NEWS AND VIEWS

A diamond age of masers

Applications for masers — the microwave equivalent of lasers — have been hindered by their extreme operating conditions and the inability to produce continuous emissions. A diamond maser overcomes these limitations.
Ren-Bao Liu is in the Department of Physics and the Centre for Quantum Coherence, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China.
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In 1954, scientists reported the first maser1 — a device similar to a laser, but operating at microwave frequencies. Although lasers were not demonstrated until six years later2, masers have not been as widely used as their optical counterparts. The bottleneck has been the need to operate masers under conditions of either high vacuum or extremely low temperature (a few kelvin). In a paper in Nature, Breeze et al.3 present, for the first time, a maser that works continuously under ambient conditions. Such a device could lead to advances in microwave metrology and communications, and in quantum many-body physics.

The key component of a maser (or a laser) is a material known as a gain medium. In ordinary materials, electrons usually exist in their lowest energy states and can absorb radiation by jumping to higher states. However, in a gain medium, the electron population is inverted: there are more electrons in higher states than in the lowest ones.

A photon passing through a gain medium can stimulate an electron in a higher state to jump to a lower state and emit an identical photon. If these photons are bounced back and forth between mirrors, or confined in a closed metal structure called a cavity, they can be copied many times before escaping from the system, generating a macroscopic quantum state of many identical photons. This process explains the name maser (laser): microwave (light) amplification by stimulated emission of radiation.

The different orientations of the tiny magnet (the spin) associated with an electron are known as electron spin states, and have energy separations of the right magnitude for microwave emission. However, these states are susceptible to the collisions, rotations and vibrations of atoms through an effect called spin–orbit coupling. Researchers have minimized atomic collisions in masers using high-vacuum conditions and dilute gain media, such as ammonia molecules (as in the first reported maser1), hydrogen atoms, free electrons and rubidium gases4. Masers have also been built using solid-state gain media, such as ruby, and sapphire ‘doped’ with iron, in which extremely low temperatures are required to suppress the atomic vibrations4.

For practical applications, a solid-state maser that operates at room temperature is highly desirable. In 2012, a breakthrough towards this goal was reported in the form of a maser whose gain medium was an organic material: a crystal of the compound p-terphenyl, doped with pentacene molecules5. Lightweight atoms (such as carbon, hydrogen and oxygen) have weak spin–orbit coupling, which means that spins in organic materials have relatively long lifetimes — they can remain in a particular energy state for a long time before jumping to a different state.

Population inversion in the p-terphenyl maser was achieved by ‘pumping’ the electrons in the pentacene molecules with optical radiation at a pump rate of more than 104 hertz. However, organic materials cannot usually withstand the intensive laser radiation required for such optical pumping — the materials often have low melting points, are evaporated by optical radiation and have poor heat conductivity. Consequently, the p-terphenyl maser could produce only microwave pulses, rather than continuous emission.

Building on previous work68, Breeze and colleagues used a millimetre-scale diamond as the gain medium for their maser (Fig. 1a). Because it is composed of carbon atoms, diamond would be expected to have long spin lifetimes. The authors housed the diamond in a copper cavity and introduced free-electron spins into the diamond by adding defects called nitrogen-vacancy centres. Such a defect comprises a nitrogen atom, which replaces a carbon atom, and the void of a nearest-neighbour carbon atom. A nitrogen-vacancy centre has two unpaired electrons and three possible spin states (denoted by –1, 0 and +1)9.

Figure 1 | A diamond maser. a, Breeze et al.3 report a microwave laser (maser) that operates continuously under ambient conditions. Their maser consists of a closed copper structure called a cavity and a millimetre-scale diamond that contains defects known as nitrogen-vacancy centres. Each of these defects has two unpaired electrons whose magnetic moments (spins; blue arrows) can point in one of two directions. Nitrogen-vacancy centres therefore have three possible spin states (denoted by −1, 0 and +1). b, The authors applied a strong magnetic field (B) to the diamond, so that the −1 state had a lower energy than the 0 state. They then used a laser to ‘pump’ the nitrogen-vacancy centres into the 0 state. The defects relaxed to the −1 state (black arrow), and in doing so produced the microwave radiation associated with a maser.

Breeze et al. applied a strong, uniform magnetic field to the diamond so that the –1 state had a lower energy than the 0 state. They then used a laser to pump the nitrogen-vacancy centres into the 0 state, to achieve population inversion. The defects produced microwave radiation as they relaxed to the –1 state (Fig. 1b).

The maser required a pump power of at least 138 milliwatts. For a power of 180 mW, the authors measured the spin lifetime of the nitrogen-vacancy centres to be roughly 50 times that of the electrons in the p-terphenyl maser and the pump rate to be only about 300 Hz. Thanks to diamond’s high thermal conductivity (10,000 times higher than that of p-terphenyl), the temperature of the gain medium increased by only 35 °C when the pump power was raised to 400 mW. At room temperature, the maser worked continuously for up to 10 hours without noticeable degradation in power.

Because the frequency of microwaves produced by masers is highly stable, these devices have applications in time-keeping, high-precision spectroscopy and microwave amplification for deep-space communication and for the detection of astronomical objects. In the absence of solid-state masers that could operate at room temperature, alternative microwave sources and amplifiers were developed that were based on, for example, electronic circuits called crystal oscillators10 and sensitive detectors known as superconducting quantum-interference devices11. These usually also require low temperatures. The authors’ room-temperature solid-state maser could therefore transform both microwave metrology and communications.

The good thermal conductivity of diamond and the persistence of long spin lifetimes at high temperatures12 mean that the authors’ maser could be pumped at higher powers than they demonstrated, improving both the intensity and the stability of the emission. However, the maser’s performance is constrained by various factors, including the requirement for a strong, uniform magnetic field, temperature fluctuations caused by laser heating and low efficiency of power conversion from the pump laser to the output. Possible approaches to address these issues could include introducing the types of defect found in similar materials to diamond8, or borrowing ideas from other areas of research, such as superradiant lasers13 and lasing without population inversion14.

Finally, Breeze and colleagues’ maser could provide a platform for studying quantum many-body physics. The spins of nitrogen-vacancy centres have not only long lifetimes, but also long coherence times9 — the length of time for which spins can be in several different energy states at the same time, known as a quantum superposition. For this reason, nitrogen-vacancy centres have been intensively studied for quantum computing9 and quantum sensing15.

The interaction between many spins and many microwave photons in the authors’ maser could result in a quantum mixture that has a half-spin, half-photon nature1618. Such a mixture might offer a way to study macroscopic quantum phenomena at room temperature. These studies could be further enriched by introducing dipole–dipole interactions between the spins, or by transforming the electron spin states into nuclear spin states.

Moreover, if the spin coherence time in the maser were longer than the photon storage time of the cavity, a photon–spin mixture could be realized in which the quantum coherence is associated mainly with the spins. The result would be a superradiant maser13 that, unlike the authors’ maser, has an emission frequency that is insensitive to the temperature fluctuations caused by laser heating. Thanks to Breeze and colleagues, a diamond age of masers can now be envisaged.

Nature 555, 447-449 (2018)

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