There’s more than one way to make a qubit. All you really need is something that can provide two different and defined quantum energy levels to serve as analogs for the 0 and 1 in a classical scheme. Many potential qubits are natural phenomena, manipulating the quantum characteristics of atomic nuclei, ions, or electrons to encode information into a quantum system. But what if you could manufacture qubits artificially with whatever properties you want them to have?
This approach has spawned an entire branch of quantum computing research that is trying to perfect the superconducting qubit. Perhaps unsurprisingly, IBM Research has emerged as one leader in this space, as the approach meshes nicely with the company’s expertise in superconductivity, microfabrication, and--perhaps most importantly--the scaling of technologies into finished products.
Stripped of a lot of complex physics, it’s easy to think of a superconducting qubit as an artificial atom. Technologically speaking, a superconducting qubit involves two superconducting materials running an oscillating current across a device called a Josephson junction, which through the magic of quantum physics allows the qubit to carve out just two oscillation frequencies of the many that the current might have and use those frequencies as the classical 0 and 1 (there’s a lot of quantum mechanics involved that we won’t get into here, but suffice it to say that controlling these oscillations satisfies the fundamental requirements for a qubit).
The main advantage of superconducting qubits is that they are manufacturable, and therefore lend themselves to customization and eventual scalability to a larger quantum computer possessing hundreds or thousands of qubits. But even the team at IBM--which recently demonstrated record-setting coherence times of up to 10-100 microseconds and gate operations with 95 percent success rates--knows that it’s far too early in the race to declare their method a winner.
“The superconducting approach has great potential and we think its the front-runner and that’s why we’re working on it,” says Mark Ketchen, a physicist helming IBM Research’s Physics of Information initiative. “But it’s early in the game and things could change. Five years from now the system could look very different.”
Tapping Electron Spin
That’s because superconducting qubits are far from the only game in town. At Harvard University, Dr. Amir Yacoby is exploring the possibility of encoding information via the spins of the electrons inside quantum dots--tiny semiconductor crystals with unique electronic characteristics. Broadly speaking, electrons have two possible spin states--call them left and right--that can represent the 0 or 1 state of a classical bit. Trapped in a quantum dot, electron spin can be measured and manipulated. But this introduces a problem that is common across quantum computing.
This is the same problem introduced by Schrodinger’s Cat, a common paradoxical problem when dealing with quantum systems (for a deeper understanding of all this, read up on the infamous cat and quantum entanglement). To create a usable qubit, researchers want something that is good at decoupling itself from its environment, something that won’t be influenced by external factors. At the same time, it’s necessary to have something that can be manipulated by external forces so the computation can be controlled.
Finding something that satisfies these contradictory needs of a viable quantum computing system isn’t particularly easy, but electron spin goes a long way toward serving both sides of the paradox. Spin lives for a long time, atomically speaking, so you can encode information in the spin and it will exist in the system for a relatively long time, contributing to better coherence. Electrons trapped in quantum dots can be coaxed into decoupling from their environments while still responding to weak magnetic fields--fields that are weak and predictable enough that even when they introduce error-producing noise into the quantum system, it’s relatively easier to correct the errors.
Still, spin isn’t immune to the problems dogging many in the quantum computing community who are trying to do very big things with very small particles. As with superconducting qubits, quantum dot computing would have to happen at very cold temperatures--something like a tenth of a degree above absolute zero. And all quantum complexity aside, the engineering challenges inherent in fabricating such a system with more than a few qubits are daunting. But Yacoby is unfazed.
“I think we’re going to encounter a lot of discovery before we have to face the engineering challenges in cooling down one thousand or ten thousand qubits,” Yacoby says. “I’m optimistic--very confident--that that level will be met within my lifetime.”single page
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