Quantum battery breakthrough: the larger the battery capacity, the shorter the charging time

An international team of researchers from the UK, Italy and Australia has used superabsorption techniques to validate a theoretical model of a quantum battery. The result was published in Science Advances on January 15 [1].
 
Using a technique called electron beam deposition, the researchers built a series of "microcavities": thin layers of light-absorbing organic molecules, a few nanometers wide and a few square centimeters in area. When they charged the cavity with a laser, they were able to observe the microcavity charging at an ultrafast rate. And the bigger the cavity, the faster the charging. The researchers say the discovery could be used to build prototype quantum batteries.

Superabsorption and quantum batteries

The superabsorption effect, the N-fold enhancement of the absorption of radiation by an ensemble of N two-level systems (TLS), has only recently been shown to work with a small number of atoms.Quantum batteries (QBs) are brand new energy storage devices that operate based on quantum mechanical principles, and their key property is the super-expansion of energy absorption.It is driven by quantum entanglement, which reduces the number of lateral states in Hilbert space, or increases the effective quantum coupling between the cell and the source through cooperative behavior. These effects mean that the charging time of a quantum battery is inversely proportional to the battery capacity.

Superabsorption and quantum batteries

In this paper, the researchers used an organic semiconductor as an ensemble of two-stage systems, coupled to closed optical modes in a microcavity. The device structure in the experiment consists of a thin (active) layer of a low-mass molecular semiconductor dispersed in a polymer matrix. This layer was deposited by spin coating and placed between two dielectric mirrors to form a microcavity, as schematically shown in Figure 1A. The microcavity consists of Lumogen-F orange (LFO) dispersed in a polystyrene (PS) matrix between distributed Bragg reflectors (DBRs). LFO is an organic semiconductor with the chemical structure shown in Figure 1B. The normalized absorption (red) and photoluminescence (blue) spectra of the 1% concentration LFO film are shown in Figure 1B. The researchers operated near the peak of absorption/photoluminescence.
 
By diluting the LFO, the intermolecular interactions leading to emission quenching are reduced, yielding high photoluminescence quantum yields of about 60% at low concentrations. The absorption peak at 526 nm and the emission peak at 534 nm correspond to the 0-0 transition, the electronic transition from the lowest vibrational state to the lowest vibrational state. The LFO molecule works around the 0-0 transition and can reasonably be thought of as a TLS.
 
To demonstrate the quantum battery, the researchers prepared samples with concentrations of 0.5%, 1%, 5%, and 10%. The LFO concentration determines the working coupling region. 0.5% and 1% of LFO cavities work in the weak coupling region, 5% work in the middle coupling region, and 10% work in the strong coupling region. Figure 1C shows the angle-dependent reflectance of the 1% cavity, matching the cavity mode shown by the blue dashed line.

Figure 1 Schematic and experimental setup of the LFO microcavity.

(A) Microcavity composed of Lumogen-F orange (LFO) in a polystyrene (PS) matrix dispersed between distributed Bragg reflectors (DBR);(B) normalized absorption of 1% concentration LFO film (red) and photoluminescence (blue) spectra, the molecular structure is shown in the inset. The researchers operate near the peak of absorption/photoluminescence;(C) angle-dependent reflectance of the 1% cavity, the blue dashed line shows the fit of the cavity mode;(D) the laser pump pulse excites the LFO molecule . The energy of the molecule is then measured with a probe pulse of delay time t, from which peak energy density (Emax), rise time (τ), and peak charging power (Pmax) can be determined;(E) Experimental setup for ultrafast instantaneous reflectance measurements. The output of a non-conjugate optical parametric amplifier (NOPA) is split to generate pump (dark green) and probe (light green) pulses. A mechanical chopper was used to modulate the pump pulses to generate alternating pump-probe and probe-only pulses.

The researchers used "ultrafast transient absorption spectroscopy" to measure charging and energy storage kinetics, allowing the measurement of femtosecond charging times. The microcavity was first excited with a pump pulse, and then the change in stored energy (that is, corresponding to the number of excited molecules) was measured with a second probe pulse with a delay time t (Fig. 1D). The probe pulses are transmitted through the top distributed Bragg reflector (DBR) of the cavity, and the reflection from the bottom DBR is measured. The differential reflectivity due to the pump pulse is given by:

The picture and picture are the reflectance of the probe with (without) pump excitation; the control film (active layer without microcavity) is measured at different transmittances ΔT∕T.

By comparing the control film and microcavity spectra shown in Figure 2A, it is shown that ultrafast transient absorption spectroscopy can monitor the number of excited molecules. It was found that the spectra of the control films at all concentrations showed two positive bands around 530 and 577 nm, which both reflect the excited state particle number.

Figure 2 Experimental demonstration of ultra-extended charging

(A) Differential transmittance (ΔT/T) spectra of the control film (1% LFO concentration) at a probe delay time of 1.0 ps and differential reflectance (ΔR/R) of the microcavity at a probe delay time of 1.25 ps spectrum;(B) The time-resolution energy density of the microcavity shows that the rise time decreases with increasing stored energy density, indicating hyperextended charging. A1, A2, and A3 labeling results for microcavities containing LFO at concentrations of 10%, 5%, and 1% when the ratio of pump photons to molecules is kept around r≃0.14. B1 and B2 labeling measurements of LFO at 1% and 0.5% concentrations, where r≃2.4.

Picture 2B shows the experimental values of the stored energy density over time. In all the microcavities studied, the energy density experienced a stage of rapid rise and then slow decay. The time scale of rapid rise varies with concentration. The researchers then adjusted the laser power to fix the photon density (r) of the whole comparable microcavity and compared the performance of different LFO concentrations. It is found that it is impossible to compare all microcavities at the same r value in order to achieve a sufficiently high signal-to-noise ratio; On the contrary, a constant R value is maintained for the matched structure.

Table 1 summarizes the rise time or time to half maximum energy (τ), peak stored energy density (Emax), and charging rate or peak charging power density [Pmax=max(dE∕dt)]. These were extracted from theoretical fits to the data in Figure 2B. We see that τ decreases with increasing number of molecules N, whereas Emax and Pmax increase with increasing N.

Table 1 In each experimental grouping A, B, the number of molecules (N) increased and the ratio of photons to molecules remained unchanged (r ≈ 0.105 and 2.4, respectively). The rise time τ is defined by the time to reach Emax/2, where Emax is the peak stored energy or energy density per molecule. The charging rate Pmax=max(dE/dt) is the peak charging power or charging power density per molecule.

The results show that as the number of molecules in the microcavity increases, its charging power density increases significantly. This means that charging a single microcavity containing N molecules takes less time than charging N single-molecule microcavities, even if the latter are charged simultaneously. Furthermore, a microcavity containing N molecules will store more energy than N microcavities (each containing one molecule).

Quantum batteries still face challenges

The researchers used ultrafast spectroscopy to provide direct experimental evidence of ultra-extended energy storage capacity and charging in organic microcavities. However, pure closed unitary dynamics is insufficient to realize a practical quantum battery. The storage of energy requires a finely tuned decoherence process that allows the battery to charge quickly but discharge much more slowly. The stabilization of this stored energy is a critical step in utilizing ultra-extended charging. Observations of decoherence show that realistic noisy environments can aid in the realization and application of quantum batteries.
 
Finally, quenching limits the performance of quantum batteries at high concentrations, and overcoming this limitation requires careful selection of materials to suppress intermolecular quenching. While research has focused on quantum advantages in charging, methods for efficiently extracting energy do exist.
 
A challenge for future work is to further explore how the concept of ratchet states could enable quantum batteries to operate in the range of higher-level energy states associated with maximum absorption enhancement.

Paper link：[1]https://www.science.org/doi/10.1126/sciadv.abk3160