultra enriched Silicon
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.
A team of NIST researchers team lead by Josh Pomeroy of PML’s Quantum Processes and Metrology Group, has created what may be the most highly enriched silicon currently being produced, more than 99.9999% pure silicon-28 (28Si). This shall pave the way to to build Quantum computer with Silicon as a platform. They also were able to utilize a much less complicated technique, similar to mass spectrometry, for producing ultra-enriched silicon. 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 naturally contains about 92% 28Si, and other isotopes including 29Si at about 4.7%, which is a dominant factor for decoherence.
The deleterious interactions in solid-state environment, between the qubit and other degrees of freedom of the crystal lattice such as the nuclear spins of surrounding atoms, or charge and spin fluctuations in defects, oxides and interfaces, leads to a drastic reduction in coherence times in solid-state spin qubits. Unenriched For silicon then, coherence time can be drastically improved through the isotopic enrichment of the spin-zero nuclear species 28Si. Researchers were able to extend the coherence time to three minutes—from a matter of seconds—the time in which scientists can manipulate, observe and measure the processes.
Currently, there is no reliable commercial source of sufficiently enriched silicon, which should consist of at least 99.99% 28Si to be useful for most quantum computing purposes. The researchers employed a technique similar to mass spectrometry to isolate 28Si. Silicon atoms from natural abundance silane gas (SiH4) are ionized, extracted at high voltage, and then shot through a magnetic field, which causes the ion trajectories to curve. The radius of their curvature depends on their mass, so 28Si and 29Si diverge into separate beams. To collect the 28Si ions, researchers direct the beam of the preferred ions onto an unenriched silicon substrate about 1 square cm. This is much less complicated than other state-of-the-art methods currently in use for producing ultra-enriched silicon.
“The real challenge now is to make this amorphous enriched silicon into a form that is equivalent to what you would get if you bought a wafer or epitaxial layer,” which will eventually be needed to build practical quantum information systems. Pomeroy’s team has already had some success in tackling this next step, which involves growing the ultra-enriched silicon as a nearly perfect crystal.
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.
Atomic-scale control of tunneling in donor-based devices
Atomically precise silicon-phosphorus (Si:P) quantum systems are actively being pursued to realize universal quantum computation and analog quantum simulation. Atomically precise control of tunneling rates is critical to tunnel-coupled quantum dots and spin-selective tunneling for initialization and read-out in quantum computation. Atomically precise donor-based quantum devices are a promising candidate for solid-state quantum computing and analog quantum simulations. However, critical challenges in atomically precise fabrication have meant systematic, atomic scale control of the tunneling rates and tunnel coupling has not been demonstrated.
A big challenge to this endeavor is in figuring out how to duplicate such small transistors, which would act like small on-off switches. Utilizing the recipe they devised, the team led by NIST became just the second ever to create a single-atom transistor and the first ever to produce a series of transistors with only a single electron each, whose geometry could be manipulated at the atomic level.
The scientists were also able to gain control over the quantum phenomenon of quantum tunneling, changing the rate at which individual electrons travelled through a physical gap or the transistor’s electrical barrier. The significance of managing this process lies in allowing the transistors to get “entangled” according to the laws of quantum mechanics. This can lead to new ways of creating quantum bits (qubits) – the basic unit of information in quantum computing.
Their methods for precisely duplicating the devices that can work as qubits featured key innovations like sealing the phosphorus atoms involved with layers of silicon to protect them and then sending electricity to the embedded atoms, as NIST researcher Richard Silver explained. “We believe our method of applying the layers provides more stable and precise atomic-scale devices,” he added.
What’s also remarkable about their achievement is that this approach of making electrical contact with the micro transistors has a nearly 100% success rate. This allows the devices to operate as part of a circuit. As Silver’s colleague on the research, Jonathan Wyrick, stated, “You can have the best single-atom-transistor device in the world, but if you can’t make contact with it, it’s useless.”
“In this study, we overcome previous challenges by uniquely combining hydrogen lithography that generates atomically abrupt device patterns10,11 with recent progress in low-temperature epitaxial overgrowth using a locking-layer technique12,13,14 and silicide electrical contact formation15 to substantially reduce unintentional dopant movement. These advances have allowed us to demonstrate the exponential scaling of the tunneling resistance on the tunnel gap separation in a systematic and reproducible manner. We suppress unintentional dopant movement at the atomic scale using an optimized, room temperature grown locking layer, which not only locks the dopant position within lithographically defined regions during encapsulation, but also improves reproducibility since the critical first few layers are always grown at room temperature.”
The researchers also included Xiqiao Wang, Michael Stewart Jr., and Curt Richter.
