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The qubit is the elementary information unit in quantum information science1. Encoding a qubit into two modes of a single photon allows the distribution of quantum information over long distances. This has enabled a range of experiments, from fundamental tests of quantum physics10,11 to applications related to quantum communication3 and quantum networks9,12. However, unavoidable absorption, diffraction and scattering losses of long transmission channels severely limit the transfer distance. It is important to emphasize that these losses typically occur independent of the state of the qubit that is encoded in the two optical modes, be it time bins or light polarization. In fact, in optical fibres the loss rate of the qubit carrier, the photon, can be many orders of magnitude larger than all decoherence rates of the encoded qubit. By contrast, material qubit carriers are rarely lost in any quantum information protocol demonstrated so far. For photons, the loss is fundamental and cannot be eliminated in any of the envisioned quantum information processing tasks. However, the loss effect could be mitigated by tracking the photon without destroying the encoded qubit, provided a nondestructive photonic qubit detector (NPQD) is available.

Once at hand, such an NPQD could communicate to both the sending and the receiving nodes of a quantum network whether or not a photonic qubit has been lost along the way (Fig. 1a). This loss monitoring has several advantages: first, it allows quantum communication schemes to stop the execution of further operations and restart the protocol when the qubit-carrying photon was found to be lost along the communication channel. This is important when these operations are time-expensive or involve the use of precious resources such as long-distance entanglement for quantum teleportation. Second, the detector can precertify the arrival of a photonic qubit immediately before a planned quantum measurement so that the latter is only conducted when the qubit is present at the measurement input. This enables us to perform loss-sensitive qubit measurements even in the presence of high transmission losses. Examples are measurements limited by the detection noise, photon-receiving repeater schemes13, or experiments where qubit loss leads to loopholes as in Bell tests6,7. The last are of key importance for device-independent quantum key distribution14.

Fig. 1: Nondestructive photonic qubit detector.

a, A quantum network including NPQDs. Detected and non-detected qubit photon events are coloured in green and red, respectively. b, The NPQD set-up showing the crossed optical-fibre cavities together with the microwave antenna and the state-detection beam (red solid line). Input photonic qubits (red arrows) are reflected by the NPQD via a highly transmittive non-polarizing beam splitter (NPBS, with a reflection of R = 0.5%) and later sent to a qubit measurement set-up (black dashed rectangle) including λ/2 and λ/4 waveplates, a polarizing beam splitter (PBS) and single-photon detectors (SPD). c, Atomic-level scheme together with the qubit coupling transitions (dashed arrows). d, The atomic state affects the entrance of the qubit photon into the qubit cavity. |ea denotes the atomic excited state that is coupling to |0a via the qubit cavity modes. e, Bloch spheres I–IV show the atomic state for different stages of the ideal NPQD scheme.

Despite recent progress on the quantum nondemolition detection of optical and microwave fields15,16,17,18,19,20,21, the nondestructive detection of the photonic energy without projection of the encoded qubit information remains an outstanding challenge. Some work related to the experimental implementation of NPQDs in the optical domain has been conducted, such as the nondestructive detection of bright two-mode light pulses using cross-phase modulation22, although this scheme operates far from the single-photon qubit regime. Another approach makes use of parametric down-conversion of the incoming qubit photon23, where one down-converted photon heralds the qubit presence and the other provides the qubit information. This approach shows a severely limited efficiency (−76 dB), owing to the small conversion efficiency of the applied photon-splitting process. Other related work includes heralded qubit amplifiers, which use two ancilla photons that interfere with the input signal24. However, interference requires prior knowledge about the pulse shape and arrival time and therefore restricts the range of possible applications. Finally, our previous work on heralded quantum memories25,26 can, in combination with readout, be used to herald photonic qubits. But the single-photon herald signal is highly sensitive to losses and the system therefore has a small detection efficiency. It is important to point out that (in contrast to the NPQD reported here) all mentioned qubit-heralding schemes destroy the original incoming photon waveform, which is important in applications where synchronization and interference matter, as for time-bin qubits or in linear optical quantum computing27.

We here demonstrate the implementation of an NPQD using a single atom strongly coupled to the modes of two independent optical resonators. Photonic polarization qubits are sent onto the qubit cavity where they are reflected while imprinting a π phase shift on an atomic superposition state. By coherent manipulation of the atomic superposition state, phase information is mapped into population information, which can be read out using the state-detection cavity. This signal heralds the nondestructive detection of the photonic qubit reflected off the qubit cavity. We show that the strong atom–photon interaction provided by both cavities allows for an efficient NPQD. We also prove that the reflection mechanism preserves the temporal waveform of the light pulse, which is of key importance for photonic time-bin qubits.

The qubit–atom interaction mechanism is, in essence, a single-rail version of the scheme proposed by Duan et al.28. In our case, it starts with a 87Rb atom prepared in state ((|{0}_{{rm{a}}}rangle +|{1}_{{rm{a}}}rangle )/sqrt{2}) (Bloch sphere I in Fig. 1e). The atomic state |0a is strongly coupled to the optical cavity modes, thus preventing a photon from entering the cavity. Instead, the photon reflects from the cavity input mirror (Fig. 1d) so that |Ψph|0a → +|Ψph|0a, where |Ψph denotes a photonic polarization qubit. By contrast, state |1a has no transition in resonance with the qubit cavity. Hence, the qubit photon enters the cavity and acquires a π phase shift upon reflection, |Ψph|1a → −|Ψph|1a. Together, the qubit photon flips the atomic superposition state to ((|{0}_{{rm{a}}}rangle -|{1}_{{rm{a}}}rangle )/sqrt{2}) (Bloch sphere II). Finally, a π/2 pulse rotates the atomic state to |0a (|1a) in the case of one (no) qubit photon (Bloch sphere III), enabling us to witness the atomic phase flip by means of deterministic atomic-state detection (Bloch sphere IV).

Compared to previous experiments18, our scheme is made possible by two key ingredients: first, we choose a state |0a |F = 1, mF = 0 so that the qubit cavity, with its two left and right circularly polarized and frequency-degenerate eigenmodes, couples strongly to two atomic transitions, |0a ↔ |F′ = 2, mF = ±1 (Fig. 1c). Here, F and F′ denote the magnitude of the total atomic angular momentum and mF denote their projection onto the quantization axis. The coupling strengths for both transitions are equal by definition but much smaller than for a cycling transition. We compensate this reduction by using a miniaturized fibre resonator29, achieving a coupling constant g = 2π × (18.6 ± 0.5) MHz and a cooperativity of C = 1.67 ± 0.09 (see Methods). Second, the compact lateral size of fibre resonators allows for the integration of a second crossed optical-fibre resonator with small mode volume. Here, it is used as the state-detection cavity which is tuned to the atomic cycling transition |F = 2 ↔ |F′ = 3, enabling a strong fluorescence signal to be observed from |1a |F = 2, mF = 0. This is used to distinguish the two atomic states |0a and |1a with a fidelity of (98.2 ± 0.2)% at a photon-number threshold of one.

The physical elements needed to implement the NPQD are shown in Fig. 1b. The photon qubit |Ψph first passes a highly transmitting non-polarizing beam splitter. Although the non-polarizing beam splitter is suitable for characterization measurements employing weak coherent pulses, for future implementations using single photons it should be replaced by an efficient optical circulator (Fig. 1a). After reflection at the non-polarizing beam splitter, the photon qubit is coupled into a single-mode optical fibre that connects to the qubit fibre cavity where the nondestructive interaction takes place. After reflection, the qubit photon is characterized by state tomography. Coherent manipulation of the atomic ground states is achieved by means of microwave fields emitted by an antenna close to the fibre cavities. The microwave flips the atomic state in about 12 μs with a probability of (97 ± 1)% (see Methods). The detector herald signal is generated by a laser that, together with the state-detection cavity, induces cavity-enhanced fluorescence9 for typically 7.5 μs. The fluorescence photons are predominantly emitted into the cavity from where they are guided towards a single-photon detector. Note that both fibre cavities are single-sided (for details, see ref. 26), so that the dominant escape channels for cavity photons are the two single-mode fibres.

We now characterize the nondestructive detection of photonic polarization qubits. For this, weak coherent pulses at the single-photon level were used. Figure 2a illustrates the NPQD temporal sequence by showing a time histogram of single-photon detector counts at the qubit measurement setup and the state-detection cavity output. The mean photon number here is |α|2 = 0.13 in front of the qubit cavity. As can be seen, the reflection of photonic qubits at the NPQD (green bars) has a large effect on the number of observed state-detection photons (red bars) compared to the situation where no qubits are sent (blue bars). This enables an efficient detection of the photonic qubits. However, the definition of the NPQD efficiency is ambiguous as it depends on the use case. Relevant parameters that relate to the NPQD efficiency include the conditional probability of detecting the atom in the qubit-heralding state |0a, either given a qubit at the qubit cavity output (1oq), P(0a|1oq) = (79 ± 3)%, or given a qubit at the NPQD input (1iq), P(0a|1iq) = (45 ± 2)%, and the photon survival probability ηsurv = (31 ± 1)%. Another figure of merit for any detector is the dark-count probability, which is pDC = (3.3 ± 0.2)% for our NPQD.

Fig. 2: Experimental NPQD implementation.

a, Time histogram showing the NPQD sequence, which includes the microwave pulses, the reflected qubit photon events (green) and the state-detection photon events. Red (blue) coloured state-detection counts represent data conditioned on a reflected photonic qubit detection (without an incoming photonic qubit). Labels I–IV refer to the Bloch spheres in Fig. 1e. b, Poincaré sphere showing the result of state tomography (coloured spheres) for reflected photonic qubits that were nondestructively detected. Labels indicate input polarizations. c, Bar chart of mean fidelities ({bar{ {mathcal F} }}_{x}) and the quantum process state fidelity ({bar{ {mathcal F} }}_{{rm{s}},chi }). The Poincaré sphere shows a rotation around the A/D axis which, if eliminated, leads to ({bar{ {mathcal F} }}_{circlearrowleft }). d, The preservation of the photonic waveform is tested by reflecting photonic qubits with three different waveforms off the NPQD (blue points). Yellow points represent empty cavity measurements as reference. The error bars in c (d) describe the 1σ confidence interval (standard deviation). The error bar of ({bar{ {mathcal F} }}_{{rm{s}},chi }) is assessed with a Monte Carlo method and represents the standard error.

As NPQDs should preserve the qubit, polarization state tomography was conducted on the outgoing photonic qubits and compared with the incoming qubits. With this we calculate the fidelity ( {mathcal F} ) = Ψin|ρout|Ψin. Six orthogonal polarization states were tomographically investigated (Fig. 2b), showing an overall mean state fidelity of ({bar{ {mathcal F} }}_{{rm{a}}{rm{l}}{rm{l}}}=(96.2pm 0.3){rm{ % }}) conditioned on a nondestructive qubit detection. Our fidelity, here for |α|2 = 0.2, greatly exceeds the upper limit of the fidelity 2/3 or 5/6 which is possible for schemes employing 1 → ∞ or 1 → 2 universal quantum cloning machines, respectively30. Note that the limits must be slightly shifted to 67.5% (ref. 31) and 84.9%, owing to higher photon number contributions of weak coherent pulses. Against this backdrop we conclude that our NPQD operates in the quantum regime.

A major contribution to the infidelity comes from a polarization rotation around the A/D axis, attributed to a residual birefringence of the qubit cavity (see Methods). The mean fidelity for rotated and non-rotated states is given by ({bar{ {mathcal F} }}_{{rm{HVRL}}}) and ({bar{ {mathcal F} }}_{{rm{AD}}}), respectively (Fig. 2c). However, taking into account this rotation in the calculation (which would be experimentally feasible by placing retardation waveplates after the qubit cavity) leads to an overall mean state fidelity of ({overline{ {mathcal F} }}_{circlearrowleft }=(98.0pm 0.3) % ) (orange bar). The remaining 2% infidelity is attributed to errors in the calibration of the qubit measurement setup and to fluctuations of the qubit cavity resonance frequency. Additionally, a quantum process is reconstructed via a maximum-likelihood fit, leading to an overall mean state fidelity of ({bar{ {mathcal F} }}_{{rm{s}},chi }=(96.3pm 0.6) % ).

Another important property of an NPQD is to preserve the temporal waveform of the photonic qubit (Fig. 2d). This is of great importance in scenarios where photons interfere or where quantum information is encoded in the temporal mode of the photon in addition to, or instead of, the polarization. To illustrate that our NPQD preserves the waveform, we compare three different envelopes of qubit pulses reflected off our atom–cavity system (blue points) with pulses reflected off an empty cavity (yellow points). For all three cases we find an intensity waveform overlap between the outgoing and the incoming photon exceeding 99.5%.

The mean input photon number |α|2 of the weak coherent pulses encoding the qubit has a role in the performance of the NPQD. Its characterization is presented in Fig. 3 where different figures of merit of our detector are shown. For the given range of |α|2 in front of the qubit cavity, the nondestructive detection probability upon qubit survival P(0a|≥1oq) (blue points in Fig. 3a) shows a maximum of (79 ± 3)% at |α|2 = 0.13. Here, the major error is attributed to a difference in intensity reflection coefficients for a coupling (({R}_{|{0}_{{rm{a}}}rangle }=0.50pm 0.02)) and a noncoupling atom (({R}_{|{1}_{{rm{a}}}rangle }=0.117pm 0.003)); see Methods. For high |α|2, the conditional probability converges towards 0.5 owing to the balanced contribution of odd and even photon numbers32, and decreases for small |α|2 values owing to dark counts of the qubit measurement set-up. The unconditional probability P(0a) (green points) shows the same convergence for high |α|2. However, in the limit of |α|2 → 0, the probability is lower-bounded by pDC (grey dashed line), which is due to imperfections in the atomic-state manipulation, in the optical pumping and in the state detection.

Fig. 3: Characterization with different mean input photon numbers.

a, Nondestructive detection probability as a function of the mean input photon number |α|2 in front of the qubit cavity. P(0a|≥1oq) is conditioned on a successful qubit reflection (≥1oq), whereas P(0a) is unconditioned. The horizontal dashed line represents the NPQD dark-count probability pDC. b, Mean photon number at the qubit cavity output conditioned on a nondestructive detection event ({bar{n}}_{{rm{oq}}}({0}_{{rm{a}}})). Inset, the autocorrelation function g(2)(τ = 0) of the nondestructively detected qubits. All solid data points are taken with Gaussian photon pulse shapes and linear near-vertical polarization. Open circle (square) data points were taken with orthogonal polarization (different pulse shapes; Fig. 2c). For all plots the solid line is given by theoretical simulations described in Supplementary Information. The dashed line in b considers pDC = 0. The error bars in a represent the 1σ confidence interval. The error bars in b and the inset to b represent the standard error and the standard deviation, respectively.

Another important figure of merit is the probability of having a photonic qubit at the NPQD output conditioned on its nondestructive detection, P(1oq|0a). Because we characterize the NPQD with weak coherent states (and not with single-photon Fock states), we instead show in Fig. 3b the conditioned mean photon number, ({bar{n}}_{{rm{oq}}}({0}_{{rm{a}}})), which is equivalent to P(1oq|0a) for small input photon numbers. We observe ({bar{n}}_{{rm{oq}}}({0}_{{rm{a}}})=0.56pm 0.02) for |α|2 = 0.2, but this value decreases for smaller |α|2, owing to the NPQD dark counts pDC. The simulation that considers pDC = 0 (dashed line), converges towards P(1oq|0a) = 52.3% for |α|2 → 0. This value is below one because of parasitic losses of the cavity mirrors, imperfect mode matching and a finite atomic decay rate. Interestingly, not only the mean photon number but also the photon statistics change after qubit reflection (Fig. 3b, inset). We prove this by measuring the second-order autocorrelation function at zero time delay, g(2)(0), of the reflected qubits conditioned on their nondestructive detection. The obtained sub-Poissonian statistics g(2)(0) < 1 originates from the distillation of single photons out of the incoming weak coherent pulse (see Supplementary Information and ref. 32).

To explore the potential of our NPQD, we discuss now four exemplary applications that would benefit from our detector (details in Methods). Figure 4a, b illustrates situations in which monitoring the qubit loss along a transmission channel saves time and precious resources. The first example (Fig. 4a) consists of an atom–photon entanglement source9 that sends photonic qubits to a heralded quantum memory26 to generate sender–receiver entanglement. The plot shows the entanglement speed-up defined as the ratio of the mean entanglement generation time without the nondestructive detector to that with the nondestructive detector, ({T}_{{rm{ent}}}/{T}_{{rm{ent}}}^{{rm{NPQD}}}), versus the channel length L. The position of the detector was chosen such that ({T}_{{rm{ent}}}^{{rm{NPQD}}}) is minimal. Our nondestructive detector (solid lines) outperforms direct transmission at channel distances greater than or equal to 14 km, whereas a perfect NPQD (dashed lines) would provide an advantage at any distance. The second example (Fig. 4b) describes a situation in which it is more efficient to perform a complex operation on an input photonic qubit only if this qubit has survived the transmission through a lossy channel. This can be accomplished by placing an NPQD right before the operating node. One example for a complex operation is quantum teleportation of the photonic qubit by means of previously established long-distance entanglement. A good figure of merit is the ratio of the probability of having a reflected qubit photon after nondestructive detection P(1oq|0a) to the probability of having an incoming qubit photon Piq, usually called qubit amplification24. The qubit amplification is substantially higher than one for Piq 1 and would notably improve in the absence of NPQD dark counts (dashed line).

Fig. 4: NPQD applications.

a, Entanglement generation between an atom–photon entanglement source and a heralded quantum memory (HQM). An NPQD along the transmission channel can speed up the mean sender–receiver entanglement time compared to the standard situation. The blue and green coloured lines represent the entanglement speed-up and the NPQD entanglement time, respectively. b, Photonic qubits are remotely sent to a receiver for a follow-up operation that involves precious resources (for example, long-distance entanglement). The qubit amplification gives the ratio of the probability of having a photon after its nondestructive detection at the NPQD to the probability of having a photon before detection (Piq). c, Photonic qubits transfer to a noisy qubit measurement set-up (QMS). Gating the QMS with the NPQD herald signal improves the SNR. d, Bell test based on precertification of the photon’s presence6. The dashed lines in the plots represent the situation of a perfect NPQD (ac) or a dark-count-free NPQD (b).

Another group of applications refers to situations in which NPQDs can improve a subsequent photonic qubit measurement. Figure 4c shows a scenario in which photonic qubits are sent to a remote receiver that uses noisy detectors for the qubit measurement. An NPQD right before the receiver enables measurement of the qubit only when it is not lost, reducing the impact of measurement noise. This is of particular interest for quantum key distribution, because the dark-count noise of classical detectors is an important limitation on the quantum key rate for large distribution distances14. The signal-to-noise ratio (SNR) gain is defined as the ratio of the SNR with an NPQD to that without an NPQD, SNRNPQD/SNR, and exceeds one for a sender–receiver distance L > 1 km. For longer distances, this ratio converges towards approximately seven, which could be raised with lower NPQD dark counts (illustrated by the dashed line, which considers perfect NPQD parameters). A last example relates to loophole-free Bell tests (Fig. 4d). As proposed previously6, the herald signals of NPQDs could allow two parties to be certain that they share an entangled photon pair right before the measurement, helping to close the detection loophole. The important parameter here is the photonic qubit reflection probability conditioned on its nondestructive detection. From our experiment we find ({bar{n}}_{{rm{oq}}}({0}_{{rm{a}}})=0.56pm 0.02) for an input |α|2 = 0.2 (Fig. 3b). This value exceeds the minimum detection efficiency of 43% (assuming no detector background noise) which is required for a detection-loophole-free asymmetric Bell test33.

In conclusion, we have demonstrated a nondestructive detector for photonic polarization qubits. We anticipate that it should also work with time-bin qubits. The main features are a conditional detection efficiency of up to (79 ± 3)% and a qubit preservation fidelity of at least (96.2 ± 0.3)%. None of the observed limitations seems fundamental. Most importantly, we present four possible applications that would benefit from our present device, to which we still expect to make improvements. This creates confidence that our detector will turn out to be a useful tool in near-future quantum communication links and fundamental tests of quantum physics.

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