One of the fundamental issues with making measurements in the quantum regime is that the act of measuring can actually destroy the fragile quantum state of such a system. This occurs despite one’s best attempts to be discreet in the characterization process. Basically, measuring messes things up.
Such sensitive measurements are especially difficult for macroscopic scale objects, and even on micrometer scales the quantum nature of objects is generally lost in the noise of the larger world.
In atomic physics and, more recently, in solid-state physics, researchers have gotten around this conundrum by using other quantum systems as probes. For example, the quantum signature of an oscillating system, like an electromagnetic cavity, can be imprinted on a probe and then inferred through an independent measurement of the probe – without destroying the quantum state of the original system. Such probes include individual atoms and nano-scale electronic quantum bits or "qubits."
A qubit is the quantum computing equivalent of the binary digital bit in classical computing. Like the “1” or “0” of its digital counterpart, the qubit has a state that can be controlled and measured.
In the new technique demonstrated by the Caltech and JPL team, the qubit is a tiny electrode in which electrons can be made to take on a single energy state, i.e., charged or uncharged. The qubit is placed in close proximity to a nanoresonator, which is a tiny beam of stiff material that vibrates at high frequency. In this configuration, the vibration frequency of the resonator is sensitive to the energy state of the quantum bit, and vice versa. The Caltech and JPL scientists used the sensitive dependence of the resonator frequency to probe the state of the qubit. In principle, though, either component can be used as a probe to determine quantum information about the other.
“We’ve demonstrated that we can use the nanomechanical resonator to probe quantum effects in the qubit. Based on these measurements, it looks we should be able to turn the experiment around and start looking for quantum effects in the mechanical system,” said Caltech’s Matt LaHaye. LaHaye is lead author of a paper describing the team’s work, published in the June 18 issue of the journal Nature.
According to quantum mechanics, the energy stored in any vibrating system – including macroscopic scale systems – should take discrete, or quantized, values. This discreteness of energy has been verified beautifully in other vibrating systems, such as atomic and electromagnetic systems. However, observation of this subtle quantum effect in ordinary mechanical structures, like the nanoresonator described above, remains a hotly pursued challenge in quantum measurement. The new nanoscale system demonstrated by the Caltech and JPL team is a very promising approach that could ultimately meet this challenge.
“What’s really exciting is that a lot of people have been thinking about how to use this coupled system for manipulating and measuring quantum states of mechanical objects. There are many possible experiments and directions in which we could head, all with the potential for very rich physics,” said LaHaye.
A tantalizing possibility for the team’s measurement technique is to observe the resonator in a quantum superposition of states, where it is simultaneously oscillating at two distinct frequencies. It may also be possible to examine the quantum imprint of the resonator on the energy state of the qubit to look for “quantum jumps” in the resonator’s energy.
The JPL team members had been developing the qubits for quantum computing applications and contributed to the research with fabrication of the structures and expertise in their measurement.
The work resulted from a collaboration between Profs. Michael Roukes and Keith Schwab at Caltech and Dr. Pierre Echternach at JPL. Measurements at milli-Kelvin temperatures were performed by Dr. Matt LaHaye and graduate student Junho Suh at Caltech. Electron beam lithography was performed by Richard Muller at JPL.
Reference: M.D. LaHaye, J. Suh, P.M. Echternach, K.C. Schwab, M.L. Roukes. (2009). “Nanomechanical measurements of a superconducting qubit.” Nature 459, 960-964.
Caltech press release: http://media.caltech.edu/press_releases/13271
JPL Microdevices Laboratory Website: http://microdevices.jpl.nasa.gov