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Doing calculations with a quantum computer is a race against time, thanks to the fragility of the quantum states at their heart. And new research suggests we may soon hit a wall in how long we can hold them together thanks to interference from natural background radiation.

While quantum computing could one day enable us to carry out calculations beyond even the most powerful supercomputer imaginable, we’re still a long way from that point. And a big reason for that is a phenomenon known as decoherence.

The superpowers of quantum computers rely on holding the qubits—quantum bits—that make them up in exotic quantum states like superposition and entanglement. Decoherence is the process by which interference from the environment causes them to gradually lose their quantum behavior and any information that was encoded in them.

It can be caused by heat, vibrations, magnetic fluctuations, or any host of environmental factors that are hard to control. Currently we can keep superconducting qubits (the technology favored by the field’s leaders like Google and IBM) stable for up to 200 microseconds in the best devices, which is still far too short to do any truly meaningful computations.

But new research from scientists at Massachusetts Institute of Technology (MIT) and Pacific Northwest National Laboratory (PNNL), published last week in Nature, suggests we may struggle to get much further. They found that background radiation from cosmic rays and more prosaic sources like trace elements in concrete walls is enough to put a hard four-millisecond limit on the coherence time of superconducting qubits.

“These decoherence mechanisms are like an onion, and we’ve been peeling back the layers for the past 20 years, but there’s another layer that left unabated is going to limit us in a couple years, which is environmental radiation,” William Oliver from MIT said in a press release. “This is an exciting result, because it motivates us to think of other ways to design qubits to get around this problem.”

Superconducting qubits rely on pairs of electrons flowing through a resistance-free circuit. But radiation can knock these pairs out of alignment, causing them to split apart, which is what eventually results in the qubit decohering.

To determine how significant of an impact background levels of radiation could have on qubits, the researchers first tried to work out the relationship between coherence times and radiation levels. They exposed qubits to irradiated copper whose emissions dropped over time in a predictable way, which showed them that coherence times rose as radiation levels fell up to a maximum of four milliseconds, after which background effects kicked in.

To check if this coherence time was really caused by the natural radiation, they built a giant shield out of lead brick that could block background radiation to see what happened when the qubits were isolated. The experiments clearly showed that blocking the background emissions could boost coherence times further.

At the minute, a host of other problems like material impurities and electronic disturbances cause qubits to decohere before these effects kick in, but given the rate at which the technology has been improving, we may hit this new wall in just a few years.

“Without mitigation, radiation will limit the coherence time of superconducting qubits to a few milliseconds, which is insufficient for practical quantum computing,” Brent VanDevender from PNNL said in a press release.

Potential solutions to the problem include building radiation shielding around quantum computers or locating them underground, where cosmic rays aren’t able to penetrate so easily. But if you need a few tons of lead or a large cavern in order to install a quantum computer, that’s going to make it considerably harder to roll them out widely.

It’s important to remember, though, that this problem has only been observed in superconducting qubits so far. In July, researchers showed they could get a spin-orbit qubit implemented in silicon to last for about 10 milliseconds, while trapped ion qubits can stay stable for as long as 10 minutes. And MIT’s Oliver says there’s still plenty of room for building more robust superconducting qubits.

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" ----jn;lsbu;dnw Insomnia www.sleephealthfoundation.org.au | Raising awareness of sleep health Important Things to Know About Insomnia • Around 1 in 3 people have at least mild insomnia. • Many poor sleepers have developed poor sleep habits. • For specialist help, cognitive behavioural therapy for insomnia is best • In the long run, taking sleeping pills isn't effective. • There are sleep specialists, clinics and on-line programs that can help. What is insomnia? Insomnia is said to be present when you regularly find it hard to fall asleep or stay asleep. It has several patterns. You may have trouble getting to sleep initially. Or even if you can fall asleep, you might not be able to stay asleep for as long as you would like. Also you may wake up during the night and not be able to go back to sleep for a long time. Many people have two of the above problems, or even all three. Because of these, you might feel tired during the day. What causes insomnia? Insomnia has many causes which can include: • Some medicines and drugs, e.g. asthma or blood pressure medication, caffeine, alcohol or smoking • Chronic pain and other uncomfortable illnesses • Stress at work or in your personal life • Depression • A friend or loved one passing away • Anxiety and worrying, including worrying about not getting enough sleep • Another sleep problem (see Ten Common Sleep Disorders) Sometimes there is no clear cause for insomnia, in which case it is called primary insomnia. How common is insomnia? Most people have experience insomnia symptoms at some time of their lives. At any given time around 10% of people have at least mild insomnia. Who is at risk? Older people with poor health have a higher risk. Also women have twice the rates compared to men. This may be related to higher rates of anxiety and depression, which can be associated with insomnia. Shift workers have a higher risk too. How does it affect people? You might feel that it's harder to focus and remember things. But most people think their memory is worse than it really is. The same goes for concentration. Your risk of a traffic accident or other injury may be higher (see Drowsy Driving). You may be more emotional and a lack of sleep can cause depressed mood. Some people feel sleepy during the day, but this can be caused by many things (see Excessive Daytime Sleepiness). How is it treated? This depends on what is causing the insomnia. • If poor sleep habits are the cause then these need to be improved (see Good Sleep Habits). SHF-Insomnia-1111 30/11/11 4:37 PM Page 1 • If your sleep habits seem to be okay but you are still having problems then you may need more specialist help. Cognitive-behavioural therapy for insomnia has been shown to be more effective in the medium and long term than sleeping tablets. (See the information under the final three subheadings.) • Stress, depression and anxiety are best treated by specialists, but taking steps to improve your sleep can also help with these. • Sometimes sleep specialists will suggest a sleep study to be sure they understand what may be causing the poor sleep and also check for sleep apnoea. What about sleeping pills? If you only take them occasionally, sleeping pills can get you a good night's sleep. However if you take them often, you will get used to them and they will stop working as effctively. Also they can be habit-forming and it can then become difficult to stop taking them. Where and when should you seek help? If you are having ongoing trouble sleeping, persistent problems with mood, restlessness in bed, severe snoring or wakening unrefreshed, make sure that you go and see your doctor. Your GP can refer you to a sleep specialist or psychologist. Insomnia treatment services in Australia are listed here: http://www.sleep.org.au/information/sleep-servicesdirectory Is there on-line help available? Yes, on-line, low cost, sleep improvement programs are available, such as: http://www.sleeptherapy.com.au/index.php?page=1 (Note: The Sleep Health Foundation does not endorse any program or product and this link is provided for information only.) Where can I find out more? http://www.counselling.cam.ac.uk/selfhelp/leaflets/ insomnia www.sleephealthfoundation.org.au | Raising awareness of sleep health Sleep Health Foundation ABN 91 138 737 854 114/30 Campbell Street, Blacktown NSW 2148 T: +61 (0) 2 8814 8655 F: +61 (0) 2 9672 3884 Disclaimer - Information provided here is general in nature and should not be seen as a substitute for professional medical advice. Ongoing concerns about sleep or other medical conditions should be discussed with your local doctor. ©Sleep Health Foundation, 2011 This information is produced by: Sleep Health Foundation ABN 91 138 737 854 www.sleephealthfoundation.org.au A national organisation devoted to education, advocacy and supporting research into sleep and its disorders Sleep Disorders Australia ABN 98 075 427 459 www.sleepoz.org.au A voluntary group offering assistance and support to people and their families living with sleep disorders Australasian Sleep Association ABN 32 172 170 561 www.sleep.org.au The peak national association of clinicians and scientists devoted to investigation of sleep and its disorders SHF-Insomnia-1111 30/11/11 4:37 PM Page 2