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Quantum optics

Quantum optics

Physicists create and measure quantized mechanical oscillations

25 Sep 2017
Artist's conception of the Vienna/Delft experiment
Good vibrations: artist's conception of the Vienna/Delft experiment

Two groups of physicists – one in the US and one in Europe – have developed two different techniques to create and measure quantized mechanical oscillations. The research has fundamental implications for the macroscopic limits of quantum mechanics and the techniques could also be useful for transferring quantum states between different systems. The latter could be very useful for creating new technologies for quantum computing and quantum communication.

Quantum mechanics is usually thought of as the physics of very small objects such as atoms or subatomic particles. It also governs larger objects, but even a relatively small macroscopic system must be cooled to extremely low temperatures for quantum effects to be observed above fluctuations related to thermal noise.

In 2010, Andrew Cleland and colleagues at the University of California, Santa Barbara observed quantum effects in an object visible to the naked eye by coupling a microwave-frequency mechanical oscillator to a superconducting circuit. John Teufel of the National Institute for Standards and Technology (NIST) says that this work was “ahead of its time”, and because it was so technically challenging there has been very little follow-up work. “In the original Cleland paper, their heroic measurement was being able to measure [the quantum state] before this thing decayed,” he says. Further work such as constructing unusual quantum states or performing intricate operations was impossible.

Simple and improved

In one of the new experiments, Robert Schoelkopf and colleagues at Yale University fabricated a conceptually similar, but technically much simplified and improved, set-up using a superconducting circuit with multiple, tunable energy levels. Team member Yiwen Chu explains that two of these energy levels can be used to create a quantum bit (qubit).

One of the circuit’s electrodes is connected to a thin disc of piezoelectric aluminium nitride at one end of a sapphire resonator cavity. This resonator contains multiple independent, quantized vibrational modes. The aluminium-nitride’s piezoelectricity allows the researchers to inject quantized vibrations (phonons) into the sapphire or to draw them out.

By adjusting the spacing between the energy levels, the researchers can tune the qubit into resonance with any of the vibrational modes. They performed several operations on one of the quantum states, such as measuring its decay rate by exciting a single phonon mode, waiting a variable amount of time and extracting the energy again to measure the probability that the phonon had been lost. They found that the lifetime of the state is approximately 17 μs – around 1000 times longer than the lifetime measured by Cleland’s team in 2010.

Schrödinger’s cat states

The Yale researchers now hope to use this extended lifetime to create more complex quantum states in the resonator. “In principle, we can, for example, put two phonons into our mechanical resonator,” says Chu, “That just involves exciting the qubit, putting in one phonon, exciting the qubit again and putting in another phonon: it’s just a matter of how many quantum operations we can do before the quantum state is lost.” They also hope to demonstrate “Schrödinger’s cat” states, in which the resonator vibrates in two independent ways at once. This would be a first for a mechanical system.

The other experiment was done by researchers at the University of Vienna in Austria and Delft University of Technology in the Netherlands. It involves firing laser pulses at a microscopic optical cavity that is cooled almost to absolute zero. Each laser photon has an energy greater than that of a photon at the optical resonance of the cavity. The excess energy is equal to the energy of a phonon at the cavity’s mechanical resonance frequency. While most of the photons are simply reflected from the cavity, about 1% create a photon and phonon inside the cavity. “If we detect a photon that’s on resonance with the cavity, we know we must have excited the mechanical system,” explains Delft’s Simon Gröblacher.

When this happens, the researchers switch to probing the cavity with much brighter laser pulses with photons of energy lower than the cavity’s optical resonance frequency. Occasionally, one of these photons will absorb energy from a phonon giving it a new energy that matches the optical resonance of the cavity.

Preserved states

By looking at the statistical properties of the photons emitted on resonance with the cavity, the researchers can show that these photons are in the same quantum state as the photons originally used to excite the mechanical resonator. The quantum state of the input photon had therefore been preserved in the single phonon they created and was then transferred back onto the output photon.

The team is now working to achieve more control over precisely when a phonon is inserted into the cavity and extracted, which would be necessary for creating a quantum memory: “At present, we get a receipt when the state has been transferred,” says Vienna’s Marcus Aspelmeyer. “We would like to know ahead of time with 100% probability that, when we push the button, the state will be transferred.”

The European researchers’ level of control over their quantum states is simpler than that achieved by the Yale group. However, their scheme has practical advantages because the input and output quantum states photons at telecommunication wavelengths and could therefore be used in quantum networking.

Quantum connections

Chu agrees: “Superconducting circuits are very good at generating complex quantum states, but photons have the advantage of being able to send information over very long distances using light,” she says. “One of the holy grails of this field is to be able to connect these two systems – for example to use a superconducting circuit to create a complicated quantum state, convert it into light and send it off down an optical fibre. One possible way of doing this is by coupling them through a mechanical oscillator: each of these two papers is demonstrating half of that conversion.”

Teufel, who was not involved in the two experiments, is impressed: “One of the frontiers of what people are trying to do [in quantum mechanics] is to measure mechanical systems at the quantum level,” he says, “These two papers, in two very different ways, are at the forefront of that.”

The research is described in two papers that are published in Science.

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