June 16, 2014
Ever since D-Wave arrived on the scene with a type of quantum computer capable of performing a problem-solving process called annealing, questions have flown thick and fast over whether or not the system really functioned — and, if it did function, whether it was actually performing quantum computing. A new paper by researchers who have spent time with the D-Wave system appears to virtually settle this question — the D-Wave system appears to actually perform quantum annealing. It would therefore be the first real quantum computer.
Up until now, it’s been theorized that D-Wave might be a simulator of a quantum computer based on some less-than-clear benchmark results. This new data seems to disprove that theory. Why? Because it shows evidence of entanglement. Quantum entanglement refers to a state in which two distinct qubits (two units of quantum information) become linked. If you measure the value of one entangled qubit as 0, its partner will also measure 0. Measure a 1 at the first qubit, and the second qubit will also contain a 1, with no evidence of communication between them.
Researchers working with a D-Wave system have now illustrated that D-Wave qubit pairs become entangled, as did an entire set of eight qubits. (The D-Wave uses blocks of eight qubits, as shown below). [DOI: http://dx.doi.org/10.1103/PhysRevX.4.021041 – “Entanglement in a Quantum Annealing Processor”]
Assuming the experimental evidence holds up, this fundamentally shifts the burden of proof from “Prove D-Wave is quantum,” to “Prove the D-Wave isn’t quantum.” Evidence of entanglement is the gold standard for whether or not a system is actually performing quantum computing.
So, now what?
Now that we have confirmation that D-Wave is a quantum computer (or at least, as close to confirmation as we can likely get), the question is, how do we improve it? As we’ve previously covered, the D-Wave isn’t always faster than a well-tuned classical system. Instead of arguing over whether or not an Nvidia Tesla GPU cluster with customized software is a better or worse investment than a supercomputer that’s cryogenically cooled and computes via niobium loops, we’re going to look at what D-Wave needs to do to improve the capabilities of its own system. As Ars Technica points out, its architecture is less than ideal — for some problems, D-Wave can only offer less than 100 effective qubits despite some newer systems having 512 qubits in total, because its architecture is only sparsely connected. Each group of eight qubits connects to itself, but each island of eight qubits has just eight connections to two other adjacent qubits.
D-Wave has stated that it intends to continue increasing the number of qubits it offers in a system, but we can’t help wondering if the company would see better performance if it managed to scale up the number of interconnects between the qubit islands. A quantum system with 512 qubits but more than just two connections to other islands might allow for much more efficient problem modeling and better overall performance.
Inevitably this kind of questioning turns to the topic of when we’ll see this kind of technology in common usage — but the answer, for now, is “you won’t.” There are a number of reasons why quantum computing may never revolutionize personal computing, many of them related to the fact that it relies on large amounts of liquid nitrogen. According to D-Wave’s documents for initial deployments, its first systems in 2010 required 140L of LN2 to initially fill and boiled off about 3L of fluid a day. Total tank capacity was 38L, which required twice-weekly fill-ups. The Elan2 LN2 production system is designed to produce liquid nitrogen in an office setting and can apparently create about 5L of LN2 per day at an initial cost of $9500. [Read: Google’s Quantum Computing Playground turns your PC into a quantum computer.]
Did I mention that you have to pay attention to Earth’s magnetic field when installing a D-Wave system, the early systems created about 75dB of noise, and it weighs 11,000 pounds? Many of these issues confronted early computers as well, but the LN2 issue is critical — quantum computing, for now, requires such temperatures — and unless we can figure out a way to bring these systems up to something like ambient air temperature, they’ll never fly for personal use. Rest assured that lots of research is being done on the topic of room-temperature qubits, though!