Research Brings Quantum Computing Closer
University of Michigan researchers used lasers to create an arbitrary initialized quantum state of solid-state qbits at rates of ~1 GHz, taking fundamental steps toward qbit programming. The researchers trapped electron spin in a "dark state" in which they can arbitrarily adjust the proportions of 0 and 1 represented by the qbit.
Alexander E. Braun, Senior Editor -- Semiconductor International, 11/10/2008 8:25:00 AM
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Working with colleagues at the University of California at San Diego and the U.S. Naval Research Laboratory (Washington, D.C.), the researchers said they have made fundamental progress toward qbit programming. While a conventional bit must be a 0 or a 1, a qbit’s advantage is that it can be simultaneously both or a proportion of each. Until now, scientists had been unable to stabilize this duality.
The researchers used lasers to stably trap the spin of one electron confined in a single semiconductor quantum dot that, in a quantum computer, acts like a transistor in a conventional computer. Spin is an intrinsic property of the electron; not true rotation, but a state comparable to the magnetic poles. Electrons have up or down spin. In quantum computing, these directions represent conventional computing’s 0s and 1s.
The researchers trapped the spin in a “dark state” in which they can arbitrarily adjust the proportions of 0s and 1s that are represented by the qbit. They call it “dark” because it does not absorb light — which can destabilize the qbit — thus avoiding a loss of coherence between the two states and loss of the data held; information can be stored without error. “We’re the first to show that you can do this to a single electron in a self-assembled quantum dot,” Steel said. “To do quantum computing, you must be able to work with one electron at a time, and we’ve accomplished this.”
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| When a pump and probe laser hit the two-photon resonance of a lambda system, a superposition of the ground states, known as the dark state, is formed. This intensity plot shows the probe absorption spectrum as a function of both probe detuning and pump Rabi frequency. The sharp line shows an absence of absorption and is a telltale sign of coherent population trapping. (Source: University of Michigan) |
The researchers set out to do this coherently, by applying “coherent trapping,” a coherent superposition state; that is, a dark state. If these two spin states couple to a high-lying optically accessible state, then if light is applied to excite from one spin state to the optical state, and a second laser beam is used to excite from the upper state down to the other spin state, a new state is created — a combination of a 0 and a 1, a spin-up and spin-down. “And that state becomes dark; it doesn’t interact with the radiation,” Steel said, adding that as long as the lasers are on, the state is fixed, preserving the data.
“We can now initialize qbits in an arbitrary state,” he said, rather than just a 0 or a 1. “In the past, we created states that were almost all 0 and a little 1, or almost all 1 and a little 0. Now we’ve been able to make an arbitrary mix all the way from all of one to all of the other while preserving the quantum phase.”
Steel’s approach uses ultrafast lasers to manipulate arrays of semiconductor quantum dots, each containing one electron. Quantum logic gates result from quantum mechanical interactions between the dots.
This capability to represent multiple states simultaneously could theoretically enable quantum computers to factor numbers several orders of magnitude faster with smaller machines than conventional computers. This could vastly improve computer security. “The National Security Agency has said that, based on our present technology, we have about a 20-year window of security,” Steel said. “That means that if we sent up a satellite today, it would take somebody about 20 years to crack the code. Quantum computers will allow you to develop a code that would be impossible to crack with a conventional computer.”
Steel claims that he and his colleagues have met one of the requirements in the DiVincenzo Criteria — the ability to initialize qbits. “And we can do it at a 1 GHz rate. The trick to doing it at this rate is to realize that because the photon cannot flip the spin, another fast interaction must be turned within the quantum dot. We did this by applying a magnetic field in a direction parallel to the chip. This mixes the states and turns on an optically induced spin flip transition that is normally forbidden. So now we can initialize a dot at a billion times a second for any calculation.”
However, for many applications, one does not want to just initialize a qbit in the 0 or the 1 state. Quantum systems allow any combination of 0s and 1s simultaneously. “That’s one of the differences between the classical computer and the quantum computer, the other being quantum entanglement,” Steel said. “Classical computers can be 0s or 1s, but a quantum computer can have 20% 0, 80% 1 — simultaneously and with a definite phase. This is what you want, but you must do it while maintaining the phase between the two states. You don’t want the phase to drift. If you think of quantum mechanics as waves, then these two spin states are like waves and therefore have a phase, and if the phase of these two fields changes, then you’re in trouble. We can now maintain that phase.”
Steel said he expects the next breakthrough to come when it becomes possible to demonstrate entanglement between electrons’ spins isolated in adjacent dots.
