Category Archives: Quantum Computing
Technique for measuring and controlling electron state is a … – UCLA Newsroom
During their research for a new paper on quantum computing, HongWen Jiang, a UCLA professor of physics, and Joshua Schoenfield, a graduate student in his lab, ran into a recurring problem: They were so excited about the progress they were making that when they logged in from home to their UCLA desktop which allows only one user at a time the two scientists repeatedly knocked each other off of the remote connection.
The reason for their enthusiasm: Jiang and his team created a way to measure and control the energy differences of electron valley states in silicon quantum dots, which are a key component of quantum computing research. The technique could bring quantum computing one step closer to reality.
Its so exciting, said Jiang, a member of the California NanoSystems Institute. We didnt want to wait until the next day to find out the outcome.
Quantum computing could enable more complex information to be encoded on much smaller computer chips, and it holds promise for faster, more secure problem-solving and communications than todays computers allow.
In standard computers, the fundamental components are switches called bits, which use 0s and 1s to indicate that they are off or on. The building blocks of quantum computers, on the other hand, are quantum bits, or qubits.
The UCLA researchers breakthrough was being able to measure and control a specific state of a silicon quantum dot, known as a valley state, an essential property of qubits. The research was published in Nature Communications.
An individual qubit can exist in a complex wave-like mixture of the state 0 and the state 1 at the same time, said Schoenfield, the papers first author. To solve problems, qubits must interfere with each other like ripples in a pond. So controlling every aspect of their wave-like nature is essential.
Silicon quantum dots are small, electrically confined regions of silicon, only tens of nanometers across, that can trap electrons. Theyre being studied by Jiangs lab and by researchers around the world for their possible use in quantum computing because they enable scientists to manipulate electrons spin and charge.
Besides electrons spin and charge, another of their most important properties is their valley state, which specifies where an electron will settle in the non-flat energy landscape of silicons crystalline structure. The valley state represents a location in the electrons momentum, as opposed to an actual physical location.
Scientists have realized only recently that controlling valley states is critical for encoding and analyzing silicon-based qubits, because even the tiniest imperfections in a silicon crystal can alter valley energies in unpredictable ways.
Imagine standing on top of a mountain and looking down to your left and right, noticing that the valleys on either side appear to be the same but knowing that one valley was just 1 centimeter deeper than the other, said Blake Freeman, a UCLA graduate student and co-author of the study. In quantum physics, even that small of a difference is extremely important for our ability to control electrons spin and charge states.
At normal temperatures, electrons bounce around, making it difficult for them to rest in the lowest energy point in the valley. So to measure the tiny energy difference between two valley states, the UCLA researchers placed silicon quantum dots inside a cooling chamber at a temperature near absolute zero, which allowed the electrons to settle down. By shooting fast electrical pulses of voltage through them, the scientists were able to move single electrons in and out of the valleys. The tiny difference in energy between the valleys was determined by observing the speed of the electrons rapid switching between valley states.
After manipulating the electrons, the researchers ran a nanowire sensor very close to the electrons. Measuring the wires resistance allowed them to gauge the distance between an electron and the wire, which in turn enabled them to determine which valley the electron occupied.
The technique also enabled the scientists, for the first time, to measure the extremely small energy difference between the two valleys which had been impossible using any other existing method.
In the future, the researchers hope to use more sophisticated voltage pulses and device designs to achieve full control over multiple interacting valley-based qubits.
The dream is to have an array of hundreds or thousands of qubits all working together to solve a difficult problem, Schoenfield said. This work is an important step toward realizing that dream.
The research was supported by the U.S. Army Research Office.
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Technique for measuring and controlling electron state is a ... - UCLA Newsroom
Quantum Computers Made Even More Powerful with New microchip generating ‘Qudits’ – TrendinTech
Before quantum computing has fully gotten off the ground, a team of researchers has made advancements to them already.Called qudits, this new configuration promises to improve the impressive power and speed of quantum computing.
Currently, quantum bits, or qubits, make up the basics of quantum computing storage with their ability to simultaneously perform as 1 and 0 or both on and off at the same time. However, the latest microchip that creates qudits surpasses this ability by being able to act in a number, at least 10 and sometimes more, states concurrently. Such a configuration could lead the pack to quantum computers even more powerful than previously imagined.
In traditional computing, classical bits can only be 1 or 0 at any given time so to solve equations only one possibility can be considered at a time. However, the superposition of qubits allows them to do two calculations at a time and therefore quicker than even the quickest of traditional computers.
Yet, the superpositions are delicate in nature and create challenges when trying to combine more than one qubit, which is necessary to build a functioning quantum computer. Qudits can eliminate this hardship by reducing the number of qubits needed for the same power and performance. For example, two qudits with 32 states each have the same computational power as 10 qubits working together while eliminating the fragility and complications that accompany a qubit entanglement.
The microchip at the center of this breakthrough can create two entangled 10-state qudits, a total of 100 dimensions, and is more than six qubits working cooperatively could generate.
We have now achieved the compact and easy generation of high-dimensional quantum states, announced a statement from Michael Kues, co-lead author and a quantum optics researcher at Canadas National Institute of Scientific Research.
In their research, scientists fired laser pulses of light at a circular resonator fixed to silica glass. A key role in the procedure, it produces pairs of entangled photons in a superposition of 10 colors or wavelengths.
For example, a high-dimensional photon can be red and yellow and green and blue, although the photons used here were in the infrared wavelength range, said Kues.
Recently published in the journal Nature, the scientists findings assert that the new microchip could support 13 qubits, a grand total of 9,000 dimensions, theoretically.
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Quantum Computers Made Even More Powerful with New microchip generating 'Qudits' - TrendinTech
Quantum Computing Record Broken – Wall Street Pit
Researchers have been chasing after the dream of quantum computing for quite awhile now. And its easy to understand why.
Currently, conventional computers, even the fastest and most powerful ones, theoretically pale in comparison with quantum computers. And its mainly because of the way the respective computers process data.
A traditional computer uses bits that can only take on the form of either 1 or 0. On the other hand, a quantum computer uses qubits (short for quantum bits) which can take on the form of either 1 or 0, or 1 and 0 simultaneously. And it can do this via the bizarre quantum behavior known as superpositioning being able to be in two states at once. This extraordinary ability is what allows a quantum computer to process data much faster than a conventional computer as it can perform more than one calculation at a time. To be more specific, while at a superposition, one qubit can do two calculations; two qubits can do four calculations; three qubits can do eight calculations; and so on.
By itself, the very idea of superpositioning is already hard to ponder. What makes it even more complicated is the thought that as soon as superpositioning is detected, measured, or simply observed in some way, the system collapses, and the quantum computer reverts to an ordinary computer. This is what makes building a quantum computer so challenging.
Theres this general belief that for a quantum computer to achieve processing power equal to todays supercomputers whats being referred to as quantum supremacy it has to be working at least 49 qubits. Anything higher and its performance will be completely out of any conventional computers reach.
While there have already been a few simulations using up to 6 qubits and one done at 42 qubits, the feat achieved by Thomas Hner and Damian S. Steiger of the Institute for Theoretical Physics at Swiss Federal Institute of Technology (ETH Zurich) tops them all as they have succeeded in simulating the largest quantum device so far a 45-qubit circuit. Its not yet the 49-qubit circuit being aspired, but its pretty close.
The simulation was done at the National Energy Research Scientific Computer Center (NERSC) using the supercomputer called Cori II. All the technical jargon aside, what this achievement proves is that a 49-qubit device is nearing our reach, and the simulation done by Hner and Steiger can be used as a standard or precedent for succeeding quantum supremacy experiments. Google, IBM and all the other quantum computer developers might all benefit from looking into this 45-qubit simulation.
The race to quantum supremacy is fierce. But theres no denying that the company who gets to be the first will not be the sole winner in this race. Considering that quantum computers are being envisioned as instruments that could help solve some of the worlds worst and most urgent problems, all of humanity will certainly benefit when the first practical quantum computer is finally built.
Hner and Steigers work are detailed in a paper entitled 0.5 Petabyte Simulation of a 45-Qubit Quantum Circuit published through arXiv.org.
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Quantum Computing Record Broken - Wall Street Pit
Alkermes and IBM’s quantum computing. Who’ll be the big winner? Malcolm Berko – Durham Herald Sun
Durham Herald Sun | Alkermes and IBM's quantum computing. Who'll be the big winner? Malcolm Berko Durham Herald Sun Dear Mr. Berko: My stockbroker wants me to sell my 100 shares of IBM which I bought in 2012 at $201 a share, meaning I now have a big loss. He wants me to buy 300 shares of Alkermes, which he thinks will double in a year because several of its drugs ... |
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Alkermes and IBM's quantum computing. Who'll be the big winner? Malcolm Berko - Durham Herald Sun
Qudits: The Real Future of Quantum Computing? – IEEE Spectrum – IEEE Spectrum
Photo: INRS University Scientists have built a microchip that can generate two entangled qudits each with 10 states, for 100 dimensions total, more than what six entangled qubits could generate.
Instead of creating quantum computers based on qubits that can each adopt only two possible options, scientists have now developed a microchip that can generate qudits that can each assume 10 or more states, potentially opening up a new way to creating incredibly powerful quantum computers, a new study finds.
Classical computers switch transistors either on or off to symbolize data as ones and zeroes. In contrast, quantum computers use quantum bits, or qubitsthat, because of the bizarre nature of quantum physics, can be in a state ofsuperpositionwhere they simultaneously act as both 1 and 0.
The superpositions that qubits can adopt let them each help perform two calculations at once. If two qubitsare quantum-mechanically linked, orentangled,they can help perform four calculations simultaneously; three qubits, eight calculations; and so on. As a result, aquantum computer with 300 qubits could perform more calculations in an instant than there are atoms in the known universe, solving certain problems much faster than classical computers. However, superpositions are extraordinarily fragile, making it difficult to work with multiple qubits.
Most attempts at building practical quantum computers rely on particles that serve as qubits. However, scientists have long known that they could in principle use quditswith more than two states simultaneously. In principle, a quantum computer with two 32-state qudits, for example, would be able to perform as many operations as 10 qubits while skipping the challenges inherent with working with 10 qubits together.
Researchers used the setup pictured above to create, manipulate, and detect qudits. The experiment starts when a laser fires pulses of light into a micro-ring resonator, which in turn emits entangled pairs of photons.Because the ring has multiple resonances, the photons have optical spectrumswitha set of evenly spaced frequencies(red and blue peaks), a process known as spontaneous four-wave mixing (SFWM).The researchers were able to use each of thefrequencies to encode information, which means the photons act asqudits.Each quditis in a superposition of 10 possible states, extending the usual binary alphabet (0 and 1) of quantum bits.The researchers also showed they could perform basic gate operations on the qudits using optical filters and modulators, and then detect the results using single-photon counters.
Now scientists have for the first time created a microchip that can generate two entangled qudits each with 10 states, for 100 dimensions total, more than what six entangled qubits could generate. We have now achieved the compact and easy generation of high-dimensional quantum states, says study co-lead author Michael Kues, a quantum optics researcher at Canadas National Institute of Scientific Research, or INRS,its French acronym,in Varennes, Quebec.
The researchers developed a photonic chip fabricated using techniques similar to ones used for integrated circuits. A laser fires pulses of light into a micro-ring resonator, a 270-micrometer-diameter circle etched onto silica glass, which in turn emits entangled pairs of photons. Each photon is in a superposition of 10 possible wavelengths or colors.
For example, a high-dimensional photon can be red and yellow and green and blue, although the photons used here were in the infrared wavelength range, Kues says. Specifically, one photon from each pair spanned wavelengths from 1534 to 1550 nanometers, while the other spanned from 1550 to 1566 nanometers.
Using commercial off-the-shelf telecommunications components, the researchers showed they could manipulate these entangled photons. The basic capabilities they show are really what you need to do universal quantum computation, says quantum optics researcher Joseph Lukens at Oak Ridge National Laboratory, in Tennessee, who did not take part in this research. Its pretty exciting stuff.
In addition, by sending the entangled photons through a 24.2-kilometer-long optical fiber telecommunications system, the researchers showed that entanglement was preserved over large distances. This could prove useful for nigh-unhackable quantum communications applications, the researchers say.
What I think is amazing about our system is that it can be created using components that are out on the market, whereas other quantum computer technologies need state-of-the-art cryogenics, state-of-the-art superconductors, state-of-the-art magnets, saysstudy co-senior authorRoberto Morandotti, a physicistatINRSin Varennes. The fact that we use basic telecommunications components to access and control these states means that a lot of researchers could explore this area as well.
The scientists noted that current state-of-the-art components could conceivably generate entangled pairs of 96-state qudits, corresponding to more dimensions than 13 qubits. Conceptually, in principle, I dont see a limit to the number of states of qudits right now, Lukens, from Oak Ridge,says. I do think a 96-by-96-dimensional system is fairly reasonable, and achievable in the near future.
But he adds that several components of the experiment were not on the microchips, such as the programmable filters and phase modulators, which led to photon loss. Kues says that integrating such components with the rest of the chips and optimizing their micro-ring resonator would help reduce such losses to make their system more practical for use.
The next big challenge we will have to solve is to use our system for quantum computation and quantum communications applications, Kues says. While this will take some additional years, it is the final step required to achieve systems that can outperform classical computers and communications.
The scientists detailed their findings in the latest issue of the journal Nature.
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Qudits: The Real Future of Quantum Computing? - IEEE Spectrum - IEEE Spectrum
Google to Achieve "Supremacy" in Quantum Computing by the End of 2017 – Big Think
In theory, quantum computers could be vastly superior to regular or classical computers in performing certain kinds of tasks, but its been hard to build one. Already a leader in this field, Google is now testing its most powerful quantum chip yet,a 20-qubit processor,which the company looks to more than double in power to 49 qubits by the end of 2017.
Google's qubit devices are built on integrated circuits and can perform calculations using the physics of quantum mechanics.Qubits(or quantum bits) are units of quantum information that can be a mix of 0 and 1at the same time,making them better suited than classical bits for encoding large amounts of data.
Last year, Google actually released a plan on how it will achieve what it called quantum supremacy - getting quantum computers to do something the classical computers cannot, like factoring very large numbers. The paper says that if the processors manage to get to 50 qubits, quantum supremacy would be possible.
One big issue for Google to resolve - figuring out how to simulate what randomly arranged quantum circuits would do. Even a small difference in input into such a system would produce extremely different outputs, requiring a great amount of computing power that doesnt currently exist.
Theyre doing a quantum version of chaos, is how Simon Devitt from the RIKEN Center for Emergent Matter Science in Japan described Googles challenge. The output is essentially random, so you have to compute everything.
Computational difficulties aside, Google and other companies like IBM are moving along quicker than expected in their development. While they figured out the science necessary to create the qubits, the next challenges lie in scaling down their systems and reducing error rates.
The engineer Alan Ho from Googles quantum AI lab revealed that his teams current 20-qubit system has the error measure also known as two-qubit fidelity of 99.5%. The goal for the 49-qubit system would be to reach 99.7% fidelity.
It might take until 2027 until we get error-free quantum computers, according to Ho, meaning that usable devices are still some time away.
For more on how quantum computing works, check out this video:
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Google to Achieve "Supremacy" in Quantum Computing by the End of 2017 - Big Think
Quantum Computing Becomes More Accessible – Scientific American
Quantum computing has captured imaginations for almost 50 years. The reason is simple: it offers a path to solving problems that could never be answered with classical machines. Examples include simulating chemistry exactly to develop new molecules and materials and solving complex optimization problems, which seek the best solution from among many possible alternatives. Every industry has a need for optimization, which is one reason this technology has so much disruptive potential.
Until recently, access to nascent quantum computers was restricted to specialists in a few labs around the world. But progress over the past several years has enabled the construction of the worlds first prototype systems that can finally test out ideas, algorithms and other techniques that until now were strictly theoretical.
Quantum computers tackle problems by harnessing the power of quantum mechanics. Rather than considering each possible solution one at a time, as a classical machine would, they behave in ways that cannot be explained with classical analogies. They start out in a quantum superposition of all possible solutions, and then they use entanglement and quantum interference to home in on the correct answerprocesses that we do not observe in our everyday lives. The promise they offer, however, comes at the cost of them being difficult to build. A popular design requires superconducting materials (kept 100 times colder than outer space), exquisite control over delicate quantum states and shielding for the processor to keep out even a single stray ray of light.
Existing machines are still too small to fully solve problems more complex than supercomputers can handle today. Nevertheless, tremendous progress has been made. Algorithms have been developed that will run faster on a quantum machine. Techniques now exist that prolong coherence (the lifetime of quantum information) in superconducting quantum bits by a factor of more than 100 compared with 10 years ago. We can now measure the most important kinds of quantum errors. And in 2016 IBM provided the public access to the first quantum computer in the cloudthe IBM Q experiencewith a graphical interface for programming it and now an interface based on the popular programming language Python. Opening this system to the world has fueled innovations that are vital for this technology to progress, and to date more than 20 academic papers have been published using this tool. The field is expanding dramatically. Academic research groups and more than 50 start-ups and large corporations worldwide are focused on making quantum computing a reality.
With these technological advancements and a machine at anyones fingertips, now is the time for getting quantum ready. People can begin to figure out what they would do if machines existed today that could solve new problems. And many quantum computing guides are available online to help them get started.
There are still many obstacles. Coherence times must improve, quantum error rates must decrease, and eventually, we must mitigate or correct the errors that do occur. Researchers will continue to drive innovations in both the hardware and software. Investigators disagree, however, over which criteria should determine when quantum computing has achieved technological maturity. Some have proposed a standard defined by the ability to perform a scientific measurement so obscure that it is not easily explained to a general audience. I and others disagree, arguing that quantum computing will not have emerged as a technology until it can solve problems that have commercial, intellectual and societal importance. The good news is, that day is finally within our sights.
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Quantum Computing Becomes More Accessible - Scientific American
Tektronix AWG Pulls Test into Era of Quantum Computing – Electronic Design
When a company calls and says they have the best widget ever, you have to be skeptical. However, you also cant help but be curious. When they talked about how it would advance the state of the art in radar, electronic warfare, and quantum-computing test, and make an engineers workspace tidier, I was smitten.
I met up with theTektronix team, led by Product Market Manager Kip Pettigrew, and wasnt disappointed: The new AWG5200 arbitrary waveform generator is a work of art and function. Physically, its both commanding and imposing. It measures 18.13 6.05 from the front, but its 23.76 inches deepso, while itll sit nicely within a test stack and help reduce clutter, the stack had better have a deep shelf (Figs. 1 and 2).
Its whats within those dimensions, and what you have to pay to get it, though, that give the AWG5200 a certain level of gravitas. For sure, its hard to ignore a price point of $82,000, but its not surprising when you understand what youre getting in return.
1. The AWG5200 measures 18.13 6.05 and comes with a 6.5-inch touchscreen, a removable hard drive (upper right), and two, four, or eight channels (bottom right). (Source: Tektronix)
Aimed squarely at military/government and advanced research applications, the system emphasizes signal fidelity, scalability, and flexibility. It can accurately reproduce complex, real-world signals across an ever-expanding array of applications without having to physically expand a test area. Its also supported by Tektronixs SourceXpress software, which lets you create waveforms and control the AWGs remotely, and has a growing library of waveform-creation plugins.
2. The AWG5200 is designed to be compact so that it can stack easily with other equipment to reduce overall space requirements, though it is 23.76 inches deep. A synchronization feature allows it to scale up beyond eight channels by adding more AWG5200s. (Source: Tektronix)
Let the Specs Tell the Story
Digging into the specs uncovers what the AWG5200 is all about. Words like powerful, precision, and solid engineering come to mind. The system can sample at 5 Gsamples/s (10-Gsamples/s with interpolation) with 16-bit vertical resolution across two, four, or eight channels per unit. Channel-to-channel skew (typical) is <25 ps with a range of 2 ns and a resolution of 0.5 ps. The analog bandwidth is 2 GHz at 3 dB) or 4 GHz at 6 dB, and the amplitude range is 100 to 0.75 V p-p, with an accuracy of 2% of setting.
The AWG5200s multi-unit synchronization feature helps scale up beyond eight channels. Note that each channel is independent, so the classic tradeoff of sample memory for bandwidth doesnt apply here. Each channel gets 2 Gsamples of waveform memory.
The precision is embodied within its ability to generate RF signals with a spurious-free dynamic range (SFDR) of 70 dBc. Combined with a software suite and support, this is critical as new waveforms and digital-modulation techniques are explored in a time of rapid wireless evolution in military and government applications, as well as 5G and even quantum-computer test. Signal fidelity isnt something you want to worry about, and the expanding library and customizable features help kickstart and then fine-tune your research and development waveforms.
Howd They Do That?
Achieving higher or improved specifications is almost always a labor of love: The test companys engineers constant urge to make things better combines with customer feedback and an analysis of where to focus energy and development to have the most impact. However, at a fundamental level, the AWG5200s advances go back to the digital-to-analog converter (DAC) technology at the heart of the system.
Advances in DAC technologies, particularly with respect to signal processing and functional integration, allow them to directly generate detailed and complex RF and electronic-warfare (EW) signals. This is an area worth digging into in more detail, so Christopher Skach and Sahandi Noorizadeh developed a feature specially for Electronic Design on DAC technology advances and how its changing signal generation for test. Its worth a look.
Rapidly Evolving Applications
Pettigrew also provided a quick run through of the newer and more interesting applications, as well as the key market trends that the system is solving for. In general electronic test, go wide technologies like MIMO need test systems that can scale as they need multiple, independent, wide-bandwidth RF streams (Fig. 3).
3. Rapid expansion in the use of techniques such as MIMO requires more advanced and flexible waveform generators to generate multiple high-fidelity, RF signals with complex modulation schemes. (Source: Tektronix)
This translates over to mil/gov, too, where systems must be tested for their ability to detect and respond to adaptive threats. The signals of interest are able to be generated on two channels, while the others can be used to generate expected noise, Wi-Fi interferers, and other MIMO channels.
However, just being able to reproduce the signals isnt enough: The AWG must be capable of enabling stress and margin testing, as well as verification and characterization.1
On the research front, it turns out that quantum computing needs advanced AWGs, too, said Pettigrew, as they lack the fidelity, latency, and scalability. In quantum computers, the qubits are often controlled using precision-pulsed microwave signals, each requiring multiple independent RF channels. This is only going to get more interesting and challenging as companies like IBM and Google, along with many independent physicists and engineers, work to scale up quantum-computing technology and applications.
For all three of these applications, cost remains a factor. So, instead of developing multiple custom solutions, the AWG5200 may be a good commercial off-the-shelf (COTS) option.
References:
1. How New DAC Technologies are Changing Signal Generation for Test
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Tektronix AWG Pulls Test into Era of Quantum Computing - Electronic Design
Purdue, Microsoft Partner On Quantum Computing Research | WBAA – WBAA
Purdue researchers are partnering with Microsoft and scientists at three other universities around the globe to determine whether theyve found a way to create a stable form of whats known as quantum computing.
A new five-year agreement aims to build a type of system that could perform computations that are currently impossible in a short timespan, even for supercomputers.
Purdue physics and astronomy professor Michael Manfra is heading up the West Lafayette team, which will work with Microsoft scientists and university colleagues in Australia, the Netherlands and Denmark to construct, manipulate and strengthen tiny building blocks of information called topological qubits."
The real win that topological quantum computing suggests is that if you devise your system in which you store your information cleverly enough, that you can make the qubit insensitive basically deaf to the noise thats all around it in the environment, Manfra says.
He says that deafness is important because of whats held quantum computing back the ease with which its disturbed.
It can interact with photons; electromagnetic fields. It can interact with vibrations of the lattice. And those interactions, what they can do is cause a decoherence of that qubit basically cause it to lose the stored information.
Manfra says its an open question whether quantum computing will ever overtake the current zeroes-and-ones system of information storing, but he says hes interested in either proving or disproving the concept.
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Purdue, Microsoft Partner On Quantum Computing Research | WBAA - WBAA
Toward mass-producible quantum computers | MIT News – MIT News
Quantum computers are experimental devices that offer large speedups on some computational problems. One promising approach to building them involves harnessing nanometer-scale atomic defects in diamond materials.
But practical, diamond-based quantum computing devices will require the ability to position those defects at precise locations in complex diamond structures, where the defects can function as qubits, the basic units of information in quantum computing. In todays of Nature Communications, a team of researchers from MIT, Harvard University, and Sandia National Laboratories reports a new technique for creating targeted defects, which is simpler and more precise than its predecessors.
In experiments, the defects produced by the technique were, on average, within 50 nanometers of their ideal locations.
The dream scenario in quantum information processing is to make an optical circuit to shuttle photonic qubits and then position a quantum memory wherever you need it, says Dirk Englund, an associate professor of electrical engineering and computer science who led the MIT team. Were almost there with this. These emitters are almost perfect.
The new paper has 15 co-authors. Seven are from MIT, including Englund and first author Tim Schrder, who was a postdoc in Englunds lab when the work was done and is now an assistant professor at the University of Copenhagens Niels Bohr Institute. Edward Bielejec led the Sandia team, and physics professor Mikhail Lukin led the Harvard team.
Appealing defects
Quantum computers, which are still largely hypothetical, exploit the phenomenon of quantum superposition, or the counterintuitive ability of small particles to inhabit contradictory physical states at the same time. An electron, for instance, can be said to be in more than one location simultaneously, or to have both of two opposed magnetic orientations.
Where a bit in a conventional computer can represent zero or one, a qubit, or quantum bit, can represent zero, one, or both at the same time. Its the ability of strings of qubits to, in some sense, simultaneously explore multiple solutions to a problem that promises computational speedups.
Diamond-defect qubits result from the combination of vacancies, which are locations in the diamonds crystal lattice where there should be a carbon atom but there isnt one, and dopants, which are atoms of materials other than carbon that have found their way into the lattice. Together, the dopant and the vacancy create a dopant-vacancy center, which has free electrons associated with it. The electrons magnetic orientation, or spin, which can be in superposition, constitutes the qubit.
A perennial problem in the design of quantum computers is how to read information out of qubits. Diamond defects present a simple solution, because they are natural light emitters. In fact, the light particles emitted by diamond defects can preserve the superposition of the qubits, so they could move quantum information between quantum computing devices.
Silicon switch
The most-studied diamond defect is the nitrogen-vacancy center, which can maintain superposition longer than any other candidate qubit. But it emits light in a relatively broad spectrum of frequencies, which can lead to inaccuracies in the measurements on which quantum computing relies.
In their new paper, the MIT, Harvard, and Sandia researchers instead use silicon-vacancy centers, which emit light in a very narrow band of frequencies. They dont naturally maintain superposition as well, but theory suggests that cooling them down to temperatures in the millikelvin range fractions of a degree above absolute zero could solve that problem. (Nitrogen-vacancy-center qubits require cooling to a relatively balmy 4 kelvins.)
To be readable, however, the signals from light-emitting qubits have to be amplified, and it has to be possible to direct them and recombine them to perform computations. Thats why the ability to precisely locate defects is important: Its easier to etch optical circuits into a diamond and then insert the defects in the right places than to create defects at random and then try to construct optical circuits around them.
In the process described in the new paper, the MIT and Harvard researchers first planed a synthetic diamond down until it was only 200 nanometers thick. Then they etched optical cavities into the diamonds surface. These increase the brightness of the light emitted by the defects (while shortening the emission times).
Then they sent the diamond to the Sandia team, who have customized a commercial device called the Nano-Implanter to eject streams of silicon ions. The Sandia researchers fired 20 to 30 silicon ions into each of the optical cavities in the diamond and sent it back to Cambridge.
Mobile vacancies
At this point, only about 2 percent of the cavities had associated silicon-vacancy centers. But the MIT and Harvard researchers have also developed processes for blasting the diamond with beams of electrons to produce more vacancies, and then heating the diamond to about 1,000 degrees Celsius, which causes the vacancies to move around the crystal lattice so they can bond with silicon atoms.
After the researchers had subjected the diamond to these two processes, the yield had increased tenfold, to 20 percent. In principle, repetitions of the processes should increase the yield of silicon vacancy centers still further.
When the researchers analyzed the locations of the silicon-vacancy centers, they found that they were within about 50 nanometers of their optimal positions at the edge of the cavity. That translated to emitted light that was about 85 to 90 percent as bright as it could be, which is still very good.
Its an excellent result, says Jelena Vuckovic, a professor of electrical engineering at Stanford University who studies nanophotonics and quantum optics. I hope the technique can be improved beyond 50 nanometers, because 50-nanometer misalignment would degrade the strength of the light-matter interaction. But this is an important step in that direction. And 50-nanometer precision is certainly better than not controlling position at all, which is what we are normally doing in these experiments, where we start with randomly positioned emitters and then make resonators.
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Toward mass-producible quantum computers | MIT News - MIT News