Aspelmeyer in Nature, “Quantum mechanics: The surf is up”

“Researchers have long wanted to be able to control macroscopic mechanical objects in their smallest possible state of motion. Success in achieving that goal heralds a new generation of quantum experiments.” Markus Aspelmeyer, “Quantum mechanics: The surf is up,” News and Views, Nature 464:685-686, April 2010.

Dead silence — and then roaring applause. I still remember that brief moment that revealed the astonishment of everyone in the audience. Andrew Cleland had just concluded his talk at the conference ‘Quantum Optics of Nano- and Micromechanical Systems’, which was held last July in Bad Honnef, Germany. He had taken us all by surprise. Andrew’s talk had begun as a review of recent results of his and John Martinis’ team — they had achieved an unprecedented degree of control over individual electromagnetic quanta (photons) in a microwave resonator by coupling the resonator to a superconducting two-state quantum system, or qubit. But the awe came with his last slide: in it, he showed that if the microwave resonator could possibly be replaced with a mechanical oscillator of similar resonance frequency, then the same qubit device could be used to attain quantum control over individual mechanical quanta (phonons).

“O’Connell et al. have opened the door for quantum control of truly macroscopic mechanical devices. The prospects are exciting. For example, superposition states of massive mechanical objects may be used to test possible deviations from quantum mechanics, which have been suggested to eliminate the ‘Schrödinger’s cat’ paradox (in which a cat concealed in a box can be both dead and alive in a superposition of states). Such tests require quantum superpositions of macroscopic spatial separation between two states of an object, literally of an object being both ‘here’ and ‘there’. In O’Connell and colleagues’ experiment, access to this regime is still hampered by the resonator’s high mechanical frequency: the actual displacement between the two motional states of the prepared superposition is six orders of magnitude smaller than the size of the unit cells of the resonator’s structural lattice.

“The difficulty in bringing a mechanical device to its quantum ground state lies in the environmental temperatures (T) needed. To suppress residual thermal phonons in the device requires T < h*fm/kB, where fm is the device’s resonance frequency and h and kB are Planck’s and Boltzmann’s constants, respectively. Typical mechanical resonators require ground-state temperatures below those achievable with standard cryogenic refrigerators. One solution is simply to increase the vibrational frequency. O’Connell et al.’s micromechanical resonator achieves ultra-high mechanical frequencies of fm ~ 6 gigahertz, for which they could prepare their system’s ground state using a conventional dilution refrigerator: at temperatures of about 25 millikelvin.

“To observe the quantum effects, the resonator must be couplet to another quantum system sufficiently strongly. Decoherence mechanisms prevented quantum effects from being observed. O’Connell et al. overcome this problem by connecting their resonator through a capacitor to a Josephson phase qubit, which consists of two superconductors coupled by an insulating (Josephson) junction. The qubit’s ground and excited states represent the two lowest energy states of the superconductors’ wavefunction phase difference across the tunnel junction. By using the piezoelectric nature of the resonator, the electromagnetic energy of the qubit can be converted into the mechanical energy of the resonator, or vice versa. In principle, this interaction allows the authors to coherently transfer an arbitrary state of the qubit to the resonator, and even to generate entanglement — a quantum effect in which the states of the qubit and the resonator are linked together in an inseparable way.

“To demonstrate coherent quantum-state transfer, O’Connell et al. prepared the qubit in the excited state and switched on the interaction between the qubit and the resonator. They then measured the occupancy of the qubit’s excited state as a function of the interaction time. The observed oscillatory behaviour of this occupancy is a clear quantum effect and indicates reversible qubit–resonator exchange of a single quantum of energy. The typical transfer times for single quanta (the Rabi swap time) was 4 nanoseconds, which is smaller than the energy decay times of 17 ns and 6 ns for the qubit and resonator, respectively. The authors observed a similar oscillatory behaviour in the excited-state occupancy when they transferred a qubit’s superposition state, one in which the system is in the ground and excited states at the same time, hence preparing a quantum superposition of the mechanical system. It is also worth noting that, after half the Rabi swap time, the authors’ transfer interaction should create an entangled state between the qubit and the mechanical resonator. They point out, however, that their current experimental performance excludes a direct test of entanglement.”

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