Ultrafast Gates for Single Atomic Qubits
A team of physicists has achieved ultrafast “switching” time in an operation central to quantum information processing, changing the state of a single trapped ion in less than 50 picoseconds (10-12, or trillionths of a second) with more than 99 percent reliability. That is the fastest time on record – by a factor of about 1,000—for ion “gates,” which may serve as a key component of an eventual quantum computer.
“Computer users are familiar with the Idea that when the wizards at semiconductor companies release a faster processor, this means more powerful computers,” says Wesley Campbell of the Joint Quantum Institute (JQI), first author on the team’s report, which will appear in Physical Review Letters.*
“What we're doing here is improving the processing speed of a quantum computer, which will not only make it faster, but also help to protect it against unavoidable nasty physical processes that work to erase our quantum information during the computation."
Quantum computing is performed on information units called qubits. Unlike the bits (binary digits) of conventional digital computing, which can have values of 0 or 1, qubits (quantum bits) can exist in a “superposition” of both values at the same time. The fragile, isolated condition necessary to sustain this superposition is called coherence, and its duration is arguably the single most important time scale in quantum information processing. The longer the coherence time, the longer the interval during which an operation can be performed on a qubit; the shorter the operation time, the greater the likelihood that the desired effect can be achieved before coherence disappears.
“In systems of the sort we used in this experiment,” Campbell explains, “it typically takes something on the order of a microsecond (one millionth of a second) to switch a qubit from the ‘0’ state to the ‘1’ state by conventional methods. Our ultrafast laser method is more than 10,000 times faster.”
Campbell and colleagues in the research group led by JQI Fellow Christopher Monroe began by confining a single ytterbium ion. (Once an atom becomes an ion by losing one or more electrons, it is no longer electrically neutral, and thus can be held in a trap by electrical fields.) Certain ions are notorious for coherence times that can last for minutes – a virtual eon in the quantum world.
The JQI scientists defined their qubit’s 0 and 1 values in terms of two closely-spaced excitation states of the Yb ion. The goal was to switch the ion from one state to the other with high fidelity, which is exactly what is necessary to use an ion as a gate that can process quantum information. To accomplish that transition as quickly as possible, the team used a “mode-locked” laser – that is, a laser that emits its beams in pulses rather than a continuous wave – and a high-speed device called a “pulse-picker” which acts like a camera shutter, allowing researchers to select individual pulses from the laser’s stream of 121 million per second, or about one every eight nanoseconds.
Each selected laser pulse had the same wavelength: 355 nm, in the ultraviolet part of the spectrum. But the team adjusted the intensity of the pulses and determined that there was a clear – and controllable – relationship between the energy content of the pulse and the probability that the transition occurred. By carefully tuning the intensity, Campbell and coworkers were able to dependably switch the ion’s state 72 percent of the time.
“To get the remaining 28 percent,” Campbell says, “we split the pulse into two parts and put a very slight delay between them” by changing the distance that one pulse traveled before reaching the ion. That delay was in the range of 40 picoseconds, corresponding to about one centimeter. Using the split-pulse method, the team was finally able to achieve transition rates better than 99 percent.
What makes that tally accurate is an exploitable physical difference between the two states: When light from a second laser is aimed at the ion, it will emit many photons if it is in one state, but remain “dark” if it is in the other, giving the researchers an unambiguous signal.
“What’s really significant here,” Campbell observes, “is that we have demonstrated that it is possible to ‘flip’ the single-ion qubit in about one-tenth of one trillionth of its coherence time. Now, of course, you can also flip it by other means, such as a beam of microwaves. But that method takes thousands of times longer.
Why does the speed matter? There are two main reasons. "First," Campbell says, "a computer can only run as fast as it can perform operations on units of information. So if it takes a microsecond to switch a qubit, your computer is limited to a clock speed of one megahertz, or one million cycles per second. That’s pretty slow. Today’s conventional desktop computers operate a thousand times faster, around two or three gigahertz. Second, and equally important, the shorter your operation time, the less you have to worry about ‘noise’ in the system, such as effects from random fluctuations in the equipment or changes in external fields. Noise is typically most problematic when it occurs on approximately the same time scale as the signal you’re looking for. So if you can do each element of your information-processing really fast, say a hundred or a thousand times faster than the normal frequency of noise, you’ll have dramatically better accuracy.”
* See reference publication.